APEX/SABOCA observations of small-scale structure of infrared-dark clouds

26 Pages·2013·3.19 MB·English

A&A559,A79(2013) Astronomy
DOI:10.1051/0004-6361/201321869 &
(cid:13)c ESO2013 Astrophysics
APEX/SABOCA observations of small-scale structure
of infrared-dark clouds
I. Early evolutionary stages of star-forming cores(cid:63),(cid:63)(cid:63)
SarahE.Ragan,ThomasHenning,andHenrikBeuther
Max-Planck-InstituteforAstronomy,Königstuhl17,69117Heidelberg,Germany
e-mail:[email protected]
Received9May2013/Accepted28August2013
ABSTRACT
Infrared-darkclouds(IRDCs)harbortheearlyphasesofclusterandhigh-massstarformationandarecomprisedofcold(∼20K),dense
(n > 104cm−3)gas.Thespectralenergydistribution(SED)ofIRDCsisdominatedbythefar-infraredandmillimeterwavelength
regime, and our initial Herschel study examined IRDCs at the peak of the SED with high angular resolution. Here we present a
follow-upstudyusingtheSABOCAinstrumentonAPEXwhichdelivers7.8(cid:48)(cid:48)angularresolutionat350µm,matchingtheresolution
weachievedwithHerschel/PACS,andallowingustocharacterizesubstructureon∼0.1pcscales.Oursampleof11nearbyIRDCsare
amixoffilamentaryandclumpymorphologies,andthefilamentarycloudsshowsignificanthierarchicalstructure,whiletheclumpy
IRDCsexhibitlittlehierarchicalstructure.AllIRDCs,regardlessofmorphology,haveabout14%oftheirtotalmassinsmallscale
core-like structures which roughly follow a trend of constant volume density over all size scales. Out of the 89 protostellar cores
weidentifiedinthissamplewithHerschel,werecover40ofthebrightestandre-fittheirSEDsandfindtheirpropertiesagreefairly
wellwithourpreviousestimates((cid:104)T(cid:105) ∼ 19K).Wedetectanewpopulationof“coldcores”whichhaveno70µmcounterpart,but
are100and160µm-bright,withcoldertemperatures((cid:104)T(cid:105)∼16K).Thislatterpopulation,alongwithSABOCA-onlydetections,are
predominantlylow-massobjects,buttheirevolutionarydiagnosticsareconsistentwiththeearlieststarlessorprestellarphaseofcores
inIRDCs.
Keywords.catalogs–stars:formation–ISM:structure–submillimeter:ISM
1. Backgroundandmotivation self-gravitatingcores,andtheirfinaldissipationareallimportant
ingredientsonourwaytounderstandingthenatureofhigh-mass
Despite the importance of high-mass stars to the energy bud-
starformation.
get of galaxies, their formation remains a major open question
InSpitzercontinuumbands,IRDCsappear“dark,”butonly
inastronomy(Zinnecker&Yorke2007).Amajorhindranceto
progress is the inherent difficulty in obtaining well-defined ini- Herschel observations showcase the transition from dust ab-
sorbing the Galactic background (shortward of ∼100µm) into
tialconditionsobservationally.Becausehigh-massstarsarerare,
optically thin emission of cold dust structures (longward of
they are on average at large distances, thus angular resolution
∼100µm).Onsmallscales,Herschelhastransformedourview
is of paramount importance. With the advent of Spitzer Space
of IRDCs by probing their deeply embedded core population,
TelescopeandHerschelSpaceObservatory(Pilbrattetal.2010),
coupledwithextensiveground-basedsurveyefforts,wenowcan a fact that has been extensively exploited in recent literature
(Henningetal.2010;Beutheretal.2010,2012;Linzetal.2010;
approachthisobservationaltaskstatistically.
Bontemps et al. 2010; Ward-Thompson et al. 2010; Könyves
Ever since their discovery in absorption in mid-infrared
etal.2010;Hennemannetal.2010;Motteetal.2010;Battersby
Galacticplanesurveys,ISOdata(Peraultetal.1996)andMSX
etal.2011;Gianninietal.2012;Raganetal.2012b;Rygletal.
observations (Egan et al. 1998), infrared-dark clouds (IRDCs)
2013; Gaczkowski et al. 2013; Pitann et al. 2013; Stutz et al.
have been subject to intense study because they are the cold
2013).Intheseworks,thespectralenergydistribution(SED)of
(T < 20K), dense (n > 104cm−3) environments believed to
compact or point-like sources have been analyzed, thus giving
be required for the formation of high-mass stars and clusters
estimates of the mass, luminosity, and average temperature of
(cf. Rathborne et al. 2006; Ragan et al. 2009; Battersby et al.
theobjects.
2010).TheyarelocatedthroughouttheGalacticplane,concen-
trated within the spiral arms (Jackson et al. 2008). The for- WhileHerschelprovidesanewlookatthestarformationin
mation of IRDCs, their kinematic structure and population of IRDCs, the properties of the cores are not strongly-constrained
by Herschel observations alone. For example, in Ragan et al.
(2012b, hereafter R12), we find the presence of a MIPS 24µm
(cid:63) Based on observations carried out with the Atacama Pathfinder
and PACS 70µm counterpart to be an important evolutionary
Experiment (APEX). APEX is a collaboration between Max Planck
Institut für Radioastronomie (MPIfR), Onsala Space Observatory indicator. On the long-wavelength (λ > 160µm) side of the
(OSO),andtheEuropeanSouthernObservatory(ESO). SED, however, there are very few constraints at matching an-
(cid:63)(cid:63) Appendicesareavailableinelectronicformat gular resolution. In order to address this issue, we obtained
http://www.aanda.org mapsofelevenIRDCsselectedfromtheEarliestPhasesofStar
ArticlepublishedbyEDPSciences A79,page1of26
A&A559,A79(2013)
Table1.Targetandobservationsummary.
IRDC RA(J2000) Dec(J2000) Distance Mb Mapsize Observation t rms N(H )sens
atlasgal int 2
namea (h:m:s) (◦:(cid:48):(cid:48)(cid:48)) (kpc) (M ) (“×”) date (s) (Jybeam−1) (cm−2)
(cid:12)
IRDC310.39–0.30 13:56:04.9 –62:13:42 5.0±1.2 1398 6×6 2011-04-03 7560 0.22 8.44E+21
IRDC316.72+0.07* 14:44:19.2 –59:44:29 2.8±0.4 3165 6×6 2011-04-14 7560 0.26 9.97E+21
0.5
IRDC320.27+0.29 15:07:45.0 –57:54:16 2.3±0.4 156 6×6 2011-04-15 7500 0.26 9.97E+21
IRDC321.73+0.05 15:18:13.1 –57:21:52 2.3±0.4 564 7×7 2011-08-16 7980 0.23 8.82E+21
IRDC004.36–0.06 17:55:45.6 –25:13:51 3.3±1.0 327 6×5 2011-08-22 7860 0.19 7.29E+21
1.6
IRDC009.86–0.04 18:07:37.4 –20:26:20 2.7±0.6 143 6×6 2011-04-14 7440 0.19 7.29E+21
0.8
IRDC011.11–0.12* 18:10:27.7 –19:20:59 3.4±0.5 5045 9×23 2010-05-11 21000 0.27 1.04E+22
IRDC015.05+0.09 18:17:40.3 –15:49:10 3.0±0.4 160 6×6 2011-08-16 3840 0.43 1.65E+22
0.5
IRDC18223* 18:25:11.5 –12:49:45 3.5±0.3 3501 4×16 2011-04-19 12060 0.21 8.06E+21
0.4
IRDC019.30+0.07 18:25:53.8 –12:06:18 2.4±0.4 624 7×7 2011-04-19 4500 0.42 1.61E+22
0.5
IRDC028.34+0.06* 18:42:50.3 –04:02:17 4.5±0.3 15011 7×8 2011-08-16 3780 0.40 1.54E+22
Notes.(a)FilamentaryIRDCsdenotedwithanasterisk.(b)MassaboveN ∼1021cm−2fromATLASGALsurvey(see Raganetal.2012b).
formation (EPoS) Herschel guaranteed time key program sam- 2.2. APEXobservationsanddatareduction
ple of IRDCs (R12)with APEX telescope using SABOCA, a
WesurveyedoursampleattheAtacamaPathfinderExperiment
bolometer operating at 850GHz, or 350µm. The angular reso-
lution, 7.8(cid:48)(cid:48) is well-matched to the Herschel/PACS resolution, (APEX) 12-m telescope using the Submillimetre APEX
Bolometer Camera (SABOCA, Siringo et al. 2010), a
sothephysicalscalescanbedirectlycompared.Thegoalofthis
37-elementarrayoperatingat350µmwith7.8(cid:48)(cid:48) angularresolu-
studyistoexaminethecloudstructureseenwithSABOCAfirst
tion.OurpilotstudytomapIRDC011.11−0.12wascompleted
independentlytheninconcertwiththeHerscheldataset.Dowe
in 2010 (project M085−0029), and the remaining ten sources
recover all Herschel cores? How reliable are the properties de-
wereobservedinAprilandAugust2011(projectM087−0021).
rivedfromouroriginal“PACS-only”SEDs?Whatisthenature
The precipitable water vapor (PWV) was required to be below
of recovered PACS sources and new sources found only with
0.5mm and ranged from 0.2mm to 0.5mm throughout the ob-
SABOCA?
servations.Theatmosphericopacityatzenithwascalculatedus-
Tothisend,wefirstemploythehierarchicalstructureiden-
ingtheskydipprocedureandwasfoundtobelessthan1.
tificationalgorithmdendrogramtoquantifytheemissionstruc-
tures. Then we correlate the structures with those known from The data were reduced using the BOA software (Schuller
Herschel observations. We model the SEDs to derive or place 2012) using the standard iterative source-masking procedure.
(upperlimitson)themass,luminosity,andtemperatureofeach Each map is comprised of a series of spiral scans. In the first
structure to place them in evolutionary context. These observa- iteration, the scans are calibrated based on the estimated opac-
tionsprovetobecriticalincharacterizingtheearlypre-andpro- ity, noisy channels are then flagged, and correlated noise is re-
tostellarphasesofstarformationinIRDCs. moved. This produces the initial map. In subsequent iterations,
we use a smoothed version of the initial map that serves as a
modelfortheactualemittingstructure.Thismodelissubtracted
2. Observationsanddatareduction
fromtheinitialmapinordertomodel(“flat-field”)theresidual
2.1. Sampleselection flux.Themodelisthenaddedbacktotheimage.Theprocessis
repeated on the new image until the flux levels plateau and the
As part of the Herschel Earliest Phases of Star formation
artefacts(e.g.“bowls”surroundingstrongemissionregions)are
(EPoS) guaranteed time key program, we investigated the
minimized.
protostellar core population in 45 massive regions (R12).
A summary of the observations is given in Table 1, which
From this sample, we selected eleven IRDCs which ex-
notesthesizeofourbolometermaps,thetotalintegrationtime,
hibit a range in morphology and protostellar populations. The
andthermsthatweachievedineachmap.Wecomputetherms
clouds, their positions and kinematic distances are listed in
from1(cid:48)×1(cid:48)sizedemission-freeregionsineachmap.Anexample
Table 1. The sample contains a mix of filamentary and clumpy
mapofasmallregioninIRDC011.11−0.12isshowninFig.1,
clouds, where “filamentary” is defined as a cloud with dense
andthefullsetofmapsarepresentedinAppendixA.
(N > 1021cm−2) material elongated preferentially along one
axis by at least an aspect ratio of 3:1. Clouds that qual-
ify as “filaments” are IRDC011.11−0.12, IRDC028.34+0.06,
IRDC316.72+0.07, and IRDC18223. These are also the most 3. Structureidentification
massiveclouds(seeTable1),buttheyhavesimilardistancesto
3.1. Method
thesevenclumpycloudsinthesample.
We have selected well-studied clouds, which has the added The defining hierarchical nature of molecular clouds has given
advantagethatexistingmolecularlinesurveys(e.g.Raganetal. rise to many characterization techniques which attempt to as-
2006; Vasyunina et al. 2011; Tackenberg et al. 2013b) have al- sign gas, traced either by molecular line emission or dust, to
ready verified that the IRDCs contained in our selected fields structure on various scales. For consistency, we adopt the size-
are coherent in velocity space. Thus, we are confident that the based terminology given to these scales in Bergin & Tafalla
clouds are each genuine cold, absorbing entities at a common (2007): a “core” ranges in size between 0.03 and 0.2pc and a
distance (all on the near side of the Galaxy) and not chance- “clump” from 0.3 to 3pc, and both entities are found within
alignedclouds. theboundariesofseveralparsecsized“clouds”.TheSABOCA
A79,page2of26
S.E.Raganetal.:IRDCcoreswithSABOCA
6
Image: 24µm
-19°23'30" Contour: ATLASGAL 875µm IRDC015.05+0.09
5
5
24'00"
2000) 1m]−4 3
ec (J 30" bea 3 1 2
D 25'00" x [Jy 7 6 8 4
Flu 2
30" 0.5 pc
1
24s 21s 18s 15s 18h10m12s
RA (J2000)
0
Image: 100µm Fig.2.ExampleofdendrogramtreestructureforIRDC015.05+0.09.
-19°23'30" Contour: SABOCA 350µm
TheindicesarethoselistedinTableB.1inorderofincreasingrightas-
cension.Theblackleaves(3,6,7,and8)havenoparenttreestructure,
24'00" andtheredleavesarenestedinseparatetrunks.Theirfluxvalueswere
0) correctedforthelocalmergelevel,whichismarkedinblue.Thehor-
00 izontalblackdashedlineshowsthe3σminimumfluxrequirementfor
2
c (J 30" thiscloud.
e
D
20
25'00" has parent, corrected
has parent, uncorrected
no parent
30" 0.5 pc
15
s
24s 21s 18s 15s 18h10m12s e
v
RA (J2000) a
e
Fig.1. Top: 24µm image of region in IRDC011.11−0.12 with of l10
ATLASGAL 875µm contours. The 19.2(cid:48)(cid:48) ATLASGAL beam at this r
e
wavelength is shown in the lower-left corner. Bottom: PACS 100µm b
m
imageofthesameregionover-plottedwithSABOCA350µmcontours. u
The7.8(cid:48)(cid:48)SABOCAbeamisshownatthelower-leftcorner.Thesquare N
5
marksthepositionofa24µm-brightprotostar;the×markstheposition
ofa24µm-darkprotostar;thetrianglemarksanIR-darkcore.
0
1.0 0.5 0.0 0.5 1.0 1.5 2.0
log (S ) [Jy]
angular resolution accesses the “core” scale in our sample of 350µm
IRDCs. Fig.3.DistributionofS forcorrected(magenta)anduncorrected
350µm
Onemethodthatquantitativelydescribesthishierarchyisa (green) leaves, compared with leaves with no parent structure (black
structure tree algorithm, popularly implemented in astronomy hatched).
with the dendrogram representation (Rosolowsky et al. 2008).
An advantage to using this technique is that it imposes no as-
example,therearefourleaveswithnoparentstructure(3,6,7,
sumptionsabouttheshapeoremissionprofileofthetreestruc-
and8)andfourleaveswhicharenestedinaparentstructure.The
tures,anditoperatesonbothtwoandthreedimensionaldatasets.
minimumcontourlevelisshownwiththedashedline.
Weusethepythonimplementation,astrodendro1.Ourmoti-
TableB.1liststhepositionsofallleavesandthetotalfluxof
vationforusingsuchanalgorithmistoquantifytherelativeflux
eachleaf,135intotalinthe11IRDCs.Iftheleafstemsfroma
contributions at 350µm from the core and from the cloud. Our
trunkorbranch(i.e.aparentstructure,theaveragefluxofwhich
methodologyisdescribedbelow.
isknownwithinthealgorithmasthe“mergelevel”),wecorrect
A dendrogram tree has three main features: a trunk,
thatfluxbysubtractingthemergelevelfluxfromeachleafpixel.
branches,andleaves,whichamounttosurfaceswithinincreas-
InFig.3weshowthefluxdistributionforleaveswithnoparent
ingcontourlevelsinourtwo-dimensionaldataset.Wedefineour
structures and both the corrected and uncorrected flux distribu-
lowest contour level, or the intensity associated with the trunk
tionsforleavesstemmingfromparentstructures2.Wedefinethe
of the dendrogram tree, to be 3σ (roughly corresponding to
correction factor, µ , as the ratio of uncorrected to corrected
a mean minimum detectable column density of 1022cm−2, see corr
totalleafflux.Weplotµ asafunctionofleafsurfacebright-
Table 1 and Sect. 4.1), and we require 1σ steps and eight con- corr
ness in Fig. 4a. The mean value for µ in child leaves is 4.8,
tiguouspixels(selectedtomatchthenumberofpixelsperAPEX corr
but ranges between 1.4 and 15 (see Fig. 4b). We find no trend
beam)todefinefurtherbranchesuptheintensitytree.Thepeaks
comprise the so-called “leaf” population. We show an exam- 2 We note that this method of separating leaf flux from branch flux
ple dendrogram tree in Fig. 2 for IRDC015.05+0.09. In this willgivelowerfluxvaluesthanwouldhaveotherwisebeenestimated.
Throughoutthefollowingcalculations,weadoptthecorrectedvaluebut
1 http://github.com/bradenmacdonald/astrodendro notetheupperlimithadtheparentfluxbeenincluded.
A79,page3of26
A&A559,A79(2013)
16 16
(a) uncorrected All PACS cores
14 corrected 14 Recovered PACS cores
12 12
10 10
r
e
orr 8 mb 8
µc u
N
6 6
4 4
2 2
0 0
10-3 10-2 10-1 100 1.0 0.5 0.0 0.5 1.0 1.5 2.0
Surface brightness [Jy arcsec−2] Predicted log(S350µm) [Jy]
25 Fig.5. Distribution of flux density at 350µm from all PACS cores in
(b) R12 for the IRDCs in this sample (gray). The red histogram shows
the portion of these cores which were recovered by our SABOCA
20 observations.
s
e
v
ea15 is a “PACS core” and may or may not have a 24µm counter-
of l part.A“coldcore”isdetectedonlyinthe100and160µmbands
r of PACS (not 24 or 70µm), but is also a recovered core with
e
b10 SABOCA3.
m
u The statistics of the PACS core recovery rate per cloud are
N
giveninTable2.Forthese11IRDCs,R12cataloguedapopula-
5 tionof89PACScoresandplusanother76candidatecoldcores.
ThislatterpopulationcouldnotbemodeledusingHerscheldata
aloneduetopoorspectralcoverageattherelevantsizescaleand
0
0 2 4 6 8 10 12 14 16 werethereforenotcatalogedatthattime.Werecover52(58%)
µ
corr PACScoresinSABOCAemission,40(45%)ofwhichareinthe
Fig.4. a) Flux correction factor, µ (see Sect. 3.1), as a function of formofleafstructures,otherwisetheycoincidewithbranchlevel
corr
theleafsurfacebrightnessinJanskypersquarearcsecond.Thegreen emission.Werecover14(18%)candidatecoldcores,12(16%)
dotsaretheuncorrectedmeasurements,andthemagentastarsarethe of which are leaves. The figures in Appendix A show filled
correctedsurfacebrightness.b)Histogramshowingµcorrfortheleaves circles for recovered cores (i.e. Herschel point sources with a
withparentfluxcontribution.Themeanvalueofµcorris4.8. SABOCA leaf counterpart) and empty circles for unrecovered
cores.
In three cases, a leaf overlaps two or more Herschel cores.
intheneededcorrectionasafunctionofsurfacebrightness,and The affected objects are noted in Table B.1. Since the leaf flux
theimplicationsofthiswillbefurtherdiscussedinSect.5.1.
has already been corrected for parent contribution, we assume
Thismethodisdesignedtoisolatethefluxofthecorestruc-
that similar to PACS wavelengths, the flux of each core scales
turesbyremovingthecontributionofthelessdensesurrounding
withthe(predicted)luminosityfrompreviousfits(seeFig.8in
cloud.TheSABOCAobservationsdofilteroutsomelarge-scale
R12). Therefore, only for the purposes of SED-fitting, we use
structure,butherewetakeadvantageofthesuperbangularreso-
theluminosityratioofthecoresthatfallintheleafareatodivide
lutionofthesedataandassumethatatthelocationsoftheemis-
theSABOCAfluxbetweenthesecorecontributions.Thefactors
sionpeaks,thesmallscalecoreisthedominantsource.Wewill
by which the fluxes are scaled are detailed in Section 4.5. We
addressthehierarchicalcloudstructureinaforthcomingpubli-
then consider the scaled 350µm flux with the closest-matching
cation.Inthispaper,weconcentrateonthecorepopulation.
HerschelcoreintheSED-fittinganalysisthatfollows.
The recovery rate of PACS cores is mainly governed by
3.2. Corerecoveryrates the sensitivity of our SABOCA observations. In Table 1, we
list the rms levels of each map, which range from 0.19 to
Wecross-matchedthepositionsoftheSABOCAleaveswiththe 0.43Jybeam−1.BasedontheoriginalSEDfitsinR12,theme-
Herschel/PACS point source catalog of R12 using the TOPCAT
dian predicted flux at 350µm is 0.58 Jy (in contrast to 2.4Jy
softwarepackagerequiringamatchwithina10(cid:48)(cid:48)radius.Wenote
medianpredictedfluxforrecoveredcores),whichdoesnotmeet
which of the SABOCA leaves have a Herschel/PACS counter-
the 3σ detection requirement. Figure 5 shows the distributions
partinTableB.1.Fortheremainderofthepaper,wedistinguish
ofpredicted350µmfluxbasedonthebest-fitSEDtothePACS
thetypesofobjectsbasedoninwhichwavelengthstheyarede-
data from R12, showing that we clearly recover the brightest
tected. All SABOCA detections are referred to as “leaves” re-
gardlessofwhethertheyhaveHerschelcounterparts.Wereferto 3 Weagainemphasizethattheuseoftheterm“core”reflectsthecom-
Herschelsourcesas“cores”anddistinguishthesubsamplethat pact size of the structures (see Sect. 3.1), but given our present res-
wedetectwithSABOCAas“recoveredcores”.Anycoreappear- olution limitations and the range of masses, the “cores” could be the
ingintheR12catalog(withcounterpartsat70,100,and160µm) precursorstoindividualstars,binaries,orboundmultiples.
A79,page4of26
S.E.Raganetal.:IRDCcoreswithSABOCA
30 value is 2.4×1022cm−2. Leaves of greater column density are
All leaves
more likely to be seen also as a Herschel core. While most of
Recov. PACS
25 Recov. cold the cold cores are below the ensemble median column density,
24ofthe40PACScoresareabovethemedianvalue.Thecolumn
density values and the tendency for the higher column density
20
structurestohavestarformation(appearingasaPACScore)are
er consistent with what is observed in local clouds (Enoch et al.
b
m15 2008).
u
N For more accurate mass estimates, we evaluate Eq. (1) at
10 the best-fit or upper-limit temperature, for recovered cores and
leaves,respectively.Thebest-fittemperatureisgiveninTable3,
and the upper-limit temperature for leaves with no Herschel
5
counterpart is calculated by fitting an SED to the 3σ detection
limitofPACSfluxesinallbandsplusthecorrected350µmflux
0 andisusedtodeterminetheleafmass,givingalowerlimit.With
21.8 22.0 22.2 22.4 22.6 22.8 23.0 23.2 23.4
log (<N(H2)> / cm−2) onlyafewexceptions,thehigh-massleaves(M350µm > 40M(cid:12))
correspond to PACS cores. The mean and median masses for
Fig.6. Distribution of mean column density of all leaf structures.
PACS cores are 239 and 16M , respectively, while for the re-
Recovered PACS cores are shown in blue, and recovered cold cores (cid:12)
areshowningreen.Themediancolumndensityof2.4×1022 cm−2 is mainingleaves,wefind28and8M(cid:12).
shownintheverticaldashedline. Weexaminetherelationshi√pbetween M350µm andtheeffec-
tive radius, defined as reff = Aleaf/π, where Aleaf is the area
of a structure, for various dendrogram structures in Fig. 7. We
cores.Inothercases,IRDC028.34andIRDC316.72especially,
plotseverallinesofreferenceinthesediagrams:lociofconstant
source confusion plays a role in some bright PACS cores not
numberdensity(dashedgray),theLarson(1981)relationinthe
beingrecovered.Clearly,ourrecoveryrateisdominatedbyour
red dash-dotted line, and the relation proposed by Kauffmann
SABOCAsensitivity.
et al. (2010), which shows their empirical dividing line in con-
Therecoveryrateforcoldcoresissubjecttothesamelimita-
centration betweenlocal clouds devoidof massive stars (below
tionsasabove,withtheaddeddifficultythattheoriginalcatalog
line) and high-mass star-forming clouds (above line). All but
of 73 cold cores was based on the criterion of only two detec-
14leavesfrompanels(a)and(c)inFig.7(10%)fallbelowthis
tionsat100and160µm,ratherthanthethreerequiredforPACS
criterion. Twelve of these 14 are found in filamentary IRDCs,
cores. The 100 and 160µm bands had the highest uncertainties
andeighthaveHerschelcounterparts.
andgreaterlargescalevariation,thusincreasingthepropensity
In plot (a) we show the child leaves corrected for the par-
forcontaminationofthecandidatecatalog.Thereforetheirpoor
ent structure flux. We indicate the leaves for which there are
recoveryrateisunsurprizing.
Herschelcounterpartsbyfillinginwithblue(forPACScores)or
green(forcoldcores).Wefindatrendofroughlyconstantvol-
umedensity(M ∝r2.9)fortheensembleofcoreswithamedian
4. Results eff
valueof4.4×104cm−3.Thetrendisslightlysteeper(M ∝r3.2)
4.1. IRDCsmall-scalestructure whenonlyleaveswithHerschelcounterpartsarefit,thoughetffhe
mediandensityisroughlythesame.
ForeachleafextractedfromtheSABOCAmap,wecalculatethe
We show the same relationship in plot (b), but in this case
massviathestandardformulation,
M isderivedfromtheuncorrectedfluxvalue.Thetrendis
350µm
S d2R shallowerinthiscase: M ∝ r2.3 forthefullensembleofleaves
M = 350µm gd (1) eff
350µm κ350µm Bν(Td) and M ∝ re2ff.6 for just those leaves with Herschel counterparts.
Themediandensitiesinthesepopulationsare1.2×105cm−3and
whereS350µmisthespecificflux,disthedistance,Rgdisthegas- 8.0×104cm−3,respectively,whichcomparefavorablytorecent
to-dustratioassumedtobe100,andBνisthePlanckfunctionat interferometricmeasurementsofIRDCcores(e.g.Beutheretal.
dust temperature, Td, that we first assume here to be uniformly 2013)4.
20K,andthecolumndensity The roughly constant volume density trend for the com-
pactobjectsshowninFig.7aisconsistentwiththeKainulainen
N =S R [Ω µm κ B (T )]−1 (2)
H2 350µm gd beam H 350µm ν d et al. (2011) extinction study of local clouds but in contrast
where Ω is the size of the beam, µ is the mean molecular to that which is observed in dust emission locally (e.g. Enoch
beam
et al. 2008), which follows a trend of constant surface density,
weight(2.36),andm isthemassofhydrogen.Weassumethe
H
dustmassopacityat350µm,κ ,tobe10.1cm2 g−1,which similar to the Larson (1981) relationship for turbulent clouds
350µm
using CO observations. This apparent discrepancy might eas-
istakenfromOssenkopf&Henning(1994)fordustgrainswith
thinicemantlesatn=106 cm−3 (Col.5,hereafterOH5),which ily be reconciled by examining the various scales identified as
iso-surfaces in lower levels of the dendrogram tree. In plot
issupportedforsimilarenvironmentsintherecentliterature(e.g.
(c) of Fig. 7, we show the relationship for all “trunks” of the
Shirleyetal.2011;Kainulainenetal.2013).Boththemassand
dendrogram, i.e. the sum of each independent flux structure.
mean column density for each leaf are reported in Table B.1.
Forexample,inFig.2,structures3,6,7,and8aretrunks,then
GiventhefluxlimitsdiscussedinSect.3.2,ourmasssensitivity
is roughly 1−2M(cid:12) depending on the leaf’s environment. Our 4 We note that due to the chopping away of flux on large scales,
typicalcolumndensitysensitivityis∼1022cm−2. the computed sizes of objects larger than the bolometer array size
We show the distribution of mean leaf column densities in (FWHM∼100(cid:48)(cid:48))shouldbetreatedwithcaution.Thiseffectwouldtend
Fig. 6, which are left uncorrected for parent flux. The median tobiasboththefluxandsizetolowervalues.
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A&A559,A79(2013)
Table2.Corepopulation.
IRDC N N N Noverlap Noverlap
PACS 70dark SABOCA PACS 70dark
name (1) (2) (3) (4) (5)
IRDC310.39-0.30 2 0 2 1(2) 0(0)
IRDC316.72+0.07 14 9 42 10(12) 6(6)
IRDC320.27+0.29 3 6 4 1(1) 1(1)
IRDC321.73+0.05 10 1 5 4(4) 0(0)
IRDC004.36-0.06 1 3 4 0(0) 0(0)
IRDC009.86-0.04 3 1 2 1(1) 0(0)
IRDC011.11-0.12 20 28 35 7(11) 2(4)
IRDC015.05+0.09 3 0 8 0(1) 0(0)
IRDC18223 13 11 10 7(9) 1(1)
IRDC019.30+0.07 6 4 3 3(3) 0(0)
IRDC028.34+0.06 14 10 20 6(8) 2(2)
Total: 89 73 135 40(52) 12(14)
Notes.(1)Detectedat70,100,and160µm,asreportedinRaganetal.(2012b)overlappingwithmappedarea.(2)Detectedat100and160µm,but
not70µm.(3)Numberof“leaves”detectedinSABOCA350µmmapwithdendrogram.(4)NumberofoverlappingcoresbetweenCols.(1)and(3).
ThenumberinparenthesesisthenumberofPACScorescoincidentwithanySABOCAemission(notjustleaves).(5)Numberofoverlappingcores
betweenCols.(2)and(3).Thenumberinparenthesesisthenumberof“70dark”corescoincidentwithanySABOCAemission(notjustleaves).
(a) (c)
Child leaves All trunks
103 (Background corrected) m3− 103 m3−
105 c 105 c
] 102 m3− ] 102 m3−
[MM350µmfl101 Kauffmann+2010 103 c [MM350µmfl101 Kauffmann+2010 103 c
100 100
10-1 10-1
10-2 10-1 100 10-2 10-1 100
effective radius [pc] effective radius [pc]
(b) (d)
Child leaves Clouds
103 (Not background corrected) m3− 104
105 c
3
[M]M350µmfl110012 Kauffmann+2010 103 cm3− [M]M875µmfl110023 105 cm − 103 cm3−
100 101 Filament: N>1021cm−2
Clump: N>1021cm−2
Filament: N>1022cm−2
Clump: N>1022cm−2
10-1 100
10-2 10-1 100 10-1 100 101
effective radius [pc] effective radius [pc]
√
Fig.7. a)Childleafmass(correctedfortheparentfluxcontribution)asafunctionofeffectiveradius(reff = Aleaf/π).Theblueandgreenfilled
circlesaretheleavesrecoveredasPACSandcoldcores,respectively,andopencircleshavenoHerschelcounterpart.Themassesarecalculated
usingthebestfittemperatures,ortheupperlimittemperaturesforleaveswithnoHerschelcounterpart.Thedashedgraylinesarelociofconstant
numberdensity,andthedashed-dottedredlineisLarson’smass-sizerelation(Larson1981),whichisapproximatelyalineofconstantcolumn
density, N = 1022cm−2. b) Same as a), but the leaf mass is not corrected for parent flux contribution. c) Total mass in each trunk, no flux
correction,H2asafunctionofeffectiveradius.d)TotalcloudmassfromtheATLASGALsurveymapsasafunctionofeffectiveradius,computed
fortheareaabove1021cm−2 (emptysymbols)and1022cm−2 (filledsymbols).Filamentarycloudsareplottedinredstarsandclumpycloudsare
plottedingreentriangles.Notethattheaxisscalesind)differfroma)throughc).
A79,page6of26
S.E.Raganetal.:IRDCcoreswithSABOCA
so are structures 1 + 2 and 4 + 5, including their parent flux. blackbody does not capture the shape of the full spectral range
This is analogous to applying a single column density contour well. We again assumed OH5 dust for both the warm and cold
toeachcloud(seeTable1).Heretherelationbecomesshallower components5 of our SED. We list the best fit parameters to the
(M ∝r2.3)withamediandensityof4.3×104cm−3. coldcomponentineachpanelofFig.8andinTable3.Inparen-
eff
Taking this idea a step further, using ATLASGAL data theses, we show the results from a fit using the 350µm flux
(Schuller et al. 2009) we plot the same again in Fig. 7d for value, which is not corrected for parent flux (blue data point at
differentcolumndensityboundarydefinitionsofthecloud,and 350µm).Thosefits,alwayswithlowertemperaturesandhigher
the result is consistent with the Larson relation. Here we also massesandluminosities,areshowninthedashedcurves.
make a distinction between the clumpy and filamentary clouds The summed (warm + cold) SED fit is shown with the
in our sample, showing that while the filamentary sources tend red line in Fig. 8. For those cases, the “full-SED” parameters
tobemoremassive,theyadheretothesamerelationshipasthe given in Table 3 are that of the cold, outer component. In most
clumpy clouds. We return to this discussion of morphology in cases, the inclusion of a second blackbody component to ac-
Sect.5.2. count for the 24µm flux causes Tfull−SED to be lower than was
cold
Isthesteepeningofthemass-sizerelationonthesmallscales fitwiththePACSdataalone.Thissupportsthepicturepresented
aconsequenceofanobservationalbiasorarealphysicaleffect? in R12 where the warm temperatures in 24µm-bright cores are
Inanidealizedcase,aconstantvolumedensitytrendisexpected attributedtotheirmarginallymoreadvancedevolutionarystage.
ifIRDCcoresareinpressurebalance(cf.thePipeNebula,Lada The warm dust component contributes slightly to the 70µm
etal.2008).However,thereareseveralobservationalissuesone emission, and with the second fit component invoked here, the
must keep in mind. First, we note that the spread in the rela- temperatureofthecoldcomponentdecreases.ThesummedSED
tion is nearly 1 dex and, as mentioned above, bolometer map- forthefitincludingtheuncorrectedvalueofS isshownin
350µm
ping may bias us toward smaller radii (see also Enoch et al. thereddashedcurve.
2006; Rosolowsky et al. 2010), which would steepen the rela- Figure 9 shows the SEDs for objects which were only de-
tion. Furthermore, the mass-radius relation has been shown to tected in PACS 100 and 160µm bands and were recovered by
sufferfromprojectioneffectsandthuspoormappingtophysical the SABOCA observations. We fit a single temperature modi-
structure (Ballesteros-Paredes & Mac Low 2002; Shetty et al. fiedblackbodytothethreedatapointsontheSED,forwhichwe
2010),whichalsocausesconsiderablescatter.Finally,thecom- listthederivedpropertiesfromthebestfitinTable3andinthe
plexdynamicsplayanimportantifnotdominantroleintheen- individualpanelsofFig.9.Wealsoshowthe350µmfluxuncor-
ergy balance of IRDCs (e.g. Ragan et al. 2012a), and we lack rected for the contribution from the parent structure (blue data
this information on the core scale for this sample. Therefore, point) and the corresponding derived properties in parentheses.
sincethechangingtrendscanbereproducedsimplybyconsid- Again,theuncorrectedvalueforS resultsinlowertemper-
350µm
ering different radii within the structure, the reader should be atures and higher masses, but given the uncertainties, it is less
cautionsindrawingconclusionsonthecorestabilityfromthese clear whether the correction, which is on the order of a factor
dataalone. 3.6±1.0,improvestheblackbodyfitsornot.
Figure 10a shows the temperature distribution for all PACS
cores, which had the mean and median of 20K. Based on the
4.2. SEDsandcoreproperties
SED fits, the 70µm-dark “cold” cores are colder on average
InR12,wefitblackbodiestotheSEDsofallcoresdetectedinall (meanandmedian16K)thanthe70µm-brightcores(meanand
three PACS bands, but left out Herschel/SPIRE data for longer median 19K and 18K). The cold core temperatures compare
wavelengthsbecausetheangularresolutionwasnotsufficientto well with the temperatures found in the starless globules mod-
isolatethe0.1pcscale.BecauseSABOCAboastssimilarangu- eled in Launhardt et al. (2013), whereas the PACS cores more
larresolutionasPACSat350µm,wearenowabletoovercome closely match the peak temperatures found for the protostellar
thatlimitation.Table3listsallcoresforwhichaSABOCAleaf cores.ThecenterandrightpanelsofFig.10showthatwhilethe
matchesaHerschel/PACScoreidentifiedinR12.Foracomplete cold cores occupy the same range in mass as the 70µm-bright
comparison, we also list in Table B.1 the M (see Eq. (1) cores, the luminosities of 70µm-bright cores tend to be higher
350µm
andSect.4.1)usingthebest-fittemperature(incasesofarecov- (median52L )than70µm-darkcores(median17L ).
(cid:12) (cid:12)
eredcore,notedinthelastcolumn)ortheupper-limittempera-
ture(incasetheleafhasnoHerschelcounterpart).Forthelatter
cases, using an upper-limit temperature will result in a lower- 4.3. Coremassestimates
limitmass.
Calculating accurate core masses is fundamental in constrain-
Figure8showsthefullSEDsforthe40overlappingobjects.
ing the initial conditions for star formation in IRDCs. With the
First, we fit a single temperature modified blackbody function
present dataset, we calculate mass in two ways; one is using
to the SED from 70 to 350µm SABOCA data, shown with the
just the 350µm flux (M : Eq. (1)) and the other is using
solidblackline,representingthe“cold”component.Wefollow 350µm
SEDfitsdescribedinSect.4.2(M ).Aweaknessinherent
themethodexactlyaswedidforfittingthePACSSEDsinR12. full−SED
in the former method is that one must assume a dust tempera-
TheSED-fittingalgorithmassumesthedustisopticallythinand
ture (in Table B.1 we assume a uniform temperature of 20K),
takes into account the frequency-dependence of the dust opac-
whereinthelattermethodthetemperatureisfitsimultaneously
ity, κ , for which we adopt OH5. We compare parameters de-
ν
rivedfromthenewfits(Mfull−SED,Tfull−SED,andLfull−SED)tothe
PACS+SABOCASEDtottohtaolsefouncdoldinR12,whiccohldusejustthe 5 WenotethatOH5dustmaynotbetheoptimalchoicefortherela-
tivelywarminnercomponentoftheSED,wheretheprotostarhaspre-
PACSdata(M ,T ,andL ),inTable3.
PACS PACS PACS sumably heated the region. If we instead adopt a dust opacity model
Next,forcoreswith24µmcounterparts,wefitthefullSED
withnoicemantles(e.g.Ossenkopf&Henning1994,Col.2,orOH2),
(from 24µm to 350µm, a total of five data points) with two forthewarminnercomponent,themeandeviationfromourreported
temperature components henceforth referred to as the “warm” massinTable3isabout7%.Therefore,forsimplicity,weadoptuni-
and “cold” components, since a single temperature modified formdustproperties(OH5)throughouttheanalysis.
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A&A559,A79(2013)
Table3.Coreproperties,methodcomparison.
IRDC ID Mb Mc Mfull−SED Tb Tfull−SED Lb Lfull−SED Notes
PACS 350µm total PACS cold PACS cold
No.a (M ) (M ) (M ) (K) (K) (L ) (L )
(cid:12) (cid:12) (cid:12) (cid:12) (cid:12)
IRDC310.39–0.30
1d 303 229(2054) 246(391) 17 18(16) 716 660(793)
IRDC316.72+0.07
6 9 3(30) 4(81) 17 18(12) 19 14(32)
7 – 93(462) 93(659) – 12(10) – 31(78)
8 6 2(17) 2(44) 20 22(14) 34 31(55)
9 8 8(36) 9(64) 18 17(13) 26 25(42)
16 1 1(5) 1(8) 20 19(14) 5 9(12)
18 – 14 12 – 14 – 7
19 – 22(50) 26(70) – 12(11) – 9(14)
22d 7 25(177) 18(72) 16 15(13) 12 19(38)
24 – 8(25) 10(40) – 15(13) – 11(17)
25 – 2(10) 2(10) – 19(14) – 10(16)
29 12 8 8 16 16 16 14
32 22 7 9 14 15 16 10
35 13 23 24 19 17 54 63
36 16 7 10 16 16 21 17
38 79 90(228) 86(201) 25 25(21) 1650 1672(1972)
42 – 17 15 – 14 – 11
IRDC320.27+0.29
3 – 2(7) 2(11) – 16(13) – 3(5)
4 5 3(10) 3(14) 18 18(15) 14 13(20)
IRDC321.73+0.05
2 3 34 28 22 15 32 62
3 1 2 1 23 22 16 21
4 24 43 36 18 17 71 84
5 4 9 8 18 16 12 15
IRDC009.86–0.04
2 2 3 2 24 22 37 70
IRDC011.11–0.12
4 5 36 30 23 17 68 119
7 25 10(35) 13(55) 20 22(17) 152 142(197)
14 2 17 14 21 15 16 27
17 4 6 5 23 21 51 72
18 80 183(302) 169(365) 24 21(17) 1444 1857(2064)
24 3 1(5) 1(10) 22 26(17) 32 32(47)
27 – 116 75 – 12 – 21
28 – 6(23) 7(34) – 18(15) – 24(34)
33 4 3 3 24 24 73 80
IRDC18223
2 10 17(44) 17(62) 21 19(15) 79 104(130)
3 9 25 23 20 17 51 62
4 61 143(240) 122(324) 20 18(15) 323 383(436)
6 12 99(211) 86(292) 24 17(13) 200 349(407)
7 185 254(450) 256(548) 22 21(18) 1979 4814(5154) IRAS18223-1243
8 51 2(15) 4(31) 18 24(18) 156 81(132)
9 – 3 4 – 20 – 22
10 7 14 14 20 17 33 41
IRDC019.30+0.07
1 12 13 13 18 17 33 34
2 0.8 4 3 25 19 19 33
3 37 36 38 20 20 219 229
IRDC028.34+0.06
2 93 782(1062) 671(1132) 28 18(16) 3577 8058(8470) MSXG028.3373+00.1189
4 – 22(67) 20(80) – 17(15) – 50(71)
7 14 37 34 20 17 85 106
8 49 1369 971 18 12 151 357
9d 534 4490(9249) 3496(6720) 20 15(14) 2950 4703(5488) MSXG028.3937+00.0757
11 265 110(286) 140(332) 14 15(13) 177 134(201)
15 9 5(18) 6(25) 20 21(16) 52 48(66)
18 – 19(71) 17(87) – 17(14) – 41(64)
Notes.(a) FromTableB.1;(b) fromRaganetal.(2012b);(c) derivedwiththebestfitdusttemperature(Col.6ofthistable);(d) leafoverlapswith
multipleHerschelcores.SABOCAfluxisscaledaccordingtoluminosityratioofcores(seeSect.3.2).
A79,page8of26
S.E.Raganetal.:IRDCcoreswithSABOCA
Fig.8. SEDs of cores with 70µm counterparts. The IRDC name and ID number from Table B.1 are shown in the upper-left corner of each
panelalongwiththebest-fitpropertiestothecoldcomponentfromthefull-SEDfit(thepropertiesfromthefittotheSEDuncorrectedforparent
fluxcontributionaregiveninparentheses).Forallobjects,thesolidblacklineshowsthesingle-temperaturefittothedataexcludingthe24µm
counterpart.Incasesinwhicha24µmcounterpartwasdetected,theredlineshowsthesummedspectrumofatwocomponentfit.Theuncorrected
350µmfluxisshowninblue,andthefitstotheuncorrectedSEDareshownindashedblack(singlecomponent)andred(doublecomponent)lines.
A79,page9of26
A&A559,A79(2013)
Fig.9. SEDsofcoreswithno70µmcounterparts.TheIRDCnameandIDnumberfromTableB.1areshownineachpanel.Thebest-fitsingle
temperaturemodifiedblackbodyfittotheSEDwiththewithS correctedfortheparentfluxisplottedinthesolidline(theparametersare
350µm
listedineachpanel),andthefittotheSEDincludingtheuncorrectedS isshownwiththedashedgrayline.
350µm
20 (a) All PACS cores 20 (b) All PACS cores 20(c) All PACS cores
Recov. PACS Recov. PACS Recov. PACS
Recov. cold Recov. cold Recov. cold
s15 15 s15
mber of core10 umber of cores10 mber of core10
u N u
N 5 5 N 5
010 15 20 25 30 00.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 00.0 0.5 1.0 1.5 2.0 2.5 3.0
Temperature [K] log (Luminosity / L ) log (Mass / M )
fl fl
Fig.10.Grayhistogram:a)temperature;b)luminosity;andc)massdistributionofallPACScoresfoundinthissamplebyR12.Bluehistogram:
subsetofthesecoreswhichmatchedSABOCAleaves.Greenhistogram:distributionsofcoldcores(seeFig.9).
with the other parameters. We compare the previous estimate the M and the corresponding cold component tempera-
full−SED
of core mass, M , from R12 to these two methods using ture(seeSect.4.2).
PACS
SABOCAdatainFig.11.Below100M ,theuseofthefull-SED A similar comparison cannot be made for cold cores, as
(cid:12)
orM resultsinahighermasstwiceasoftenasitresultsina it was impossible to model their SEDs with PACS data only.
350µm
smallermass.Themassincreasesaremoreextremeforthemost However, there is good agreement between their M and
full−SED
massivecores,thoughthisisaresultofthelargeangularextent M , within 50% for all twelve cores. The maximum mass
350µm
oftheseobjectsinSABOCA. of a cold core is 93M . Since these cores have lower temper-
(cid:12)
Wecancomparethethreemethodsforthe40coresthatap- ature than the PACS cores, any inaccuracy in the temperature
peared in R12 that were recovered by SABOCA. In 25 of the resultsinlargererrorsinthemassthanforwarmercores.
40 cases (63%), the inclusion of the SABOCA data point al- SinceM agreesfairlywellwithM ,wecancon-
350µm full−SED
tered the mass estimate by less than a factor 2, and in 33 cases fidently estimate masses for the 83 SABOCA leaves that have
(83%),themassadjustmentwasunderafactor5.FromFig.11 no Herschel counterpart using Eq. (1). We stress that this en-
weseethatinmost(68%)ofthecasesthepreviousPACS-only tirelynewpopulationofcoresareofparticularinterestbecause
mass estimate was lower than the full-SED mass. We also plot with no Herschel counterpart, they could be cold, dense, star-
M evaluated at the full-SED fit temperature result values lessstructures,representingthepristineearliestphaseofthestar
350µm
typically within 20% of M . In the following, we adopt formationprocess.
full−SED
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