Sequential and Spontaneous Star Formation Around the Mid-Infrared Halo HII Region KR 140

0.38 MB·English

by  C. R. Kerton

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed2February2008 (MNLATEXstylefilev2.2)
Sequential and Spontaneous Star Formation Around the
Mid-Infrared Halo H II Region KR 140
1⋆ 1 2 3
C. R. Kerton , K. Arvidsson , Lewis B. G. Knee , and C. Brunt
8 1DepartmentofPhysicsandAstronomy,IowaStateUniversity,Ames,IA50011,USA
0 2AtacamaLargeMillimeterArray,AvenidaElGolf40,Piso18,LasCondes,Santiago,Chile
0 3SchoolofPhysics,UniversityofExeter,StockerRoad,ExeterEX44QL,UK
2

n
a 2February2008
J

0
1 ABSTRACT
We use2MASSandMSX infraredobservations,alongwithnewmolecularline(CO)obser-

] vations, to examine the distribution of young stellar objects (YSOs) in the molecular cloud
h
p surroundingthehaloHII regionKR140inordertodetermineiftheongoingstar-formation
- activityinthisregionisdominatedbysequentialstarformationwithinthephotodissociation
o region(PDR)surroundingtheHIIregion.WefindthatKR140hasanextensivepopulationof
r
YSOsthathave“spontaneously”formedduetoprocessesnotrelatedtotheexpansionofthe
t
s HIIregion.MuchoftheYSOpopulationinthemolecularcloudisconcentratedalongadense
a filamentarymolecularstructure,tracedbyC18O,thathasnotbeenerasedbytheformationof
[
the exciting O star. Some of the previouslyobservedsubmillimetreclumpssurroundingthe

1 HII region are shown to be sites of recent intermediate and low-mass star formation while
v other massive starless clumps clearly associated with the PDR may be the next sites of se-
5
quentialstarformation.
2
6 Keywords: stars:pre-main-sequence–stars:formation–HIIRegions–infrared:stars
1
.
1
0
8
1 INTRODUCTION sociationregion(PDR)surrounding theHII regionasameansof
0
gaugingtherelativeimportanceofspontaneousandsequential,or
: Thisstudy is the fourth in a series of papers exploring the struc-
v triggered,starformationinthisregion.Toachievethiswehavean-
i tureoftheHIIregionKR140anditsassociatedstar-formingac- alyzedacombinationofsubmmdatafromKertonetal.(2001)and
X tivity. In Kerton,Ballantyne,&Martin (1999) VES 735, the O-
Mooreetal.(2007),2MASS(2MicronAllSkySurvey)andMid-
r star powering the HII region was examined. A second paper,
a courseSpaceExperiment(MSX)infrareddata,andnewlyacquired
Ballantyne,Kerton&Martin(2000),wasamultiwavelengthstudy 12CO,13COandC18OmolecularlinedatafromtheFiveCollege
of thestructure, energetics,and kinematicsof theHII region. Fi-
RadioAstronomyObservatory(FCRAO).Inthenextsectionwere-
nally, Kertonetal. (2001) presented an analysis of submillimetre
viewthepertinentparametersofKR140andprovideinformation
(submmhereafter)observationsoftheregionat450and850µm.
onthenewobservationsoftheregion.In§3and§4wefirstshow
ThemainresultofKertonetal.(2001)wasthediscoveryofnumer-
how 2MASS datacan be utilizedtoidentify theYSO population
oussubmmdustcoreslocatedwithinthemoleculargassurrounding
andthenexamineitsspatialdistribution.Ourresultsarediscussed
theHIIregion,includingalargenumberofcoresthatwereclearly
in§5andconclusionsarepresentedin§6.
locatedattheinterfacebetweenionizedandmoleculargas,alikely
locationforstar-formationinducedor“triggered”bytheexpansion
oftheHIIregion(Elmegreen1998).Twoofthemoreisolatedcores
wereclearlyassociatedwithIRASsources,butthelowresolution
ofIRAScombinedwiththeextensivediffuseemissionofdustasso-
2 KR140:PROPERTIESANDOBSERVATIONS
ciatedwiththeHIIregionmadeitimpossibletodetermineifany
oftheothercoreswereassociatedwithstar-formationactivity. KR 140 (l = 133◦.425, b = +0◦.054; Kallas&Reich 1980) isa
Thegoalofthisstudyistodeterminethedistributionofyoung 5.7pcdiameterHIIregionlocatedatadistanceof2.3±0.3kpc.It
stellarobjects(YSOs)throughouttheKR140molecularcloud.We iscloseto,butapparentlyisolatedfrom,thelargeW3/W4/W5star-
focus especially on the embedded stellar content of the submm formation complex. The region is ionized by a single O8.5 V(e)
cores as these are potentially the youngest star-forming regions. star VES 735. Foreground extinction is AV = 5.7 ± 0.2 mag-
WeusethespatialdistributionoftheYSOsrelativetothephotodis- nitudes toward VES 735 and ranges between AV ∼ 6−7 over
theHII region. Molecular lineobservations (12CO J=1−0) indi-
catethattheHIIregionisassociatedwithamolecularcloudwith
⋆ E-mail:[email protected] amassintherange 103.7 –104.0 M⊙.A detaileddiscussionand
2 Kertonet al.
Table 1. Spatial correspondence between Kertonetal. (2001) and
Mooreetal.(2007)submmclumps(KMJBandM07respectively).
KMJB M07 KMJB M07
1 37 11 14
2 ··· 12 11
3 32 13 8&10
4 33 14 5
5 26&29 15 ···
6 25 16 6
7 24 17 ···
8 23 18 4
9 17 19 3
10 ··· 20 ···
derivationofthesepropertiescanbefoundinKertonetal.(1999) Figure1.MSXBandA(8.3µm)imageoftheKR140region.Notethedis-
andBallantyneetal.(2000). tinctivehalomorphologyofthePDR.Whitecrossesindicatethepositions
ofsubmmclumpsapparently associated withthePDRandblackcrosses
indicatethesubmmclumpsfoundawayfromthePDR.
2.1 SubmillimetreObservations
2.3 MolecularLineObservations
Forthisstudyweusedacombinedsampleofsubmmclumpsiden-
tified in the Kertonetal. (2001) and Mooreetal. (2007) 850 µm We obtained new molecular line observations of KR 140 in or-
SCUBA scan-maps of the KR 140 region. There are only minor der todeterminethefullspatial extent of thesurrounding molec-
disagreementsbetweenthetwosampleswheretheyoverlapdueto ularcloudandtoidentifyanyhighcolumndensityregions.Maps,
the different ways in which structures were identified in the two covering a 0◦.8 square region around KR 140, were obtained in
studiesandthecorrespondencebetweenthetwosamplesisshown the J=1−0 transition of 12CO (115.3 GHz), 13CO (110.2 GHz)
inTable1.Intheareacoveredbyourmolecular-lineobservations and C18O (109.8 GHz) using the SEQUOIA array on the 14 m
(2h18m15s 6α2000 62h22m30sand60◦54′ 6δ2000 661◦20′, FCRAOmillimetre-wavetelescope(Ericksonetal.1999).There-
see§2.3)thereareatotalof39submmclumps:KMJB1-KMJB sulting data cubes have a spatial resolution of ∼ 45′′ (20′′ pixel
20 from Kertonetal. (2001) and clumps 7, 9, 12-13, 21, 39-40, grid)andavelocityresolutionof0.065kms−1.Integratedinten-
42-45and47-54fromMooreetal.(2007). sity(zeroth moment) 12CO and 13COmaps of emissionbetween
−54 < VLSR < −44 km s−1 are shown in Fig. 2. Contours of
integrated intensity C18O emission are also shown on each map
highlightingthetwomainregionsofhighcolumndensityfoundto
2.2 Mid-InfraredMSXObservations
theeastandwestoftheHIIregion.
Mid-infrared emission from polycyclic aromatic hydrocarbons We find a mass of 8500 ± 1000 M⊙ for the cloud based
(PAHs)canbeusedtodetectthePDRsthatariseattheinterfacebe- on the integrated 12CO data. A value of X = 1.9×1020 from
tweenionizedgasinanHIIregionandsurroundingmolecularma- Strong&Mattox (1996) to convert between integrated intensity
terial(Giardetal.1994).WeobtainedMSXBandA(λo=8.3µm, (K km s−1) and H2 column density and using a mean molecular
λ=7−11µm)imagesofKR140with20′′resolutiontakenaspart weightof2.29toaccountforthemolecularnatureofthehydrogen
oftheMSXGalacticPlaneSurvey(Priceetal.2001).TheBandA andthepresenceofHe.Thisestimateislargerthanthatquotedin
emissionisdominatedbyintenselineemissionfromPAHscaused Ballantyneetal.(2000)mainlybecauseweareusingalargerspa-
by various C−C stretching and C−H bending modes. An up-to- tialrangetodefinetheassociatedcloud.
datelistingofthenumerousPAHfeaturesinthemid-infraredcan
befoundinDraine&Li(2007).
Thedistinctivemid-infraredmorphologyexhibitedbyKR140 2.4 Near-infraredData
(seeFig.1)andinothersimilarHIIregions(e.g.Cohen&Green Photometric(J,H,Ks1)dataonYSOsassociatedwithKR140were
2001; Deharvengetal. 2003) arises from changes in the relative
obtainedfromthe2MASSAll-SkyPointSourceCatalog(2MASS
balancebetweenultraviolet(UV)photonswhichpreferentiallyde-
PSChereafter)usingtheon-lineGATORquerytoolattheInfrared
stroy PAHs and those which excite their emission. Deep within
ScienceArchive(IRSA).Detaileddiscussionofthesurveyandthe
the HII region, hard UV radiation destroys PAHs and are them- various data products are available in Skrutskieetal. (2006) and
selves destroyed. Outside the PAH-free zone, the remaining UV
Cutrietal. (2003). Here we note that the overall quality of each
photonsmoveintothesurroundingPDRandexcitePAHemission
2MASS photometric measurement is indicated by a photometric
in a layer around the HII region. This layer isrelatively thin be- qualityflag(“ph qual”) inthe2MASS PSC.Validmeasurements
cause UV fluxes continue to drop through the PDR, resulting in
are indicated by a ph qual value of A, B, C, or D for each filter
a drop in PAH emission. The overall result isa striking ring-like
withthesignal-to-noiseofthedetectiondecreasing asonemoves
or“halo”morphology. InFig.1wealsoshow thelocationofthe
fromph qual=Athroughph qual=D.Non-detectioninafilteris
submmclumpsdiscussedin§2.1:thecorrespondencebetweenthe
locationofsomeofthecoresandtheinterfaceregiontracedbythe
mid-infraredemissionisevident. 1 InthisstudyweuseKsandKbandphotometryinterchangeably
StarFormationAroundKR 140 3
Figure3.Near-infrared colour-colour diagram. TTauriandHAeBe data
from Kenyon&Hartmann (1995) and The´etal. (1994) respectively are
shownshiftedtoadistanceof2300pcandwithAV =5.5offoreground
reddening.Thesolidlinesshowtheintrinsiccoloursofmain-sequence(V)
andgiant(III)stars(Koornneef1983;Bessell&Brett1988).Thedash-dot
lines show AV = 20reddening vectors fora K5V and O9V star de-
rivedusingtheinfraredextinctionlawE(J−H)/E(H−K)=1.7±0.4from
Rieke&Lebofsky(1985).
Figure2.Integrated12CO(top)and13CO(bottom)mapsoftheKR140
molecularcloud.ContoursinbothpanelsshowintegratedC18Oemissionat
the1.2and1.4Kkms−1levels.Whitecrossesinthelowerpanelindicate
thepositionsofalldetectedsubmmclumps.
typicallyindicatedbyph qual=U,inwhichcaseanupperbright-
nesslimitisgiven.
3 USING2MASSTODEFINETHEYSOPOPULATION
3.1 JHKColour-ColourandColour-MagnitudeDiagrams
Since we know the distance to KR 140 and the amount of fore-
groundextinction,ourapproachistoscaleobservationsofdiffer-
enttypesofYSOsinlocalstar-formingregionstothedistanceof
KR140,includingtheeffectofforegroundextinction,thusdefin- Figure4.Colour-magnitudediagramforobjectsatadistanceof2300pc.
DataarethesameasinFig.3.Themain-sequence (V)andgiant branch
ingregionsontheJHK colour-colour(CC)andcolour-magnitude
(III)areshown(solidlines)withsomerepresentativespectraltypeslabeled.
(CM) diagrams where we would expect to find YSOs associated
Reddeningvectors(dash-dotlines)aredrawnforAV = 20foranO9V
withtheKR140molecularcloud.
andK5Vstar.
InFig.3wehaveplottednear-infraredcoloursofTTauristars
(approximatelysolar-massYSOswithopticallythickdisksofcir-
cumstellar material) from the Kenyon&Hartmann (1995) study sented in Robitailleetal. (2006), once the appropriate amount of
of the Taurus-Auriga region (distance ∼ 140 pc). Intermediate- foregroundextinctionisappliedtothemodel-basedCCdiagrams.
mass YSO colours are illustrated using 2MASS photometry for The near-infrared CM diagram showing scaled T Tauri and
a subset of the The´,deWinter,&Pe´rez (1994) catalogue of Her- HAeBedataisshowninFig.4.TheutilityofhavingboththeCC
big Ae and Be stars (HAeBe) that have distance estimates from and CM diagrams available for data analysis is readily apparent.
Finkenzeller&Mundt (1984). One sees that the majority of the Forexample,intheCMdiagramtheHAeBesampleisclearlydis-
sample lies outside of the band of reddened stellar photospheres, tinctfromthelowerluminosityTTauristars.Also,possibleconfu-
withaportionoftheTTaurisamplebeingthemainexception.The sionbetweenHAeBestarsandgiantscanbeavoidedbyusingthe
regionoccupiedbyourYSOsampleagreesnicelywiththeCCdi- CCdiagram,wherethetwosamplesarewellseparated.
agrams constructed using an extensive grid of YSO models pre- Table2showstheaveragemagnitudeandstandarddeviation
4 Kertonet al.
Table2.JHKmagnitudesofYSOsamplesat2300pc.
Sample Band Avg. σ min max 10σa Db
HAeBe J 12.73 1.8 9.30 16.41 15.8 17.2
H 11.28 1.5 8.62 14.59 15.1 16.1
K 10.03 1.4 8.16 13.14 14.3 15.3
TTauri J 18.21 2.0 13.91 25.73 15.8 17.2
H 16.50 1.6 12.52 21.36 15.1 16.1
K 15.44 1.3 11.22 19.12 14.3 15.3
a2MASScompletenesslimits.
b2MASSphqual=DlimitsforKR140region.
cataloguesearchesandthecriteriausedtoidentifyYSOsdiscussed
belowaresummarizedinTable3.Figs.5and6showboththeYSO
andnon-YSOpopulationoftheregionforcomparisonwhileFig.7
showsthespatiallocationoftheYSOsample.
We first searched for all stars with valid photometry (i.e.,
Figure5.2MASSColour-colourdiagram.Coloursforthe4144starswith ph qualvaluesofA,B,C,orDinallthreebands2).Thissearchre-
valid JHK photometry are plotted (dots) along with colours of the P1 turned4144starsfromthecatalogue.Thesurfacedensityofstarsis
(crosses),P1+(diamonds),andP2(squares)YSOsamples.TheP2(J−H) fairlyuniformacrossthesearchareaat∼4.7stars/⊔⊓′(1σ=1.3).
coloursarelowerlimits(indicated bythearrows).Note,forclarity,error
WithinthissampleofstarswesearchedforYSOsdefinedasstars
barsarenotshown.
with(J−H)>1and(J−H)<1.7(H−K)−0.075.Thefirstlimit
on (J−H) reflects the fact we expect there to be sufficient fore-
groundextinctiontopushanyYSOabovethe(J−H)>1line.The
secondcriteriaselectsstarslyingredwardofthereddeningvector
associatedwithanO6Vstar.Thissearchfinds72starswhichwe
calltheP1sample.
Wedidanadditionalsearchforstarsthatmaybelocatedinthe
overlapregionofTTauristarsandreddenedstellarphotospheres.
Thisregionislocatedbetween1.3 <(J−H)< 1.6andbetween
the two main-sequence reddening vectors. We also required that
K > 14.5 in order to reduce contamination from giant stars and
distantearlymain-sequencestars.Thisresultsinanadditional 38
stars, which wecall the P1+ sample. We note the conclusions of
this paper are not sensitive to whether or not one uses the more
conservativeselectioncriteriaoftheP1sampleorthecombinedP1
andP1+sampleof110stars.Weexpectthatanycontaminationof
the larger sample will arise from distant A and F main-sequence
starsascloser,laterspectraltypestarswilltendnottohaveenough
Figure6.2MASSColour-magnitudediagram.AsFig.5butalsoshowing extinctiontomatchthecolourcriteria.
the P3 (diamond) YSO sample. TheP3 (H−K)colours are lower limits TheKR140regionwasthenre-examinedforstarswithanup-
(indicatedbythearrows). perbrightnesslimitatJandvalidHandKphotometry(ph qual=
[U][A-D][A-D]).Thiscataloguesearchreturned133stars.Possible
YSOsweredefinedusing(H−K)>1and(J−H)<1.7(H−K)−
alongwiththemaximumandminimummagnitudeforeachofthe 0.075.Thelatterconstraintplacesthe(J−H)lowerlimitbelowthe
YSOsamples.Alsotabulatedarethe2MASSlimitingmagnitudes reddenedstellarphotosphereregionoftheCCdiagram.Intotal33
for a complete sample (“10σ”) and for ph qual =D quality pho- starsmatchthesecriteriaandweidentifythemastheP2sample.
tometry(“D”;see§2.4).WeseethatHAeBestarswillbedetected Finally we searched the region for stars with only upper
inallthreebandswithinthe10σ limits.TTauristarswillalsobe brightnesslimitsinJ andH alongwithvalidK band photometry
detectedinallthreebandsalthoughthesamplewillbeincomplete, (ph qual=[U][U][A-D]).Ofthe37starsfound,10ofthemwere
missingveryheavilyembeddedsources defined as possible YSOs by requiring the lower limit of (H−K)
>1.
3.2 TheYSOPopulationofKR140
UsingtheintegratedCOmapsasaguideweinvestigateda0◦.5×
0◦.4 region centered on the KR 140 HII region (2h18m15s 6
α2000 6 2h22m30s and60◦54′ 6 δ2000 6 61◦20′).The2MASS
PSC was queried using the GATOR software at IRSA. All of the 2 phqualMATCHES[A-D][A-D][A-D]inSQLusedbyGATOR
StarFormationAroundKR 140 5
tionalassociations. FortheKertonetal.(2001)sampleweuseda
searchradiusofDeff/2,whereDeff isthedeconvolvedFWHMofa
Gaussianfittoanazimuthally-averagedprofileofeachsource.The
averagesearchradiuswas0.18pc.Sincewedonothavesizeinfor-
mationfor the Mooreetal. (2007) sample, weused a20′′ search
radius(0.22pcat2.3kpc)foreachsourceinthatsample.Table4
showstheresultsofthissearch.Column1givestheclumpIDnum-
ber.Columns2through5containthe2MASSPSCsourcenumber
forassociatedYSOsfromtheP1,P1+,P2,andP3samplesrespec-
tively. Column 7contains the column density of the submm core
expressed in termsof visual extinction, AV. For the Mooreetal.
(2007) samples masses were calculated for a distance of 2.3 kpc
andaclumptemperatureof20Kassumingthatalltheclumpshad
adiameterof0.36pc.ThisresultsinAV ∼10S850 whereS850is
theintegratedfluxdensityat850µm.AlloftheseAV estimatesare
veryroughasthecolumndensitywillvarywiththesquareofthe
estimateddiameterandthetemperatureentersinthroughtheexpo-
nentialinthePlanckfunction.Valuesfortheremainingcoreswere
taken from Kertonetal. (2001). Finally, Column 7 denotes if the
Figure7.ThespatialdistributionoftheP1(crosses)P1+(diamonds),P2
coreisapparently isolatedor associated withthePDR.Thetable
(squares)andP3(triangles)YSOsamplesareshownagainsttheintegrated
13COemissionassociatedwithKR140.Theboxesshowthelocationofthe includestwosubmmclumps,3and53m,thathaveonlyassociated
IRASsourcesandarepossibleClass0/IYSOs.
fourconcentrationsofYSOsdiscussedinthetext,RegionA-D,fromleftto
right.Whiteorblacksymbolsareusedinterchangeably dependingonthe InFig.8andFig.9weplottheCCandCMdiagramsshow-
backgroundlevel. ingtheYSOsassociatedwiththesubmmclumps.TheCMdiagram
clearlyseparatestheintermediate-massYSOs(HAeBestars)asso-
ciatedwithclumps 1, 4 and 19, from thelower luminosity (solar
Table3.YSOsamples. massorTTauri)YSOs.Theformerobjectsprovideanimportant
constraint on the timing of star-formation throughout the molec-
Sample Number phqualvalues Colourlimitsa ular cloud asHAeBestarsclear substantial portions of surround-
ingmolecularmaterialontimescalesof∼106years(Fuenteetal.
P1 72 [A-D][A-D][A-D] (J−H)>1
1998).
(J−H)<1.7(H−K)−0.075
P1+ 38 [A-D][A-D][A-D] 1.3<(J−H)<1.6 TheTTauriYSOsfoundassociatedwiththesubmmclumps,
(J−H)<1.7(H−K)−0.075 and those associated with the dense C18O filament (see § 4.2),
(J−H)<1.7(H−K)+0.3805 arealsolikelytracingstarformationonsimilartimescales.Given
K>14.5 thatthetypicalradiusfor thesubmmclumps isonly0.25 pcand
P2 33 [U][A-D][A-D] (J−H)L<1.7(H−K)−0.075 the velocity dispersion of T Tauri stars is typically 1–2 km s−1
(H−K)>1.0 (Neuhauseretal. 1998) it will only take the T Tauri star 0.2–0.4
P3 10 [U][U][A-D] (H−K)L>1.0 Myrtonolonger beassociatedwiththeclumpasseenbyanob-
a(J−H)Land(H−K)Lindicatelowerlimits. server(assuming,forexample,thatthestarismovingat45◦tothe
line-of-sight). SimilarlyYSOsassociated withthe C18O filament
4 THEYSOPOPULATIONOFTHEKR140 havenothadenoughtimetodispersefarfromtheirpointoforigin.
MOLECULARCLOUD Thereisgrowingobservationalevidencefromtheanalysisof
large-scale maps of molecular clouds that star-formation is more
Usingthe2MASSsampledefinedabove,wenowexploretwoques- likelytooccurinhighcolumndensityregionsofmolecularclouds
tions:1)WhatistheassociationbetweentheYSOsandthesubmm wherealowerionizationfractioncanreducetheefficiencyofmag-
sourcesidentifiedbyKertonetal.(2001)andMooreetal.(2007), netic support (McKee 1999) and/or turbulent magnetohydrody-
and2)WhatistheoverallspatialdistributionoftheYSOsthrough namic support (Ruffleetal. 1998). Many studies have found an
themolecularcloud? extinction threshold (AtV) where regions with AV > AtV are
abletocollapseandfragmenttoformstellar-sizedstructures;ob-
servedvaluesrangefromAV ∼ 4forTaurus(Onishietal.1998),
4.1 YSOsandSubmmSources
through AV ∼ 6 for Perseus (Kirketal. 2006) to values around
The submm clumps identified by Kertonetal. (2001) have typ- AV ∼ 15forOphiuchus(Johnstone,DiFrancesco,&Kirk2004).
ical diameters of 0.5 pc (105 AU), which is about an order of TheHatchelletal. (2005) study of Perseusdoes not show such a
magnitude larger than the 0.05 pc (104 AU) size scale typi- clear threshold, but does findan increasing probabilityof finding
cally associated with the smaller core structures within molec- cores at increasing levels of AV. We note that all of these stud-
ular clouds thought to be forming individual stars, for exam- iesrefer totheconditions required for coreformationanddo not
ple, the Kirk,Johnstone,&DiFrancesco (2006) observations of considertheYSOcontentofthecore.
the Perseus molecular cloud and the Class 0/I YSO models of InFig.10ahistogramshowshowthenumberofclumpswith
Robitailleetal.(2006).Thusitisquitepossiblethatasingleclump andwithoutassociatedYSOsvariesasafunctionofAV.Whilewe
willhavemultiplesitesofstar-formationassociatedwithitandthat donotseeanobvioussinglethresholdvalueitdoesappearthatstar-
these regions need not correspond with the peak position of the formationisveryunlikelytooccurintheverylowcolumndensity
submm emission. We examined our entire YSO sample for posi- clumps. There are 14 clumps with AV < 4, of which 11 (79%)
6 Kertonet al.
Figure 10. Plot showing the number of clumps with (black) and with-
out (white) YSOs as a function of clump extinction for the combined
Kertonetal.(2001)andMooreetal.(2007)sample.Forexample,thereare
5clumpswith1 6AV < 2ofwhich2haveassociatedYSOsand3do
not.TherightmostbinincludesallclumpswithAV >20.
Figure8.AsinFig.5butshowingthepositionofYSOsspatiallyassociated
withsubmmclumps.Thesize ofthesymbols indicate theuncertainty in
colour.YSOsareidentifiedusingthenomenclaturefromTable4,e.g.,the 4.2 GeneralYSOSpatialDistribution
YSOfromthe“P2”sampleassociatedwithclump13islabeled“13.P2”.A
lowerlimittothe(J−H)colourisindicatedbyanarrowsymbol. Visually there appear to be four concentrations of YSOs in the
KR140molecularcloud(fordiscussionwewilldenotetheseRe-
gions A through D) along with a noticeable lack of YSOs in the
northeast portion of the molecular cloud (see Fig. 7). Region A
is at the eastern end of the molecular filament near 2h22m, Re-
gionBisassociatedwithsubmmclump1at2h21m,RegionCis
nearthecentralPDRat2h20m,andfinallyRegionDisassociated
with clumps 17, 18 and 19 near 2h19m30s. To quantify this vi-
sualimpressionwedividedthemolecular cloudareainto2′ ×2′
boxesandcountedthenumber ofstarsfallingwithineachboxas
aroughmeasureoftheYSOsurfacedensity.Theaveragenumber
ofYSOs/boxis0.7±1.4(±1σ)andboxesnearthefourregions
inquestion hadsurface densitiesof7, 7,6and11for AtoDre-
spectively.Therewerenootherregionsinthemolecularcloudwith
similar>3σdeviationsfromtheaveragevalue.
RegionAconsistsof17YSOs,fourofwhichareassociated
withsubmmclumps.AlloftheYSOsinthisregionareTTauri-like
withanaverage(H−K)valueof0.92±0.25.NoneoftheYSOsare
coincidentwithIRAS02186+6033(MSX6CG133.6890+00.1643),
strengtheningitsidentificationasaClass0/IYSO.
Figure9.AsinFig.6butshowingthepositionofYSOsspatiallyassociated
withsubmmclumps.Thesize ofthesymbols indicate theuncertainty in The focal point of Region B is the submm clump KMJB 1
magnitude and colour. YSOs are identified using the nomenclature from which was identified as a Class I YSO (IRAS 02174+6052)
Table4,e.g.,theYSOfromthe“P2”sampleassociatedwithclump13is in Kertonetal. (2001). The higher resolution of the MSX and
labeled“13.P2”.Alowerlimittothe(H−K)colourisindicatedbyanarrow 2MASSdatarevealthatClump1containstwoYSOs,includingan
symbol. intermediate-massHAeBewhichisalsoaMSX pointsourcewith
averyredmid-infraredspectrum(F8.3 = 0.22Jy,F14.65 = 0.73
Jy). Just to the east of the clump there are two very red YSOs.
donothaveassociatedYSOsanditisunlikelythatstarformation 2MASS02210851+6105582 isdetectedonlyinone2MASSband
willoccurintheseclumps.Attheotherendoftheextinctionrange withK= 14.306 andalowerlimitcolour of (H−K)L > 3.26. It
allthreeoftheclumpswithAV > 20haveYSOs.Theremaining isalsoabright MSX point source (MSX6CG133.5561+00.0919)
22 clumps with 4 < AV < 20 areessentially evenly split, with with an extremely red spectrum (F8.2 = 0.34 Jy, F14.65 = 1.4
10havingassociatedYSOsand12nothavingYSOs.Basedonthe Jy). The other YSO, 2MASS 02210610+6106043, is detected in
studiesofotherstar-formingregionsmentionedaboveweinterpret boththeHandK bandsandhas(H−K)= 3.63andK = 11.899.
theclumpsinthisextinctionrangewithoutYSOsasbeingpotential BothoftheseYSOsareprobablymorehighlyembeddedversions
star-formationsites.ThelackofdetectableYSOswiththeseclumps of1.P1.Inaddition, justtothewestof KMJB1,thereisanother
could reflect their relative youth, or it may be the case that any highlyembeddedYSOwithK = 15.208and(H−K)L > 2.30.It
YSOsthathaveformedmaybeverylowmassandthusfallbelow isquite likelythat theseobjects haveformed together asasingle
the2MASSdetectionlimitsevenforourP3sample. groupingofstars.
StarFormationAroundKR 140 7
Table4.YSOsassociatedwithsubmmclumps.
2MASSYSOsample PDR(P)
ClumpIDa P1 P1+ P2 P3 AV orIsolated(I)
1 02210449+6106045b ··· 02210217+6105496 ··· 5.1 I
2 ··· ··· ··· 02205994+6107173 1.6 I
3c ··· ··· ··· ··· 4.5 I
4 02205329+6108215 ··· ··· ··· 15.4 I
6 ··· ··· 02203428+6107425 ··· 13.1 P
12 ··· ··· 02195506+6105211 ··· 68.2 P
13 ··· ··· 02195384+6102192 ··· 1.4 P
16 ··· ··· 02194584+6101426 ··· 1.1 P
17 ··· ··· ··· 02193257+6107399 8.7 P
19 02192184+6107069 ··· 02192272+6107157 ··· 21.7 P
45m ··· ··· ··· 02214126+6105457 37.7 I
02214085+6105439
47m 02215369+6106275 ··· ··· ··· 6.8 I
49m ··· ··· 02215837+6106405 ··· 5.4 I
51m 02220660+6107253 ··· ··· ··· 6.5 I
52m ··· 02222067+61073111 ··· ··· 15.7 I
53md ··· ··· ··· ··· 13.4 I
aMooreetal.(2007)IDnumbershaveatrailingm(e.g.7m),otherIDnumbersfromKertonetal.(2001)
b2MASS02210449+6106045=MSX6CG133.5492+00.0900
cIRAS02171+6058
dIRAS02186+6033=MSX6CG133.6890+00.1643
RegionCisinterestingbecauseofitsassociationwiththePDR beyondthedirectinfluenceoftheHIIregion,andthePDR,which
ridgeseenintheMSXimages.Themajorityofthestarsarefound istheregiondirectlyinfluencedbytheexpansionandUVfluxof
tothe northeast of thePDR,while5 of the remaining 6starsare theHIIregionandwhichcanbeclearlytracedbyPAHemission.A
foundalongthePDRwithoneoftheYSOsbeingcoincidentwith comparisonoftheYSOpopulationfoundineachoftheseregions
asubmm clump. Thispattern suggests that star-formationispro- thengivesameasureof therelativeimportance ofsequential and
gressingfromnortheasttosouthwestwiththesubmmclumpsfound spontaneousstarformationwithinthemolecularcloud.
alongthePDRbeingthelatestroundofstarformation. To delineate these two regions we use the 1.5 × 10−6
Region D contains the submm clumps 17, 18, and 19. W m−2 sr−1 contour in the MSX Band A image to (generously)
Intermediate-mass and solar-mass YSOs are associated with define the extent of the PDR. We find that 42% of the full YSO
clump 19 and, just to the north of this clump, there are an addi- sample (P1, P1+, P2 and P3) lie within the PDR as defined and
tional six T Tauri YSOs. All three of these clumps make up the 58% lieoutside thePDR. Theexact percentages willvary some-
infrared source IRAS 02157+6053 which MSX and 2MASS now what depending on how the PDR is defined. For example, using
clearly show is actually a concentration of YSOs combined with a more conservative definition for the PDR region (1.75×10−6
extendedemissionfromthePDR. Wm−2sr−1)resultsin31%oftheYSOswithinthePDRand69%
outsidethePDR.Thepercentages,inbothcases,arethesamefor
theYSOpopulationbothwithandwithoutincludingtheP1+sam-
ple. In all cases it is clear that a substantial fraction of the YSO
5 DISCUSSION
populationliesbeyondthePDRinthisstar-formingregionandis
5.1 SpontaneousversusSequentialStarFormation thusnottheresultofsequentialstarformationassociatedwiththe
HIIregion.
When examining star formation processes within a molecu-
lar cloud containing an HII region, a key distinction is be-
tween sequential star formation, where the trigger for star for-
5.2 AScenarioforStarFormationinKR140
mation is directly related to the presence of the HII region (e.g.,
Deharveng,Zavagno&Caplan2005),andspontaneousstarforma- TheexcitingstaroftheHII region,VES735,isabout2Myrold
tion, which occurs with no apparent causal relationship with the (Kertonetal. 1999), and kinematic models of the HII region set
HIIregion.Itiswellknownthatforbothclassicaldensity-bounded expansiontimescalesof1-2Myr(Ballantyneetal.2000).Ourob-
HII regions and blister/champagne-flow HII regions, the ioniza- servationsandanalysisofKR140areconsistentwithamolecular
tion front (IF) is preceded by a shock front as it propagates into cloudundergoingcontinuousstarformationoverthisperiod.There
the molecular cloud (Osterbrock 1985; Bedijn&Tenorio-Tagle isnoclearsignatureofasignificantolderpopulationofstarsseen
1981).Time-dependentmodelsofPDR/IFdevelopmentaroundOB inthe2MASSphotometryoftheregion(seeFigs.5and6).Lower
stars(Roger&Dewdney1992;Diaz-Miller,Franco&Shore1998) massYSOsthatformedatthesametimeasVES735havehadtime
show that the IF and PD front merge rapidly, i.e., on time scales todispersethroughthemolecularcloudandmayformsomeofthe
lessthanthemain-sequencelifetimeoftheexcitingstar,fortypical morespatiallydistributedpopulationofYSOsseeninoursample.
molecularclouddensities.Withthisinminditisusefultoconsider TheexpansionoftheHIIregionhassweptupmaterialwhich
two distinct environments within the molecular cloud; the region hassubsequentlycollapsedtoformthesubmmclumpsthatweob-
8 Kertonet al.
serveinthePDR.Atthesametimetheelongatedmorphology of Bessell,M.S.,Brett,J.M.,1988,PASP,100,1134
the densest portion of the molecular cloud has resulted in ongo- Cohen,M.,Green,A.J.,2001,MNRAS,325,531
ing star formation inregions well away from thedirect influence Cutri, R.M. et al., 2003, Explanatory Supplement to the
oftheHIIregion.Asdiscussedearlier,theassociationofHAeBe 2MASS All Sky Data Release (Washington: NASA),
andTTauristarswithdenseregionstracedinthesubmmandC18O http://www.ipac.caltech.edu/2mass/releases/allsky/doc/explsup.html
supportagesof. 106 years.ThepresenceoftheClass0/IYSOs, Diaz-Miller,R.I.,Franco,J.,Shore,S.N.,1998,ApJ,501,192
IRAS02171+6058andIRAS02186+6033showstherecontinuesto Deharveng, L., Zavangno, A., Salas, L., Porras, A., Caplan, J.,
be very recent star formation activity on the timescale of ∼ 105 Cruz-Gonzalez,I.,2003,A&A,399,1135
years(Robitailleetal.2006). Deharveng,L.,Zavagno,A.,Caplan,J.,2005,A&A,433,565
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guishbetweendifferentYSOevolutionarystages(Robitailleetal. 1999,IEEETrans.MicrowaveTheoryTech.,47,2212
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6 CONCLUSIONS Giard, M., Bernard, J.P., Lacombe, F., Normand, P., Rouan, D.,
1994,A&A,291,239
Whilemuchofthefocusofstudiesofstarformationinandaround Hatchell, J., Richer, J.S., Fuller, G.A., Qualtrough, C.J., Ladd,
HII regions is on the interface regions, our study of KR 140 il- E.F.,Chandler,C.J.,2005,A&A,440,151
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ACKNOWLEDGMENTS 1998,ApJ,502,296
Osterbrock, D.E., 1985, Astrophysics of Gaseous Nebulae and
ThisresearchhasmadeextensiveuseoftheNASA/IPACInfrared
ActiveGalacticNuclei,UniversityScienceBooks,Sausalito,CA
Science Archive (IRSA), the Canadian Astronomy Data Centre
Price, S.D., Egan, M.P., Carey, S., Mizuno, D.R., Kuchar, T.A.,
(CADC),anddatafromtheTwoMicronAllSkySurvey(2MASS).
2001,AJ,121,2819
IRSAisoperatedbytheJetPropulsionLaboratory,CaliforniaIn-
Rieke,G.H.,Lebofsky,M.J.,1985,ApJ,288,618
stituteofTechnology,undercontractwithNASA.2MASSisajoint
Robitaille,T.P.,Whitney,B.A.,Indebetouw,R.,Wood,K.,Denz-
projectoftheUniversityofMassachusettsandtheInfraredProcess-
more,P.,2006,ApJS,167,256
ingandAnalysisCenter/CaliforniaInstituteofTechnology,funded
Roger,R.S.,Dewdney,P.E.,1992,ApJ,385,536
byNASAandtheNSF.TheCADCisoperatedbytheNationalRe-
Ruffle, D.P., Hartquist, T.W., Rawlings, J.M.C.,Williams, D.A.,
searchCouncilofCanadawiththesupportoftheCanadianSpace
1998,A&A,334,678
Agency.FCRAOwassupportedbyNSFgrantAST0540852. CB
Rydgren,A.E.,Vrba,F.J.,1983,AJ,88,1017
issupported byanRCUKFellowshipattheUniversityofExeter,
Skrutskie,M.F.etal.,2006,AJ,131,1163
UK.TheauthorswouldliketoacknowledgetheworkdonebyCar-
Strong,A.W.,Mattox,J.R.,1996,A&A,308,21
olinePomerleauintheinitialstagesofthisprojectsupportedbythe
The´,P.S.,deWinter,D.,Pe´rez,M.R.,1994,A&AS,104,315
WomeninEngineeringandScienceprogramofNRCCanada.
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