Investigating the formation of isolated
massive YSOs in the LMC

ALMA/45m/ASTE$Users$
Meeting$2017
Dec 26W 27,$2017

Ryohei Harada1,$Toshikazu$Onishi1,$Annie$Hughes7,$Margaret$Meixner2,9,$Marta$Sewilo8,$Remy$Indebetouw3,$Omnarayani Nayak9,$Kazuki Tokuda1,$5,$Sarolta Zahorecz1,$5,$Syota Takada1,$Aya Homma1,$Yasuo Fukui4,$Akiko$Kawamura5,$Kazuya$
Saigo5,$Tony$Wong6,$JeanWPhilippe$Bernard7
1. Osaka Prefecture$University,$2.$Space$Telescope$Science$Institute,$3.$University$of$Virginia,$Charlottesville,$4.$Nagoya$University,$5.$National$Astronomical$Observatory$of$Japan,$
6.$University$of$Illinois,$7.$IRAP, 8.$NASA$Goddard$Space$Flight$Center,$9.$Department$of$Physics$&$Astronomy,$Johns$Hopkins$University

Introduction

ALMA$Observations

HighWmass stars usually form in giant molecular clouds (GMCs) as part of ayoung stellar cluster, but some isolated O/B stars are observed. The aim of thisstudy is to measure the distribution and basic physical properties of the neutral
gas associated with isolated intermediateW and highWmass young stellar objects(YSOs) in the Large Magellanic Cloud (LMC).
As part of the Surveying the Agents of a Galaxy's Evolution (SAGE) Spitzer
Legacy program for the LMC (SAGE; Meixner et al. 2008), their team has
cataloged 1800 unique YSO candidates in the LMC (Whitney et al. 2008; seealso Gruendl & Chu 2009). By examining the spatial coincidence between theYSOs and 12CO(1W0) emission detected by the NANTEN mapping survey (FWHM
beam = 2.6’, Fukui et al. 2008), we identified intermediate/massive YSOs in theLMC that appear to be isolated, i.e. not associated with CO emission. All of ourtargets are separated by more than 200 pc from known CO clouds or the edgesof HI supershells to exclude potential “runaway” sources.
We carried out deeper followWup CO observations with the higher resolution
Mopra telescope (beam=30”). In some cases, the CO emission was only
detected exactly coincident with the YSO position (Fig. 1). The molecular clouds
asscociated with these YSOs must therefore have sizes smaller than 7 pc andmasses less than a few thousand solar masses.

Our$final$target$sample$is$spatially$distributed$across$the$LMC$gas$disk$(see$Fig.$2),$and$consists$of:i)$Three$YSOs$with$CO$emission$that$was$unresolved$by$Mopra.$The$molecular$clouds$associated$with$these$high$mass$YSOs$must$have$sizes$smaller$than$7$pc$and$masses$less$than$a$few$thousand$solar$
masses.
ii)$Seven$YSOs$where$CO$emission$was$detected$by$Mopra;$CO$mapping$observations$were$not$
attempted$for$these$sources$but$the$morphology$of$the$350$µm$emission$suggests$that$the$molecular$
clouds$are$likely$to$be$spatially$compact.

Table 1':'List'of'observation'parameters
Fig. 1: CO(1-0) distribution for thetargets we selected as compact
sources with Mopra 22m telescope.
The YSO is located at the center
position. The beam size and gridsize are ~33 arcsec and 30 arcsec,
respectively.

Observed line

13CO,C18O(J=1W0)

12CO,13CO,C18O(J=2W1),

CS(J=2W1),$continuum$(band3)
Ang.$res.

3”.2$ 2”.4$(band3:$12CO(J=1W0))
1”.9$ 1”.1$(band6:$12CO(J=2W1))

Vel. res.

0.2$km/s

The$rms noises$

20$mJy/beam$(band3)
50$mJy/beam$(band6)

Obs. mode

12m$array, ACA

Observed time

21.4$hFig. 2: Positions of the isolated intermediate/massive YSOs for theALMA observations. The numbers in red show the target number.
Gray scale shows integrated intensity of HI (Kim et al. 1999). Blue
contour shows CO integrated intensity from NANTEN (Fukui et al.
2008). The yellow lines show the region observed with NANTEN.

We carried out mosaic observations with ACA for Band 6 torecover the largest angular structure of ~20 arcsec for thecomparison with Band 3 data to precisely derive thephysical properties.

ALMA$Results
target$1

Band3

target$2

target$3

target$4

target$5target$613CO(1W0)

6W1

6W2 target$7

target$8

target$9

target$10

CS(2W1)
continuum
Band612CO(2W1)
13CO(2W1)
Fig.$3 <top>:$Yellow and$red$contours$show$the$13CO(1W0) and$CS(2W1)$integrated$intensity$observed$by$ALMA,$respectively.$White$contour$:$100$GHz$
continuum$emission.$Color:$Spitzer$8μm$image.$White$crosses:$positions$of$intermediateW and$highWmass$YSOs.$The$angular$resolution$is$indicated$at$
the$lowerWleft$corner.$White$line$shows$the$edge$of$the$primary$beams.
<bottom>:$Yellow and$red$contours$show$the$12CO(2W1)$and$13CO(2W1)$integrated$intensity,$respectively.$Color:$Spitzer$8μm$image.

LVG Analyses
target$1

target$2

target$5• Association$of$compact$molecular$gas! size$:$a$few$pc! mass$:$hundreds$ thousands solar$masses• Most$of$the$highWmass$YSOs$are$found$to$be$located$at$the$peak/center$of$the$compact$clouds.
W>$The$natal$cloud$has$not$experienced$the$significant$dissipation$by$the$formed$stars.

We$used$the$combination$of$the$ratios$of$13CO(J$=$2–1)$to$12CO(J$=$2–1)$and$
13CO(J$=$2–1)$to$13CO(J$=$1–0).
LVG$parameters[12CO]/[H2]$=$1.610W5 (Mizuno$et$al.$2010)12CO/13CO$abundance$ratio$:$50$(Mizuno$et$al.$2010)velocity$gradient$:$dv/dr =$2$km$sW1pcW1

Fig.$6$:$Map$of$the$gas$kinetic$temperature$calculated$by$the$LVG$analyses$

Turbulent$Motion$&$Velocity$Structure
Fig. 4 shows that the enhancements of the line width toward theYSOs (offset = 0) with some shifts by about 1 pc. The enhancement is
not peaky, but somewhat gradual, maybe reflecting the initialconditions of the star formation therein.
We investigated that the enhancements of the line width whether
are derived from massive YSOs or is as initial condition. Themomentum of each cloud is 1 ~ 20  103 M kmsW1 (Table 2). The
Fig.$4:$Cross$cut$of$the$line$width$of$
momentum supplied by outflows amounts to 1 W 300 M kmsW1 for
12CO(2W1)$emission$toward$the$targets.$$
YSOs with a luminosity of 102 W 104 L (e.g., Zhang et al. 2005). In thiscase, these momenta amount to ~ 10% of momentum of each cloud,target$5
which indicates the molecular gas may have large line width as initial
228.4$– 230.6$km/s
232.6$– 234.6$km/s
condition.
The molecular clouds associated with some targets have complex
velocity structure. Fig. 5 shows that two filamentary clouds withdifferent velocities, and they are merging at the region where theYSOs is located. There is a possibility that the turbulent motion of thegas may be enhancing the star formation activity, for example,
Fig.$5:$12CO(2W1)$intensity$map$integrated$over$a$velocity$range$
228.4W230.6$and$232.6W 234.6,$respectively$toward$target$5.
through cloudWcloud interaction there.
Table$2.$YSO$and$Cloud$properties
Target' YSO'mass'
No.
[Mo]
1Luminosity[104 Lo]
1cloud'mass[102Mo]'2

cloud'size'[pc]'3

6.0

0.6

virialratio
4

XCO factor
5

H2 density[ 103'cmJ3]
6

kinetic'temperature[K]'6

1.3

3.5

6.8

12CO(2J1')'

peak'Tr*[K]Momentum[ 103'M km'sJ1]72132

1.5

110

0.32

213

0.62

7.2

0.6

0.6

6.4

5.4

22$

27

2.7

3

130.38

6.4

0.7

1.4

2.8

5.7

26$32

1.94

14

1.1

48.5

1.6

1.7

3.7

5.4

24

9.8

5

1719

64.8

2.0

2.1

1.6

7.1

30

43

9.6

6W1

17

1.7

27.6

1.2

1.3

2.3

6.3

30

59

9.7

6$W 2

13

1.1

5.6

0.6

1.5

2.6

6.3

25

24

1.0

7

14

1.0

2.8

0.6

0.9

2.9

6.2

23

0.8

8

16

2.5

45.4

1.6

0.6

5.3

3.9

21.9$

23

20

9

15

1.2

4.6

0.6

0.9

3.2

5.1

31.0$

48

1.8

10

15

1.1

3.6

0.5

0.6

5.4

9.7

25.1

34

1.6

1:$In$order$to$derive$the$characteristics$of$the$YSOs,$we$use$the$YSO$model$grid$of$Robitaille et$al.$(2006)$and$the$spectral$energy$distribution$(SED)$fitter$to$determine$physical$parameters$of$YSO$from$available$photometry$data$Robitaille et$al.$(2007)$with$Spitzer(and$Herschel(fluxes$(1.2$W 500$μm)$included$in$our$modeling.
2:$Assuming$LTE.$N(H2)=50/1.610W5 N(13CO)$ (Fujii et$al.$2014)
3:$Radius$of$a$circle$having$the$same$area$above$the$half$of$peak.$4:$LTE$mass$divided$by$virial$mass.
5:$Xco factor$based$on$virial$masses$and$CO$luminosities.
6:$The$average$H2 density$and$gas$kinetic$temperature$of$the$cloud$calculated$by$the$LVG$analyses
7:$The$momentum$of$the$clouds$associated$with$YSOs.

! The$tentative$kinetic$temperature$and$H2 density$are$20$– 40$K,$3–9$ 103 cm−3$! The$density$is$higher$than$that$of$the$Orion$molecular$cloud$(~ 103 cm−3) that$is$
typical$highWmass$star$formation$region$in$the$galaxy.$
W>$UV$radiation$deeply$penetrates$into$the$cloud$and$CO$molecule$may$be$heavily$photodissociated in$the$lowWmetallicity$environment.
! We$cannot$find$the$enhancement$of$temperature$toward$some$massive$YSOs$(Fig.$6).
W>$These$clouds$have$intrinsic temperature$which$are$not$yet$much$heated$by$
the$massive$YSOs.

CI$observations
In$lowWmetallicity$environment,$[CI](1W0)$is$suggested$to$be$another$good$probe$for$the$molecular$mass$(e.g.,$Papadopoulos$and$Greve 2004,$Glover$and$Clark$2016).$• There$is$no$significant$difference$in$the$distribution$of$[CI]$and$CO.
• Massive$stars$are$formed$in$compact$molecular$clouds$even$in$CI$emission.
target$1

target$6

target$8

Fig. 7: CI integrated intensity map with ALMA 7m array. White cross show YSO positions.

Summary
The ALMA observations detected compact clouds toward the 10 isolated YSOs (Fig. 3). The typical cloud
radius and mass are a few pc and ~103 M, respectively. This result confirmed that most of our selected
YSOs are just formed or being formed in a compact molecular cloud in an isolated environment, andALMA spatially resolved the gas distribution. In spite of the fact that the compact clouds are even lessmassive than Taurus molecular cloud, the star formation therein is high. Our analysis indicates that thevelocity distribution seems not to be always quiet; the clouds are rather turbulent. There is a possibility
that the turbulent motion of the gas may be enhancing the star formation activity. LVG analyses indicatesthat the density is higher than that of the Orion molecular cloud, which indicate that CO molecule may beheavily photodissociated. The natal clouds associated with some of the targets have not experienced thesignificant dissipation and have not heated by the formed stars, which the natal cloud may have initialconditions of the massive star formation.