A&A 646, A25 (2021)
https://doi.org/10.1051/0004-6361/202038421© S. Zhang et al. 2021

Astronomy&
Astrophysics

H II regions and high-mass starless clump candidates
II. Fragmentation and induced star formation at ∼0.025 pc scale: an ALMA
continuum study
S. Zhang ()1 , A. Zavagno1,2 , A. López-Sepulcre3,4 , H. Liu5,6,7 , F. Louvet8 , M. Figueira9 , D. Russeil1 ,Y. Wu10 , J. Yuan11 , and T. G. S. Pillai12,13

1

Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
e-mail: sijuzhangastro@gmail.com
2
Institut Universitaire de France (IUF), Paris, France
3
Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 38000 Grenoble, France
4
Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-D’Hères, France
5
Department of Astronomy, Yunnan University, Kunming 650091, PR China
6
CASSACA, China-Chile Joint Center for Astronomy, Camino El Observatorio #1515, Las Condes, Santiago, Chile
7
Departamento de Astronomía, Universidad de Concepción, Av. Esteban Iturra s/n, Distrito Universitario, 160-C, Chile
8
Departamento de Astronomía de Chile, Universidad de Chile, Santiago, Chile
9
National Centre for Nuclear Research, ul. Pasteura 7, 02-093 Warszawa, Poland
10
Department of Astronomy, Peking University, 100871 Beijing, PR China
11
National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, PR
China
12Institute for Astrophysical Research, 725 Commonwealth Ave, Boston University Boston, MA 02215, USA
13
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Received 14 May 2020 / Accepted 24 November 2020
ABSTRACT
Context. The ionization feedback from H II regions modifies the properties of high-mass starless clumps (HMSCs, of several hundred
to a few thousand solar masses with a typical size of 0.1–1 pc), such as dust temperature and turbulence, on the clump scale. The
question of whether the presence of H II regions modifies the core-scale (∼0.025 pc) fragmentation and star formation in HMSCs
remains to be explored.
Aims. We aim to investigate the difference of 0.025 pc-scale fragmentation between candidate HMSCs that are strongly impacted by
H II regions and less disturbed ones. We also search for evidence of mass shaping and induced star formation in the impacted candidate
HMSCs.
Methods. Using the ALMA 1.3 mm continuum, with a typical angular resolution of 1.300 , we imaged eight candidate HMSCs, including four impacted by H II regions and another four situated in the quiet environment. The less-impacted candidate HMSCs are selectedon the basis of their similar mass and distance compared to the impacted ones to avoid any possible bias linked to these parameters.
We carried out a comparison between the two types of candidate HMSCs. We used multi-wavelength data to analyze the interaction
between H II regions and the impacted candidate HMSCs.
Results. A total of 51 cores were detected in eight clumps, with three to nine cores for each clump. Within our limited sample, we did
not find a clear difference in the ∼0.025 pc-scale fragmentation between impacted and non-impacted candidate HMSCs, even though
H II regions seem to affect the spatial distribution of the fragmented cores. Both types of candidate HMSCs present a thermal fragmenthtation with two-level hierarchical features at the clump thermal Jeans length λth
J,clump and 0.3λJ,clump . The ALMA emission morphologyof the impacted candidate HMSCs AGAL010.214-00.306 and AGAL018.931-00.029 sheds light on the capacities of H II regions toshape gas and dust in their surroundings and possibly to trigger star formation at ∼0.025 pc-scale in candidate HMSCs.
Conclusions. The fragmentation at ∼0.025 pc scale for both types of candidate HMSCs is likely to be thermal-dominant, meanwhile
H II regions probably have the capacity to assist in the formation of dense structures in the impacted candidate HMSCs. Future ALMA
imaging surveys covering a large number of impacted candidate HMSCs with high turbulence levels are needed to confirm the trendof fragmentation indicated in this study.
Key words. stars: formation – stars: massive – HII regions – ISM: structure – submillimeter: ISM – techniques: interferometric

1. Introduction
The formation process behind high-mass stars (>8 M ) is still amatter of debate due to the unsolved questions coming from boththe observational and theoretical sides. Apart from the mechanisms that lead to the formation of high-mass stars, the roleof the environment and its impact on the future star-formation
properties are still poorly known.
Observationally, high-mass star-formation (HMSF) regions
are often situated at further distances (several kpc) and deeply
embedded in the molecular clouds. In these regions, the studyof embedded structures smaller than 0.1 pc commonly requires
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Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A&A 646, A25 (2021)

high-resolution interferometric (sub)millimeter or centimeter
observations.
Theoretically, the two widely discussed HMSF models,
namely, monolithic collapse (MC) and competitive accretions(CA), propose a different initial mass for the cores embeddedin the massive clumps (Zinnecker & Yorke 2007). The MCmodel proposes that massive stars form by disk accreting masses
from the hosted turbulent massive cores. This picture is qualitatively similar to the scale-up version of low-mass star formation(McKee & Tan 2003; Krumholz et al. 2009). The CA model
proposes that the massive clumps fragment into a number ofcores with Jeans mass, around one to a few solar masses, inthe thermal-dominant case. Furthermore, the cores located in thegravitational potential well center of the hosted massive clumps
are expected to more easily accrete masses to form massive stars(Bonnell et al. 2001, 2004). There are a number of the Atacama
Large Millimeter/submillimeter Array (ALMA) results that have
proven to be consistent with the CA model (Cyganowski et al.
2017; Fontani et al. 2018).
One of the key methods in differentiating the mechanismof initial HMSF is to study the fragmentation of massive
clumps (typical size of 0.1–1 pc) at their earliest evolution stage.
Whether prestellar cores (typical size of 0.01–0.03 pc) with several dozen solar masses exist and whether the cores hosted in thehigh-mass starless clumps (HMSCs) have a mass significantly
larger than the Jeans mass have been investigated by many studies. Louvet et al. (2019) summarized the only five high-mass
prestellar core candidates that had been detected up to the publication of their paper, namely CygX-N53-MM2 (Duarte-Cabral
et al. 2014), G11.92-0.61-MM2 (Cyganowski et al. 2014), G11.11P6-SMA1 (Wang et al. 2014), G028CA9 (Kong et al. 2017),
and W43-MM1-#6 (Nony et al. 2018; Molet et al. 2019). Thesesources would favour the existence of high-mass prestellar cores,
although a more specific check to see whether they are driving
outflows is needed. Anyhow, the scarcity of high-mass prestellar cores is still significant, suggesting that the MC is not likely
to be the common path leading to the formation of high-massstars. Recent observations with ALMA and Submillimeter Array(SMA) toward 70 µm quiet massive clumps (>300 M ) showthat most of the fragmented cores (0.02 pc scale) are low- tointermediate-mass objects (Li et al. 2019; Sanhueza et al. 2019;
Svoboda et al. 2019). For example, Sanhueza et al. (2019) identified a total of 294 cores in 12 candidate HMSCs (600 M to a
few thousand M ) and 210 of them are probably prestellar candidates, whereas only eight cores (two prestellar candidates + six
protostellar candidates) have masses of >10 M .
Both the MC and CA models of HMSF are expected to bemerged into a consistent and unified picture with the progress ofobservations and theories. The evolutionary scenario proposedby Motte et al. (2018) suggests that the large-scale (∼0.1 pc)
gas reservoirs, known as starless massive dense cores (MDCs) or
starless clumps, could replace the high-mass analogs of prestellar
cores (about 0.01 to 0.03 pc). The phase of high-mass prestellar
core could be skipped because massive reservoirs concentrate
their mass into high-mass cores at the same time the stellar
embryos are accreting. Louvet et al. (2019) tested this recent scenario by observing nine 70 µm quiet MDCs (0.1 pc, ∼100 M )in NGC 6334 using ALMA, with a resolution of 0.03 pc. They
reveal a lack of high-mass prestellar cores in these MDCs; therefore, this result supports the “skipped” prestellar phase and thescenario proposed by Motte et al. (2018).
We are interested in the impact of H II regions on the formation of high-mass stars. In particular, we want to study how theseregions impact and modify future (high-mass) star formation that
A25, page 2 of 28

occurs in their immediate surroundings. Previous studies showedthat at least 30% of HMSF in the Galaxy are located at theedges of the H II regions (Deharveng et al. 2010; Kendrew et al.
2016; Palmeirim et al. 2017). In our first paper (hereafter Paper I,
Zhang et al. 2020), we investigated the feedback of H II regions
on candidate HMSCs that are quiet from ∼1 to 70 µm andwithout any star-formation signposts (Yuan et al. 2017). Higher
resolution interferometric observations show that many of thesecandidate HMSCs are not absolutely starless because low- tointermediate-mass protostellar objects usually exist there (Fenget al. 2016; Traficante et al. 2017; Contreras et al. 2018; Sanhueza
et al. 2019; Svoboda et al. 2019; Pillai et al. 2019; Li et al.
2019). Using single-dish data observed by the Herschel andthe Atacama Pathfinder EXperiment (APEX), Paper I exploredhow H II regions modify the candidate HMSCs properties on
clump scale (0.1–1 pc). The results presented in Paper I showthat more than 60% of the candidate HMSCs are associatedwith H II regions and 30–50% of these associated HMSCs show
the impacts from H II regions. The heating, compression, andturbulence injection by H II regions make the impacted candidate HMSCs have a higher dust temperature (T dust , increment is
3–6 K), a higher ratio of bolometric luminosity to envelope mass
(L/M, from . 0.5 L /M increases to &2 L /M ), a steeper profile of radial column density NH2 ∝ rα (power-law index α at
approximately −0.12 decreases to approximately −0.27), and ahigher turbulence (from supersonic to hypersonic).
We consider whether these differences, created by
H II regions, could modify the early HMSF in massive
clumps. The findings of Paper I suggest that higher turbulenceand steeper clump radial density profile probably favour theformation of massive fragments by limiting fragmentation (forthe effect of turbulence, we refer to Mac Low & Klessen 2004;
for the effect of clump density profile, we refer to Girichidis
et al. 2011; Palau et al. 2014). In this paper, we study theproperties of fragmentation at a typical scale of ∼0.025 pc inthe candidate HMSCs under different environments (impacted
or non impacted by an H II region) to see whether differences
exist or not. We take advantage of the candidate HMSCs studied
in Paper I. In order to limit the distance and mass biases (similar
distance and mass for the pair of impacted and non-impacted
candidate HMSCs), we have compiled a relatively low numberof sources for this study, as explained in Sect. 2.
This paper is presented as follows: Sect. 2 describes the sample selection including a short description of the previous studies
for each source. Section 3 presents the ALMA continuum data.
We analyze the fragmentation in Sect. 4. In Sect. 5, we explore
the signposts of mass shaping exerted by H II regions on candidate HMSCs with the ALMA data. In Sects. 6 and 7, wediscuss and summarize the results. This paper mainly presents
the ALMA continuum results. The spectral analyses will bepresented in a forthcoming paper.

2. Sample selection
2.1. Sample selection

Through a process of cross-matching with star formation indicators such as infrared point sources from ∼1 to 70 µm, Yuan et al.
(2017) identified 463 candidate HMSCs in the inner Galactic
plane (|l| < 60◦ , |b| < 1◦ ) with 870 µm images from the APEX
Telescope Large Area Survey of the GALaxy (ATLASGAL,
Schuller et al. 2009). In Paper I, we cross-matched H II regions
and these candidate HMSCs (90% of them are more massive
than 100 M ) according to two main criteria: (1) the projected

S. Zhang et al.: H II regions and high-mass starless clump candidates

separation between the center of H II region and HMSC is lessthan double radii of H II region. (2) The velocity difference|∆vlsr | between the H II region and HMSC is <7 km s−1 , which
is derived from the standard deviation of the velocity difference between H II regions and HMSCs (see details in Sect. 4of Paper I). More than 60% of the candidate HMSCs are associated with H II regions and termed as AS candidate HMSCs or
AS. For the candidate HMSCs faraway from H II regions, they
are simply termed as NA (Non-Associated) candidate HMSCs orNA.
We made use of the ALMA data observed in ALMA Cycle
4 by project 2016.1.01346.S1 (PI: Thushara Pillai, project title:
Galactic census of all massive starless cores within 5 kpc). This
project, covering the entire inner Galactic plane, surveyed theGalactic massive starless core candidates within 5 kpc. Their
selected candidates are dark from near IR to 70 µm. We crossmatched candidate HMSCs in Paper I and sources for which their
ALMA data are public. A total of four AS candidate HMSCs
impacted by H II regions were extracted. To have a basis for acomparison with the non-impacted candidate HMSCs, we tentatively searched for additional four NA candidate HMSCs in thesame ALMA observing project source pool. To limit the bias andindependent variables, these NA are required to have the mostsimilar single-dish derived clump mass (Mclump ), column density(NH2 ), size (FWHM), ellipticity (e), and distance (D) comparedto AS counterparts in the same ALMA project. Other single-dishderived properties, such as T dust , L/M, turbulence (line width),density profile (power-law index α), and virial ratio are not considered when searching for the possible counterparts. The reason
is that the modifications of these physical properties made by the
impacts of nearby H II regions have been proven in Paper I. The
environment and single-dish properties of the selected candidate
HMSCs are shown and listed in Figs. 1, 2, and Table 1, respectively. The clump mass of the sample is approximately equal to
the necessary clump mass to form at least one high-mass starwith a mass of 8 M , which is 260 M –320 M , derived by
Sanhueza et al. (2017, 2019), using the initial mass function of
Kroupa (2001) and a star formation efficiency of 30% (Alveset al. 2007).
2.2. Single-dish derived properties and environment

Compared to the selected NA, the selected AS are warmer, moreluminous, steeper in density profile, more turbulent, and probably less gravitationally bounded as indicated by Table 1. These
differences are in line with the interpretations in Paper I, whichassert the compression and energy injection by H II regions playa role in modifying the clump-scale properties.
Pair 1: AS1 is located at the edge of IR dust bubble N1(Deharveng et al. 2010). N1 bubble is very close to the populous star cluster W 31-CL hosted in the W 31 giant H II region(see Fig. 1). The age of W 31-CL is about 0.5 ± 0.5 Myr
derived with near-IR data by Bianchin et al. (2019). With MAGPIS 20 cm continuum (Helfand et al. 2006) and H II region
expansion model of Tremblin et al. (2014), the estimated ageof N1 is about 0.4 ± 0.2 Myr with a pressure of about(7 ± 2) × 10−10 dyne cm−2 . NA1 has a similar single-dish massand beam-averaged NH2 to AS1. The FWHM of AS1 (2000 ) is
close to the resolution of ATLASGAL (19.200 ). The smallest size,
the highest surface density Σ, and small α clearly reflect the morecompact nature of AS1 compared to others.
1

http://almascience.eso.org//aq/?project_code=2016.1.
01346.S

Pair 2: AS2 is hosted in a filamentary cloud at the edgeof an open H II region mainly created by an O8.5 star
(Tackenberg et al. 2013). The question of whether triggered
star formation is happening in this clump has been exploredby the single-dish studies of Tackenberg et al. (2013). Theirresearch reveals the possible existence of gas shocked by the
H II region. These authors did not find the features of compression in the density profile with a resolution of 3700 . The
appearance of the “champagne flow” in the H II region prevents us from validly applying the expansion model of Tremblin
et al. (2014) on age estimation of the H II region. Alternatively,
the ionized gas pressure on the interacting filament is estimatedwith the electron density ne derived from 20 cm continuum
integrated flux (Martín-Hernández et al. 2005). The pressure is.(4.5 ± 0.5) × 10−10 dyne cm−2 .
A 24 µm bright source is in the neighborhood of NA2 (seeFig. 1). This IR bright source is probably an H II region considering the tight relation between 24 µm hot dust emission andcentimeter free-free continuum (Ingallinera et al. 2014; Makai
et al. 2017). No significant emission of MAGPIS 6 and 20 cm
continuum is detected toward this 24 µm bright source. Furthermore, the associated exciting OB star BD−15 4928 has a Gaia
parallax distance of 1.6 kpc (0.6148 mas, Gaia Collaboration
2018). Therefore, we propose that the 24 µm bright source is justa projected contamination.
Pair 3: AS3 is associated with H II region G022.495-00.261(Anderson et al. 2014) and only very weak (1–3 rms) radio continuum is detected towards this region. Dust temperature T dust of
AS3 is similar to its NA counterpart. As a result, AS3 is probably only weakly affected by the H II region. We note that AS3 is
probably not a starless clump owing to a 24 µm point source situated at an off-center position (see Fig. A.2). Svoboda et al. (2016)
identified it as a protostellar clump whereas Yuan et al. (2017)
and ALMA project of Pillai identified it as a starless candidate. With ALMA imaging, we are able to check the protostellar
properties of AS3.
A bright MAGPIS 90 cm continuum source is located nearNA3 with a separation of ∼2 pc. The limited size (<0.3 pc) andthe detection of CH3 OH maser (Avison et al. 2016) reveal thatthe centimeter bright source is an UCH II region thus it does nothave significant impacts on distant NA3.
Pair 4: AS4 is likely located in a bright rimmed structure
traced by 8 µm PAH2 emission (see Fig. 2). AS4 is in the giant
molecular cloud – massive star formation complex G333.0-0.5,
which hosts many bubbles and pillars. The SUMSS 35 cm continuum shows that AS4 is adjacent to two interacting H II regions
located on both western and eastern sides (Bock et al. 1999).
NA4 is probably located in the hub of three converging filaments
far away from any H II regions. NA4 is more massive than AS4
by a factor of 1.5, whereas their deconvolved surface density isquite similar.

3. ALMA dust continuum
3.1. ALMA band 6 data

We made use of the ALMA band 6 data (225 GHz, equal to1.33 mm) of 12 m main array and 7 m Atacama Compact Array(ACA) in the ALMA Cycle 4 project 2016.1.01346.S. Table 2
lists the main ALMA observing parameters. The sources wereobserved between March 2017 and June 2018 for the 12 m array,2

Polycyclic aromatic hydrocarbon (PAH) commonly exits in the photodissociation region (PDR; Fleming et al. 2010).
A25, page 3 of 28

A&A 646, A25 (2021)−0◦ 160−0◦ 200
170

Galactic Latitude

Galactic Latitude

210

180

190

220

230

200
240

1.0 pc
150

140

130

120

10◦ 1101.0 pc590

580

Galactic Longitude

570

560

9◦ 550Galactic Longitude−0◦ 230

◦

+0 00

0

◦

0

−0 01

Galactic Latitude

Galactic Latitude

240

020

250

260
030

270
04

1.0 pc

0

580

570

560

550

18◦ 540

Galactic Longitude

1.0 pc
320

310

300

290

15◦ 280

Galactic LongitudeFig. 1. Environment of candidate HMSCs. Each panel shows ATLASGAL 870 µm contours (blue, levels are [0.2, 0.3, 0.4, 0.5, 0.7,
0.9, 1.3, 1.8, 2.5, 4] Jy Beam−1 ) overlaid on RGB images constructed from Spitzer 24 µm (in red), 8 µm (in green), and 4.5 µm (in blue) images.
The candidate HMSCs are indicated by white crosses. Red contours indicate radio continuum emission, which are MAGPIS 20 cm for most of
the regions except for NA3 and AS4 (MAGPIS 90 cm and SUMSS 35 cm, respectively; see Fig. 2.). The root mean square (rms) of the radio
images are indicated in the bottom center of each panel. The radio contour levels are the radio image rms × [3, 4, 6, 8, 10, 15, 20, 30, 40, 60, 80]. The
beams of radio and ATLASGAL images are shown in bottom left by red and blue ellipses, respectively. The used survey data include the: Galactic
Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE, Churchwell et al. 2009); Multiband Imaging Photometer Galactic Plane Survey(MIPSGAL, Carey et al. 2009); Multi-Array Galactic Plane Imaging Survey (MAGPIS, Helfand et al. 2006), and Sydney University Molonglo Sky
Survey (SUMSS, Bock et al. 1999).

and between May and August 2017 for the 7 m array. The typicalprecipitable water vapor (PWV) is similar for all sources, except
for the main array observation of AS3, which is twice as large.
The time on source for each clump is 0.81 min for the main arrayand 3.53–6.05 min for the ACA.
3.2. ALMA data reduction

The data reductions were conducted using the same versions ofCommon Astronomy Software Applications (CASA, McMullin
et al. 2007) as those used in the QA2 pipeline products3 . Theband 6 receiver is equipped with lower and upper sidebands centering at around 217.83 and 232.00 GHz, respectively. The total
used continuum bandwidth is around 7.2 GHz.
3https://almascience.eso.org/processing/science-pipeline
A25, page 4 of 28

The 12 m main array and 7 m ACA data are combined by
CASA task CONCAT in order to create the 12 m + 7 m arraycombined continuum images (referred to as combined images or
combined continuum, hereafter). The line-contaminated single
polarization (XX) continuum is regridded to a channel width of
9 MHz in the frequency axis to accelerate the data process. The
CASA task TCLEAN is then performed on the line-free channels to finally obtain the cleaned images. The pixel size “cell”
in the cleaned image is set to be 0.300 , which is about one fifthof the synthesized beam '1.300 . A deconvolver using multi-term(multi-scale) multi-frequency synthesis is applied. The Briggs
weighting with a robust of 0.5 is chosen. The threshold for stopping clean process is set to be 0.4 mJy Beam−1 , about 2 to 3 rms
noise. Self-calibrations are not applied because the strongest
emission is just around a few dozen rms. The rms measured inthe 7 m + 12 m combined cleaned continuum images is around0.15 mJy Beam−1 (for NA2, AS3, and NA3) to 0.55 mJy Beam−1

S. Zhang et al.: H II regions and high-mass starless clump candidates−0◦ 160−0◦ 100

Galactic Latitude

Galactic Latitude

170
110

120

180

190
13

0

1.0 pc

1.0 pc

200

140
340

330

320

310

22◦ 300

500

490

Galactic Longitude

480

470

18◦ 460

Galactic Longitude−0◦ 430130

440

Galactic Latitude

Galactic Latitude

120

450

110

460
100

470

1.0 pc
030

020

010

333◦ 000

1.0 pc+0◦ 090332◦ 590

350

Galactic Longitude

340

330

320

29◦ 310Galactic LongitudeFig. 2. Environment of candidate HMSCs. The same as Fig. 1, but for another four candidate HMSCs.
Table 1. Single-dish derived properties of HMSC candidates.
r (a)
pc

Distance (b)kpc

Mclump (c)M

NH2 (c,d)1022 cm−2

Σ (e)g cm−2T dust (c)
K

L/M
L /M

α(f)

00

AGAL010.214-00.306 (AS1)
AGAL009.951-00.366 (NA1)

20
31

0.13
0.36

3.1 ± 0.2
3.1 ± 0.2

310 ± 90330 ± 906.6 ± 1.6
5.9 ± 1.5

1.29
0.16

16.6
12.0

2.1 ± 0.8
0.3 ± 0.1

−0.21 ± 0.18
−0.11 ± 0.06

3.6 ± 0.2S−C O
18
1.6 ± 0.4S−C O

0.8 ± 0.3
0.4 ± 0.3

AGAL018.931-00.029 (AS2)
AGAL015.503-00.419 (NA2)

42
38

0.60
0.51

3.3 ± 0.2
3.1 ± 0.4

330 ± 90320 ± 1103.2 ± 0.8
3.7 ± 0.9

0.06
0.08

18.2
14.0

4.2 ± 1.9
0.8 ± 0.3

−0.14 ± 0.08
−0.12 ± 0.08

3.6 ± 0.4F−C O
18
2.8 ± 0.8F−C O

3.6 ± 1.7
1.8 ± 1.6

AGAL022.531-00.192 (AS3)
AGAL018.801-00.297 (NA3)

31
34

0.60
0.66

5.0 ± 0.6
4.9 ± 0.3

900 ± 300
1160 ± 330

5.2 ± 1.3
7.0 ± 1.8

0.17
0.18

12.4
12.3

0.4 ± 0.1
0.3 ± 0.1

−0.15 ± 0.19
−0.12 ± 0.06

5.2 ± 1.3F−C O
18
2.7 ± 1.1F−C O

2.5 ± 2.0
0.6 ± 0.5

AGAL333.016-00.751 (AS4)
AGAL029.556+00.186 (NA4)

33
31

0.44
0.53

3.4 ± 0.4
4.5 ± 0.3

240 ± 80360 ± 100

3.8 ± 0.9
3.3 ± 0.8

0.08
0.08

15.6
12.8

1.3 ± 0.5
0.4 ± 0.1

−0.25 ± 0.25
−0.14 ± 0.08

3.8 ± 0.7H CO
13
2.8 ± 0.2F− CO

4.0 ± 3.0
1.8 ± 0.7

SourceFWHM (a)δv (g)
km s−1

Virial ratio
18

18

18

13

+

Notes. (a) FWHM is measured with ATLASGAL 870 µm continuum by Csengeri et al. (2014). r is derived from FWHM and distance but deconvolved with the beam of ATLASGAL image (19.200 ). See details in Yuan et al. (2017) and Paper I. (b) Distance is taken from Paper I. (c) Clump
mass Mclump , H2 column density NH2 , and dust temperature T dust are derived by fitting SED pixel-by-pixel to Herschel 160, 250, 350, 500 µm, andATLASGAL 870 µm by Yuan et al. (2017) but updated with the new distance by Paper I. (d) NH2 is the beam-averaged H2 column density takenfrom SED fitting resulted maps with a beam of 36.400 . (e) Surface density Σ is derived by the clump mass divided by π × r2 . The r is the deconvolved
radius in the third column. ( f ) α is the power-law index of H2 column density radial profile NH2 (r) ∝ rα , see details in Paper I. (g) The FWHM line
widths for the same pair of sources are taken from the same survey as possible as much. The abbreviation “S − C18 O” means SEDIGISM C18 O
J = 2−1 transition (Structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium survey, Schuller et al. 2017). “F −13 CO” and“F − C18 O” represent FUGIN 13 CO J = 1−0 and C18 O J = 1−0 transitions, respectively (FOREST Unbiased Galactic plane Imaging survey with
the Nobeyama 45 m telescope, Umemoto et al. 2017). “H13 CO+ ” is MALT90 H13 CO+ J = 1−0 transition (Millimetre Astronomy Legacy Team90 GHz, Jackson et al. 2013).
A25, page 5 of 28

A&A 646, A25 (2021)
Table 2. Information about the ALMA observations in this work.
Sourcerms (a,c)mJy Beam−1

Mass rms (a,d)
M Beam−1

Beam (a)00
× 00

Resolution (a)pc × pc

Config.

Baseline (b)mAntennas (b)Time (b,e)
mObservation period (b)MM/YYYYPWV (b)mm

Main array calibrators ( f )Flux & PhaseAS1

C: 0.55
A: 2.0

C: 0.15
A: 0.55

C: 1.80 × 0.99
A: 6.76 × 3.90

C: 0.027 × 0.015
A: 0.102 × 0.059

C43-2

M: 15–314
A: 9–49

M: 46
A: 8–11

M: 0.81
A: 6.05

M: 05/2018
A: 07&05/2017

M: 1.0
A: 0.9Titan & J1832-2039

NA1

C: 0.20
A: 1.0

C: 0.09
A: 0.45

C: 1.98 × 0.98
A: 6.98 × 3.96

C: 0.030 × 0.015
A: 0.106 × 0.060

C43-2

M: 15–314
A: 9–49

M: 46
A: 8–11

M: 0.81
A: 6.05

M: 05/2018
A: 07&05/2017

M: 1.0
A: 0.9Titan & J1832-2039

AS2

C: 0.16
A: 1.7

C: 0.04
A: 0.43

C: 1.46 × 1.02
A: 6.39 × 4.73

C: 0.023 × 0.016
A: 0.102 × 0.076

C43-2

M: 15–314
A: 9–48

M: 47
A: 8–10

M: 0.81
A: 5.04

M: 05/2018
A: 08/2017

M: 1.1
A: 0.9Titan & J1832-1035

NA2

C: 0.15
A: 1.2

C: 0.05
A: 0.40

C: 1.44 × 1.02
A: 6.35 × 4.70

C: 0.022 × 0.015
A: 0.097 × 0.072

C43-2

M: 15–314
A: 9–48

M: 47
A: 8–10

M: 0.81
A: 5.04

M: 05/2018
A: 08/2017

M: 1.1
A: 0.9Titan & J1832-1035

AS3

C: 0.15
A: 1.5

C: 0.16
A: 1.6

C: 1.29 × 1.04
A: 7.48 × 4.07

C: 0.031 × 0.025
A: 0.183 × 0.099

C43-2

M: 15–314
A: 9–49

M: 46
A: 9–12

M: 0.81
A: 4.03

M: 06/2018
A: 07/2017

M: 1.1
A: 0.8

Titan & J1832-1035

NA3

C: 0.15
A: 1.2

C: 0.15
A: 1.2

C: 1.48 × 1.04
A: 6.36 × 4.75

C: 0.035 × 0.025
A: 0.151 × 0.113

C43-2

M: 15–314
A: 9–48

M: 47
A: 8–10

M: 0.81
A: 5.04

M: 05/2018
A: 08/2017

M: 1.1
A: 0.9Titan & J1832-1035

AS4

C: 0.20
A: 1.0

C: 0.07
A: 0.35

C: 1.63 × 1.31
A: 6.19 × 4.55

C: 0.027 × 0.022
A: 0.102 × 0.075

C40-2

M: 15–287
A: 8–44

M: 45
A: 10–12

M: 1.01
A: 3.53

M: 03/2017
A: 05/2017

M: 2.7
A: 1.8

Titan & J1603-4904

NA4

C: 0.16
A: 1.2

C: 0.13
A: 0.98

C: 1.36 × 1.06
A: 7.18 × 4.26

C: 0.030 × 0.023
A: 0.157 × 0.093

C43-2

M: 15–314
A: 9–49

M: 46
A: 9–12

M: 0.81
A: 4.03

M: 06/2018
A: 07/2017

M: 1.1
A: 0.8

Titan & J1832-1035

Notes. (a) “C” and “A” in the second to fifth columns represent the information derived from cleaned continuum images of 12 m + 7 m combined
data and 7 m data, respectively. (b) “M” and “A” in the seventh to eleventh columns represent the observational information of 12 m main array and
7 m ACA, respectively. (c) The rms noise in the cleaned continuum images, see Sect. 3.2. (d) The rms mass sensitivity is estimated with the rms noisein cleaned continuum images, using Eq. (1). (e) Time on source. It is estimated by the CASA Analysis Utilities task “TimeOnSource”, see details
in https://casaguides.nrao.edu/index.php?title=TimeOnSource. ( f ) The 7 m ACA calibrators are different for different observations.
Flux calibrators: Titan, Neptune, and J1733-1304 for AS1 and NA1; Neptune and J1733-1304 for AS2, NA2, AS3, NA3, and NA4; Titan, Callisto,
and Ganymede for AS4. Phase calibrators: J1833-210B and J1832-2039 for AS1 and NA1; J1833-210B and J1733-1304 for AS2, NA2, and NA3;
J1851+0035 for AS3 and NA4; J1650-5044 for AS4.

(AS1), with a median value of 0.16 mJy Beam−1 . The imaged
field is down to 20% power point of the primary beam. The spatial resolution for each source is about 0.018 pc (∼3800 AU forNA2) to 0.029 pc (∼6100 AU for NA3) with a median value of∼0.025 pc.
Considering the larger synthesized beam (∼5.500 ) of the 7 m
ACA observations, we created 7 m ACA continuum images(written as 7 m images or 7 m continuum hereafter) to moreclearly show potential larger structures. The 7 m images are processed in a way similar to the combined images. The pixel size
and threshold for stopping clean process in task TCLEAN are
set to be 100 and 4 mJy Beam−1 , respectively. Other TCLEAN
parameters are similar to those for combined images. The rms
noise in the final cleaned 7 m images is around 1.5 mJy Beam−1 .
The information about ALMA observations is listed inTable 2. At 225 GHz, 12 m array and 7 m ACA have a primarybeam of 25.200 and 44.600 , sensitive to the structure smaller than
1100 and 1900 , respectively. Interferometric observations lose theemission from larger-scale structures. Assuming a dust emissivity spectral index β ' 1.5, we estimate the missed 1.3 mm fluxin 7 m + 12 m array observations by comparing with singledish ATLASGAL 870 µm flux in the ALMA imaged field. Our
7 m + 12 m combined ALMA data set recovers 10–20% of thesingle-dish integrated flux (minimum is 10% for NA2, NA3, andAS4, maximum is 19% for NA4). This value range is similar
to other ALMA observations toward HMSC candidates such asContreras et al. (2018); Beuther et al. (2018a); Sanhueza et al.
(2019), which resulted in a value from 10 to 30%. Most of thelarge-scale emission detected in single-dish telescope is lost byinterferometer, which implies that most of the masses in the earliest high-mass clump are more likely to be in large-scale diffuse
envelope rather than in compact cores.
A25, page 6 of 28

3.3. Core extraction and physical properties

We extracted the cores in the cleaned 7 m + 12 m combined
continuum images with the technique of Astrodendrograms4(Rosolowsky et al. 2008). Astrodendrogram algorithm uses atree diagram to describe the hierarchical structures over a rangeof scales. The cores we extracted correspond to the “leaves” inthe dendrogram, which means that there are no substructures forthese leaves. We set the minimum emission to be considered in
the dendrogam as 3 rms and the steps to differentiate the leaves
as 1 rms. The minimum size considered as an independent leaf
is set to be half of the synthesized beam. Table A.1 lists theprimary beam corrected properties of the cores identified with
Astrodendrogram.
Assuming that the dust continuum emission is optically thin,core mass is estimated with (Hildebrand 1983):
Mcore = Rgd

F ν D2,
κν Bν (T dust )(1)

where Fν , κν , and Bν (T dust ) are the measured integrated flux corrected by primary beam, dust opacity per gram, and the Planck
function at the frequency ν. The gas-to-dust mass ratio Rgd isassumed to be 100 in this work. The κν is set to be 0.9 cm2 g−1 ,corresponding to the opacity of the dust grains with thin ice mantles at a gas density of ∼106 cm−3 (Ossenkopf & Henning 1994).
The uncertainties of Fν are derived by multiplying image rms
noise with the area of the core. Distance uncertainties are takenfrom Table 1, which stem from the uncertainties in the distanceestimator developed by Reid et al. (2016). The combined uncertainties of Rgd /κν are around 30% according to the estimationsof Sanhueza et al. (2019).
4

https://dendrograms.readthedocs.io/en/stable/

S. Zhang et al.: H II regions and high-mass starless clump candidates

The uncertainties of T dust should be carefully treated. A
unified T dust for all cores embedded in one HMSC, which normally comes from single-dish clump-scale observations, is not
solid due to T dust gradients of the clump and different evolutionary stages of the cores. Dust temperature is expected to
decline towards the center of starless clump (Guzmán et al. 2015;
Svoboda et al. 2020; Zhang et al. 2020). The temperature could
drop to .10 K for the starless center (Lin et al. 2020). Besides,
protostellar cores or clumps are warmer than starless counterparts. The high-mass clumps which have mid-IR emission but
for which the embedded H II region has not developed yet, havean average T dust of 18.6 ± 0.2 K, about 2 K higher than that of thehigh-mass clumps quiet in mid-IR (Guzmán et al. 2015). In theabsence of T dust measurement at high resolution (∼100 ), we simply set three T dust values, which are single-dish derived T dust andT dust ± 3 K. A difference of 3 K is estimated from Guzmán et al.
(2015) and the T dust difference between candidate HMSCs close
to and faraway from H II regions in Paper I. The core mass for
cold
warm
lower and higher T dust is estimated (Mcoreand Mcore). The difcold
warm
ferences between Mcore and Mcore or Mcore could reach a levelof about 30–50%.
With spherical assumption, we calculate the core mass surface density Σcore by Mcore /πr2 . Further assuming a molecularweight per hydrogen molecule of µH2 = 2.8 (Kauffmann et al.
2008), the H2 number density (nH2 ) and column density (NH2 )are also estimated. All resulted physical parameters are listed inTable A.2. The identified cores and their mass are shown in the
middle panels of Figs. 3 and 4.

4. Fragmentation at ∼0.025 pc scale
4.1. Calculation

Fragmentation is a multi-scale process existing in the interstellar medium from ∼10 kpc-scale spiral arm of the galaxy
to ∼100 AU-scale protostellar system (Efremov & Elmegreen1998). Taking advantage of the ALMA spatial resolution of∼0.025 pc, in this work, we explore how the pc scale candidate
HMSCs fragment into ∼0.025 pc-scale dense cores.
In the simplest case, fragmentation is dominated by thermal
support in a non-magnetic, isothermal, homogeneous, and selfgravitating clump. Without turbulence, HMSCs are expected tofragment into cores with a mass around thermal Jeans mass (MJth )and a core separation around thermal Jeans length (λthJ ) (Jeans
1902; Mac Low & Klessen 2004):
MJth = 3

4πρ  λth
π5/2 σth 3
 J  =
,
p
3
2
6
G3 ρ

λth
J = σth

π
Gρ

(2)

!1/2
,

(3)

where ρ is the mass density and σth is the thermal velocity
dispersion:
kB T
σth =
µmH

!1/2
.

(4)

The mean molecular weight per free particle µ is set to be 2.37
because σth is governed by H2 and He (see Appendix B). The
sound speed cs = σth in this case.
In the case that takes into account both thermal and nonthermal motions, σth is replaced by the total velocity dispersion

σtot which is derived from the non-thermal velocity dispersionof the observed lines σnth,line and the sound speed cs by
σtot = (σ2nth,line + c2s )1/2 ,

(5)
1/2

where σnth,line = (σ2 − σ2th,line )(Palau et al. 2015; Li et al.
2019). Here, σ is derived from the observed line width δv in
Table 1 by σ = δv/(8ln2)1/2 . The σth,line is the thermal velocity
 k T 1/2
B
dispersion of the observed line σth,line = µmole. For µmole ,mH
13
the weight for different molecules, is 29 for CO and 30 for
C18 O and H13 CO+ .
The corresponding Jeans parameters are (Palau et al. 2015):!1/2
ππ5/2 σtot 3
tot
tot
,
λJ = σtot
.
(6)
MJ =
p
6
Gρ
G3 ρ
More specifically, considering the cases that the compressionof large scale supersonic flow creates density enhancement bya factor of Mach number square, M2 , the corresponding Jeansparameters are (Mac Low & Klessen 2004; Palau et al. 2015):
MJcom,flow =3

π5/2 σnth,lineπ, λcom,flow= σnth,line
p
J
3
6
Gρ
eff
G ρeff

!1/2
,

(7)

where ρeff √is the effective density ρeff = ρM2 and M is equal to
vflow /cs = 3σnth,line /cs .
The comparison between Mcore and Jeans mass is probably
more straightforward than that between core separations S coreand Jeans length. The actual separations are longer than or equal
to those separations derived from the images owing to the projection effect, which is the main uncertainty when using separations
to analyze the fragmentation. On average, the projected separations are shorter than the actual separations by a factor of 2/π(see math derivations in Appendix B). A unified clump-scale
T dust derived from the SED fitting for each core embedded in
the clump is a loose approximation for Mcore . Some cores havealready presented molecular emission, such as the outflow tracer
SiO J = 5−4 (see Sect. 4.5), indicating star-forming activities.
As a result, these cores are actually warmer and the core massderived at the clump-scale T dust is an upper limit. The warm corewarm
mass Mcoreis probably more appropriate for these cores. Bothcore mass and separation estimations have pros and cons andtherefore we analyze the fragmentation by combining both MJthand λthJ to attain a more consistent conclusion.
We derived the nearest neighbouring separations for all coresin each clump using the minimum spanning tree (MST) method.
It works by connecting a set of points with a set of straight lines
and ending up with a minimum sum of line lengths. The MST isfrequently used in the research of the spatial distribution of starforming objects, such as YSOs (Panwar et al. 2019; Pandey et al.
2020a,b) and dense cores (Dib & Henning 2019; Sanhueza et al.
2019). In particular, Clarke et al. (2019) have demonstrated that
MST is able to robustly detect the characteristic fragmentation
scales. We plot the core MST for each clump in the middle panelsof Figs. 3 and 4.
4.2. General trend shown by combined images

We show the core MST separations and mass scaled by clump
thermal Jeans parameters in Fig. 5. Most of the scaled core massth(Mcore /MJ,clump, the ratio of core mass to clump thermal Jeansmass) are less than one except for the most massive cores andA25, page 7 of 28

A&A 646, A25 (2021)−0◦ 180 0600

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4800

0600

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1200

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4200

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30600

Flux (mJy Beam−1 )Galactic Latitude

250 0000

1800

5

0
24

00

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2400

1800

1200

0600

300 0000 15◦ 290 5400

Galactic Longitude

0.1 pc
2400

1800

1200

0600

300 0000 15◦ 290 5400

Galactic Longitude

0.1 pc2000

1000

300 000015◦ 290 5000

Galactic LongitudeFig. 3. 7 m + 12 m combined 1.3 mm continuum. The presented images are uncorrected for the primary beam for displaying a homogeneous noisebackground. Four rows of panels: AS1, NA1, AS2, and NA2, respectively. ALMA emission (grayscale) is overlaid with the color-filled contours
starting from 3 rms to 21 rms with a step of 1 rms. Each row has three panels. Left panel: extracted cores (blue ellipses) and their Astrodendrogram“leaves” (red contours). Green dashed contours show the negative emission starting from −3 rms with a step of −1 rms. ATLASGAL emission
contours (black) are the same as Fig. 1. The synthesized beam (black ellipse) is shown in the bottom left. Central panel: core mass is shown by theorange circle whose area is in proportion to mass. The numbers close to cores are core indexes ranking from the highest to the lowest mass. The
highest and the lowest masses are indicated in the bottom left. MST short and long separations are shown in red and green lines, respectively, seemore in Fig. 5. Blue lines indicate λth
J,clump . Magenta arrows show the impacted direction. Right panel: ALMA 1.3 mm color contours overlaid on
GLIMPSE 8 µm background. See Sect. 5.1 for the meanings of yellow stars, crosses, triangles, and associated circles.
A25, page 8 of 28

S. Zhang et al.: H II regions and high-mass starless clump candidates−0◦ 110 1800

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42333
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1800

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3600

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0.1 pc
3600

3000

2400

1800

1200

Galactic Longitude29◦ 330 0600 4000

0.1 pc
3000

2000

1000

29◦ 330 0000

Galactic LongitudeFig. 4. 7 m + 12 m combined 1.3 mm continuum. The meanings of lines and marks are similar to those in Fig. 3 but for AS3, NA3, AS4, and NA4.

AS1, suggesting the dominant role of the thermal motions in the
fragmentation. Similar trend is also implied by the scaled core
MST separations (S core /λth
J,clump , the ratio of core separation toclump thermal Jeans length), which are less than one for mostsources except for AS1.
The distribution of S core /λth
J,clump presents a bimodal profilethwith S core /λJ,clump peaked at ∼0.3 and ∼0.8 in our limited sample.

Svoboda et al. (2019) found a similar bimodal distribution peaking at around 0.3 and 1 when surveying twelve candidate HMSCs(400 M –3600 M with a median mass of 800 M ) by ALMABand 6 observations. The physical scale they studied (0.015 pc
in 0.5 pc-scale clump) is similar to our cases. Their bimodal profiles that are corrected and uncorrected for projection effects areshown in Fig. 5. By checking with 7 m + 12 m combined images
in Figs. 3 and 4, the reason for the bimodal profile could be theA25, page 9 of 28

A&A 646, A25 (2021)(a). Length Ratio
corrected
uncorrected

10

Number

8

6

4

2

0
0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Score/λth
J,clump(b). Mass Ratio
AS1
NA1
AS2
NA2
AS3
NA3
AS4
NA4

10

Number

8

6

4

2

0

10−2

10−1

100
th
Mcore /MJ,clump

101

Fig. 5. Core separations and mass scaled by clump thermal Jeans parameters. Histograms with different colors represent different candidate
HMSCs. Panel a: scaled core separations S core /λth
J,clump . The hatchedhistograms indicate short separations that two cores are in the sameensembles (see Sect. 4.3). Black and red solid lines represent the distributions derived from Svoboda et al. (2019) by multiplying theirprobability density functions (PDFs) with the total number of coresin our cases. Black and red lines show the projected nearest neighborseparations and the projection-corrected nearest neighbor separations,
respectively, see details in Svoboda et al. (2019). Blue and red dash lines
are median values of short separations for NA and AS, respectively.
Similarly, dash-dotted lines indicate median values of long separations.
thPanel b: scaled mass Mcore /MJ,clump. The red and blue lines indicate
median values for AS and NA, respectively.

hierarchical fragmentation at two scales &0.1 pc and .0.05 pc.
The most significant case is AS2, which shows four groups ofcores with a separation of 0.1–0.2 pc and each group contains
two to three cores with separations of .0.05 pc, corresponding
to the double peaks in the bimodal profile.
4.3. Hierarchical fragmentation shown by 7 m data

To confirm the hierarchical nature of fragmentation, the
7 m + 12 m combined continuum overlaid with 7 m ACA
continuum contours are presented in Fig. 6. It is obvious thata number of very close cores are hosted in the same 7 m
iso-contours, indicating the sub-fragmentation from structures
outlined by iso-contours to cores. Here we term these 7 m
ACA emission structures as “ensembles”. We first identify“leaves” within Astrodendro in 7 m images uncorrected for the
A25, page 10 of 28

primary beam. The criteria of Astrodendro are looser than thoseused in the combined images (minimum emission = 2.5 rms,
step = 1 rms, and minimum size = half of the 7 m ACA synthesized beam). The information of all identified 7 m image “leaves”
could be found in Figs. A.3, A.4, and Table A.3. These 7 m image“leaves” are possible candidate ensembles, however, we classify
the “ensembles” in a more rigorous way here:
First, if the identified 7 m “leaves” have a major-to-minor
ratio less than 2.5, they are directly selected as ensembles. The
ensemble mass and density are calculated in a manner that issimilar to that in Eq. (1).
Next, for the identified 7 m “leaves”, which present a prolonged shape (the major-to-minor ratio >2.5), they are probably made of more than two individual candidate ensembles
if the ensemble peaks are too similar to be differentiated byAstrodendro. We carefully check the 7 m ACA continuum contours to search for the peak and subpeak for these prolonged“leaves” and then simply assume that these peaks and subpeaks
are the positions of the candidate ensembles. Taking into accountthe roughly equal peaks, we directly use the density of the“leaves” as the estimated density of each ensemble.
Lastly, we note that some 7 m ACA emission structures
are not identified in Astrodendero due to their low signal-tonoise ratio (S/N), whereas they host one or few low-mass cores
detected in the 7 m + 12 m combined images, such as the candidate ensemble W1 in NA2. We mark these weak structures by
eye and assume that they are also candidate ensembles. The massand density are not calculated due to their low S/N.
All the identified ensembles, their density, and thermal Jeansparameters are listed in Table 3. With regard to the calculationof the ensemble thermal Jeans length, λth
J,ens , we can see that
AS3 and NA3 are special because a number of their cores arenearly equally spaced in a prolonged leaf, invoking the cylindrical fragmentation. For an isothermal gas “cylinder”, it could
fragment into equally spaced cores under gravitational instability. The typical separation and mass in the case of cylindricalthermal fragmentation is:
−1/2
th
λth
, MJ,cylinder=J,cylinder = 22σth (4πGρc )22 σth 3,pπ1/2 G3 ρc(8)where ρc is the density at the center of the “cylinder” in virial
equilibrium (Chandrasekhar & Fermi 1953; Jackson et al. 2010;
Wang et al. 2014). We corrected the λth
J,ens of AS3 main leaf
according to Eq. (8). The ρc is assumed to be the minimum
density of the cores embedded in the filament, which is about
one magnitude larger than the filament average density, a resultthat is similar to other methods of estimations in studies of filament fragmentation, such as in Jackson et al. (2010), Wang et al.
(2014), and Lu et al. (2018).
Ensemble identification is not clear for NA3. The identified
7 m leaf has a major-to-minor axis ratio of >2.5 but the presented three subpeaks E2-S1?, E2-S2?, and E2-S3? cannot cover
all cores (see Fig. 6). NA3 is the most distant source in our sample (4.9 kpc compared to 3.1–3.4 kpc of other sources) except forAS3. In addition, the major axis of 7 m image synthesized beamroughly aligns with the filament. This evidence may shed lighton the possibility that its longer core separations correspond to
separations between ensembles rather than to core separationsin the same ensembles. To cover all possibilities, we considered
two cases for NA3: (1) the identified 7 m image “leaf” is mergedby few ensembles and, thus, it represents the fragmentation fromclump to ensemble. (2) The “leaf” is just one filamentary ensemble and, therefore, it represents the fragmentation from ensemble

S. Zhang et al.: H II regions and high-mass starless clump candidates−0◦180 0600

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130 0000

5400
4800
Galactic Longitude

4200

1200

−1

0.1 pc−210◦120 3600

1800

1200

0600
570 0000
Galactic Longitude5400

9◦560 4800

5−0◦010 3000

−0◦240 5400
4
1

3600

0

25 00

00

0600
2
1200

0

Flux (mJy/beam)

Flux (mJy/beam)

4800

Galactic Latitude

Galactic Latitude

3
4200

1

5400

1800

020 0000

2400

00.1 pc0600

560 0000

5400
4800
Galactic Longitude4200

0.1 pc18◦550 3600

2400

1800

1200
0600
Galactic Longitude

300 000015◦290 5400

4
−0◦110 1800−0◦170 3600

2

3600
1

4200

4800
1

54

00

180 0000

Flux (mJy/beam)

3000

Flux (mJy/beam)

4200

Galactic Latitude

Galactic Latitude

2

3

2400

0

0
48

00

0600

0.1 pc0600

320 0000

5400
4800
Galactic Longitude4200

0.1 pc22◦310 3600

1800

1200

0600
480 0000
Galactic Longitude5400

18◦470 4800

2400

−0◦440 4800

3

450 0000

0600
0

Flux (mJy/beam)

1

2

1200

0600

1200

110 00001800+0◦100 5400

100.1 pc333◦010 1800

1200

0600

0000
000 5400
Galactic Longitude

4800

4200

0.1 pc

−1
3600

3000

(filament) to core. In the second case, we apply the calculationof λthJ,ens that is corrected with Eq. (8).
Figure 7a shows the core separations scaled by ensemble thermal Jeans length (S core /λth
J,ens ) for the cores in thesame ensembles. The median S core /λth
J,ens is around 1–1.5. The

2400
1800
Galactic Longitude

1200

29◦330 0600

Flux (mJy/beam)

1800

Galactic Latitude

Galactic Latitude

54

00

Fig. 6. Hierarchical fragmentation.
Grayscale shows the primary beam
uncorrected 7 m + 12 m combined
images, overlaid with primary beam
uncorrected 7 m ACA continuum inblack (positive) and green (negative)
contours with levels of ±7 m image
rms × [3, 31.25 , 31.50 , 31.75 , 32.0 , 32.25 , 32.50 ,32.75 , 33.0 ]. Blue ellipses and yellow
contours show the “leaves” and theirshapes derived by Astrodendro. Largeand small triangles mark the positionsof candidate ensembles and cores,
respectively. Green and red lines are
MST core separations the same as thosein Figs. 3 and 4. Blue lines indicate theclump thermal Jeans length, λth
J,clump .

S core /λth
J,ens for AS3 and NA3 is significantly reduced from >2 to<2 in the case of cylindrical fragmentation. A maximum factorof 1.5λth
J means that there is a non-thermal velocity dispersion,
σnth , playing a role equivalent to thermal dispersion, σth . Even if
the ensemble-scale (<0.1 pc) line measurement is not presented
A25, page 11 of 28

A&A 646, A25 (2021)
Table 3. Jeans parameters for clumps and ensembles.
Sourceλth
J,clumppc

λtot
J,clumpλth
J,clumpλcom,flow
J,clumpλth
J,clumpth
MJ,clump

M

M tot
J,clumpM thJ,clumpM com,flow
J,clumpM thJ,clumpEns. (a)

M

pc

N
N
N

3.6
4.1
−

.7

0.013 ± 0.003
0.012 ± 0.002−∼2.5

0.030 ± 0.006 0.5 ± 0.2 1, 3
0.029 ± 0.006 0.5 ± 0.2 2
0.021 ± 0.005 0.3 ± 0.1 −0.026 ± 0.006 0.4 ± 0.2 4

1
2
3
4

0.9 ± 0.3 1, 3
1.1 ± 0.4 2, 8, 9
1.2 ± 0.5 4, 6
1.2 ± 0.5 5, 7

1
2
3
3

Subfrag. (b)Ens. nH2 (c)
6

10 cm

−3

Mens (c)

λth
J,ens

AS1

0.034 ± 0.007

6.8

0.58

0.8 ± 0.3

311

26

E1
E2
W1

NA1

0.14 ± 0.03

2.7

0.58

2.2 ± 0.8

19

3.5

E1
E2
E3
E4

Y
N
N
N

0.50
0.52
0.98
0.66

Y
Y
Y
Y

0.51
0.34
0.27
0.27

∼3.5

0.036 ± 0.007
0.044 ± 0.010
0.049 ± 0.012
0.049 ± 0.012

thMJ,ensCores (d)7 m Leaves (e)

M

ALMAn

ACAn

0.3 ± 0.1
0.3 ± 0.1
−

2
1
3

1
2
−

AS2

0.36 ± 0.07

5.7

0.58

8.6 ± 2.9

181

18

E1
E2
E3-S1
E3-S2

NA2

0.25 ± 0.07

5.7

0.58

4.6 ± 1.8

182

18

E1
E2
W1

Y
N
N

0.53
0.65
−

.4.9

0.031 ± 0.007
0.028 ± 0.007−

0.6 ± 0.2 1, 3
0.5 ± 0.2 −
2

1
2
−

AS3

0.18 ± 0.05

10.7

0.58

2.9 ± 1.1

1216

65

E1-S1
E1-S2
E2-S1
E2-S2

Y
N
N
N

0.41
0.41
0.25
0.25

.13

0.034 ± 0.007
0.034 ± 0.007
0.043 ± 0.010
0.043 ± 0.010

0.6 ± 0.2 1, 3, 4, 6, 8, 9
0.6 ± 0.2 7
0.7 ± 0.3 2
0.7 ± 0.2 5

1
1
2
2

NA3

0.18 ± 0.04

6.8

0.58

2.9 ± 1.0

320

26

E2-S1?
E2-S2?
E2-S3?
E2-U?

−
−
−
−

0.65

−

0.043 ± 0.01

0.7 ± 0.2

1?
2?
3, 4?
6, 7?

2

AS4

0.24 ± 0.07

9.4

0.58

5.0 ± 2.0

819

50

E1
W1

Y
N

0.2
−

∼5

0.054 ± 0.012−

1.1 ± 0.4−

1, 2, 4, 5, 6, 7, 8
3

1
−

NA4

0.24 ± 0.05

6.3

0.58

4.0 ± 1.4

251

22

E1-S1
E1-S2
E1-S3

Y
Y
N

0.37
0.37
0.49

∼15

0.036 ± 0.007
0.036 ± 0.007
0.031 ± 0.007

0.6 ± 0.2 2,3,5,7
0.6 ± 0.2 4, 6
0.5 ± 0.2 11
12

Notes. (a) Candidate ensembles. The nomenclature: (1) En. Candidate ensemble shown by 7 m image leaf ACAn in Table A.3. (2) En-Sm. Subpeak
m of 7 m image leaf ACAn in Table A.3. (3) Wn. Weak-emission ensemble. See details about three types of ensembles in Sect. 4.3. E2-S1?, E2-S2?,
and E2-S3? are uncertain subpeaks for NA3, as explained in Sect. 4.3. E2-U? is just a potential candidate because of the existence of two close
cores ALMA 6 and 7, but this candidate is highly uncertain. (b) Whether sub-fragmentation is detected. (c) Estimated ensemble number density andaverage mass, see details in Sect. 4.3. (d) Embedded cores identified in combined images. The “?” following the core index for NA3 means theassociation between cores and ensembles is uncertain, see Sect. 4.3. (e) 7 m image “leaves”, listed in Table A.3, correspond to candidate ensembles.

in this continuum study, other studies toward embedded structures with similar scale in HMSCs reveal a line dispersion of&0.5–2 km s−1 , which is twice larger than σth at least (Li et al.
2019). Another potential bias is that the mass and density of most
massive 7 m “leaves” are likely overestimated due to the existing star-formation activities (see Sect. 4.5). A dust temperature
improvement of 3 K in ensemble mass calculation could reducethe derived mass and density √by one third, leading to a new ther3/2λth
mal Jeans length λth,warm>
J,ens . Therefore, the thermalJ,ens
motions are more effective in sub-fragmentation from ensembles
to cores although other effects such as turbulence and magnetic
field may play a limited assistant role in the sub-fragmentation
process.
Figure 7b shows ensemble separations scaled by clump
thermal Jeans length (S ens /λth
J,clump ). The median value ofthS ens /λJ,clump is around 1. Nearly all S ens /λth
J,clump are less than
1.5 except for AS1 and NA1, confirming the dominance of thethermal motions in fragmentation from clumps to ensembles
for most candidate HMSCs. The mass of NA1 ensembles is inthlow-mass range (1–3 M ), similarly to MJ,clump. The turbulent
fragmentation model is inappropriate for most HMSCs because
both ensemble mass and separations are significantly smaller
tot
than MJ,clumpand λtot
J,clump . Taking the compression of supersonicflow into account, the resulting λcom,flow
J,clump , which is even smaller
A25, page 12 of 28

than λth
J,clump , gives a more contradictory picture than the one
given by the thermal fragmentation.
As a result, the early-stage fragmentation in the high-mass
clumps probably takes place in a hierarchical fashion: clumps
fragment into ensembles with separations around clump thermal
Jeans length, λth
J,clump , and then some of these ensembles continuously sub-fragment into cores with separations slightly larger
than ensemble Jeans length, λth
J,ens .
4.4. Considering possible differences between AS and NA

In this limited statistic, there is no clear overall trend that fragmentation between AS and NA has a significant difference. Thethth
S core /λth
J,clump , S ens /λJ,clump , S core /λJ,ens in Figs. 5 and 7 only show
a slight difference between AS and NA. The median values ofthMcore /MJ,clump
in Fig. 5 indicate that the cores in NA are probably more massive than AS, but we note that AS2 has much more
low-mass cores than NA2. The nine cores of AS2 in the lowmass end could be the reason for which the cores in type ASthhave a lower median Mcore /MJ,clump
when compared to those inNA.
For thermal fragmentation, a higher T dust results in a larger
MJth and λth
J if density ρ is roughly kept constant. A higher T dust
(3–6 K) and a similar ρ for AS, as compared to NA, have been

S. Zhang et al.: H II regions and high-mass starless clump candidates(a). Cores in Ensembles(b). Ensemble & Long separation

3.5

3.5
3.0
3.0
2.5

Sens /λth
J,clump

Score /λth
J,ens

2.5

2.0

2.0

1.5

1.5

1.0

1.0

0.5

0.5

0.0
0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

Separations for Cores in the Same Ensemble Score (pc)

0.0
0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Ensemble Separation Sens (pc)Fig. 7. Thermal dominant fragmentation. Red and blue filled stars represent the median values for AS and NA, except for AS3 and NA3, while
the stars with blank facecolor represent the median values for all AS and NA. The labels near the data points mark the ALMA cores (in the sameensembles) or ensembles making up the separations. Panel a: separations for the cores in the same ensembles, scaled by ensemble thermal Jeans
length λth
J,ens . The color-coding for different clumps is similar to that of Fig. 5. The circles with cyan and blue facecolors are the recalculations in
the case of cylindrical fragmentation for AS3 and NA3, respectively. Panel b: ensemble MST separations scaled by clump thermal Jeans length
λthJ,clump .

revealed with the single-dish results in Paper I. Taking advantage
of 58 candidate HMSCs located in the 3–5 kpc range given inthPaper I, the expected MJ,clumpand λthJ,clump for strongly impacted
AS (S-type AS, 31 in total, see Paper I) and non-impacted NA
thHMSCs (27 in total) are shown in Fig. 8. The peak of MJ,clump
distribution shifts from about 2 M for NA to 6 M for AS.
If most AS and NA HMSCs follow thermal fragmentation, thefragmented cores in AS are expected to be more massive thanNA on average. The study of eight candidate HMSCs in thispaper is a pilot and tentative exploration due to the very limited sample. An ALMA survey covering several dozens of AS
and NA candidates observed with the similar spatial resolution
is crucial to resolving the trend suggested in this work.
4.5. Diversity of fragmentation

In this section, we discuss in greater detail more the fragmentation of the candidate HMSCs, presenting diverse properties
besides the general hierarchical fragmentation found. A scaled
separation S core /λth
J,clump of ∼3 suggests that cores in AS1 probably result from not only thermal motions. Meanwhile, massive
cores probably exist in some candidate HMSCs. The most massive cores in AS1, AS3, NA3, and NA4 have a scaled massth
Mcore /MJ,clumpof 20, 6, 3, and 3, respectively. As mentioned in
Sect. 3.3, the evolutionary stage of cores is an important sourceof uncertainty when estimating Mcore because a large scaled mass
could be due to an underestimated T dust .
Notable case AS1. AS1 fragments into three massive
cores with mass from 10–17 M and a separation of '0.1 pc.
thBoth MJ,clumpand λthJ,clump show that its fragmentation cannot be
explained by thermal motions alone. The turbulent Jeans paramtot
eters in total support case (MJ,clumpand λtot
J,clump ) are much larger
than the observed parameters, whereas taking compressible flowcom,flow
into account gives a closer MJ,clump(∼20 M ) with a more con-th
tradict λcom,flow
J,clump (∼1/2λJ,clump ). It implies that turbulence alone
is equally insufficient to fully explain AS1 fragmentation. Introducing more physical factors, such as magnetic field support, andconsidering a different relative significance for each factor in the
fragmentation process could probably lead to the building of amore suitable scenario to explain this process (Tang et al. 2019).
A potential bias is that the worse rms mass sensitivity(0.15 M Beam−1 ) of AS1 combined image compared to that ofother candidate HMSCs would make us leave out more low-masscores. We image AS1 continuum with only 12 m array dataset,
resulting in an image rms of 0.2–0.3 mJy Beam−1 , which is better than combined image of AS1 (0.55 mJy Beam−1 ) and close
to combined images of other candidate HMSCs. Using the same
Astrodendro parameter, the number of identified cores in AS1
12 m image is the same as the one in the combined image. Therefore, we propose that the number of missed cores in AS1 is likely
at a level similar to other candidate HMSCs.
High-velocity components (>20 km s−1 ) of outflow tracer
SiO J = 5−4 (Louvet et al. 2016; Matsushita et al. 2019) aredetected within our data set toward cores ALMA1 and ALMA2,
indicating the stars are forming and still accreting mass there.
Meanwhile, thermal methanol lines CH3 OH 4(2, 2)–3(1, 2)–E at
218.44 GHz (Eu = 45.46 K), which probably trace the shockedgas (Zapata et al. 2012; Johnston et al. 2014), are also detectedwarm
toward both two cores. When taking the warm mass Mcore
as
warm
th
core mass, the scaled mass Mcore /MJ,clump is still around 8–13.
Combining it with the mass accretion and very early evolutionary stage of AS1 (70 µm dark), AS1 cores probably have thepotential to form high-mass stars. The highest clump surface
density compared to other candidate HMSCs, 1.3 g cm−2 , also
meets the high-mass star formation density criterion of 1 g cm−2
suggested by Krumholz & McKee (2008).

A25, page 13 of 28

A&A 646, A25 (2021)
0.5−0◦ 110 1000(a) Separations

0.4

0.2

0.1

15

3000

10
40

ALMA flux S/N

20

0.3
Galactic Latitude

Separation (pc)

2000

00

5

0.0

10

12

14

16

18

20

Tdust (K)5000

0.1 pc

1.5

AS1
NA1
AS2
NA2
AS3
NA3
AS4
NA4

(b) Core Mass

log10(M )

1.0

0.5

1000

320 0000

5000

4000

22◦ 310 3000

Galactic LongitudeFig. 9. AS3 24 and 70 µm emission. Green contours, star, and circle
indicate the 70 µm emission, its peak and beam, respectively. Background shows 24 µm emission with a peak at the red star. The beam of
24 µm image is shown in white circle. Color-filled contours show 7 m +12 m combined 1.3 mm emission with levels similar to those in Fig. 4.

0.0

−0.5

−1.0

10

12

14

16

18

20

22

Tdust (K)thFig. 8. Expected λth
J,clump and MJ,clump for 58 candidate HMSCs locatedin the 3–5 kpc distance range. Red and blue contours show the distrith
butions of MJ,clumpand λthJ,clump for all strongly impacted AS (31 S-type
AS, see details in Paper I) and all NA (27), respectively. Contours witha heavier color mean a larger number of points. Red and blue stars
mark the peaks of corresponding distributions. Panel a: distributions of
λthJ,clump . The triangles, crosses, and circles represent ensemble MST separations, core short MST separations, and core long MST separations for
the ALMA sources in this paper, respectively. Panel b: distributions ofthMJ,clump. Circles and triangles represent core mass and average ensemble
mass for the ALMA sources in this paper, respectively.

Star formation and warm gas indicators. A check onwhether massive cores show star formation signposts is beneficial to understand the overestimated mass and whether highmass prestellar cores exist. We will present a complete spectral
study of this ALMA data set in a forthcoming paper. Here we
only describe a few cases of significant spectral detection in our
regridded spectra.
Recent studies show that H2 CO mainly traces denser andwarmer gas (&30 K, ∼105 cm−3 ), such as warm gas in OMC-1(Tang et al. 2018) and cometary globule (bright-rimmed cloud
impacted by H II regions, Mookerjea et al. 2019). The detection of H2 CO is also related to shock chemistry corresponding
to outflow (Contreras et al. 2018; Gieser et al. 2019). The most
massive ensemble E1 in AS2 (see Fig. 6), containing cores
ALMA1 and ALMA3, is just in front of the PDR traced by PAH
8 µm emission, showing the gas and dust distribution shaped
A25, page 14 of 28

by the H II region. This structure will be discussed in moredetailed in Sects. 5.2 and 6.3. The small organic molecular thermometer H2 CO is only detected toward this ensemble while
there is no detection for other ensembles in AS2, suggestingthat ALMA1 and ALMA3 are in a warmer environment. H2 CO
is also detected in the most massive cores of NA2 (ALMA1,
Mcore ' 6 M ), NA3 (ALMA1, Mcore ' 10.3 M ; ALMA2,
Mcore ' 6.9 M ), indicating shocked gas or warmer gas, orboth. As a result, the mass of these massive cores could beoverestimated.
There are high-velocity components (&10 km s−1 ) of
SiO J = 5−4 detected toward the most massive core of NA4(ALMA1, Mcore ∼ 13 M ) meanwhile c-H2 COCH2 (Ethylene
Oxide, hot core tracer, see Ikeda et al. 2001) is detected towards
ALMA2 (Mcore ∼ 10 M ), showing that massive cores of NA4
are already at the protostellar stage.
AS3 presents a filamentary structure in ALMA 1.3 mm;
H2 CO is detected towards ALMA1, 3, 4, 8, and 9, indicatingthat the shocked gas or warm gas is present along the filament.
Meanwhile, SiO J = 5−4 is detected towards ALMA1. AS3 hasbeen identified as a 70 µm quiet clump with single-dish (sub)mm
images by Yuan et al. (2017); Traficante et al. (2018); Urquhart
et al. (2018), and the ALMA project of Pillai but a comparison
between ALMA image, MIPSGAL 24 µm (Beam ∼ 600 , Careyet al. 2009), and Herschel 70 µm (Beam ∼ 8.500 , Molinari et al.
th2010) reveals that ALMA1 (Mcore /MJ,clump∼ 6) is probably associated with an IR source at 24 µm (20.984 mJy, FWHM ' 6.2400 ,Gutermuth & Heyer 2015) and 70 µm (278 mJy, FWHM ' 10.100 ,Molinari et al. 2016) with an offset of 2.500 and 3.800 , respectively,
as shown in Fig. 9. The possible reason for ignoring the far IR
source for these authors could be the significant offset between
the IR source and the single-dish (sub)mm emission peak (seeFig. A.2).
To sum up, the fragmentation in most of the candidate
HMSCs in our sample is thermal-dominated except for AS1
which shows that only thermal motions or turbulence cannot

S. Zhang et al.: H II regions and high-mass starless clump candidates

fully explain its fragmentation. The most massive cores (Mcore >8 M ) in our candidate HMSCs commonly present star-forming
signposts, implying that there is no high-mass prestellar core in
our sample. Besides, we see that a large part (&60%) of coresthwith Mcore > MJ,clump
may already show star-forming signposts.
One possible reason could be the evolutionary bias that the
protostellar cores accrete more mass with evolution; another reason is that the T dust of these star-forming cores is likely to beinaccurate that is, in this case, underestimated and leads to an
overestimation of the core mass.

5. Mass distribution shaping revealed by ALMA
In the work discussed up through the last section, we did not
find a clear difference of fragmentation between AS and NA at∼0.025 pc scale in our limited sample. However, this does notmean that there is no implication of the impacts of H II regions
on dense ensembles or cores. In this section, we use multi wavelength data to search for signs of impacts from H II regions andcompare these with ALMA 1.3 mm emission.
5.1. Morphology of cores and clumps

Compared to millimeter interferometric observation, which isonly sensitive to compact emission, single-dish observation
could detect more diffuse and larger-scale emission from clump
envelope. ALMA observations recover 10–20% of the corresponding single-dish flux (see Sect. 3.2), showing that mostmasses of 70 µm quiet high-mass clumps exist in the envelope
rather than in the compact structures. Due to its lower density,
the clump envelope is expected to be more easily modified by
the compression of H II regions. Zhang et al. (2019) studied asample of eight impacted high-mass protocluster clumps witha mass and resolution similar to the ones in our study. Theseauthors proposed that the affected clumps present a morphology for which the embedded compact structures observed byinterferometer are expected to be closer to the interface of interaction because the clump envelope is compressed more distinctly
towards the interface.
The ATLASGAL flux peak, flux weighted clump center,and the ATLAGSAL pointing error are shown in the right panels of Figs. 3 and 4 with yellow crosses, triangles, and circles(radius = 400 , Schuller et al. 2009), respectively. The uncertainty
in pixel flux makes the flux peak taken from the sole pixel not
reliable as the flux weighted center. AS2 and AS4, which are
deeply impacted by the H II regions, present an asymmetrical morphology as shown by the systematic offset between flux
weighted clump center and mass weighted core center (marked
as yellow triangles and stars, respectively, in the right panels of
Figs. 3 and 4). The reason why deeply impacted AS1 does notpresent a clear such offset could be the density improvement
due to the clump evolution (the densest clump in eight candidate HMSCs) overwhelming the density structure modified by
the compression of H II region. AS3 is an exceptional case where
the compact structures are faraway from the interaction interface.
AS3 is only weakly affected as described in Sect. 2.2 therefore no significant compression of the clump envelope is seen.
The 3D distribution of the envelope may bias the conclusion by
projection effect, which could be found for the irregular center
positions for some NA sources.
Single-dish H2 column density NH2 maps are shown withpink contours in Figs. 10, 11, and 12 for AS1, AS2, and AS4,
respectively. The NH2 is derived from SED fitting of Hi-GAL 70,

160, 250, 350, and 500 µm emission (Herschel Infrared Galactic
Plane Survey, Molinari et al. 2010) with the techniques of point
process mapping (PPMAP) by Marsh et al. (2016), which utilize
the full instrumental point source functions to reach a spatial resolution of 1200 (Marsh et al. 2015, 2017). The Hi-GAL PPMAP
column density contours suggest that AS1 and AS2 probably
have an asymmetrical mass distribution while this asymmetry
is not clearly seen in the column density profiles cut along thedirection of the interaction (see panels named as “PPMAP Normalized NH2 ” in Figs. 10, 11, and 12). The PPMAP column
density peak and column density weighted clump center aremarked in Figs. 10, 11, and 12 and they show a similar trendthat AS2 and AS4 core centers are closer to the interaction
interference compared to the weighted clump center.
The PPMAP column densities are equally spaced in log
space between 8 K and 50 K (see panels named as “MultiT dust Normalized NH2 ” in Figs. 10, 11, and 12). Cold dust
(T dust < 21 K) normally shows a density profile similar to thatof the cumulative density of all dust temperature. Warm dust
(T dust & 21 K) profile presents a characteristic dip at the clump
center and a peak close to H II regions. Warm dust at 20–30 K
is located in the PDR surrounding the H II regions (Marsh &Whitworth 2019), thus, the asymmetrical warm dust distributions suggest stronger photodissociation, which is as expectedfrom the incoming radiation from ionizing stars.
5.2. Shaping structures in candidate HMSCs at ∼0.025 pc
scale

Among the four AS clumps of our sample, AS1 and AS2 areprobably most deeply impacted by H II regions, according to
the clump and cm free-free continuum morphology. The lack ofhigh-resolution cm continuum data for AS4 environment does
not allow for a detailed analysis of its associated ionized gas.
Evidence for gas and dust shaping in AS1. The 7 m +12 m combined 1.3 mm emission of AS1 likely emerges on a
dip of 20 cm continuum emission as shown in Fig. 10b. The
20 cm continuum of H II region is optically thin and dominatedby free-free emission of the ionized gas. Therefore, this dip is
not due to the extinction. To highlight the ionized gas and cold
dust distribution, we created a map of normalized 20 cm continuum intensity minus normalized combined 1.3 mm intensity,
as presented in the color scale in Fig. 13. It clearly shows an
overpressured ionized front (IF) that surrounds more than halfof the compact ALMA 1.3 mm emission, meanwhile, a morphology of cometary globule with short tail following the interacting
direction is clearly presented by 7 m data in Fig. 13 (Lefloch &Lazareff 1994; Tremblin et al. 2012). The 20 cm continuum profile in Fig. 10e reveals that an intensity plateau (with a widthof ∼ three beams) of 20 cm continuum profile appears on theinterface, implying that the expanding ionized gas is jammedon the clump envelope. A small fraction of ionized gas continues to leak into the envelope but stops at more compact inner
regions traced by combined 1.3 mm emission. A blue shift peakand strong blue wing of MALT90 optically thick HCO+ J = 1−0(beam ∼3800 ) are detected as shown in Fig. 14. The blue component may trace the gas shocked by H II regions or potential
infalling envelope of AS1 (Mardones et al. 1997; Evans 1999;
Zhang et al. 2016; Traficante et al. 2017).
We test whether AS1 is formed through the collect and collapse (C&C) mechanism, which describes a scenario in which
the supersonic expansion drives a shock front (SF) in front of the
IF and then a shell is collected between SF and IF. When the shell
A25, page 15 of 28

A&A 646, A25 (2021)
Jy/Beam
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8 K, 0.09
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18.4 K, 6.31
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25.7 K, 110.56
30.3 K, 3.15
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ALMA

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12

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0.0
−1.00 −0.75 −0.50 −0.25 0.00
0.25
Distance (arcmin)

0.50

0.75

1.00

Fig. 10. Impacted candidate HMSC AS1. Panel a: large-scale 20 cm continuum overlaid with the Hi-GAL PPMAP NH2 contours starting from1022 cm−2 with a step of 1022 cm−2 . The red bar indicates the pixels used to create intensity profile. Panel b: closer view of the black box in panel a,overlaid with 7 m +12 m combined 1.3 mm emission contours (white) starting from 3 rms with a step of 1 rms. The 20 cm continuum is indicatedby red contours starting from 3 rms with a step of 1 rms. The black dashed circle indicates the field of view (20% power point) of the 1.3 mm image.
The white bar indicates the pixels used for ALMA intensity profile. The yellow star, cross, and triangle are similar to those in Figs. 3 and 4, but for
the PPMAP NH2 . Yellow circles and white ellipse indicate the PPMAP and cm continuum beams, respectively. Panel c: normalized PPMAP NH2
and the 7 m + 12 m combined 1.3 mm intensity profiles cut along the direction indicated in panels a and b. The gray region highlights the ALMA
imaging field. Panel d: normalized profiles of NH2 with different T dust derived by PPMAP. The numbers after T dust values in the legend are the S/Nof the corresponding profiles. The S/N is set to be zero when <0.01. Panel e: 20 cm emission profile. The H II region, interface, and plateau are
indicated by different labels.

is too dense to be gravitational stable, the shell fragments into
several clumps (Elmegreen & Lada 1977; Zavagno et al. 2007).
The gravitational fragmentation of the collected shell is expected
7/11 −1/11 −5/11
to happen at time tfrag = 1.56C0.2
N49 n3Myr, where C0.2
is the sound speed inside the shell in unit of 0.2 km s−1 , which
is set as AS1 sound speed 0.24 km s−1 . N49 and n3 are the ionizing photon flux in unit of 1049 s−1 and the initial H number
density in unit of cm−3 , respectively (Whitworth et al. 1994; Liu
A25, page 16 of 28

et al. 2016). Taking the same N49 and n3 as those used in theestimations of bubble age and pressure, tfrag ' 0.9 Myr. The tfragis larger than bubble age '0.4 Myr, revealing that AS1 is a preexisting clump rather than a C&C triggered clump. Taken as awhole, AS1, presenting a cometary globule morphology, is likely
to be a pre-existing massive clump under the feedback of bubble
N1. Additional pressure from ionized gas possibly accelerates
the formation of protostellar cores in AS1.

S. Zhang et al.: H II regions and high-mass starless clump candidates
DN
100

150

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300

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Jy/Beam
0.0020 0.0025

0.00300.00350.0040
PPMAP
ALMA

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Galactic Latitude+0◦ 000−0◦ 010

0.6

0.4

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020

0.0
8 K, 0.11
9.5 K, 0.16
11.2 K, 0.29
13.2 K, 0.77
15.6 K, 2.69
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21.7 K, 57.27
25.7 K, 60.97
30.3 K, 0.89
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1.0

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580

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560 0000

4800

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18◦ 550 2400

0.0
−1.0

−0.5

0.0

0.5

1.0

Distance (arcmin)

Fig. 11. Impacted candidate HMSC AS2. Panel a: 8 µm emission. The pink contours have the same meanings as in panel a of Fig. 10. Black dashedrectangles indicate two PDRs. The red dashed contour shows the smoothed 20 cm continuum with a level of 3.3 rms. The yellow bar marks the
pixels used to create the intensity profile. Panel b: close view of panel a, overlaid with 7 m + 12 m combined 1.3 mm continuum contours (white)
starting from 3 rms with a step of 1 rms. Black solid and dashed circles represent the 7 m + 12 m combined fields of view cut at 20 and 10% power
points of primary beam, respectively. The white bar shows the pixels used for ALMA 1.3 mm profile. The light pink triangles mark the ALMA
cores. The lines connecting ALMA1, 3, 7, 4, and 6 indicate a wall-like morphology. The yellow star, cross (large), triangle, and circles are similar
to Fig. 10. Panel c: 20 cm continuum. The red bar indicates the pixels used to show intensity profile. Panel d: closer view of panel c. The red solid
contours indicate the 20 cm continuum with levels starting from 2 rms with a step of 1 rms. Panels e–g are similar in content to Fig. 10.

Evidence for gas and dust shaping in AS2. AS2 andits associated several-pc scale filament are proposed to be preexisting structures by Tackenberg et al. (2013). Whether triggered star formation is happening in AS2 is not clearly revealedby the single-dish research of Tackenberg et al. (2013). Our
ALMA mapping shows that the most massive core, ALMA1, isexactly located in front of the PDR traced by PAH 8 µm emission
as shown in Figs. 11b and g. We also note that several ALMA
cores are situated close to the PDR edge such as ALMA3, 4, 6,
and 7. These cores and ALMA1 reside in a wall-like morphology (indicated by the connected lines in Figs. 11b and d) whose
shape resembles the IF. The shape of the IF is indicated by thesmoothed contour of 20 cm continuum with a level of 3.3 rms in
Fig. 11. Considering the consistency between the IF shape andthe wall-like structure, the rapid decline of PAH 8 µm emission
towards ALMA1 is not only due to the IR extinction but also thedense photodissociation region situated at the interface between
the H II region and AS2.

Noisy 20 cm continuum (S /N . 4) prevents us from detecting tiny structures of the ionized gas surrounding the clump as
observed in Fig. 13. The profile of 20 cm continuum intensity cut
along the interaction direction does not present a clear plateau or
rapid decline at the interface between H II region and clump.
The smoothly decreasing intensity profile, without significant
fluctuation, is explained with the leaking ionization radiation
by Luisi et al. (2019). Leaking ionization radiation is the possible formation mechanism for the two PDR layers indicatedby 8 µm emission in Fig. 11a. The ionization radiation leaked
from the closer layer and then acted on further layer. These twolayers have an average velocity difference of 1–2 km s−1 , suggesting the possible kinematic difference driven by the feedbackof H II region.
The strongest 8 µm emission surrounding ALMA1 may
reveal the strongest impact of the H II region on ALMA1, whichis also suggested by the sole significant detection of warm orshocked gas tracer H2 CO in ALMA1. We check the 7 m + 12 m
A25, page 17 of 28

A&A 646, A25 (2021)
50

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ALMA

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460

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0.25
Distance (arcmin)combined 1.3 mm continuum in a slightly extended field of viewby lowering the cut limit of the primary beam from 20% to 10%
power point, as shown by the black circles in Figs. 11b and d. The
rms mass sensitivity in 20–10% power point of the primary beam
is around 0.2 M Beam−1 to 0.4 M Beam−1 . The minimum coremass that could be identified by Astrodendro is ∼0.6 M . No new
structure is found in the extended field of view, which meansthat the potential undetected structures in front of the most massive core, ALMA1, should be smaller than 0.6 M . Therefore,
we propose that ALMA1 and the other cores in the wall-likestructure are probably the closest to the IF. In combining all theevidence, some of the cores in the wall-like structure are likely
to have a triggered origin, especially for the most massive core,
ALMA1.

6. Discussion
6.1. Fragmentation mechanism

In our limited sample, a large population (>70%) of cores are
low-mass objects with a mass >2 M , meanwhile, the most massive cores (>8 M ) in candidate HMSCs commonly present starformation activities. The absence of high-mass prestellar coresin candidate HMSCs is fully revealed by a series of studies
with similar sampled scale of 0.02 pc to 0.03 pc, for example,
A25, page 18 of 28

0.50

0.75

1.00

Fig. 12. Impacted candidate HMSC
AS4. Panel a: 8 µm emission overlaid with PPMAP NH2 contours
with levels similar to Fig. 10. The
red bar shows the pixels used tocreate the profile. Panel b: closer
view of panel a, overlaid with 7 m +12 m combined 1.3 mm contours
with levels starting from 3 rms witha step of 1 rms. Panels c–e: similar
to those in Fig. 10.

Sanhueza et al. (2017, 2019); Contreras et al. (2018); Svoboda
et al. (2019); Louvet et al. (2019). It is not only core mass, butalso core separations that present a hierarchical thermal fragmentation, favouring the CA model for high-mass star formation.
The pc-scale candidate HMSCs fragment into several ensembles
with separations of clump Jeans length λth
J,clump and then some ofthe ensembles continue to fragment into cores with separationsof ensemble Jeans length, λth
J,ens .
Even if the core spatial distributions present a diverse morphology, from cluster-like (AS1 and AS4) to filamentary (AS3
and NA3) or simply irregular (AS2 and NA4), a common pattern
between the core and ensemble separations seems to exist within
the diverse morphology. The typical ensemble separation S ens
is ∼0.18 pc within a range of about 0.1–0.3 pc meanwhile thetypical core separation S core in the ensemble is ∼0.06 pc withina range about 0.04–0.08 pc, as shown in Fig. 7. It casts lighton the typical thermal Jeans length ratio of ensemble to clumpth
λth
J,ens /λJ,clump is . 1/3.
We collect the information from previous 7 m + 12 m
combined band 6 observations toward 31 high-mass clumps at
3–5 kpc (Contreras et al. 2018; Sanhueza et al. 2019; Svoboda
et al. 2019), which are quiet from ∼1 to 70 µm and have a massfrom 240 to 5200 M with a median of 760 M . The resolution is around 0.01 to 0.03 pc. The sensitivity and coverage of

S. Zhang et al.: H II regions and high-mass starless clump candidates

0.6

60

0.4

50

Long Separation
Short Separation
Total & Larger Bin
Sanhueza et al. (2019)2000

Svoboda et al. (2019)
Contreras et al. (2018)
This Paper

40

0.2

NumberGalactic Latitude−0◦ 180 1000

0.0

30

3000

20
−0.2
0.1 pc4000
10◦ 130 1000

0000

120 5000

104000

Galactic LongitudeFig. 13. Normalized intensity of AS1. This map shows the value of
normalized 20 cm intensity minus normalized 7 m + 12 m combined
1.3 mm continuum. White contours start from 3 rms with a step of
1 rms. The gray curve indicates the IF or the interface of interaction. Red
and black contours represent the 7 m + 12 m combined and 7 m alone
1.3 mm continuum with the levels similar to Figs. 3 and 6, respectively.

Fig. 14. Single-dish spectra of AS1. Red and black spectra show the
single-dish optically thick MALT90 HCO+ J = 1−0 and optically thin
MALT90 H13 CO+ J = 1−0 transitions, respectively. The spatial andvelocity resolutions are 3800 and 0.11 km s−1 , respectively. To improvethe depiction, the spectra have been regridded to 0.45 km s−1 . Greenlines are fitted Gaussian and its center.

these observations are all different. Contreras et al. (2018) and
Sanhueza et al. (2019) are mosaic observations covering fullclump with a sensitivity of ∼0.1 mJy Beam−1 whereas Svoboda
et al. (2019) and our data are single pointing observations witha sensitivity of ∼0.05 mJy Beam−1 and ∼0.15 mJy Beam−1 ,respectively. This data collection is statistically significant for
obtaining a universal fragmentation mechanism for candidate
HMSCs. Notably, nearly all collected candidate HMSCs are NA
besides the AS sample in this paper and G341.039–00.114 from

0
0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

MST Score /λth
J,clump
Fig. 15. Statistics of core MST separations for 31 candidate HMSCs.
The data have been taken from Contreras et al. (2018); Svoboda et al.
(2019); Sanhueza et al. (2019), and from the sources in this paper. Clump
properties for the candidate HMSCs in the single pointing mapping of
Svoboda et al. (2019) are taken from Paper I when possible, to improvethe consistency of the data. Therefore, the number distributions for thesources of Svoboda et al. (2019) here are slightly different from the raw
ones. There are a total of 363 separations. The size of the bin is set to be
0.075 (color-filled histogram) and 0.15 (black-edge histogram) to avoid
bias. Red and blue lines mark the two peaks of the distribution.

Sanhueza et al. (2019) because these authors usually set an upper
limitation on T dust , causing a bias of preference selection of NA.
Figure 15 shows the number distribution for total 363 MST
core separations scaled by clump thermal Jeans length λth
J,clump of
the collected sources. More than 70% of MST separations areless than λth
J,clump . The maximum and sub-maximum are peakedthat around 0.38λth
J,clump and 0.9λJ,clump , respectively. Firstly, theassumption of the unimodal distribution is excluded for Fig. 15,
by applying Hartigan’s Dip test (Hartigan & Hartigan 1985).
Then we test multimodal distributions by ACR methods (seedetails about ACR in Ameijeiras-Alonso et al. 2019) using the“modetest” function in the R package “multimode”5 . The test’s
results support the assumption that the bimodal distribution is
the most likely distribution in the scaled separation range of 0
to 1 in Fig. 15 and therefore prove the reliability of maximum
and sub-maximum marked in Fig. 15. Similar bimodal fragmentation nature presented in Fig. 15 for the samples of Svoboda
et al. (2019), Sanhueza et al. (2019), and this paper reveals thatdifferent coverage and sensitivity of ALMA observations do notlargely impact on the result of the bimodal fragmentation nature.
Although some low-mass cores could be lost in our singlepointing, slightly more noisy observation, the bimodal nature offragmentation revealed here is reliable.
As a result, most of the NA clumps fragment in a hierarchicaland thermal way, consistent with several studies of the high-mass
5

See more details on the “multimode” package in Ameijeiras-Alonso
et al. (2018).
A25, page 19 of 28

A&A 646, A25 (2021)

clumps at a later evolutionary stage. Klaassen et al. (2018) analyzed nine hypercompact H II regions with ALMA at a resolutionof .0.01 pc and found that the embedded cores are in line withthe consequences of thermal fragmentation. This result coincides
with that revealed in the CORE program (Beuther et al. 2018b),
which surveyed 20 young high-mass star-forming regions containing high-mass protostellar objects (HMPOs) and revealed
that turbulent fragmentation is less effective than thermal fragmentation at a resolution of .0.01 pc.
With the assumption that most of the candidate HMSCs are
dominated by thermal fragmentation, candidate HMSCs with asimilar density ρ but higher T dust are expected to fragment tocores with a higher mass and longer separation. In our limited
sample, it is only AS1 that obviously deviates from thermal fragmentation. The core separations and mass are at least three times
larger than the determined thermal values. Compared to othercandidate HMSCs, AS1 is probably the most evolved considering that its clump surface density Σ ∼ 1 g cm−2 is one magnitudelarger than others (∼0.1 g cm−2 ). A higher core mass is probably due to evolutionary effects. An observational comparison
between high-mass prestellar clumps and high-mass protostellar clumps with ALMA at a resolution of 0.02 pc shows that thefraction of total mass in cores to clump mass increases from .0.1
to &0.2 (Neupane et al. 2020), agreeing with the simulationsof Molinari et al. (2019). Notably, core separations in AS1 aresimilar to that in NA1 or other candidate HMSCs, at a scale of.0.1 pc, therefore the plausible conclusion of multi-factors dominated fragmentation in AS1 derived from equations in Sect. 4.1
is probably biased. AS1 contracts more significantly due to thepressure of H II regions or the evolution, increasing clump Σ toth
reduce the derived λth
J,clump , making it quite different from λJ,clumpat an earlier stage. Even if T dust also increases with feedback of
H II region or evolution, the change fraction is only ∼30% to50%, much smaller than the increase in Σ. It is possible that AS1
had a thermal fragmentation when observed at an earlier stage.
6.2. Transition of fragmentation mechanism

Fragmentation is a process that is determined by a multitudeof factors. The relative importance between gravity, thermal
motion, magnetic field, and turbulence can lead to a different fragmentation (Tang et al. 2019). The dynamics of 70 µm
quiet massive clumps are believed to change from turbulent togravity-dominant when they approach a critical Σ of ∼0.1 g cm−2
(Traficante et al. 2020). One problem is that the turbulence levelof AS is obviously increased by the feedback of H II regions,as shown in Paper I. Therefore Σ needs to be higher for AS
in order to keep thermal fragmentation effective. AS2 and AS4
still present a thermal fragmentation when the turbulence level
increases by less than 60% (traced by line width, see Table 1),compared to their counterparts NA2 and NA4, which implies
that turbulent fragmentation at ∼0.025 pc scale requires hugeenergy injection from H II regions.
Rebolledo et al. (2020) likely found the transition from thermal fragmentation to turbulent fragmentation under the effect of
H II regions by comparing a pair of pc-scale massive star forming clumps in the Carina region, one is severely impacted by
the H II region and the other is less disturbed. Mach number
M derived from ALMA + ATLASGAL combined images withthe techniques of densities probability distributions (N-PDF)
are 1.37 (supersonic) and 0.81 (subsonic) for impacted one andless disturbed one, respectively. The 0.02 pc-scale fragmented
cores in the more impacted clump are less numerous and five
A25, page 20 of 28

times more massive than the cores in the less disturbed clump.
The differences are explained with the different fragmentation
mechanisms (turbulent and thermal) by Rebolledo et al. (2020).
Notably, our previous single-dish spectral analyses in Paper I
suggest that most of candidate HMSCs tend to be supersonicwhile the impacted ones prefer a hypersonic status (M & 5). At
this stage, it is not clear whether this turbulence increment couldchange a fragmentation from thermal-dominated to turbulentdominated. The fragmentation at 0.01 pc scale in the most
massive star-forming clump (M ∼ 4) impacted by the H II region
RCW 120 (Kirsanova et al. 2019; Zavagno et al. 2020) is investigated by Figueira et al. (2018) and they suggested that turbulent
fragmentation is dominant due to the H II region feedback. Evenfor the cases where the fragmentation at around 0.02 pc scale inthe clumps is still thermal-dominated under the vigorous feedback of H II regions, the fragments could be more massive or
less numerous (Liu et al. 2017).
Magnetic fields make the fragmentation more complex. The
presence of filamentary structure in candidate HMSCs such as
AS3 may arise from a highly magnetised environment according to Fontani et al. (2016, 2018). Some studies have revealed theimportant role of the magnetic field in the evolution of clumps
impacted by H II regions, such as in bright rimmed clouds (Soamet al. 2017, 2018). Eswaraiah et al. (2020) studied the B fields ofmassive clumps located at the waist of the bipolar H II region
S201. These authors found that the B fields of clumps are compressed and enhanced by the impact of the H II region, leading
to a status characterized by the magnetic pressure dominatingover the turbulent and thermal pressure. Busquet et al. (2016)
explored the fragmentation of twin filament hubs at a scale of
0.03 pc. These two hubs are similar in mass, density, turbulence,and even temperature (2 K difference). The hub more impacted
by the H II region shows a lower fragmentation level comparedto the less impacted one. Busquet et al. (2016) explained it asa stronger magnetic field suppressing the fragmentation in the
more impacted hub.
To summarize, the fragmentation of candidate HMSCs
at ∼0.025 pc scale is dominated by thermal fragmentation.
Turbulent or magnetic fragmentation (or both) may happen only
in some AS HMSCs that are highly impacted by H II regions.
A specific follow-up ALMA survey towards AS HMSCs withbroad single-dish line widths, which imply an extremely turbulent status, is needed to answer whether H II regions drivea turbulent fragmentation at ∼0.025 pc scale. Besides, ALMA
observations of polarized dust emission are also needed to ascertain the role of magnetic field in the fragmentation of AS
HMSCs (Beuther et al. 2018a; Dall’Olio et al. 2019; Cortes et al.
2019).
6.3. Considering induced star formation in HMSCs at∼0.025 pc scale

AS1 and AS2 clearly show how the H II regions shape the morphology of dense material distribution in pre-existing candidate
HMSCs. The cometary globule morphology of ALMA dense
emission of AS1 that is located just at the edge of the H II region
probably reveals the classical triggered star-formation mechanism, with radiation driven implosion (RDI) at work here. The
RDI presents a picture where the UV photons ionize the surface of a pre-existing clump to form an overpressured ionizedlayer and then help the clump to collapse (Lefloch & Lazareff
1994; Kessel-Deynet & Burkert 2003; Miao et al. 2009). The
young associated bubble N1 (∼0.4 ± 0.2 Myr) likely came across
AS1 during the expansion and then the ionized gas of the bubble

S. Zhang et al.: H II regions and high-mass starless clump candidates

interacted with the recently ionized clump surface to form theionized layer observed in Fig. 13. Additional pressure from theionized layer probably promotes the collapse. IR-quiet AS1 is
expected to have an age of 0.05–0.1 Myr according to the statistical summary of the starless MDCs by Motte et al. (2018).
We assume that the IF of the bubble just propagated beyond
half of AS1 with an averaged bubble expanding velocity of.2 km s−1 , which is estimated from bubble size divided by age.
The interaction time between the H II region and AS1 is around
0.04–0.13 Myr. This roughly agrees with the expected age of
AS1. Following all the evidence found here, we suggest that thecores in AS1 probably have a triggered origin.
AS2 presents an impacted morphology different from AS1.
H II region is shaping a pre-existing 2–3 pc scale filament.
The alignment between IF and dense gas from several-pc-scalefilament, one-pc clumps to 0.02 pc-scale cores, reveals the multiscale shaping effect of the H II region. All cores in AS2 are
low-mass cores (.2 M ). Tackenberg et al. (2013) found that
AS2 single-dish H13 CO+ spectra have two velocity components
with a shift of ∼2 km s−1 (see Fig. 11 in their paper) andsuggested that these two components are emission from preexisting clump gas and shocked gas, respectively. The intensity
ratio between the two components indicates that the shock does
not propagate beyond the single-dish density peak of AS2. The
stopped shock is explained by Tackenberg et al. (2013) with theresistance of the highest density region. With ALMA images,
the shock probably has propagated beyond the wall-like structureand stopped at the further ensemble of cores shown in Fig. 11.
We propose that some of the cores in wall-like structure probably have an induced origin. Follow-up ALMA spectral mapping
towards these cores would help us to determine the evolutionary stage of each core and, thus, to demonstrate their triggered
origin. For example, if the cores in the wall-like structure are
more evolved than other less impacted cores such as ALMA 2, 8,
and 9, which may indicate that the impacted cores formed earlier
under the impacts of H II regions.
In short, ALMA data of AS1 and AS2 reveal that H II regions
are able to modify the dense structures at ∼0.025 pc scale incandidate HMSCs, which may help the formation of stars in thiskind of early stage clumps.

7. Conclusion
Our study is based on an ALMA continuum investigation of theionization feedback on ∼0.025 pc-scale fragmentation in candidate high-mass starless clumps (HMSCs), in which we exploredfour pairs of candidate HMSCs, including four clumps affectedby H II regions (referred to as AS HMSCs) and another four thatwere located in quiet environment (referred to as NA HMSCs).
The AS and NA in each pair are required to be similar in massand distance to avoid any possible bias related to these quantities. Using the ALMA 1.3 mm continuum, with a resolution of∼0.025 pc, we drew the following conclusions:
1. We did not find a clear difference in the fragmented ALMA
core mass and separations between AS HMSCs and NA
HMSCs in our limited sample, although H II regions seemto affect the morphology of the cores’ spatial distribution.
2. At ∼0.025 pc scale, thermal Jeans fragmentation with hierarchical nature dominates the fragmentation of most of theparsec-scale candidate HMSCs. The candidate HMSCs fragment into ensembles with separations of clump Jeans length,
λthJ,clump , and then some of the ensembles continue to thermally fragment into cores with separations of about one thirdof the λth
J,clump .

HII Region Feedback

Cores

Induced starformation?
Shaping core
distribution?
Turbulentfragmentation?

Ensembles
Clump

Core Separation Score
Ensemble Separation Sens

Diagram of Fragmentation in HMSCs

Fig. 16. Proposed scenario for ∼0.025 pc-scale fragmentation in theimpacted candidate HMSCs. Σclump and ρclump are the clump surface
density and volume density, respectively, while Σens and ρens are the corresponding values for ensemble. Clump and ensemble thermal Jeansthlengths are λth
J,clump and λJ,ens , respectively. Whether and how induced
star formation and turbulent or magnetic fragmentation take place inextremely impacted HMSCs need more studies.

3. AGAL010.214-00.306 (AS1), which is affected by the
H II region, is the only candidate HMSC in our eight sources
where other factors beside thermal motions could play animportant role in the fragmentation process. The fragmented
core mass (10–17 M ) and separations (∼0.1 pc) are at least
three times larger than the thermal-determined values.
4. H II regions are able to modify the dense material distributions embedded in candidate HMSCs, which probably helps the formation of next-generation stars. AS1 and
AGAL018.931-00.029 (AS2) are the two most-likely casesin our sample which indicate signs of impact from the
H II region. ALMA continuum reveals that AS1 presents acometary globular morphology meanwhile the fragmented
cores in AS2 show a spatial distribution aligned with theionization front.
Figure 16 tentatively draws a picture of hierarchical, thermaldominated fragmentation for candidate HMSCs under the
impacts of H II regions. Future ALMA polarized imaging towarda large sample of highly-turbulent candidate HMSCs (with thebroadest lines in single-dish spectra) that are strongly impactedby H II regions will ascertain the role of thermal fragmentation in candidate HMSCs and shed light on whether turbulent
or magnetic fragmentation could take over in extreme cases.
Acknowledgements. S.Z. thanks to the funding of ALMA Region Center
Grenoble and IRAM. A.Z. thanks the support of the Institut Universitaire deFrance. T.G.S.P. gratefully acknowledges support by the National Science Foundation under grant No. AST-2009842. We want to thank the anonymous referee
for the constructive comments that helped to improve the quality of the paper.
Herschel Hi-GAL data processing, map production and source catalogue generation is the result of a multi-year effort thanks to Contracts I/038/080/0 andI/029/12/0 from ASI (Agenzia Spaziale Italiana). This paper makes use of thefollowing ALMA data: ADS/JAO.ALMA#2016.1.01346.S ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan),
together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA
Observatory is operated by ESO, AUI/NRAO and NAOJ.

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S. Zhang et al.: H II regions and high-mass starless clump candidates

Appendix A: Supplementary data

Figures A.3 and A.4 show the 7 m array continuum andAstrodendro leaves extracted in the 7 m images. The parametersof leaves are listed in Table A.3.

Figures A.1 and A.2 present GLIMPSE 8 µm, MIPSGAL
24 µm and Hi-GAL 70 µm emission of the candidate HMSCs’
region. The cores identified with Astrodendro and the derived
physical parameters are listed in Tables A.1 and A.2, respectively.

−0◦ 170 2000

Galactic Latitude4000

180 0000

2000

4000

190 0000

0.1 pc0.1 pc0.1 pc

2000
4000
◦

0

−0 21 002000

130 0000

4000

2000

10◦ 120 0000

4000

2000

130 0000

4000

2000

10◦ 120 0000

4000

2000

130 0000

4000

2000

10◦ 120 0000

00

Galactic Latitude

2000

4000

220 0000

2000

4000

0.1 pc580 0000

4000

2000

570 0000

4000

0.1 pc9◦ 560 2000

580 0000

4000

2000

570 0000

4000

0.1 pc9◦ 560 2000

580 0000

4000

2000

570 0000

4000

9◦ 560 2000

Galactic Latitude−0◦ 010 0000

2000

4000

020 00002000

0.1 pc

4000
40

00

20

00

0

56 00

00

40

00

20

00

◦

0

18 55 00

0.1 pc
00

40

00

20

00

0

56 00

00

40

00

20

00

◦

0

18 55 00

0.1 pc
00

40

00

20

00

0

56 00

00

40

00

20

00

18◦ 550 0000

−0◦ 240 2000

Galactic Latitude

4000

250 0000

2000

4000

260 0000

0.1 pc
310 0000

4000

2000

300 0000

4000

Galactic Longitude15◦ 290 2000

0.1 pc
310 0000

4000

2000

300 0000

4000

Galactic Longitude15◦ 290 2000

0.1 pc
310 0000

4000

2000

300 0000

Galactic Longitude

4000

15◦ 290 2000

Fig. A.1. IR emission of candidate
HMSCs’ region. Each row shows
8, 24, and 70 µm emission foreach source. ATLASGAL 870 µm
emission is shown with black contours with levels the same as
Fig. 1. The black circles indicate
the beam size of IR images, except
for 8 µm images, owing to theirsmall beam size ('200 ).

A25, page 23 of 28

A&A 646, A25 (2021)−0◦ 100 4000

Galactic Latitude

110 0000

2000

4000

120 00002000

0.1 pc4000

2000

320 0000

4000

2000

0.1 pc22◦ 310 0000

4000

2000

320 0000

4000

2000

0.1 pc22◦ 310 0000

4000

2000

320 0000

4000

2000

22◦ 310 0000

−0◦ 170 0000

Galactic Latitude

2000

4000

180 0000

2000

4000

0.1 pc490 0000

4000

2000

480 00004000

18◦ 470 2000

0.1 pc490 0000

4000

2000

480 00004000

18◦ 470 2000

0.1 pc490 0000

4000

2000

480 00004000

18◦ 470 2000

Galactic Latitude−0◦ 440 2000

4000

450 0000

2000

4000

0.1 pc0.1 pc0.1 pc460 0000
4000

2000

010 0000

4000

2000

333◦ 000 0000

4000

2000

010 0000

4000

2000

0 0000333◦ 00
020 00
00

4000

2000

010 0000

4000

2000

333◦ 000 0000

120 0000

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4000

2000

110 0000

4000+0◦ 100 2000

0.1 pc2000

340 0000

4000

2000

330 0000

Galactic Longitude

A25, page 24 of 28

4000

00 00
29◦ 32200 200.1 pc
340 0000

4000

2000

330 0000

Galactic Longitude

4000

29◦ 32020
2000000.1 pc
340 0000

4000

2000

330 0000

Galactic Longitude

4000

29◦ 320 2000

Fig. A.2. IR emission of candidate HMSCs’ region. The meanings of each panel are similar to
Fig. A.1 but for different HMSCs.

S. Zhang et al.: H II regions and high-mass starless clump candidates
Table A.1. Astrodendrogram results for the combined images.
Clump

Core (a)

p

p

l
( )

b
( )

major (b)(00 )minor (b)
(00 )

PA
(◦ )

R (b)
(00 )

F (c)
(mJy)

F1 (d)(mJy)

F2 (d)(mJy Beam−1 )

S/N

◦

◦

AGAL010.214-00.306
(AS1)

ALMA1
ALMA2
ALMA3

10.2134
10.2153
10.2153−0.3049−0.3057−0.3038

1.66
1.39
1.75

0.96
1.00
1.13

90.8
64.6
142.5

1.26
1.18
1.41

49.80 ± 8.13
42.32 ± 6.39
38.34 ± 8.89

0.59 ± 0.026
0.48 ± 0.027
0.28 ± 0.033

13 ± 0.59
11 ± 0.60
6.3 ± 0.74

6.1
6.6
4.3

AGAL009.951-00.366
(NA1)

ALMA1
ALMA2
ALMA3
ALMA4

9.95039.95259.94969.9481−0.3671−0.3660−0.3670−0.3633

1.29
1.07
0.68
0.45

0.62
0.63
0.42
0.27

80.7
54.5
118.0
70.4

0.90
0.82
0.54
0.35

4.96 ± 1.01
4.00 ± 0.92
1.36 ± 0.37
0.98 ± 0.28

0.081 ± 0.009
0.063 ± 0.010
0.041 ± 0.009
0.067 ± 0.015

2.0 ± 0.22
1.5 ± 0.23
1.0 ± 0.23
1.6 ± 0.37

4.9
4.3
3.7
3.5

AGAL018.931-00.029
(AS2)

ALMA1
ALMA2
ALMA3
ALMA4
ALMA5
ALMA6
ALMA7
ALMA8
ALMA9

18.931218.9287
18.931718.933918.932318.934418.9326
18.9296
18.9289−0.0246−0.0326−0.0248−0.0297−0.0289−0.0302−0.0283−0.0329−0.0318

0.80
1.02
0.40
0.67
0.54
0.50
0.61
0.44
0.32

0.47
0.63
0.24
0.32
0.49
0.38
0.33
0.22
0.23

154.5
82.5
–150.8
73.3
104.6
93.4
166.5
119.1
74.7

0.61
0.80
0.31
0.46
0.51
0.44
0.45
0.31
0.27

9.12 ± 1.30
6.51 ± 1.45
2.02 ± 0.32
1.87 ± 0.43
1.81 ± 0.37
1.54 ± 0.42
1.32 ± 0.29
0.87 ± 0.25
0.63 ± 0.15

0.27 ± 0.025
0.12 ± 0.019
0.17 ± 0.023
0.087 ± 0.014
0.064 ± 0.010
0.085 ± 0.017
0.057 ± 0.010
0.071 ± 0.018
0.066 ± 0.014

5.1 ± 0.47
2.3 ± 0.35
3.1 ± 0.43
1.6 ± 0.26
1.2 ± 0.18
1.6 ± 0.33
1.1 ± 0.20
1.3 ± 0.33
1.2 ± 0.26

7.0
4.5
6.3
4.3
4.8
3.7
4.5
3.5
4.1

AGAL015.503-00.419
(NA2)

ALMA1
ALMA2
ALMA3

15.5040
15.501415.5033−0.4184−0.4190−0.4179

0.99
0.51
0.48

0.73
0.44
0.22

47.2
179.1
58.3

0.85
0.48
0.32

16.62 ± 1.41
1.44 ± 0.28
0.54 ± 0.14

0.43 ± 0.009
0.073 ± 0.009
0.041 ± 0.009

7.9 ± 0.17
1.4 ± 0.16
0.76 ± 0.17

11.7
5.1
4.0

AGAL022.531-00.192
(AS3)

ALMA1
ALMA2
ALMA3
ALMA4
ALMA5
ALMA6
ALMA7
ALMA8
ALMA9

22.532722.5284
22.5315
22.5318
22.5305
22.5333
22.529622.530722.5302−0.1928−0.1931−0.1921−0.1928−0.1942−0.1929−0.1906−0.1924−0.1919

1.03
0.66
0.46
0.54
0.70
0.23
0.38
0.47
0.40

0.54
0.55
0.35
0.19
0.26
0.21
0.30
0.23
0.30

61.8
171.6
83.3
–147.058.4–178.6–149.6–144.1
75.1

0.74
0.60
0.40
0.32
0.43
0.22
0.34
0.33
0.35

15.97 ± 1.02
3.46 ± 0.65
2.14 ± 0.24
1.24 ± 0.14
0.99 ± 0.26
0.77 ± 0.08
0.73 ± 0.20
0.70 ± 0.12
0.69 ± 0.17

0.34 ± 0.010
0.12 ± 0.012
0.13 ± 0.009
0.094 ± 0.009
0.046 ± 0.010
0.12 ± 0.012
0.055 ± 0.012
0.058 ± 0.009
0.049 ± 0.009

5.8 ± 0.18
2.0 ± 0.20
2.2 ± 0.16
1.6 ± 0.16
0.77 ± 0.17
2.0 ± 0.20
0.93 ± 0.20
0.98 ± 0.15
0.83 ± 0.16

15.7
5.3
9.0
8.8
3.8
9.1
3.7
5.6
4.2

AGAL018.801-00.297
(NA3)

ALMA1
ALMA2
ALMA3
ALMA4
ALMA5
ALMA6
ALMA7
ALMA8

18.8000
18.800818.802518.802218.7989
18.7992
18.7988
18.8018−0.2954−0.2971−0.2989−0.2982−0.2967−0.2943−0.2941−0.2962

1.18
0.79
0.53
0.51
1.25
0.49
0.40
0.51

0.87
0.68
0.39
0.38
0.43
0.28
0.21
0.23

158.2
100.1
–164.5–150.5
95.1
112.4
115.1
128.0

1.01
0.73
0.45
0.44
0.73
0.37
0.29
0.34

9.62 ± 1.63
6.43 ± 0.78
1.56 ± 0.32
1.44 ± 0.28
1.25 ± 0.36
1.24 ± 0.30
0.80 ± 0.22
0.47 ± 0.13

0.14 ± 0.010
0.19 ± 0.008
0.074 ± 0.010
0.073 ± 0.009
0.038 ± 0.009
0.075 ± 0.014
0.076 ± 0.017
0.041 ± 0.009

2.7 ± 0.19
3.7 ± 0.15
1.4 ± 0.19
1.4 ± 0.17
0.74 ± 0.18
1.5 ± 0.28
1.5 ± 0.32
0.79 ± 0.17

5.9
8.2
4.9
5.1
3.5
4.1
3.7
3.6

AGAL333.016-00.751
(AS4)

ALMA1
ALMA2
ALMA3
ALMA4
ALMA5
ALMA6
ALMA7
ALMA8

–26.9824–26.9834–26.9861–26.9834–26.9838–26.9829–26.9831–26.9821−0.7504−0.7510−0.7511−0.7493−0.7502−0.7489−0.7522−0.7515

1.20
0.82
0.92
0.86
0.47
0.45
0.66
0.54

0.53
0.53
0.52
0.31
0.38
0.37
0.45
0.37

57.1
83.4
76.1
60.0
147.8
170.8
48.0
115.6

0.80
0.66
0.69
0.52
0.42
0.41
0.55
0.45

4.23 ± 0.64
2.01 ± 0.47
1.98 ± 0.54
1.66 ± 0.27
1.09 ± 0.19
1.08 ± 0.22
1.07 ± 0.30
1.01 ± 0.23

0.07 ± 0.008
0.051 ± 0.008
0.054 ± 0.010
0.064 ± 0.008
0.053 ± 0.008
0.054 ± 0.009
0.037 ± 0.008
0.046 ± 0.008

1.9 ± 0.22
1.4 ± 0.20
1.5 ± 0.27
1.7 ± 0.22
1.4 ± 0.20
1.4 ± 0.24
0.98 ± 0.22
1.2 ± 0.23

6.6
4.3
3.7
6.3
5.8
4.8
3.6
4.4

AGAL029.556 + 00.186
(NA4)

ALMA1
ALMA2
ALMA3
ALMA4
ALMA5
ALMA6
ALMA7

29.5567
29.5590
29.559729.5575
29.5580
29.558129.5580

0.18260.18470.18390.18700.18560.18720.1850

0.83
0.84
0.77
0.69
0.56
0.33
0.38

0.61
0.50
0.32
0.48
0.30
0.29
0.30

62.7
50.4
47.7
152.9
89.8
135.1
165.0

0.71
0.65
0.50
0.57
0.41
0.31
0.34

15.34 ± 1.65
11.48 ± 1.26
6.13 ± 0.92
3.91 ± 0.58
0.97 ± 0.25
0.94 ± 0.18
0.71 ± 0.20

0.49 ± 0.015
0.36 ± 0.016
0.25 ± 0.024
0.13 ± 0.011
0.053 ± 0.011
0.078 ± 0.013
0.049 ± 0.012

8.9 ± 0.27
6.5 ± 0.29
4.5 ± 0.43
2.3 ± 0.20
0.96 ± 0.21
1.4 ± 0.24
0.88 ± 0.21

9.3
9.1
6.6
6.8
3.9
5.1
3.5

Notes. (a) Cores are ranked with their mass, e.g., ALMA1 is the most massive core while ALMA2 is the second massive core. (b) Major, minor
semi-axes, and equivalent radius, respectively. (c) Primary beam corrected integrated flux and its error. (d) Primary beam corrected pixel maximum
flux and its error. The units are mJy and mJy Beam−1 for F1p and F2p , respectively.
A25, page 25 of 28

A&A 646, A25 (2021)
Table A.2. Physical parameters of cores.

Clump

Core

Mcore (α)(M )cold (β)
Mcore
(M )

warm (β)
Mcore
(M )

r (γ)
(AU)

Σ
(g cm−2 )NH2(1023 cm−2 )nH2(106 cm−3 )AGAL010.214-00.306(AS1, T dust = 16.6 K)

ALMA1
ALMA2
ALMA3

18.6 ± 7.2
15.8 ± 6.0
14.4 ± 6.0

11.3 ± 4.3
9.6 ± 3.6
8.7 ± 3.6

3940
3700
4400

2.57 ± 0.92
2.51 ± 0.89
1.58 ± 0.63

5.47 ± 1.96
5.35 ± 1.89
3.38 ± 1.33

6.96 ± 2.54
7.30 ± 2.63
3.85 ± 1.54

AGAL009.951-00.366(NA1, T dust = 12.0 K)

ALMA1
ALMA2
ALMA3
ALMA4

14.1 ± 5.4
12.0 ± 4.5
10.9 ± 4.5

AGAL018.931-00.029(AS2, T dust = 18.2 K)

ALMA1
ALMA2
ALMA3
ALMA4
ALMA5
ALMA6
ALMA7
ALMA8
ALMA9

3.3 ± 1.2
2.3 ± 0.9
0.7 ± 0.3
0.7 ± 0.3
0.7 ± 0.3
0.6 ± 0.2
0.5 ± 0.2
0.3 ± 0.1
0.2 ± 0.1

2.1 ± 0.8
1.5 ± 0.6
0.5 ± 0.2
0.4 ± 0.2
0.4 ± 0.2
0.4 ± 0.2
0.3 ± 0.1
0.2 ± 0.1
0.2 ± 0.1

2000
2700
1000
1500
1700
1400
1500
1000
900

1.78 ± 0.62
0.73 ± 0.28
1.53 ± 0.55
0.64 ± 0.25
0.50 ± 0.19
0.59 ± 0.25
0.48 ± 0.19
0.64 ± 0.28
0.64 ± 0.26

3.80 ± 1.33
1.56 ± 0.61
3.26 ± 1.16
1.36 ± 0.54
1.06 ± 0.40
1.25 ± 0.52
1.02 ± 0.40
1.37 ± 0.59
1.36 ± 0.55

9.48 ± 3.36
2.94 ± 1.16
15.98 ± 5.77
4.46 ± 1.77
3.13 ± 1.20
4.34 ± 1.83
3.45 ± 1.36
6.64 ± 2.87
7.70 ± 3.11

AGAL015.503-00.419(NA2, T dust = 14.0 K)

ALMA1
ALMA2
ALMA3

2.6 ± 0.9
1.8 ± 0.7
0.6 ± 0.2
0.5 ± 0.2
0.5 ± 0.2
0.4 ± 0.2
0.4 ± 0.2
0.2 ± 0.1
0.2 ± 0.1

AGAL022.531-00.192(AS3, T dust = 12.4 K)

ALMA1
ALMA2
ALMA3
ALMA4
ALMA5
ALMA6
ALMA7
ALMA8
ALMA9

27.7 ± 11
6.0 ± 2.6
3.7 ± 1.5
2.2 ± 0.9
1.7 ± 0.8
1.3 ± 0.5
1.3 ± 0.6
1.2 ± 0.5
1.2 ± 0.6

13.1 ± 5.2
2.8 ± 1.2
1.8 ± 0.7
1.0 ± 0.4
0.8 ± 0.4
0.6 ± 0.3
0.6 ± 0.3
0.6 ± 0.3
0.6 ± 0.3

3700
3000
2000
1600
2100
1100
1700
1700
1700

3.60 ± 1.17
1.20 ± 0.44
1.67 ± 0.56
1.53 ± 0.52
0.68 ± 0.28
2.06 ± 0.70
0.79 ± 0.33
0.80 ± 0.29
0.72 ± 0.29

7.67 ± 2.50
2.55 ± 0.95
3.55 ± 1.20
3.27 ± 1.11
1.45 ± 0.60
4.40 ± 1.49
1.69 ± 0.71
1.70 ± 0.62
1.53 ± 0.61

10.26 ± 3.54
4.22 ± 1.64
8.83 ± 3.16
10.25 ± 3.67
3.38 ± 1.45
20.35 ± 7.26
4.95 ± 2.14
5.10 ± 1.95
4.41 ± 1.84

AGAL018.801-00.297(NA3, T dust = 12.3 K)

ALMA1
ALMA2
ALMA3
ALMA4
ALMA5
ALMA6
ALMA7
ALMA8

17.9 ± 7.1
3.9 ± 1.7
2.4 ± 1.0
1.4 ± 0.6
1.1 ± 0.5
0.9 ± 0.4
0.8 ± 0.4
0.8 ± 0.3
0.8 ± 0.4

AGAL333.016-00.751(AS4, T dust = 15.6 K)

ALMA1
ALMA2
ALMA3
ALMA4
ALMA5
ALMA6
ALMA7
ALMA8

1.2 ± 0.5
0.6 ± 0.3
0.6 ± 0.3
0.5 ± 0.2
0.3 ± 0.1
0.3 ± 0.1
0.3 ± 0.2
0.3 ± 0.1

2700
2200
2400
1800
1400
1400
1900
1500

1.27 ± 0.45
0.89 ± 0.35
0.79 ± 0.33
1.18 ± 0.42
1.17 ± 0.42
1.24 ± 0.47
0.68 ± 0.29
0.96 ± 0.38

2.35 ± 0.88
1.98 ± 0.82
1.68 ± 0.73
3.36 ± 1.27
4.06 ± 1.56
4.47 ± 1.79
1.84 ± 0.81
3.18 ± 1.31

AGAL029.556 + 00.186(NA4, T dust = 12.8 K)

ALMA1
ALMA2
ALMA3
ALMA4
ALMA5
ALMA6
ALMA7

2.1 ± 0.9
1.0 ± 0.5
1.0 ± 0.4
0.8 ± 0.4
0.5 ± 0.2
0.5 ± 0.2
0.5 ± 0.3
0.5 ± 0.2

2.2 ± 0.9
1.8 ± 0.8
0.6 ± 0.3
0.4 ± 0.2

6.0 ± 2.5
0.5 ± 0.2
0.2 ± 0.1

3.6 ± 1.4
2.9 ± 1.2
1.0 ± 0.4
0.7 ± 0.3

8.7 ± 3.6
0.8 ± 0.3
0.3 ± 0.1

10.3 ± 4.0
6.9 ± 2.5
1.7 ± 0.7
1.5 ± 0.6
1.3 ± 0.6
1.3 ± 0.6
0.9 ± 0.4
0.5 ± 0.2

16.1 ± 6.2
10.7 ± 3.9
2.6 ± 1.0
2.4 ± 1.0
2.1 ± 0.9
2.1 ± 0.9
1.3 ± 0.6
0.8 ± 0.4

13.1 ± 4.7
9.8 ± 3.6
5.2 ± 2.0
3.3 ± 1.3
0.8 ± 0.4
0.8 ± 0.3
0.6 ± 0.3

19.8 ± 7.2
14.8 ± 5.4
7.9 ± 3.0
5.1 ± 1.9
1.3 ± 0.5
1.2 ± 0.5
0.9 ± 0.4

1.6 ± 0.7
0.7 ± 0.3
0.7 ± 0.3
0.6 ± 0.3
0.4 ± 0.2
0.4 ± 0.2
0.4 ± 0.2
0.4 ± 0.2

1.6 ± 0.7
1.3 ± 0.6
0.4 ± 0.2
0.3 ± 0.1

4.6 ± 1.9
0.4 ± 0.2
0.2 ± 0.1

7.5 ± 2.9
5.0 ± 1.8
1.2 ± 0.5
1.1 ± 0.5
1.0 ± 0.4
1.0 ± 0.4
0.6 ± 0.3
0.4 ± 0.2

9.7 ± 3.5
7.2 ± 2.6
3.9 ± 1.5
2.5 ± 0.9
0.6 ± 0.3
0.6 ± 0.2
0.4 ± 0.2

2800
2600
1700
1100

2700
1500
1000

5000
3600
2200
2100
3600
1800
1400
1700

3200
2900
2200
2600
1800
1400
1500

0.81 ± 0.31
0.77 ± 0.30
0.62 ± 0.26
1.07 ± 0.46

2.38 ± 0.79
0.66 ± 0.25
0.53 ± 0.22

1.19 ± 0.43
1.52 ± 0.52
0.95 ± 0.36
0.95 ± 0.36
0.30 ± 0.13
1.13 ± 0.45
1.18 ± 0.50
0.50 ± 0.21

0.59 ± 0.21
0.42 ± 0.16
0.37 ± 0.16
0.55 ± 0.20
0.55 ± 0.20
0.58 ± 0.22
0.32 ± 0.14
0.45 ± 0.18
3.60 ± 1.21
3.24 ± 1.10
2.97 ± 1.05
1.42 ± 0.50
0.69 ± 0.29
1.17 ± 0.44
0.74 ± 0.32

1.73 ± 0.66
1.65 ± 0.65
1.32 ± 0.55
2.28 ± 0.97

5.08 ± 1.68
1.40 ± 0.53
1.13 ± 0.46

2.53 ± 0.92
3.25 ± 1.11
2.03 ± 0.77
2.02 ± 0.76
0.63 ± 0.27
2.40 ± 0.97
2.53 ± 1.06
1.06 ± 0.45

7.67 ± 2.59
6.91 ± 2.34
6.34 ± 2.24
3.03 ± 1.07
1.48 ± 0.61
2.50 ± 0.93
1.59 ± 0.68

3.10 ± 1.19
3.22 ± 1.29
3.97 ± 1.68
10.58 ± 4.58

9.53 ± 3.36
4.69 ± 1.85
5.57 ± 2.37

2.56 ± 0.94
4.55 ± 1.58
4.57 ± 1.76
4.71 ± 1.79
0.88 ± 0.38
6.60 ± 2.68
8.84 ± 3.74
3.17 ± 1.36

12.01 ± 4.13
11.87 ± 4.09
14.25 ± 5.13
5.91 ± 2.12
4.06 ± 1.69
9.00 ± 3.42
5.27 ± 2.27

Notes. (α) Core mass and its error at clump dust temperature T dust . (β) Warm (T dust +3 K) and cold (T dust −3 K) core masses and their errors. (γ) Core
radius.

A25, page 26 of 28

S. Zhang et al.: H II regions and high-mass starless clump candidates−0◦ 180 0000

40

−0◦ 210 3600

20
2400

10

130 0000

4800

3600

2

220 0000

0
−2
2400

0.3 pc
1200

4

1200

0

4800

4800

0.3 pc10◦ 120 2400

2400

1200

Galactic Longitude

570 0000

4800

9◦ 560 3600

Galactic Longitude−0◦ 010 2400

12.5

−0◦ 240 4800

15

Galactic Latitude

3600

7.5
5.0

4800

2.5
0.0

020 0000−2.5

Flux (mJy Beam−1 )10.0
250 0000

10
1200

5

Flux (mJy Beam−1 )3600

Flux (mJy Beam−1 )Galactic Latitude

1200

Flux (mJy Beam−1 )

6
30

0

2400

−5.0
1200

3600

0.3 pc1200

560 0000

4800

3600

18◦ 550 2400

0.3 pc3600

2400

Galactic Longitude

1200

300 000015◦ 290 4800

Galactic LongitudeFig. A.3. 7 m ACA continuum
images uncorrected for primarybeam. ATLASGAL 870 µm emission is shown with black contours
with levels the same as Fig. 1.
Red (positive) and green (negative) contours show the 7 m ACA
emission with levels of ± rms × [3,31.25 , 31.50 , 31.75 , 32.0 , 32.25 , 32.50 , 32.75 ,33.0 ]. Blue ellipses and orange
contours indicate the leaves andtheir pixels derived by Astrodendro,
respectively. The red numbers markthe ranking of leaves’ mass inTable A.3.

Table A.3. Astrodendrogram results for 7 m ACA images.
Clump

Leaves (α)

l
(◦ )

b
(◦ )

major (β)(00 )minor (β)
(00 )

PA
(◦ )

R (β)
(00 )

F (γ)
(mJy)

F p (γ)
(mJy)

S/N

r (β)
pc

M
M

M coldM

M warm
M

AS1
AS1

ACA1
ACA2

10.2151
10.2135–0.3053–0.3049

1.47
1.39

1.10
0.99

58.2
72.1

1.27
1.17

26.08 ± 1.44
23.49 ± 1.25

1.4 ± 0.068
1.5 ± 0.069

18.2
18.8

0.02
0.02

7.4 ± 2.6
6.6 ± 2.3

9.8
8.8

5.9
5.3

NA1
NA1
NA1
NA1

ACA1
ACA2
ACA3
ACA4

9.94999.95259.95669.9485–0.3669–0.3661–0.3638–0.3629

2.55
2.34
1.48
1.47

1.52
1.41
1.05
1.01

–167.4–178.1–144.5
51.0

1.97
1.81
1.24
1.22

8.55 ± 1.76
6.91 ± 1.54
4.21 ± 1.28
2.67 ± 0.83

0.24 ± 0.033
0.22 ± 0.033
0.26 ± 0.061
0.17 ± 0.041

4.9
4.5
3.3
3.2

0.03
0.03
0.02
0.02

3.9 ± 1.6
3.1 ± 1.3
1.9 ± 0.9
1.2 ± 0.6

6.1
4.9
3.0
1.9

2.8
2.2
1.4
0.9

AS2
AS2
AS2

ACA1
ACA2
ACA3

18.931618.9286
18.9338–0.0247–0.0320–0.0295

3.04
2.55
3.13

1.72
1.98
1.06

–166.4176.8–159.4

2.29
2.25
1.82

25.95 ± 5.73
16.47 ± 4.31
7.07 ± 2.34

0.58 ± 0.073
0.36 ± 0.062
0.2 ± 0.057

4.5
3.8
3.0

0.04
0.04
0.03

7.3 ± 2.9
4.6 ± 2.0
2.0 ± 0.9

9.3
5.9
2.5

6.0
3.8
1.6

NA2
NA2

ACA1
ACA2

15.5041
15.4999–0.4182–0.4242

2.69
1.82

2.23
1.09

–157.7
157.0

2.45
1.41

21.86 ± 3.45
5.13 ± 1.66

0.58 ± 0.038
0.23 ± 0.061

6.3
3.1

0.04
0.02

7.9 ± 3.4
1.9 ± 1.0

11.4
2.7

6.1
1.4

AS3
AS3

ACA1
ACA2

22.532222.5295–0.1926–0.1936

4.93
3.20

1.74
1.29

–142.6–143.02.93
2.03

38.95 ± 6.26
7.87 ± 2.26

0.66 ± 0.046
0.22 ± 0.046

6.2
3.5

0.07
0.05

43.6 ± 18.5
8.8 ± 4.3

67.6
13.7

31.9
6.4

NA3
NA3

ACA1
ACA2

18.8052
18.8004–0.3043–0.2961

2.00
4.63

1.80
1.82

52.4
49.2

1.90
2.90

28.99 ± 5.83
22.22 ± 4.85

0.99 ± 0.11
0.3 ± 0.038

5.0
4.6

0.05
0.07

31.0 ± 12.4
23.8 ± 9.7

48.4
37.1

22.6
17.3

AS4

ACA1

–26.9832–0.7496

4.30

2.67

–137.03.39

27.63 ± 5.36

0.27 ± 0.033

5.2

0.06

10.1 ± 4.5

13.7

8.0

NA4
NA4

ACA1
ACA2

29.5586
29.5566

0.18530.1827

5.42
3.18

1.59
1.53

55.4
59.0

2.94
2.20

32.93 ± 5.41
18.43 ± 3.19

0.48 ± 0.041
0.47 ± 0.04

6.1
5.8

0.06
0.05

28.0 ± 10.7
15.7 ± 6.1

42.5
23.8

20.7
11.6

Notes. (α) Leaves are ranked with their mass, e.g., ACA1 is the most massive leaf while ACA2 is the second massive leaf. (β) Major, minor semi-axes,
and equivalent radius, respectively. (γ) F is the primary beam corrected integrated flux. F p is the primary beam corrected pixel maximum flux. The
unit has been transformed from mJy Beam−1 to mJy by dividing the flux in the unit of mJy Beam−1 with the beam size in the unit of pixel.

A25, page 27 of 28

A&A 646, A25 (2021)−0◦ 170 2400−0◦ 110 1200

20

10
3600

5

6
4

4800

2
0

180 0000

−2

0

00

−4

1200
120 0000

0.3 pc1200

320 0000

4800

3600

0.3 pc22◦ 310 2400

2400

1200

Galactic Longitude

480 00004800

18◦ 470 3600

Galactic Longitude3600−0◦ 440 3600

8

12.5
2400

4800

10.0

4
450 0000

2
0

1200

Flux (mJy Beam−1 )Galactic Latitude

6

7.5
1200

5.0
2.5
110 0000

0.0

−2

−2.5

4800

2400

−5.0

0.3 pc0.3 pc+0◦ 100 3600

333◦ 010 2400

1200

0000

000 4800

3600

4800

Galactic Longitude

Appendix B: Math derivations
B.1. Mean ratio of projected separation to actual separation(Sanhueza et al. 2019)

Setting α as the angle between actual separation S actu andprojected separation S proj , we have

The mean ratio is
Z π
2

− π2

2
cos (x)dx = .
π

B.2. Molecular weight per hydrogen molecule µH2 and mean
molecular weight per free particle µ (Kauffmann et al.
2008)

Assuming a mass ratio of M(H)/M ≈ 0.71, M(He)/M ≈ 0.27,
M(Z)/M ≈ 0.02 for hydrogen, helium, and metals, respectively,
where M = M(H) + M(He) + M(Z). The molecular weight perhydrogen molecule is:
µH2 =

M
M
=
mH × N(H2 ) mH × 21 N(H)

2M
2M=mH × N(H) M(H)≈ 2.8,=

A25, page 28 of 28

2400

1200

29◦ 330 0000

Galactic LongitudeFig. A.4. 7 m ACA continuum
images uncorrected for primarybeam. The meanings of lines andmarks are similar to Fig. A.3
but for another four candidate
HMSCs.

where N(H), N(H2 ), and mH are the numbers of H, H2 , and themass of H atoms, respectively.
In the calculation of thermal velocity dispersion, σth isrelated to the number of free particles, thus, we need to calculate
the mean molecular weight per free particle, which is:µ=S actu × cos (α) = S proj .
1
S proj /S actu = cos(α) =π

3600

Flux (mJy Beam−1 )48

Flux (mJy Beam−1 )Galactic Latitude

24

Flux (mJy Beam−1 )

8

3600

15

00

M
M
≈
mH × N mH × N(H2 ) + mH × N(He)
M
= 1
M(H)
+
mH × N(He)
2
M
= 1
1
2 M(H) + 4 M(He)≈ 2.37.