Star Formation NewsletterNo. 302 14-21
高橋実道（工学院大学）

The structure and spectrum of the accretion shock in the atmospheres of young stars
Alexandr Dodin1
1

Sternberg Astronomical Institute, Moscow M.V. Lomonosov State University, Universitetskij pr., 13, Moscow, 119992,
RussiaE-mail contact: samsebedodin at gmail.com
The structure and spectrum of the accretion shock have been self-consistently simulated for a wide range of parameters
typical for Classical T Tauri Stars (CTTS). Radiative cooling of the shocked gas was calculated, taking into accountthe self-absorption and non-equilibrium (time-dependent) eﬀects in the level populations. These eﬀects modify thestandard cooling curve for an optically thin plasma in coronal equilibrium, however the shape of high-temperature(T > 3 × 105 K) part of the curve remains unchanged. The applied methods allow us to smoothly describe thetransition from the cooling flow to the hydrostatic stellar atmosphere. Thanks to this approach, it has been foundthat the narrow component of He II lines is formed predominantly in the irradiated stationary atmosphere (hotspot),i.e. at velocities of the settling gas < 2 km s−1 . The structure of the pre-shock region is calculated simultaneously withthe heated atmosphere. The simulation shows that the pre-shock gas produces a noticeable emission component in
He II lines and practically does not manifest itself in He I lines (λλ 5876, 10830 Å). The UV spectrum of the hotspot is
distorted by the pre-shock gas, namely numerous red-shifted emission and absorption lines overlap each other forminga pseudo-continuum. The spectrum of the accretion region at high pre-shock densities ∼ 1014 cm−3 is fully formedin the in-falling gas and can be qualitatively described as a spectrum of a star with an eﬀective temperature derived
from the Stefan-Boltzmann law via the full energy flux.
Accepted by MNRAS
https://arxiv.org/pdf/1801.02404

VLBA Determination of the Distance to Nearby Star-forming Regions. VIII. The LkHα
101 cluster
S.A. Dzib1 , G.N. Ortiz-León1,2 , Laurent Loinard3,4 , A.J. Mioduszewski5 , L.F. Rodrı́guez3 , S.-N.X.
Medina1 , R.M. Torres6
1

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany; 2 Humboldt Fellow; 3
Instituto de Radioastronomı́a y Astrofı́sica, Universidad Nacional Autónoma de México, Morelia 58089, Mexico; 4
Instituto de Astronomı́a, Universidad Nacional Autnoma de México, Apartado Postal 70-264, CdMx C.P. 04510,
Mexico; 5 National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801, USA; 6 Centro Universitariode Tonalá, Universidad de Guadalajara, Avenida Nuevo Periférico No. 555, Ejido Dan José Tatepozco, C.P. 48525,

A Model for Protostellar Cluster Luminosities and the Impact on the CO-H2 Conversion
FactorBrandt A.L. Gaches1 and Stella S.R. Oﬀner1,2
1

Department of Astronomy, University of Massachusetts - Amherst, USA; 2 Department of Astronomy, University ofTexas at Austin, USA
E-mail contact: neil.vaytet at nbi.ku.dk

We construct a semi-analytic model to study the eﬀect of far-ultraviolet (FUV) radiation on gas chemistryc(y)-4(c(y)9.2

COlower dust extinction within the cloud. These clouds
have significantly less CO emission compared to their H2column density, i.e., they have a larger fraction of “CO星質量、星形成効率に対するXcoの依存性
dark” gas (i.e. Wolfire et al. 2010). Typically, CO-darkclouds are assumed to have faint CO emission due to
白線：典型的なXco,
点線が30%error
their
low densities, but the gas here
is CO deficient due
to dissociation caused by the embedded sources. Conse白帯が典型的な星形成効率
quently,
the MW average values occupy only a narrow
band across the parameter space. Within the range oftypical star formation efficiencies, XCO is only consistentfor clusters with masses between 103 - 104 M .
XCO：大きなClusterではMstar大で微減
Prior work has also found that XCO is sensitive tothe gas density (Bell et al. 2006; Shetty et al. 2011).
Mstar大でFUV大Densities
higher than 103 cm 3 make dust extinction
more efficient, while lower densities enhance the e↵ect
COのdissociation と温度の上昇がcancel してXcoの変化は小さい
embedded clusters since more of the cloud is influ14
Gaches & ofOffner
enced by photochemistry. Our models predict XCO is
一定、Mstar大ならビリアル速度大mosteﬃciency
sensitive to density
for clouds forming small clusFigure 10.
Contour plot of XCO as a function of the fiters with high star formation efficiencies. However, thisnal star cluster mass, M⇤ , and the efficiency, "g , for model→線幅が広がりCOの積分強度が増加、Xcoが減少
trend may not be evident in observations since di↵use
CE 1000D V 1⇠. The color scale indicates the logarithm of
XCO normalized by 1020 . The white solid contour is the typclouds are less likely to form stars with high efficiencies.
ical Milky Way XCO and the white dotted contours are the
30% error bars (Bolatto et al. 2013). The horizontal whiteband marks the star formation efficiency estimated for local
Gould Belt clouds (Dunham et al. 2013).

vey of Gould Belt clouds. Within the band, a significant
amount of the parameter space is consistent with thelocal Milky Way average XCO (in white contours). Furthermore, XCO is nearly constant for moderate cluster
sizes, so our model predicts the Milky Way average isrepresentative of local molecular clouds. The model also
predicts XCO decreases by a factor of 5-10 in the largest
clusters due to the increase in turbulent linewidth.
3.3.4. Mean Gas Density

3.3.5. Turbulent Velocity Dispersion
The turbulent内からのFUVを考慮、下では無視
velocity dispersion is an important fac上がCluster

tor in the calculated CO emission due to its influenceon the line width and, hence, the optical depth. The
典型的な値までXcoを上げるには中からのFUVの寄与が必要
line opticaldepth for a given line of sight is inversely
proportional to the velocity dispersion. There is ample evidence
that higher mass clouds have greater ve星からの寄与が最大で10倍程度Xcoを増加させる
locity dispersions and that many clouds are close tovirial equilibrium (Heyer & Dame 2015). Although aconstant linewidth model is unphysical, it is useful to
examine the importance of velocity information. In thissection, we study the e↵ects of the turbulent velocity dis-

20

An ALMA study of the Orion Integral Filament: I. Evidence for narrow fibers in a
massive cloud
A. Hacar1 , M. Tafalla2 , J. Forbrich3,4 , J. Alves5 , S. Meingast5 , J. Grossschedl5 , and P.S. Teixeira5,6
1

Leiden Observatory, Leiden University, P.O. Box 9513, 2300-RA Leiden, The Netherlands; 2 Observatorio Astro重い Filament (Integral Shape Filament (ISF), M̲line~500Msun/pc)のnomico Nacional (IGN), C/ Alfonso XII, 3, E-28014, Madrid, Spain; 3 Centre for Astrophysics Research, University
4
substructure
A&A proofs: manuscript no
A&A proofs:9AB,
manuscriptUK;
no. Hacar_ISF_ALMAof Hertfordshire,(ﬁber)の観測→55本同定
College Lane, Hatfield, AL10
Harvard-Smithsonian Center for Astrophysics, 60 Garden
St, Cambridge, MA 02138, USA; 5 University of Vienna, Türkenschanzstrasse 17, A-1180 Vienna, Austria; 6 Scottishw+
Universities Physics Alliance (SUPA), School of Physics and Astronomy, University of St. Andrews, North Haugh, St.
tofi
H
Andrews, Fife KY16 9SS, UK

A. Hacar et al.: An ALMA study of the Orion Integral Filament: I. Evidence for narrow fibers in a massive cloudE-mail contact: hacar at strw.leidenuniv.nl
Are all filaments bundles of fibers? To address this question, we have investigated the gas organization within theparadigmatic Integral Shape Filament (ISF). We combined two new ALMA Cycle 3 mosaics with previous IRAM 30m
observations to produce a high-dynamic range N2 H+ (1–0) emission map of the ISF tracing its high-density material
and velocity structure down to scales of 0.009 pc. From the analysis of the gas kinematics, we identify a total of 55
dense fibers in the central region of the ISF. Independently of their location, these fibers are characterized by transonicinternal motions, Fig.
lengthsof ∼0.15 pc, and masses per-unit-length close to those expected in hydrostatic equilibrium.
5. Statistical properties of the ISF fibers (orange shaded area): (Left) Total fiber length; (Centre) average line-of-sight, non-thermal velocity
as a function of the local sound-speedc (T ); and (Right) total mass-per-unit length. The individual fiber propertiesmass
in OMC-1
左：乱流の速度分散/音速，右line
The ISF fibers aredispersion
spatiallyorganized
formingadensebundlewithmultiple
hub-like
associations likely shaped by the
and OMC-2 are highlighted by blue and red dashed lines, respectively.
local gravitational potential. Within this complex network, the ISF fibers show a compact radial emission profile with
NT

s

K

Table 1. Dense gas properties

ISF
OMC-1
N2 H+ fits (S/N 3)
25078
13305
TK (K)(a)24.8±10.3 29.0±12.5
0.294
0.318
hc s (TK )i (km s 1 )V (km s 1 )
0.607
0.722
0.240
0.288
h NT i (km s 1 )
0.81
0.91
h NT /c s (T K )i
76.4%
68.4%
NT /c s (T K )  1
1 < NT /c s (T K )  2
20.0%
25.7%
/c s (T(Left)
2K ) >SCUBA-850
Fig. 4. Main axis of the 55 fibers identified by our HiFIVE analysis along the ISF (red NT
segments).
µm3.6%
continuum emission 5.9%

OMC-2
11773
20.1±2.2
0.267
0.477
0.185
0.69
85.5%
13.6%
0.9%

ple gas components and local velocity gradients typically unresolved within a single-dish beam. We illustrate this e↵ect in
Fig. 6 by comparing the emission in a single position recovered
using di↵erent e↵ective beam sizes. At the native ALMA resolution (red spectrum; upper panel), the observed N2 H+ emission
shows three independent narrow lines, clearly separated in thehyperfine isolated component (grey dashed segments). Due tothe complex gas kinematics within this region, the above line
substructure is progressively smeared out when convolved withneighbouring positions to the resolution of previous single-dishstudies (e.g., blue spectrum; central panel). Dilution and blending e↵ects give the appearance of a single broad component at

14・亜音速乱流が効いている。
・約半分が重力で束縛されている。

+

Fig. 8. (Upper) Mass-per-unit-length (Mlin ) of the OMC-1 (blue dots)
and OMC-2 (red dots) fibers as function of their internal non-thermalvelocity dispersion normalized by the local sound speed ( NT /c s (T K )).
This plot includes the expected critical masses (mcrit ) for an infinite filament in hydrostatic equilibrium at temperatures of 10 K (thick dashedline), 20 K (dashed-dotted line), and 30 K (thick dotted line), respectively (see Eq. 3). (Lower) Individual critical mass ratio (Mlin /mcrit ) for
all the fibers in OMC-1 (blue) and OMC-2 (red). For every single fiber
the critical mass ratio is calculated relative to its local mcrit (T K , NT ).
The error bars indicate the factor of two uncertainties associated to our
Mlin estimates.

Critical line massとの比較

p
te
p
p
a
a
ti
IS
sh
th

d
to
E
e
IS
0
g
o
ti
ra
d
&
a
a
W
ti
u
g
In
a
o

3

A&A proofs: manuscript no. Hacar_ISF_ALMA

Fig. 10. Observed fiber FWHM along the ISF. (Left) Median (red solid line) and minimum-maximum (red shaded area) FWHM values obtainedin each fiber. (Right) Total FWHM values (576 data points; red histogram) measured along the ISF fibers. The total median value of 0.035 pc(blue dashed line) in comparison with the ALMA 0.009 pc beamsize (grey dotted line) are indicated in both panels.

highly idealized, it is instructive to compare the expected variations of the scale height of a filament in hydrostatic equilibrium(H0 ) as function of the gas temperature (TK ), non-thermal motions ( NT ), and density (n(H2 )) (Ostriker 1964):ss
2 2e f f
c s (T K )2 + 2NT
H0 (T K , n(H2 )) =/.
(4)
⇡ G n(H2 )n(H2 )
Following this equilibrium solution, denser filaments are thus
expected to show narrower radial configurations. For the ISF
fibers, and consistent with our line opacity measurements (seeAppendix B.3), we find typical gas densities of n(H2 ) ⇠ 107
108 cm 3 assuming a cylindrical symmetry and the previously
derived Mlin and FWHM values (see Table 2). These estimatesare (at least) 2 orders of magnitude higher than the densities measured for typical low-mass filaments in clouds like IC5146, Polaris, or Taurus. According to Eq. 4, the observed radii in both the
ISF fibers (TK ⇠ 25 K; NT = c s (25K); n(H2 ) = 108 cm 3 ) andHerschel filaments (e.g., Herschel: TK =10 K; NT = c s (10K);n(H2 ) = 5 ⇥ 105 cm 3 ) are expected to show a size dependency
such that H0H(Herschel)⇠ 9. Although deviating from these sim0 (ISF)
FWHM(Herschel)unit-lengths (right) in both OMC-1 (blue) and OMC-2 regions(red). We also list the mean and 1- dispersion values in Table 2(see col. 2 and 3). Overall, we find no systematic di↵erences between these properties along the ISF. On average, both OMC-1
and OMC-2 regions have fibers with statistically similar length,
velocity dispersion, and mass per-unit-length. Minor variations
are, however, apparent in Figures 5 (left) and 5 (centre). The
OMC-1 fibers appear to show larger velocity dispersions andgradients than those in OMC-2 (see additional parameters in Table 2). While appealing, these di↵erences are found within the
1- dispersion estimates and can only be considered as tentative. Despite these local variations, the observed ISF fibers arefound to present roughly uniform properties regardless of theirlocal environment.
Morphologically speaking, the ISF fibers are distributed in
multiple hub-like associations (Myers 2009) in both OMC-1 andOMC-2 regions. As seen in Fig. 4, most OMC-1 fibers are oriented radially converging towards the OMC-1 South region (seeRodriguez-Franco et al. 1992; Wiseman & Ho 1998). Althoughpartially recovered in our fiber analysis, similar fan-like arrangements of fibers can also be recognized at smaller scales towards

On the Rotation of Supermassive Stars
Lionel Haemmerlé1, Tyrone E. Woods2 , Ralf S. Klessen3,4 , Alexander Heger2 and Daniel J. Whalen5
1

Observatoire de Genève, Université de Genève, chemin des Maillettes 51, CH-1290 Sauverny, Switzerland; 2 Monash
Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800, Australia; 3 Universität
Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, D-69120 Heidelberg, Germany; 4 Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer
Feld 205, D-69120 Heidelberg, Germany; 5 Institute of Cosmology and Gravitation, University of Portsmouth, Dennis
Sciama Building, Portsmouth PO1 3FX, UK
E-mail contact: lionel.haemmerle at unige.ch
Supermassive stars born from pristine gas in atomically-cooled haloes are thought to be the progenitors of supermassive
black holes at high redshifts. However, the way they accrete their mass is still an unsolved problem. In particular,
for accretion to proceed, a large amount of angular momentum has to be extracted from the collapsing gas. Here,
we investigate the constraints stellar evolution imposes on this angular momentum problem. We present an evolution
model of a supermassive Population III star including simultaneously accretion and rotation. We find that, forsupermassive stars to form by accretion, the accreted angular momentum has to be about 1% of the Keplerian angularmomentum. This tight constraint comes from the ΩΓ-limit, at which the combination of radiation pressure andcentrifugal force cancels gravity. It implies that supermassive stars are slow rotators, with a surface velocity less than
10 – 20% of their first critical velocity, at which the centrifugal force alone cancels gravity. At such low velocities, thedeformation of the star due to rotation is negligible.
Accepted by ApJL
https://arxiv.org/pdf/1711.09916.pdf

The stellar IMF from Isothermal MHD Turbulence

1

Sternberg Astronomical Institute, Moscow M.V. Lomonosov State University, Universitetskij pr., 13, Moscow, 119992,
The structure and spectrum of the accretion shock in the atmospheres of young stars
14 Russia
Alexandr Dodin1
E-mail contact: samsebedodin at gmail.com
1
Sternberg Astronomical Institute, Moscow M.V. Lomonosov State University, Universitetskij pr., 13, Moscow, 119992,
The structure and spectrum of the accretion shock have been self-consistently simulated for a wide range of parameters
Russiatypical for Classical T Tauri Stars (CTTS). Radiative cooling of the shocked gas was calculated, taking into accountE-mailcontact: samsebedodin
at gmail.com
theself-absorption
and non-equilibrium(time-dependent) eﬀects in the level populations. These eﬀects modify thestandard
cooling
for anthinshockplasmain beencoronalequilibrium, simulated
however theof high-temperature
The structure
andcurve
spectrumof opticallythe accretionhave
self-consistently
for ashapewide rangeof parameters
5
(T
>
3
×
10
K) partof theStars
curve(CTTS).
remainsRadiative
unchanged.
Theofapplied
methods
allow
us to smoothly
describethetypical for ClassicalT Tauri
cooling
the shockedgas wascalculated,taking into
accounttransition
from the andcooling
flow to the hydrostatic
stellar eﬀects
atmosphere.
to this approach,it has modify
been foundthe self-absorption
non-equilibrium(time-dependent)
in the Thanks
level populations.
These eﬀects
thethat
the narrowof He
II linesthin
is formed
in the irradiated(hotspot),
standard
coolingcomponent
curve for an
optically
plasmapredominantly
in coronal equilibrium,
howeverstationary
the shapeatmosphereof high-temperaturei.e.
of the< 2remains
km s−1 . unchanged.
The structureof thepre-shockregionis calculated
simultaneously(T at> velocities
3 × 105 K)
partsettlingof the gas
curve
The
appliedmethods
allow
us to smoothly
describe withthetheheated fromatmosphere.
Theflowsimulationshows that stellar
the pre-shockgas produces
a noticeable
emissiontransition
the coolingto the hydrostatic
atmosphere.
Thanks tothis approach,it hascomponent
been foundinthat
the narrowcomponentof He
lines is formedin the10830
irradiatedstationary
atmosphereHe
II lines
and practically
does
notIImanifest
itself inpredominantly
He I lines (λλ 5876,
Å). TheUV spectrumof the(hotspot),
hotspot is
i.e. at velocitiesof the settling< 2 kms−1 . The
structure emissionof the pre-shock
region is lines
calculated
simultaneously
withdistortedby the pre-shockgas, gas
namelynumerous
red-shifted
and absorption
overlap
each other forming
14
heated atmosphere.
simulation
showsthat thepre-shockgas
producesdensities
a noticeable
componentinathe
pseudo-continuum.
TheThespectrumof theaccretion
regionat highpre-shock∼ 10emission
cm−3 isfully formed
II lines
and practicallydoesmanifest itself
in He Iaslines(λλ 5876,of10830
TheanUV
spectrumof the hotspot
isinHethein-falling
gas and canbe notqualitatively
described
a spectrum
a starÅ).
with
eﬀectivetemperature
deriveddistortedby the pre-shock gas,emission and absorption lines overlap each other forming
fromthe Stefan-Boltzmann
law namely
via the numerous
full energyred-shifted
flux.
a pseudo-continuum. The spectrum of the accretion region at high pre-shock densities ∼ 1014 cm−3 is fully formed
Accepted by MNRAS
in the in-falling gas and can be qualitatively described as a spectrum of a star with an eﬀective temperature derived
https://arxiv.org/pdf/1801.02404
from the Stefan-Boltzmann law via the full energy flux.
Accepted by MNRAS

VLBA
Determination of the Distance to Nearby Star-forming Regions. VIII. The LkHα
https://arxiv.org/pdf/1801.02404
101 cluster
S.A. Dzib1 , G.N. Ortiz-León1,2 , Laurent Loinard3,4 , A.J. Mioduszewski5 , L.F. Rodrı́guez3 , S.-N.X.

A spectroscopic survey of the youngest field stars in the solar neighborhood. II. The
optically faint sample
A. Frasca1 , P. Guillout2 , A. Klutsch1 , R. Freire Ferrero2 , E. Marilli1 , K. Biazzo1 , D. Gandolfi3 and D.
Montes4
1

INAF - Osservatorio Astrofisico di Catania, via S. Sofia, 78, 95123 Catania, Italy
2
Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France
3
Dipartimento di Fisica, Università di Torino, via P. Giuria, 1, 10125 Torino, Italy
4
Departamento de Astrofı́sica y Ciencias de la Atmósfera, Universidad Complutense de Madrid, 28040 Madrid, Spain
E-mail contact: antonio.frasca at oact.inaf.it
Star formation in the solar neighborhood is mainly traced by young stars in open clusters, associations, and in thefield, which can be identified, for example, by their X-ray emission. The determination of stellar parameters forthe optical counterparts of X-ray sources is crucial for a full characterization of these stars. This work extends the
spectroscopic study of the RasTyc sample, obtained by the cross-correlation of the TYCHO and ROSAT All-Sky
Survey catalogs, to stars fainter than V = 9.5 mag and aims to identify sparse populations of young stars in the solar
neighborhood. We acquired 625 high-resolution spectra for 443 presumably young stars with four diﬀerent instrumentsin the northern hemisphere. The radial and rotational velocity (v sin i) of our targets were measured by means of thecross-correlation technique, which is also helpful to discover single-lined (SB1), double-lined spectroscopic binaries(SB2), and multiple systems. We used the code ROTFIT to perform an MK spectral classification and to determine
the atmospheric parameters (Teﬀ , log g, [Fe/H]) and v sin i of the single stars and SB1 systems. For these objects, weused the spectral subtraction of slowly rotating templates to measure the equivalent widths of the Hα and Lii 6708 Å
lines, which enabled us to derive their chromospheric activity level and lithium abundance. We made use of Gaia
DR1 parallaxes and proper motions to locate the targets in the Hertzsprung-Russell (HR) diagram and to computethe space velocity components of the youngest objects. We find a remarkable percentage (at least 35 %) of binaries
and multiple systems. On the basis of the lithium abundance, the sample of single stars and SB1 systems appears to
be mostly (∼ 60 %) composed of stars younger than the members of the UMa cluster. The remaining sources are inCA 91109, USA; Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA;
Institute for Research on Exoplanets, Université de Montréal, Département de Physique, C.P. 6128 Succ. Centre-ville,
6
BANYAN.
XI.3J7,The
BANYANΣ multivariate
Bayesianalgorithm
to identify
18 Montréal,
QC H3C
Canada;Space Telescope
Science Institute,
3700 San
Martin Drive,
Baltimore, members
MD 21218,
7of young
associations
within150 pcMuseum of Natural History, Central Park West at 79th St., New York,USA;
Departmentof Astrophysics,
American8 1,2
3,4THETA, Univ.5 Bourgogne Franche-Comté,NY10024,USA;
OSU
Besançon,6 ,France
Jonathan Gagné Institut, Eric UTINAM,
E. Mamajek, Lison Malo , Adric Riedel6 , David Rodriguez
David Lafrenière5 ,
7
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JacquelineK.
Faherty,
Olivier
Roy-Loubier,
Laurent
Pueyo
,
Annie
C.
Robin
,
and
René
Doyon
E-mail contact: jgagne at carnegiescience.edu
12Carnegie
Institutionof
Washington
DTM,
5241
Broad
BranchRoad
NW,
Washington,DC
20015,USA;
BANYAN Σ is a new Bayesian algorithm to identify members of young stellar associations within 150 pc of theNASA
Sun.
3
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Jet Propulsionof Technology,
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Oak Grove Drive,
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It includes 27 young
associationsLaboratory,
with ages inCalifornia
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Gaussians
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5
CA
91109,
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andAstronomy,
UniversityofRochester,Rochester,NY
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dimensional XYZUVW space. It is the first such multi-association classification tool to include the nearest sub-groups
Institutefor Research
on Exoplanets,
Universitéde Montréal,
Physique,
C.P. 6128
ofthe Sco-Cen
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regions.Science
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7
USA;
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New
York,
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8
NY10024,
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Franche-Comté,Besançon,
Francewith only sky coordinates
proper motion,
but can Univ.
also include
parallax
and radial velocity
measurements,
as wellas
spectrophotometricconstraints from sequences in color-magnitude or spectral type-magnitude diagrams.
E-mailcontact: jgagnedistanceat carnegiescience.edu
BANYAN Σ benefits from an analytical solution to the Bayesian marginalization integrals that makes it more accurate
BANYANΣisanew
Bayesianalgorithm
toidentify
membersofyoungstellarassociations
within
150
pc
of
the
Sun.
and significantly faster than its predecessor BANYAN II. A contamination versus hit rate analysis is presented andIt
includes27
young
associations
with
ages
in
the
range
∼1–800Myr,modelled
withmultivariate
Gaussiansin6demonstrates that BANYAN Σ achieves a better classification performance than other moving group classification
dimensional
XYZUVW
space.
It
is
the
first
such
multi-association
classification
tool
toincludethenearestsub-groups
tools, especially in terms of cross-contamination between young associations. An updated list of bona fide membersof thethe 27
Sco-Censtar-forming
region, bythetheIC Gaia-DR1
2602, IC 2391,
Pleiadesand Platais
clusters,
andmakethe ρit Ophiuchi,
inyoung OB
associations,
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possibleCoronaAustralis,
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toanalyzelarge datasuch star-formation
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young
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withGalactic
this publication.
with only sky coordinates and proper motion, but can also include parallax and radial velocity measurements, as well
Acceptedby ApJSas spectrophotometricdistance constraints from sequences in color-magnitude or spectral type-magnitude diagrams.
BANYAN Σ benefits from an analytical solution to the Bayesian marginalization integrals that makes it more accurate
http://arxiv.org/pdf/1801.09051
and significantly faster than its predecessor BANYAN II. A contamination versus hit rate analysis is presented anddemonstrates that BANYAN Σ achieves a better classification performance than other moving group classification
Distributed
starformation
throughout
theGalactic
Centercloud
SgrB2
tools, especially in terms of cross-contamination between young associations. An updated list of bona fide members

19

Astronomie der Universität Heidelberg, Mönchhofstrasse 12-14, 69120 Heidelberg, Germany; 15 National Astronomical
Observatory of Japan, 2-21-1 Osawa, Mitaka,Tokyo, 181-8588, Japan; 16 I. Physikalisches Institut, Universiät zu Köln,Distributed
starformation
throughout
theGalactic
Center
cloud
Sgr
B2
17
Zülpicher Str. 77, 50937 Köln, Germany;
San Jose State University, One Washington Square, San Jose, CA
1,2
3
4
4
5,6
7
18
Adam
Ginsburg
,
John
Bally
,
AshleyBarnes,Nate
Bastian,
Cara
Battersby,
HenrikBeuther
95192, USA;
Boston University Astronomy Department, 725 Commonwealth Avenue, Boston, MA 02215, USA; ,
8
9
8,10
3
19
20De Pree11 , Roberto
CrystalBrogan,YanettContreras,
JoannaCorby,
JeremyDarling,
Chris
National Radio Astronomy Observatory, 1003 Lopezville Rd, Socorro, NM 87801 USA;
Max-Planck-Institut
12
13
21 7 , Todd Hunter8 , J.M. Diederik Kruijssen14 ,Galván-Madrid
,
Guido
Garay
,
JonathanHenshawfür extraterrestrische Physik, D-85748 Garching, Germany;
Joint ALMA Observatory, Alonso de Córdova 3107,
4
22 Lu15 , Fanyi Meng16 , Elisabeth A.C. Mills17,18 , Juergen Ott19 , Jaime E.
StevenLongmore,
Xing
Vitacura, Santiago, Chile;
National Astronomical Observatory of Japan, Alonso de Córdova 3788, 61B Vitacura,
20
16
16
16,20
4,21,22
Pineda,ÁlvaroSánchez-Monge,
Peter
Schilke,
Anika
Schmiedeke,
DanielWalker, andSantiago, Chile
5
David
WilnerE-mail contact: aginsbur at nrao.edu
1

2
Janskyfellowofthe
National
Radio
Astronomy
Observatory,
1003
Lopezville
Rd,
Socorro,
NM
87801
USA;
We report ALMA observations with resolution ∼0.′′ 5 at 3 mm of the extended Sgr B2 cloud in the Central Molecular3
EuropeanSouthern
Observatory,Karl-Schwarzschild-Straße
2,D-85748
Garchingbei
München,Germany;
CASA,
Zone (CMZ). We detect 271 compact sources, most of which
are smaller than 5000 AU. By ruling out alternative
4
UniversityofColorado,
389-UCB,
Boulder,CO
80309,USA;
Astrophysics
Research
Institute,
LiverpoolJohn
Moorespossibilities, we conclude that these sources consist of a mixof hypercompact HII regions and young stellar objects
5
University,146ofBrownlow
Hill, Liverpool
L3 are5RF,UK;withHarvard-Smithsonian
Astrophysics,
60 Garden(YSOs).
Most
the newly-detectedsources
YSOs
gas envelopes which,Center
based foron theirluminosities,
must
contain objects with stellar masses M∗ >∼ 8 M⊙ . Their spatial distribution spread over a ∼ 12×3 pc region demonstrates
that Sgr B2 is experiencing an extended star formation event, not just an isolated ‘starburst’ within the protocluster
regions. Using this new sample, we examine star formation13thresholds and surface density relations in Sgr B2. While
23
−2
2
×
10
cm
), not all regions of high column
all of the YSOs reside in regions of high column density (N (H2 ) >∼density contain YSOs. The observed column density threshold for star formation is substantially higher than thatin solar vicinity clouds, implying either that high-mass star formation requires a higher column density or that any
star formation threshold in the CMZ must be higher than in nearby clouds. The relation between the surface densityof gas and stars is incompatible with extrapolations from local clouds, and instead stellar densities in Sgr B2 follow
a linear Σ∗ –Σgas relation, shallower than that observed in local clouds. Together, these points suggest that a higher
volume density threshold is required to explain star formation in CMZ clouds.

Accepted by ApJ
http://arxiv.org/pdf/1801.04941