Hierarchical fragmentation and differential star formation in the Galactic "Snake": infrared dark cloud G11.11-0.12

1.5 MB·

by  Ke Wang

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed7February2014 (MNLATEXstylefilev2.2)
Hierarchical fragmentation and differential star formation in the
Galactic “Snake”: infrared dark cloud G11.11–0.12
Ke Wang,1,2,3,4,⋆ Qizhou Zhang,2 Leonardo Testi,1,5,6 Floris van der Tak,3,7
Yuefang Wu,4 Huawei Zhang,4 Thushara Pillai,8 Friedrich Wyrowski,9
4 Sean Carey,10 Sarah E. Ragan,11 and Thomas Henning11
1
0
2
1EuropeanSouthernObservatory,Karl-Schwarzschild-Str.2,85748GarchingbeiMu¨nchen,Germany

b 2Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,CambridgeMA02138,USA
e 3KapteynAstronomicalInstitute,UniversityofGroningen,Landleven12,9747ADGroningen,TheNetherlands
F 4DepartmentofAstronomy,SchoolofPhysics,PekingUniversity,Beijing100871,China
5ExcellenceClusterUniverse,Boltzmannstr.2,85748GarchingbeiMu¨nchen,Germany
6
6INAF–OsservatorioastrofisicodiArcetri,LargoE.Fermi5,50125Firenze,Italy

7SRONNetherlandsInstituteforSpaceResearch,Landleven12,9747ADGroningen,TheNetherlands
]
A 8CaliforniaInstituteofTechnology,1200ECaliforniaBlvd,Pasadena,CA91125,USA
9Max-Planck-Institutfu¨rRadioastronomie,AufdemHo¨gel69,D-53121Bonn,Germany
G 10SpitzerScienceCenter,CaliforniaInstituteofTechnology,Pasadena,CA91125,USA
. 11Max-PlanckInstitutefu¨rAstronomie,Ko¨nigstuhl17,D-69117Heidelberg,Germany
h
p
Accepted2014January16.Received2014January15;inoriginalform2013October31
-
o
r
t ABSTRACT
s
a We presentSMAλ = 0.88and1.3mmbroad-bandobservations,andVLAobservationsin
[ NH3 (J,K) =(1,1)upto(5,5),H2OandCH3OHmaserlinestowardthetwomostmassive

molecularclumpsinIRDCG11.11–0.12.Sensitivehigh-resolutionimagesrevealhierarchical
2
fragmentation in dense molecular gas from the ∼1 pc clump scale down to ∼0.01 pc con-
v
7 densationscale. Ateachscale,the massofthefragmentsisordersofmagnitudelargerthan
5 theJeansmass.ThisiscommontoallfourIRDCclumpswestudied,suggestingthatturbu-
1 lence plays an important role in the early stages of clustered star formation. Masers, shock
4 heated NH3 gas, and outflows indicate intense ongoing star formation in some cores while
. nosuchsignaturesarefoundinothers.Furthermore,chemicaldifferentiationmayreflectthe
1
0 differenceinevolutionarystagesamongthesestarformationseeds.We findNH3 ortho/para
4 ratiosof1.1±0.4,2.0±0.4,and3.0±0.7associatedwiththreeoutflows,andtheratiotends
1 to increase along the outflows downstream. Our combined SMA and VLA observations of
: severalIRDCclumpspresentthemostindepthviewsofaroftheearlystagespriortothehot
v
corephase,revealingsnapshotsofphysicalandchemicalpropertiesatvariousstagesalongan
i
X apparentevolutionarysequence.
r
a Key words: ISM: individual (G11.11–0.12)– ISM: jets and outflows – stars: formation –
stars:early-type–masers–accretiondisks
1 INTRODUCTION coined “infrared-dark clouds” (IRDCs; Perault et al. 1996; Egan
etal.1998; Hennebelleet al.2001; Simonetal.2006a,b; Peretto
Becausehigh-massstarsformdeeplyembeddedindensegasand
&Fuller2009).ThecolddustinIRDCstransitionstoemissionat
in distant clustered environments, observational studies face se-
longer wavelengths, from far-infrared to sub-millimeter and mil-
verelimitationsofopticaldepthandspatialresolution.Heavydust
limeter wavelengths (Molinari et al. 2010; Schuller et al. 2009;
extinction (N ∼ 1023cm−2, A > 100 mag) obscures the
H2 v Rosolowskyetal.2010;Aguirreetal.2011).Onekeyoutcomeof
forming young protostars and also the Galactic background radi-
therecentHerschelsurveyshasbeentheidentificationofapopula-
ation even at mid-infrared wavelengths (Churchwell et al. 2009;
tionofdeeplyembeddedprotostellarcores,whichappearaspoint
Carey et al. 2009), when viewed against the Galactic plane. This
sources(∼ 0.1pcattypical3kpcdistancetoIRDCs)intheHer-
iswhat ledtothecloud complexes containing such regions tobe
schelPACSbands(Henningetal.2010;Raganetal.2012a).How-
ever, detailed case studies beyond the core scale, the key to un-
derstandtheearlyfragmentationthat directlyinitiatessubsequent
⋆ E-mail:[email protected]
(cid:13)c 0000RAS
2 KeWanget al.
-00.05°
e
d
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t
ti
a
L 9(P1)
tic -00.10°
c
a
al 18(P6)
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-00.15°
5pc
11.30° 11.25° 11.20° 11.15° 11.10° 11.05° 11.00°
Galactic Longitude
Figure1.ASpitzercompositeimage(red/green/blue=24/8/4.5 µm)ofthe‘Snake’nebula.TheSpitzerdataaretakenfromtheGLIMPSEandMIPSGAL
legacyprojects(Churchwelletal.2009;Careyetal.2009).Thescalebarindicatesthespatialextentof5pcatthesourcedistanceof3.6kpc.Thearrowspoint
totwomid-IRsources,orprotostars,asidentifiedbyHenningetal.(2010).TheirIDnumbers(9,18)andassociatedclumps(P1,P6)arelabelledaccordingly.
NotethisfigureisplottedintheGalacticcoordinatesystem,whileotherfiguresinthispaperareintheJ2000Equatorialsystem.
clustered star formation, are only possible by deep interferomet- isstructuredasfollows.Afteradescriptionofthetargets(§2)and
ricimaging.Tomaximizethemasssensitivity,thepreferredwave- observations(§3),wepresent resultsin§4onhierarchicalstruc-
length of observations isinthesub-millimeter regime, accessible tures (§4.1), masers (§4.2), outflows (§4.3), chemical differenti-
bytheSubmillimeterArray(SMA)andbytherecentlyinaugurated ationof the cores (§4.4), and NH emission (§4.5), followed by
3
AtacamaLargeMillimeter/submillimeterArray(ALMA). discussion in §5 on hierarchical fragmentation (§5.1), shock en-
Comparedtothenumerousinterferometricstudiesonmassive hanced NH3 ortho/pararatio(§5.2),apossible proto-binarywith
proto-clusters(e.g.,Rathborneetal.2008;Hennemannetal.2009; anoutflow/discsystem(§5.3),andaglobalevolutionarysequence
Beuther&Henning2009;Longmoreetal.2011;Palauetal.2013) ofcoresandclumps(§5.4).Finally,wesummarizethemainfind-
andNH observationsofIRDCs(Wangetal.2008; Devineetal. ingsin§6.
3
2011;Raganetal.2011,2012b;Wangetal.2012),therehavebeen
fewhighangularresolutionstudiesdedicatedtopre-clusterclumps
in the literature (Rathborne et al. 2007; Swift 2009; Zhang et al. 2 TARGETS:DENSECLUMPSINIRDCG11.11–0.12
2009;Busquetetal.2010;Zhang&Wang2011;Wangetal.2011;
G11.11–0.12,alsoknownasthe“Snake”nebula,isoneofthefirst
Pillai et al. 2011; Beuther et al. 2013; Lee et al. 2013). Among
IRDCsidentifiedbyEganetal.(1998)fromtheMidcourseSpace
these,onlyasmallportionreachedaresolutionbetterthanthe0.1
Experiment imagesowingtoitsremarkablesinuousdarkfeatures
pccorescale.Therefore,inthepastyearswehaveusedSMAand
in the mid-IR (see Fig. 1 for an overview). Shortly after the dis-
VLAtopeerintoseveralIRDCclumpstostudytheirfragmentation
coveryof Eganetal.(1998), Careyet al.(1998) observed H CO
(Zhangetal.2009;Zhang&Wang2011;Wangetal.2011,2012, 2
lineemission,atracerofdensegas,inthecentralpartoftheSnake,
2013). We use SMA dust continuum emission to resolve hierar-
andthusdirectlyconfirmed(inadditiontotheinfraredextinction)
chicalstructures,andVLANH inversiontransitionstoprecisely
3
the existence of dense gas in the IRDC. A kinematic distance of
measurethegastemperature.Westrictlylimitoursampletodense
3.6kpcwastheninferredbasedontheradialvelocityoftheH CO
molecularclumpsthatrepresenttheextremeearlyphases(priorto 2
line, putting theSnake on the near side of theScutum-Centaurus
theappearanceofhotmolecularcores).Thismakesourprogramme
arm[seeTackenbergetal.(2012)andGoodmanetal.(2013)1 for
uniqueinprobingtheearlyfragmentation.
a Galactic illustration]. Later, Carey et al. (2000) and Johnstone
In one of our studies of G28.34-P1, we found hierarchical
et al. (2003) obtained JCMT 450 and 850µm continuum images
fragmentation where turbulent pressure dominates over thermal
fortheentireSnake,andidentifiedsevenmajoremissionclumpsP1
pressure.Thisisincontrastwithlow-massstarformationregions
throughP7.Pillaietal.(2006a,b)mappedtheentirecloudinNH
wherethermalJeansfragmentationmatcheswellwithobservations 3
using the Effelsberg 100-m Radio Telescope and found a consis-
(e.g., Lada et al. 2008), and isconsistent withstudies that turbu-
tentV around29.8kms−1alongtheSnake,thustheelongated
lencebecomesmoreimportantinhigh-massstarformationregions LSR
(aspectratio28pc/0.77pc=36:1)cloudisindeedaphysicallyco-
(Wangetal.2008,2009,andreferencestherein).Whetherthiskind
herententity,notachancealignment.
of fragmentation isa common mode of the initial fragmentation,
andhowthefragmentsgrowphysicallyandchemically,areofgreat
importance,yetremainunexplored.Inthispaper,weaddressthese 1 See a paper in preparation at https://www.authorea.com/
questions byextending our studytotwoearlyclumps. Thepaper users/23/articles/249
(cid:13)c 0000RAS,MNRAS000,000–000
HierarchicalfragmentationinIRDC G11.11–0.12 3
AsademonstrationcasefortheHerschelkeyproject“Earliest GHzrangesfrom0.05to0.12duringthefourtracks.Thefullwidth
Phases of StarFormation” (EPoS),Henning et al. (2010) studied athalf-maximum(FWHM)primarybeamisabout 52′′ attheob-
G11.11–0.12 withdeepHerschel imagesinmultiplewavelengths servedfrequencies.Table1summarizestheobservations.
and identified 18 protostellar “cores” along the Snake filament, The visibility data were calibrated using the IDL superset
which they call “seeds of star formation”. By fitting the spectral MIR3.Calibratedvisibilitydatawerethenexportedoutforimaging
energy distributions (SEDs) of individual cores, Henning et al. andanalysisinMIRIAD4(Saultetal.1995)andCASA5(Petry&
(2010) obtained physical parameters including dust temperature, CASADevelopmentTeam2012).Datafromdifferenttrackswere
mass,andluminosity.Amongallthesecores,two(#9and#18;see calibrated separately, and then combined in the visibility domain
Fig. 1 for their locations) massive and luminous cores stand out. forimaging.Continuumemissionwasgeneratedbyaveragingline
Withmassesof240and82M⊙andluminositiesof1.3×103and freechannelsinthevisibilitydomain.Table2liststhesynthesized
1.4×102 L⊙ respectively, the two cores distinguish from other beamand1σRMSnoiseoftheimages.
cores of much lower mass and luminosity, and therefore are the
most likely sites to form massive stars in the entire cloud. The
3.1.2 345GHzBand
twocoresresideinclumpsP1andP6respectively,coincidentwith
twomid-IRpoint sourceswhichdominatetheluminositiesofthe In2011, werevisited P1and P6withSMA at the345GHzband
clumps.P1andP6lieinthecentreandtheheadoftheSnakere- in two tracks, one in subcompact and another in extended ar-
spectively. Theseclumps, withsizes lessthan 1pc, arelikely re- ray configuration. The two tracks used the same correlator setup
sultsofglobalfragmentation(see§5.1.2),whilefurtherfragmen- which covers rest frequencies 333.7–337.7GHz in the LSB, and
tationtowardssmallerscalesarelessaffectedbytheglobal envi- 345.6–349.6GHzintheUSB,withauniformspectralresolutionof
ronmentbutratherdependonlocalpropertiesoftheclumpsthem- 0.812MHz (or 0.7 km s−1) across the entire band. System tem-
selves(Kainulainenetal.2013).Bothclumpshaveamassreservoir peratures were in the range of 200–300 K, and the zenith opac-
of ∼ 103M⊙ within 1 pc (§5.1). Therefore, P1 and P6 are two ityτ225GHz wasstableat0.06duringtheobservations. Otherpa-
massive, relatively low-luminosity molecular clumps that are the rametersarelistedinTable1.Thedatawerereducedandimaged
mostlikelysitesofhigh-massstarformationinthe“Snake”IRDC. in the same way as the 230GHz data. Additionally for P1, we
Hence,resolvingtheinitialstarformationprocessesinP1andP6 made an image using data from the extended configuration only
isofgreatinterest. andachievedahigherresolution(see§4.3.1).Imagepropertiesare
AlthoughG11.11–0.12isoneofthemostwellstudiedIRDCs, tabulatedinTable2.
previousstudiesaremostlylimitedtoangularresolutionachieved
withsingledishtelescopes(Careyetal.1998,2000;Johnstoneetal.
2003; Pillaietal.2006a; Tackenbergetal.2012). Theonlyinter- 3.2 VeryLargeArray
ferometricstudiesarestillyettoresolveunderlyingfinestructures The Karl G. Jansky Very Large Array (VLA) of NRAO6 was
(Pillaietal.2006b;Go´mezetal.2011).Here,wepresentnewSMA
pointedtowardsP1initsDconfigurationintwoobservationruns
andVLAobservationsofP1andP6whichresolvegreatdetailsof
in 2001 to observe the NH (J,K) = (1,1) transition (project
3
the star formation activities that capture the growth of these star
AW571,PI:FriedrichWyrowski).The3.125MHzbandwassplit
formationseedsinaction.
into 128 channels with a channel spacing of 24.4 kHz (or 0.3
km s−1). In 2004, P1 was revisited with another phase centre to
obtain 22 GHz H O maser and NH (2,2) and (3,3) transitions
2 3
3 OBSERVATIONS (projectAP475,PI:ThusharaPillai).TheH Omaserwasobserved
2
inasingleIF,dualpolarizationmode,withthesamebandwidthand
3.1 SubmillimeterArray
channelspacingasthe2001observations.TheNH (2,2)and(3,3)
3
3.1.1 230GHzBand wereobservedina2IF,dualpolarizationmode,splittingthe3.125
MHzbandinto64channels,eachwitha0.6 kms−1channelwidth.
TheSubmillimeterArray2 (SMA;Hoetal.2004)waspointedto-
ObservationsofP6werecarriedoutusingtheexpandedVLA,
wards G11.11–0.12 P1 and P6 to obtain continuum and spectral
orEVLA,initsCconfigurationinthreeobservationrunsduringthe
line emission in the 230 GHz band during four tracks in 2010,
EVLAearlysciencephasein2010–2011,andtworunsinthe2013
whenSMAwasinitssubcompact,compact,andextendedconfig-
Dconfiguration.Thankstotheflexibilityofthenewcorrelator,we
urations. Time-dependent antenna gains were monitored by peri-
observedtheNH (1,1),(2,2),(3,3),(4,4),and(5,5)transitionsas
odicobservationsofquasarsNRAO530andJ1911-201;frequency- 3
well as 22 GHz H O maser and 25 GHz class I CH OH maser
dependent bandpassresponseswerecalibratedbyquasars3C273, 2 3
lines, with various bandwidth and spectral resolutions (see Table
3C279and3C454.3;andabsolutefluxwasscaledbyobservedcor-
1).
relatorcountswithmodelledfluxesofSolarsystemmoonsTitan,
For all the experiments, gain, bandpass, and flux variations
Callisto, and Ganymede. The empirical flux uncertainty is about
werecalibratedby strong, point-like quasars. SeeTable1for de-
15%.Forthefourtracks,weusedthesamecorrelatorsetupwhich
tails.ThevisibilitydatawerecalibratedusingCASA,andthenim-
covers 4 GHz in each of the lower and upper sidebands (LSB,
aged and analyzed in MIRIAD and CASA. Data from different
USB),withauniformchannelwidthof0.812MHz(equivalentve-
locity1.1 kms−1at230GHz)acrosstheentireband.Systemtem-
peratures varied from 80 to 150K, and the zenith opacity at 225 3 http://www.cfa.harvard.edu/˜cqi/mircook.html
4 http://www.cfa.harvard.edu/sma/miriad,http:
//www.astro.umd.edu/˜teuben/miriad
2 TheSubmillimeterArrayisajointprojectbetweentheSmithsonianAs- 5 http://casa.nrao.edu
trophysical Observatory andtheAcademiaSinica Institute ofAstronomy 6 TheNationalRadioAstronomyObservatoryisafacilityoftheNational
and Astrophysics and is funded by the Smithsonian Institution and the Science Foundationoperated undercooperative agreement byAssociated
AcademiaSinica. Universities,Inc.
(cid:13)c 0000RAS,MNRAS000,000–000
4 KeWanget al.
Table1.Observations
Telescope Date Pointinga Lines Calibratorb Bandwidth Chan.widthc Pol. Int.Time
Config. (UT) Gain Flux Bandpass (MHz) (kms−1) (min)
(E)VLAKband(primarybeam2′)
VLA-D 2001-Oct-27 P1.I NH3(1,1) Q1 Q6 Q5 3.125 0.3 1 10
VLA-D 2001-Nov-11 P1.I NH3(1,1) Q3 Q6 Q5 3.125 0.3 1 22
VLA-D 2004-Aug-24 P1.II H2O Q1 Q6 Q1,Q6 3.125 0.3 2 20
VLA-D 2004-Aug-24 P1.II NH3(2,2),(3,3) Q1 Q6 Q1,Q6 3.125 0.6 2 50
VLA-D 2004-Aug-29 P1.II NH3(2,2),(3,3) Q1 Q6 Q1,Q6 3.125 0.6 2 50
EVLA-C 2010-Dec-24 P6.I H2O,CH3OH Q1 Q6 Q4 4 0.8 4 10,6
EVLA-C 2010-Dec-28 P6.I NH3(1,1),(2,2) Q1 Q6 Q4 4 0.2 2 7,8
EVLA-C 2011-Jan-18 P6.I NH3(2,2),(3,3) Q1 Q6 Q4 4 0.4 2 20
EVLA-D 2013-Mar-17 P6.I NH3(1,1)to(5,5),H2O Q1 Q8 Q7 4/8 0.1/0.2 2 45
EVLA-D 2013-Apr-18 P6.I NH3(1,1)to(5,5),H2O Q1 Q8 Q7 4/8 0.1/0.2 2 45
SMA230GHz(1.3mm)band(primarybeam52′′)
SMA-Sub 2010-Mar-19 P1.III,P6.II Many,seeTable5 Q1,Q2 M1 Q4 2×4000 1.1 1 5.8/6.2hrd
SMA-Com 2010-Jun-15 P1.III,P6.II Many,seeTable5 Q1,Q2 M1 Q5,Q7 2×4000 1.1 1
SMA-Ext 2010-Aug-27 P1.III,P6.II Many,seeTable5 Q1,Q2 M2,M3 Q7 2×4000 1.1 1
SMA-Ext 2010-Sep-20 P1.III,P6.II Many,seeTable5 Q1,Q2 M2,M3 Q7 2×4000 1.1 1
SMA345GHz(880µm)band(primarybeam34′′)
SMA-Sub 2011-Mar-15 P1.IV,P6.III Many,seeTable5 Q1,Q2 M1 Q5 2×4000 0.7 1 2.2/3.5hrd
SMA-Ext 2011-Jul-22 P1.IV,P6.III Many,seeTable5 Q1,Q2 M3 Q5 2×4000 0.7 1
Note:
aPhase centres in J2000 Equatorial coordinates: P1.I= 18:10:28.3, -19:22:29; P1.II=18:10:30.475, -19:22:29.39; P1.III= 18:10:28.4, -19:22:38; P1.IV=
18:10:28.21,-19:22:33.34;P6.I=18:10:07.42,-19:29:07.7,P6.II=18:10:07.2,-19:28:59;P6.III=18:10:07.38,-19:29:08.00.
bCalibratorsareQuasarsandMoons:Q1=NRAO530(J1733-130),Q2=J1911-201,Q3=J1743-038,Q4=3C273,Q5=3C279,Q6=3C286,Q7=3C454.3,
Q8=3C48;M1=Titan,M2=Callisto,M3=Ganymede.
cNativechannelwidth,subjecttosmoothingforsomelines(§3.2).
dTotalon-sourceintegrationtimecombiningdatafromallSMAarrayconfigurationsforP1andP6,respectively.
Table2.ImageProperties
P1 P6
Imagea Beam RMSb Beam RMSb
1.3mmcontinuum 2′′.2×1′′.6 0.9 2′′.1×1′′.5 0.9
1.3mmspec.linesc 2′′.7×1′′.7 25–30 sameasP1
880µmcontinuum 1′′.6×1′′.2 3.3 1′′.2×1′′.0 2.3
0′′.8×0′′.6 1.7
880µmspec.linesa 2′′.1×1′′.3 110 sameasP1
NH3(1,1) 5′′×3′′ 14 7′′.1×2′′.8 4
NH3(2,2) 5′′×3′′ 3.5 6′′.1×2′′.7 2.5
NH3(3,3) 5′′×3′′ 6.5 6′′.9×2′′.9 2.8
NH3(4,4) Nodata 7′′.7×3′′.0 2.3
NH3(5,5) Nodata 6′′.7×3′′.0 2.5
H2Omaser 5′′×3′′ 13 2′′.4×1′′.0 2.7
7′′.0×3′′.2 4
7′′.2×3′′.2 2
CH3OHmaserclassI Nodata 2′′.0×0′′.9 2.2
Note:
aAllimagesaremadewithnaturalweightingtoachievethehighestsensitivity.
b1σRMSnoisein mJybeam−1.
cForspectrallineimages,beamvariesslightlyfromlinetoline,henceatypical
beamislisted.
(cid:13)c 0000RAS,MNRAS000,000–000
HierarchicalfragmentationinIRDC G11.11–0.12 5
Figure2.HierarchicalstructuresinG11.11-P1(a–c)andG11.11-P6(d–f),asseenbyJCMTat850 µm,SMAat1.3mm,andSMAat880 µm(contours),
superposedonSpitzer8 µmimage(colorscale).TheJCMTcontinuumemissioniscontouredat0.3Jybeam−1.TheSMA1.3mmemissioniscontouredat
±(3,6,9,...)σ,whereσ=0.9mJybeam−1forbothP1andP6.TheSMA880 µmemissioniscontouredat±(3,5,7,...)σ,whereσ=3.3mJybeam−1
forP1andσ=2.3mJybeam−1forP6.Theshadedellipseinthebottomleftcornerofeachpanelrepresentsthebeamsizeofthecontouredimage.Thered
starsmarkdominantcondensationsidentifiedfromtheSMA880 µmimages.Thescalebarsrepresentaspatialscaleof0.1pcatthesourcedistanceof3.6
kpc.Negativecontoursaredashedthroughoutthispaper.
observationrunswerecalibratedseparately,andthencombinedin of∼1pc,acoreasastructurewithasizeof∼0.1pc,andacon-
thevisibilitydomainforimagingwiththeexceptionformaserfor densationasasubstructureof∼0.01pcwithinacore.Aclumpis
which we make individual images from different observing runs, capableofformingaclusterofstars,acoremayformoneorasmall
toinvestigatepotentialtimevariations.Thefinalimagecubeskeep groupofstars,andacondensationcantypicallyformasinglestar
thenativechannelwidthlistedinTable1,exceptfortheNH (1,1), or amultiple-star system. These structuresare dense enough that
3
(2,2),and(3,3)imagesofP6wherethefinalchannelwidthis0.4 gasand dust arewellcoupled (Goldsmith2001), thusweusethe
kms−1inordertoincludeboththeCandDconfigurationdata.Ob- NH gastemperatureasadirectmeasureofthedusttemperature
3
servationalparametersaresummarizedinTable1andimageprop- throughoutthispaper.
ertiesarelistedinTable2.
Dust continuum images at various resolutions reveal hierar-
chicalstructuresinP1andP6.Figure2plotsJCMT850µm,SMA
1.3mm,andSMA880µmimagessuperposedonSpitzer8µmim-
4 RESULTS ages.TheJCMTimagesoutlineIRDCclumpsP1andP6,whereP1
exhibitsabrightMIRsource(protostar#9inFig.1)initscentre,
4.1 HierarchicalStructure
whileP6showsalessbrightMIRsource(protostar#18inFig.1)
Intheliterature,therehavebeendifferentdefinitionsforaclump, initssouthernpart.Astheresolutionincreases,structuresatdiffer-
core, and condensation when describing the spatial structure of entscalesarehighlighted:from∼1pcscaleclumpsseeninJCMT
molecular clouds. For consistency, we adopt the terminology of 850µm images,tothe∼0.1pcscalecoresresolved bytheSMA
clump, core, and condensation suggested by Zhang et al. (2009) 1.3mmimages,andtothe∼0.01pcscalecondensationsresolved
andWangetal.(2011).Wereferaclumpasastructurewithasize by the SMA 880µm images. These structures show in general a
(cid:13)c 0000RAS,MNRAS000,000–000
6 KeWanget al.
Table3.PhysicalParametersoftheCondensations
Source RA Dec Fluxa T Massb Sizec
ID (J2000) (J2000) (mJy) (K) (M⊙) Maj.(′′) Min.(′′) PA(◦)
P1-SMA
1 18:10:28.27 -19:22:30.9 205.0 25 14.1 2.1 1.3 114
2 18:10:28.19 -19:22:33.2 46.7 19 4.7 1.9 0.4 86
3 18:10:28.09 -19:22:35.7 197.0 17 23.7 3.4 1.7 69
4 18:10:28.15 -19:22:27.0 57.6 18 6.3 2.5 1.2 99
5 18:10:28.56 -19:22:31.9 53.5 18 5.9 2.1 1.1 60
6 18:10:28.27 -19:22:39.7 54.5 15 8.0 2.3 1.6 156
P6-SMA
1 18:10:07.80 -19:29:01.2 109.7 11 27.9 1.1 0.9 147
1b 18:10:07.83 -19:28:59.3 83.8 11 21.3 3.4 1.3 48
2 18:10:07.39 -19:28:58.3 78.8 10 24.2 1.5 1.1 135
2b 18:10:07.58 -19:28:57.9 33.0 10 10.1 1.5 0.7 167
2c 18:10:07.22 -19:29:00.6 38.2 10 11.7 2.1 0.6 131
2d 18:10:07.30 -19:29:00.8 54.9 10 16.9 2.5 1.0 91
2e 18:10:07.39 -19:29:01.3 35.2 10 10.8 2.5 0.7 109
3 18:10:07.12 -19:29:04.3 66.4 14 10.9 1.7 1.2 55
4 18:10:07.10 -19:29:06.8 35.3 15 5.2 1.0 1.0 83
5 18:10:07.25 -19:29:09.0 82.1 21 7.2 1.6 0.8 143
5b 18:10:07.32 -19:29:10.8 30.9 21 2.7 1.4 1.2 16
5c 18:10:07.27 -19:29:11.0 21.5 21 1.9 1.7 1.1 23
6 18:10:07.25 -19:29:17.6 157.0 19 15.9 1.4 1.0 105
6b 18:10:07.51 -19:29:13.5 29.0 19 2.9 1.3 0.7 50
6c 18:10:07.46 -19:29:14.3 44.5 19 4.5 2.0 0.9 41
6d 18:10:07.39 -19:29:15.4 28.7 19 2.9 1.3 1.0 35
6e 18:10:07.06 -19:29:18.4 32.7 19 3.3 1.5 0.6 36
Note:
aIntegratedfluxobtainedfrom2DGaussianfittingandcorrectedforprimarybeamattenuation.
b Masscomputedassumingdustopacityindexβ = 1.5.Themassscales withβ inaformof
M ∝3.5β.Forreference,ifβ=2,themasswillbe1.87timeslarger.
cDeconvolvedsourcesize.
goodspatialcorrelationwiththeHerschel70µmemissionandthe exceptone(P6-SMA4)“major”condensations(redstarsinFig.2)
Spitzer8/24µmextinction,wheretwoIR-brightprotostarshaveal- coincidewiththecoresresolvedintheSMA1.3mmimages.Con-
readydeveloped(Fig.2;Henningetal.2010;Raganetal.2012a). densation P1-SMA1 is coincident with protostar #9 identified by
Weidentifythesmalleststructure,condensations,basedonthe Henningetal.(2010)frommulti-bandHerschelimages,andcon-
highestresolutionSMA880µmimagesasshowninFig.2(c,f).All densationP6-SMA6iscoincidentwiththeprotostar#18.
featuresabove5σ rmsareidentifiedasinWangetal.(2011) and Dustmassisestimatedwiththeassumptionthatdustemission
Zhangetal.(2009).Wefirstidentify“major”emissionpeakswith isopticallythin(whichisvalidat0.88and1.3mm),following
fluxes> 9σ (the4thcontour) andassignthemasSMA1,SMA2, S d2
SMA3,...,inorderfromeasttowestandfromnorthtosouth.Three Mdust = B (Tν )κ , (1)
majorpeaksareidentifiedinP1andsixidentifiedinP6,denotedby ν dust ν
redstarsinFig.2.Thenweidentify“minor”emissionpeakswith where M is the dust mass, S is the continuum flux at fre-
dust ν
fluxes>5σ(the2ndcontour).Threeminorpeaksareidentifiedin quencyν,disthesourcedistance,B (T )isthePlanckfunction
ν dust
P1, and we assign them as SMA4, SMA5, SMA6, from north to atdusttemperatureT ,andκ = 10(ν/1.2THz)β cm2g−1is
dust ν
south.InP6,11minorpeaksareidentified.Theseemissionpeaks the dust opacity (Hildebrand 1983). In the calculation we adopt
arerelativelyweakandareassociatedinpositionwiththe6major thetemperaturemeasuredfromNH (§4.5.2) andthedustopac-
3
peaks. We thus assign these minor peaks to the associated major ity index β = 1.5. If β = 2 the mass would be a factor of 2
peaks.Forinstance,thetwominorpeaksassociatedwithP6-SMA5 larger. Dust mass is then translated to gas mass accounting for a
areassignedasP6-SMA5bandP6-SMA5c.Alltheidentifiedmajor gas:dustratioof100.Thecomputedtotalmassforeachcondensa-
andminorpeaksareofthesizeofcondensations.Associatedcon- tionisreportedinTable3. Notethatinterferometricimagesfilter
densationsmayhavebeenfragmentedfromacommonparentcore. out relatively smooth emission due to imperfect (u,v) sampling,
Insummary,weidentify6condensationsinP1whichmaybelong leading to“missing flux”. Thus thetotal mass of thedense cores
to6cores(P1-SMA1,2,3,4,5,6),respectively,and17condensations orcondensationsrevealedbySMA(Fig.2)islessthantheclump
inP6whichmaybelong to6cores (P6-SMA1,2,3,4,5,6),respec- massdeterminedfromsingledishJCMTobservations.Ouranalysis
tively.Foreachcondensation,wefita2DGaussianfunctiontothe (§5.1)doesnotrelyonthesmoothemissionbutonclumpystruc-
observed SMA 880µm image and list the results in Table 3. All tures,thereforeisnot affectedbythemissingflux.Forreference,
(cid:13)c 0000RAS,MNRAS000,000–000
HierarchicalfragmentationinIRDC G11.11–0.12 7
theSMA880µmimagesrecover7%and14%ofthetotalJCMT of outflow tracers (Carey et al. 2000; Pillai et al. 2006b; Leurini
850µmfluxinP1andP6,respectively.Thisisconsistentwiththe etal.2007;Cyganowskietal.2008;Go´mezetal.2011).Ourhigh-
factthatP6containsmorecompactstructuresthanP1(Fig.2). sensitivity,high-resolution,broad-bandSMAobservationsdirectly
Thesensitivityinthe880µmimages(Figure2,Table2)cor- revealthisoutflowinmultipletracersforthefirsttime,providing
respondsto0.2–0.5M⊙forthe15–25KgastemperatureintheP1 criticalsupportforapreviouslyspeculatedoutflow-discsystemin
clump(§4.5.2),and0.2–0.7M⊙ inP6fora10–21Ktemperature P1(§5.3).
range(§4.5.2).Therefore,theidentifiedcondensationiscomplete The SiO outflow extends 0.3 pc away from the protostar in
toa5σmasslimitof1–3.5M⊙,dependingonthetemperature. theeast-westdirection(Fig.3).Whilethebluelobefurtherpropa-
gatestowardstheeasternedgeofthemainfilament,itinducedthe
NH emissionpeaksA,BandD,andprobablyexcitedthewater
3
4.2 H2OandCH3OHMasers maserW1(seefirstparagraphin§4.5.1).Theprojectedseparation
oftheNH peaksarel = 0.3pc,andl = 1pc(Fig.6),i.e.,
No H O or CH OH maser line emission was detected above 3σ 3 AB AD
2 3
themolecularoutflowextendsatleast1pcawayfromthedriving
inP6.InP1,wedetecttwo22GHzH Omaserswhichwename
2
sourceintheeasternlobe.Furthertotheeast fromD,thereisno
asW1andW2,indecreasingorderofbrightness(whiteandblack
densegastobeheatedandshocked.Inthewesternlobe,however,
crossesinFig.6).W1islocatedat(RA,Dec) =(18:10:32.902,
J2000
thereisnodensegasbeyondtheredshiftedSiOlobewhichis0.3
-19:22:23.339), close to the eastern border of the dark filament.
It has a flux of 3 Jy at V = 30.5 km s−1. W2 is located at pcfromtheprotostar.Becausethepoweringsourceislocatednear
LSR
theedgeofthedensefilament,thereismoredensegasintheeastern
(RA, Dec) = (18:10:28.298, -19:22:30.759), coincident with
J2000
lobetobeheated(andthereforebeingdetected)thaninthewestern
P1-SMA1. It has a flux of 0.25 Jy and is seen at V = 37.5
LSR
kms−1.BothW1andW2show atypicalmaser spectrumwitha lobe.Thespeciallocationandorientationofthisoutflowprovides
auniquecasetostudyenvironmentdependenceofoutflowchem-
singlevelocitycomponentandhasanarrowlinewidth(FWHM<
1 kms−1).Thespectralprofileisnotresolvedatthe0.3 kms−1 istry,whichdeservesfurtherstudy.
WecomputeoutflowparametersassumingLTEandoptically
channelwidth.
thinSiOemissioninthelinewings,followingtheformulasgiven
In addition to the two water masers, Pillaiet al. (2006b) re-
in Wang et al. (2011). We adopt an abundance of [SiO/H ] =
porteda6.7GHzclassIICH OHmaserusingtheAustraliaTele- 2
3 5×10−10 based on recent chemistrysurveys towardsIRDCsby
scopeCompactArray(ATCA),whichweassignasM1.Locatedat
(RA,Dec) =(18:10:28.248,-19:22:30.45),M1is0′′.7(2500 Sanhueza et al. (2012). The inclination angle of this outflow can
J2000
beinferredbasedontheCH OHmasersdiscoveredbyPillaietal.
AU)westofW2(Figs.3,10,11).Unlikeasinglevelocitycompo- 3
(2006b).Themasersoutline(partof)anellipsewithaneccentric-
nent seen in the water masers, M1 has multiple velocity compo-
nents ranging from 22 to 34 km s−1. Pillai et al. (2006b) noted ity of ∼ 0.2. Suppose the masers trace a circular disc, we infer
an inclination angle of ∼ 77◦ between the axis of the disc (also
thatM1consistsofsixmaserspotswhicharespatiallydistributed
alonga0′′.3north-southarc,andexhibitsanorderedvelocityfield theoutflowjet)andthelineofsight.Therefore,thediscisalmost
edge-ontousandtheoutflowjetisalmostparalleltotheplaneof
red-shiftingfromnorthtosouth.Basedonthepositionandvelocity
sky.Table4showsthederivedoutflowparameterswithandwith-
distribution,Pillaietal.(2006b)suggestedthattheCH OHmaser
3
outcorrectionforinclination.TheP1-SMA1outflowisamassive
spotstracearotatingKepleriandiscseenedge-on.Ourdetectionof
outflowjudgingfromitsenergetics,incomparisonwithotherout-
anEast-Westmolecular outflow centred onW2stronglysupports
flowsemanatingfromhighmassprotostellarobjects(Beutheretal.
thisspeculation(see§4.3.1).
2002;Zhangetal.2005).
4.3 ProtostellarOutflows
4.3.2 OutflowsDrivenbyP6-SMA2,5,6
4.3.1 OutflowdrivenbyP1-SMA1
WealsodetectmolecularoutflowsinP6.Theseoutflowsareseen
Our SMA observations clearly reveal a bipolar outflow oriented inmanymoleculartracersincludingCO,SO,SiO,H CO,butare
2
east-west in P1. The outflow is seen in many molecular tracers best seen in CH OH. Fig.4 shows channel maps of CH OH (4–
3 3
including CO,SO, SiO,H CO, and CH OH, but only inSiO do 3), where one can see that SMA2,5,6 drive relatively collimated
2 3
both the blue and red lobes appear; other tracers only reveal the outflows. Outflows associated with SMA2 and SMA6 show both
blue(eastern)lobe.Fig.3plotstheblueshiftedSiOemission(18– blueandredshiftedlobesextendingabout0.1–0.3pc,whereasthe
28km s−1, blue contours) to the east, the SiO emission close to SMA5outflowonlyshowsabluelobeapproximately0.3pclong.
the systemic velocity (29–31km s−1, gray background), and red All these outflowsare oriented in aNE-SWdirection, coincident
shiftedSiOemission(32–39kms−1,redcontours)tothewest,in withemissionfromtheNH (3,3)andhighertransitions(Fig.7).
3
comparisonwiththeSMA880µmcontinuum(blackcontours).A The outflow parameters (Table 4) are also calculated in a similar
bipolarSiOoutflowisclearlydefinedbytheblue/redlobeswithre- wayasfor P1.Theinclinationanglesfor theP6outflowsareun-
specttothedustcontinuum,whichisprobablyproducedinshocks known.Welistoutflowparameterswithoutcorrectionandwithcor-
by an underlying jet. We schematically draw the axis of the out- rectionfor aninclination of 57.3◦,the most probable value for a
flow on the plot, and measure aposition angle of 94±12◦. The randomdistributionofoutfloworientations(Bontempsetal.1996;
geometriccentreoftheoutflowisclosetoP1-SMA1,andwespec- Semeletal.2009).ComparedwithoutflowP1-SMA1,theP6out-
ulate that the outflow driving source is a protostar embedded in flowsareslightlylessenergeticbutarestillcomparablewithother
the dust condensation P1-SMA1, likely the #9 protostar identi- high-massoutflows(Beutheretal.2002;Zhangetal.2005).
fied by Henning et al. (2010). (See more discussion on the driv- ThecoresinG11.11-P1andG11.11-P6exhibitsimilarcharac-
ingsourcelaterin§5.3).Previousstudieshaveshownindirectev- teristics(mass,size,andglobalmassreservoir)tothoseinG28.34-
idence of an outflow associated withP1, for example broadening P1.Thecoresdrivingpowerfuloutflowsareundergoingaccretion
oflinewings,possibleextended4.5µmemission,andenrichment to build up stellar mass. With a typical star formation efficiency
(cid:13)c 0000RAS,MNRAS000,000–000
8 KeWanget al.
Red
Blue
Figure 3. The SiO(5–4) outflow in G11.11-P1. Blue/red contours show the blue-/red-shifted SiO emission integrated over [18, 28]km s−1 and [32,
39]kms−1,respectively,whereastheblackcontoursrepresentSiOemissionnearthesystemicvelocityintegratedover[29,31]kms−1.TheSiOcontours
are±(3,4,5,...)σ,whereσ = 85mJykms−1.Forcomparison,theSMA880µmcontinuumimageisshownasgreyscaleinthebackground.Labelled
symbolsdenoteH2OmaserW2(+),classIICH3OHmaserM1(×),andthethree2MASSpointsources(◦).Inthebottomrightcornertheopen/filledellipses
representsynthesizedbeamsoftheSiO/880µmimages.Thethickgreylinemarksschematicallytheunderlyingbipolarjetthatpropelstheobservedmolecular
outflow.ThegreylinecentresonW2andextends0.3pceastboundandwestbound,respectively.
Table4.OutflowParameters
Parametera P1-SMA1 P6-SMA2 P6-SMA5 P6-SMA6
Blue Red Blue Red Blue Blue Red
Tracer SiO SiO CH3OH CH3OH CH3OH CH3OH CH3OH
Fractionalabundance[X/H2]b 5×10−10 5×10−10 5×10−8 5×10−8 5×10−8 5×10−8 5×10−8
ExcitationtemperatureTex(K) 25 25 21 21 21 21 21
Inclinationangleθ(degree)c 77 77 57.3 57.3 57.3 57.3 57.3
Velocityrange(kms−1) [18,28] [32,39] [22,29] [31,35] [22,29] [22,29] [31,35]
TotalmassM(M⊙) 2.0 1.1 1.1 0.4 2.1 1.2 0.1
MomentumP (M⊙ kms−1) 10.8/48.2 4.6/20.7 1.8/3.3 0.8/1.5 6.1/11.3 4.1/7.5 0.3/0.5
EnergyE(M⊙km2s−2) 40.0/791.0 12.4/245.6 1.8/6.2 1.0/3.4 12.3/42.3 8.8/30.3 0.4/1.4
LobelengthLflow(pc) 0.30/0.31 0.30/0.31 0.17/0.20 0.07/0.08 0.24/0.29 0.28/0.34 0.08/0.10
Dynamicalagetdyn(103yr) 24.9/5.7 31.9/7.4 20.8/13.3 12.5/8.0 30.2/19.4 35.7/22.9 15.8/10.1
Outflowrate M˙out(10−5 M⊙yr−1) 8.0/34.8 3.3/14.3 5.4/8.4 3.2/5.1 6.9/10.8 3.5/5.4 0.7/1.1
Note:
aParameterscorrectedforinclinationfollowafterthe“/”.
bAdoptedabundancesarebasedonSanhuezaetal.(2012)andLeurinietal.(2007)forSiOandCH3OH,respectively.
cAnglebetweenoutflowaxisandthelineofsight,see§4.3.1and§4.3.2.
of 30% in dense gas and a standard stellar initial mass function, P6.Table5listsalldetectedlinesandFig.5plotsspectraofthenine
the∼103 M⊙clumpswilleventuallyformmassivestarsinsome “major”cores.BesidesCOisotopologues,P1andP6showseveral
of the cores once the protostellar accretion is completed. At that complexmolecularlineslikeOCS,HNCO,CH CN,andCH OH,
3 3
time,theseclumpswillbecomemassivestarclusters.Onekeydif- and molecules enriched by outflow shocks like SOand SiO.The
ferenceisthatwedetectcopiousmolecularemissioninthecoresin number of detected lines vary from core to core and may reflect
G11.11-P1andG11.11-P6,whichwecanusetoassesstheirevolu- chemicalevolution,despitethatthecoresarefragmentedfromthe
tionarystate(§4.4). sameparentclumps.TypicalhotcorelinesCH CNand/orCH OH
3 3
areseentowardscoresP1-SMA1,2,P6-SMA6,2,5,suggestingtheir
protostellar nature. Thisisconsistent withthedetection of proto-
4.4 ChemicalDifferentiation stellaroutflowsemanatingfrommostofthesecores.DeepHerschel
70µmimagerevealedpointsourcescoincidentwithP1-SMA1and
Our SMA broad-band observations covered a total of 16GHz in P6-SMA6andrelativelydiffuse70µmemissioningoodagreement
twoobservingbands,anddetectedmanymolecularlinesinP1and
(cid:13)c 0000RAS,MNRAS000,000–000
HierarchicalfragmentationinIRDC G11.11–0.12 9
Figure4.ChannelmapsofCH3OH(4−3)inG11.11-P6.Allpanelssharethesamecoordinatesaslabelledonthelower-leftpanel.Centralvelocity(kms−1)
ofeachchannelislabelledontheupperleftcorner.Thecontoursare±(3,4,5,6,...)σ,whereσ = 25mJykms−1.Thestarsdenotesixcoresidentified
inG11.11-P6(SMA1–6,labelledinthelastpanel).SMA2,SMA5,andSMA6driveeitherbipolaroruni-polaroutflows,asoutlinedbyschematiclines.The
filledellipserepresentssynthesizedbeam.
withour SMA 1.3mm images of P1 and P6, suggesting that P1- 4.5 NH EmissionandTemperature
3
SMA1andP6-SMA6havedevelopedanincreasedluminositythan
theirfellowcores(Raganetal.2012a). 4.5.1 ShockHeatedNH
3
The diagnosis in outflow, hot core lines, and 70µm emis-
sion show that P1-SMA1,2, P6-SMA6,2,5 are protostellar cores, We detect ammonia emission in all the observed transitions both
whiletheothercoresP1-SMA3,P6-SMA1,4,3arelikelyofprestel- inP1andP6.Figure6(leftpanel)showstheNH integratedim-
3
larnature.Moreover,thelinerichnessandstrengthrevealdetailed ages of P1 superposed on Spitzer 24µm and JCMT 850µm im-
chemicaldifferentiation,likelyreflectinganevolutionarysequence ages.ThesensitiveSpitzer24µmimagerevealsdetailsinthecen-
fromcoretocore,asweplotinorderinFig.5.Ideally,onewould tralpartoftheSnakeNebula:afilamentarysystemthatconsistsofa
compare cores with the same/similar mass, as a lower mass core mainNE-SWorienteddarkfilamentandtwominorfilamentsjoin-
can be more evolved but does not have detectable line emission. ing from the South. Thisconfiguration resembles the filamentary
However, this does not seem to affect our results, since the most system discussed by Myers (2009) and may arise from compres-
massivecoreP6-SMA1isnotmostevolved,andthemostevolved sionofanelongatedclumpembeddedinathincloudsheet,asseen
coreP1-SMA1isnotmostmassive.P1-SMA1isina“warmcore” inIRDCG14.225–0.506(Busquetetal.2013).Densegastracedby
phase because it has not yet reached the hot core phase defined theJCMTdustimageismostlyconcentratedonthemainfilament,
by Cesaroni (2005): temperature > 100 K, size < 0.1 pc, mass andNH appearsonlyonthemainfilament.TheNH (1,1)emis-
3 3
10−1000M⊙, and luminosity > 104 L⊙. The prestellar cores sionisrelativelycontinuous,whereasNH3(2,2)and(3,3)emission
all show H CO, a mid-product along the sequential hydrogena- arehighlyclumpy.WeidentifyfourrepresentativeNH emission
2 3
tion from CO to CH OH (Zernickel et al. 2012), thus they are peaks A, B,C, and D, and plot thecorresponding spectra inFig.
3
slightlymore evolved than the cores inIRDC clumps G28.34-P1 6(rightpanel).PeakAshowsthestrongestNH emission,andis
3
andG30.88-C1 whereonly12COisdetected(Zhangetal.2009; associatedwiththeIRpointsource(#9),dustcoreP1-SMA1,and
Zhang&Wang2011;Wangetal.2011).Distanceeffectcannotex- masersW2andM1.FollowingA,peaksB,C,andDareroughly
plainthedifference,seediscussionin§5.4.Withineachcore,the alignedonalineeastwardinsidethemainfilament,withDlocated
groupedcondensationsshowsimilarchemicaldifference. neartheeasternedgeofthedensefilamentclosetothewatermaser
(cid:13)c 0000RAS,MNRAS000,000–000
10 KeWanget al.
Figure5.SMA230GHzspectraextractedfromthe“major”cores,plottedininverseorderofevolution.Thebrightnesstemperaturehasbeencorrectedfor
theprimarybeamattenuation.Thesmallgapsaround218.75and230.75 GHzareduetoourcorrelatorsetup,whilethelargergapseparatesLSBandUSB.
Detectedmoleculesarelabelled.
(cid:13)c 0000RAS,MNRAS000,000–000