ASSOCIATION OF INFRARED DARK CLOUD CORES WITH YSOS:
STARLESS OR STARRED IRDC CORES
GwanjeongKim 1;2;5
, ChangWonLee 1;2;6
, JongsooKim 1;2
,YoungungLee 1
, JavierBallesteros-Paredes 3
,
PhilipC.Myers 4
, andS.Kurtz 3
1
InternationalCenter forAstrophysi s,Korea AstronomyandSpa e S ien eInstitute,61-1Hwaam-dong,
Yuseong-gu,Daejeon305-348,Korea
2
DepartmentofAstronomyandSpa eS ien e, UniversityofS ien e&Te hnology,113Gwahangno,Yuseong-gu,
Daejeon305-333,Korea
3
CentroDeRadioastronomayAstrofsi a,UniversidadNa ionalAutonomaDeMexi o,Morelia,Mi hoa an
58089,Mexi o
4
Harvard-SmithsonianCenterforAstrophysi s,60GardenStreet,Ms42,Cambridge,Ma02138,USA
5
E-mail: ar her81kasi.re.kr & wlkasi.re.kr
(Re eivedDe ember 04,2009;A eptedDe ember 28,2009)
ABSTRACT
In this paper we examinedthe asso iation of InfraRedDark Cloud (IRDC) oreswith YSOs and
thegeometri propertiesoftheIRDC ores. Forthisstudyatotalof13,650IRDC oreswere olle ted
mainlyfromthe atalogsoftheIRDC orespublishedfromotherstudiesandpartiallyfromour atalog
ofIRDC ores ontainingnew789IRDC ore andidates. TheYSO andidatesweresear hedforusing
the GLIMPSE, MSX, and IRAS point sour es by the shape of their SED or using a tivity of water
or methanol maser. The asso iation of the IRDC ores with these YSOs was he ked by their line-
of-sight oin iden e within the dimension of the IRDC ore. This work found that a total of 4,110
IRDC oreshaveYSO andidateswhile9,540IRDC oreshavenoindi ationoftheexisten eofYSOs.
Consideringthe12,200IRDC oreswithin theGLIMPSEsurveyregionforwhi htheYSO andidates
weredeterminedwithbettersensitivity,wefoundthat 4,098IRDC ores(34%)haveatleastoneYSO
andidateand1,072 oresamongthemseemtohaveembeddedYSOs,whiletherest8,102(66%)haveno
YSO andidate. Therefore,theratioof[N(IRDC orewithprotostars)℄/[N(IRDC orewithoutYSO)℄
for12,200IRDC ores isabout0.13. Taking into a ountthis ratioand typi al lifetime ofhigh-mass
embeddedYSOs,wesuggestthattheIRDC oreswouldspendabout10 4
10 5
yearstoformhigh-mass
stars. However,weshouldnotethattheGLIMPSEpointsour eshaveaminimumdete tableluminosity
ofabout1.2 L
at atypi alIRDC ore's distan e of4 kp . Therefore, theratio given hereshould
bealowerlimitandtheestimatedlifetime ofstarlessIRDC ores an beanupperlimit. Thephysi al
parametersoftheIRDC oressomewhatvarydependingonhowmanyYSO andidatestheIRDC ores
ontain. TheIRDC oreswithmoreYSOstendtobelarger,moreelongated,andhavebetterdarkness
ontrastthantheIRDC oreswithfeweror noYSOs.
Keywords: atalogs|dust, extin tion|galaxy: general|infrared: ISM |ISM: louds|ISM:
stru ture
I. INTRODUCTION
Howdomassivestarsform? Dotheyforminasim-
ilarwayto howlow-massstarsdo? Thisis aquestion
that many astronomers would like to frequently ask,
but an hardly answer for. One of suggested s enar-
iosintheearlystageofhigh-massstarformation(e.g.,
Beutheret al. 2007)isthatsomeoflow/intermediate-
massprotostarsforminHigh-MassStarlessCores(HM-
SCs) through an a retion pro ess suggested by the
standardtheoryoflow-massstarformation(e.g.,Shuet
al. 1987)andbe omemassive(>8M
)eitherthrough
massive a retion (10 4
10 3
M
yr
1
) (e.g., Nor-
CorrespondingAuthor:G.KIM
berg & Maeder 2000) or through oales en e pro ess
(e.g.,Bally&Zinne ker2005). ThenHigh-MassCores
harboringLow/IntermediateprotostarsturntobePro-
tostellar Obje ts (HMPOs). When the pro esses ob-
tainingmatterstop,theHMPOsareexpe tedto nally
be ome high-mass stars. In this s enario, however, it
ismostlyunknownhowtheHMSCsform andhowthe
a reting low/intermediate-mass stars really grow to
high-massstars,partlybe ausetherearerelativelyless
studieson theHMSCs andthe High-MassCores with
protostars omparedwithlowmass ores.
Lastde ade,however,hasseenasigni antimprove-
mentonthe studiesofhigh-mass starforming regions
due to thesurveysmadewith the InfraredSpa e Ob-
servatory (ISO; Perault et al. 1996; Hennebelle et al.
2001), theMid- ourse Spa eeXperiment(MSX; Pri e
1995; Egan et al. 1998), and the SpitzerSpa e Tele-
s ope (SST; Benjamin et al. 2003) whi h dis overed
a numberof the InfraRedDark Clouds(IRDCs) seen
in absorption against the bright, di use mid-infrared
Gala ti ba kground.
TheSST,espe ially,providedneweyestoenableto
penetratetheIRDCsofafewhundredsA
v
toseenewly
born starsthere. TheIRDCs havelarge dust olumn
densitiesresultingin highextin tionandthustheydo
not exhibit dete table emission at 8-25 m (Careyet
al. 1998). While nearby dark dense mole ular ores
are often the site of thelow-massstar formation, the
IRDCs are believed to be the natal sites of mostly
massive stars. Their overall features are known from
re ent studies of dust ontinuum and mole ular line
emission at (sub)mm wavelengths: They are usually
distant (1 8kp ),large (1-3p ), dense (10 2
10 4
m 3
),andmassive(10 2
-10 4
M
)(e.g.,Rathborneet
al. 2006;Simonet al. 2006b).
Inmost asetheIRDCshavesubstru turesshowing
lo ally mid-IRextin tionpeakduetotheirhigher ol-
umndensity omparedwithotherregionoftheIRDC,
whi h are alled as IRDC ores. These IRDC ores
have typi al sizes of 0.020.8 p , masses of 1010 3
M
, and densities of 10 3
10 7
m 3
(Rathborne et
al. 2006). Some IRDC ores without an embedded
sour ehavelowertemperature(T
NH
3
16K),narrow
line-width(V
NH
3
1.6km/s),la kofstrong omplex
mole uleemission,andlowerdete tionrate(12%)of
water masers, representing a less a tive environment
than HMPOs and Ultra ompa t H II regions (Srid-
haran et al. 2002, 2005; Wang et al. 2006; Chur h-
well et al. 1990). On the other hand other IRDC
oreswithembeddedsour eshavewarmertemperature
(25 K), sho ked gas (e.g. H
2
or SiO), broad line-
width(V
HCN(4 3)
10 km/s), ri h omplex mole u-
lar spe trum (e.g. CH
3 CH
2
CN), and morenoti eable
methanol/watermasera tivities(Redmanetal. 2003;
Ormelet al. 2005;Rathborne et al. 2005,2007, 2008;
Pillaietal. 2006;Raganetal. 2006;Wangetal. 2006).
These properties remind us that the IRDC ores
without any embedded sour e are analogous to low-
mass starless ores, while the IRDC ores with em-
beddedsour esmimi thepropertiesofstar- ontaining
low-mass ores. The hara teristi sofIRDCs aresim-
ilar tothose of HMSCsor High-Mass Coreswithpro-
tostars on the ore s ale (sub-parse s ale), or those
of Massive Starless Clumps or Proto luster on the
lump/ lusters ale(afewparse s ale)(Rathborneet
al. 2006; Beuther et al. 2007). The HMSCs are be-
lievedtopreservetheinitial onditions(e.g., oremass
fun tion) forhigh-mass starformationuntil theyturn
tobetheHigh-MassCores ontainingprotostarswhi h
mostlylosetheirinitialinformationbythestrongradi-
ationandstellarwindofembeddedmassiveprotostars.
Hen e,itis learthattheIRDC oresareofprimeim-
portan etostudytheearlypro essesofhigh-massstar
formation.
Constru tingafullypossibleset ofIRDC oresand
examininganasso iationoftheIRDC oreswithYSOs
shouldbeanimportant rststepforasystemati study
oftheirrolein high-massstarformation. Forthispur-
posewe olle tallpossibleIRDC oresfrompreviously
published atalogsin ludingoursubsetofIRDCsthat
wefoundinthisstudyandexamineasso iationofthose
withanypossibleembeddedYSOs
.
Re entGala ti Lega yInfrared Mid-PlaneSurvey
(GLIMPSE) made with SST was sensitive enough to
dete tabout70millionpointsour esalongtheline of
sightofGala ti regionshavingtheIRDC ores. Com-
bining atalogsoftheIRDC oresandGLIMPSEpoint
sour es,itseemsnowpossibletomakeasystemi study
oftheasso iationoftheIRDC oreswithYSOswhi h
willbeabasisoffurther studyontheearlypro essin
high-massstarformation.
Inthispaperwe olle tmostoftheIRDC oresstud-
ied previously by other resear hers with a subset of
our new IRDC ores whi h are missed in other ata-
logs to examinein all these IRDC oresthe existen e
of YSO andidates su h as the GLIMPSE,MSX, and
IRASpointsour es,andwaterandmethanolmasers.
In xII we des ribe how we olle t the IRDC ores
knownandhowweidentifynewIRDC ores. InxIIIwe
examinetheasso iationof theIRDC oreswith YSOs
and then in xIV we dis uss various statisti s of geo-
metri propertiesoftheIRDC oreswith andwithout
YSOs. WesummarizeourresultsinxV.
II. COLLECTIONOFTHEINFRAREDDARK
CLOUDCORES
(a) The IRDC Coresfrom Literatures
One of the most omprehensive olle tion of the
IRDC oresisa atalogbySimonet al. (2006a,here-
after SJRC), onsisting of 12,744IRDC ores. Before
theSJRC atalogwas published, therehavebeenalso
manyotherstudiesontheIRDCsandtheir oresusing
(sub)mm mole ular line and ontinuum observations
(e.g., Carey et al. 1998, 2000; Teyssier et al. 2002;
Fiege et al. 2004; Garay et al. 2004; Ormel et al.
2005; Beutheret al. 2005; Pillai etal. 2006). Sridha-
ran et al. (2005) observedover50 IRDC oresin 1.2
mm ontinuum and NH
3
mole ular line, nding that
mostofthem werehigh-massstarless ores. Re ently,
Rathborne et al. (2006)and Ragan et al. (2006)sig-
ni antlyin reasedthenumberofIRDC oresstudied
to over150. Fromallthestudies in (sub)mmmole u-
larlinesor ontinuum,we olle tasampleof87IRDC
oreswhi harenotlistedintheSJRC atalog.
A vast atalog of Spitzer Dark Clouds was re ently released
(Peretto & Fuller 2009) after thiswork was being ompleted.
Although it listsmoredark loudsthan we olle ted, our ol-
le tionofIRDCshavetwi ewider overageofthe galaxythan
SSTGLIMPSEsurveyareaandwillbea reasonablesampleto
Table 1
Statisti softheasso iationofIRDC oreswith YSO andidates
# ofYSOs #of IRDC ores # ofIRDC ores
/YSO andidates (AllSample) (GLIMPSEsample)
morethan2YSOs 2,108 2,104
1YSO 2,002 1,994
None 9,540 8,102
GLIMPSEPointSour e 4,036 4,028
IRASPointSour e 172 168
MSXPointSour e 69 68
H
2
Omaser 8 8
Methanolmaser 33 33
EYSO 1,077 1,072
PMS 3,601 3,593
Fig. 1.| An example of how to sele t our new IRDC ore andidates. The image of an IRDC ore andidate
(MSXIRDC18.89-0.48) on the left is an8.28 m image of 50 0
50 0
, the ontrast of whi h is enhan ed with the fun tion
of histogramequalization. Theother imageonthe right is ablow-upof thegreen box in entralregion ofthe leftpanel.
TheFWHMinbrightnessandtheba kgroundbrightnesslevelsare 1:210 5
and1:610 5
Wm 2
sr 1
,respe tively. An
ellipseoverlaysontheFWHMin ontour.
(b) NewlySele tedIRDC Core Candidates
TheIRDC oresintheSJRC ataloghavebeense-
le tedusinganautomati algorithm ndingextin tion
regions(theIRDC ores)ofhigh ontrastwithrespe t
tothe modelba kgroundin theMSX8.28mi ronim-
ages. However,eventhoughtheSJRC atalogwasable
to in lude most ofthe IRDC oresseenin theGala -
ti plane, theiralgorithm resultedin missing ompa t
darksour eshavingpoor ontrastorsmallregionssur-
roundedbytheverybrightmid-infraredemissionofthe
ba kground(Simonet al. 2006). Here wepresentour
additionale orts toaddmoreIRDC oreswhi h were
missed in the SJRC atalog, by making the ontrast
enhan ementofimageswiththefun tionofhistogram
equalization y
on the IDL program z
. Our IDL s ripts
using this te hniqueenableto loadMSX8.28m im-
ages and enhan e the darkness ontrast of the IRDC
ore andidatesembeddedinthebrightGala ti ba k-
groundtoidentifythembyeyeinspe tion. Wesear hed
forallMSXAbandimagesfortheentireGala ti plane
(jlj<180 Æ
,jbj<5 Æ
)whi hareretrievablein tsformat
up to size of 6 Æ
6 Æ
from the NASA/IPAC InfraRed
S ien e A hieve (IRSA) x
. A typi al sear hing size of
imageswas50ar min50ar min(Figure1). First,
adarkregionin thebrightba kgroundis sear hedfor
by eyes. Thenits areais hosenby li kingtwodiag-
onal orners withamouse buttonand the ontrastof
theregionsisenhan edwiththefun tionofhistogram
equalizationin IDLs ripts. On e adarkregion isse-
le ted as an IRDC ore andidate sample (Figure 1),
somegeometri alpropertiesoftheIRDC oresaremea-
sured. The shapes of the identi ed IRDC ores are
y
Histogramequalization isthe te hniquebywhi hthe dynami
rangeofthehistogramofanimageisadjusted.Histogramequal-
ization assigns the intensity valuesof pixels in the input im-
age su h that the output image ontains a uniform distribu-
tion of intensities. It improves ontrast and the goal of his-
togram equalization is to obtain a uniform histogram. See:
http://www. odersour e.net/ sharphistogramequalization.aspx
z
IDL is a kind of ommer ial produ ts of ITT Visual Infor-
mation Solutions and is a software for data analysis, data
visualization, and software appli ation development. See
http://www.ittvis. om/
x
The IRSA is web-based and ma hine-friendly tools for
eÆ ient a ess to alibrated s ien e data sets obtained
from infrared and submillimeter missions of NASA. See
http://irsa.ipa . alte h.edu/
Fig. 2.|Apro le- utobtainedby rossingtheminimum
brightness intensity(I
M
) of MSXIRDC18.89-0.48. This is
usefultoestimatetheba kgroundbrightnessintensity(I
B )
oftheIRDCwhi hinturnenablesustodeterminetheFull
WidthatHalfMinimum(FWHMin) ontouroftheIRDC.
des ribedbytheFullWidthatHalfMinimum(FWH-
Min) ontours of theirintensities. TheFWHMin on-
tourlevelof ea hregionis determined fromthe ba k-
ground brightness level and the minimum brightness
level. Pro le- utspassingthroughtheminimuminten-
sity position wereusefulin thispro ess, tospe ifythe
ba kgroundbrightnesslevel(SeeFigure2). Wedeter-
mined the ba kground brightness level of ea h IRDC
oreby averagingthebrightnessoverasmall ir ular
regionatroughly onstantba kgroundbrightnessnear
thepartofboundaryof IRDC ores. Typi allythedi-
ameter of these regions was 5 00
. The sele tion of the
ba kgroundregionsinthiswayissomewhatarbitrary.
We hose sevenpositions forthree IRDC oresas the
ba kground andidatestoestimatetheintensitylevels
ofthesele tedregionsandexaminetheirpossiblevari-
ation. We found a typi aldeviation of about 10%in
theba kgroundintensityvaluethatweadopt. Thede-
termination of the ba kgroundbrightness level of the
IRDC oreenablestospe ifytheleveloftheFWHMin
ontourand alsothe DarknessContrast (DC)de ned
as
(IB IM)
I
B
whereI
M
istheminimumintensityandI
B is
ba kgroundintensityof the IRDC ore(Lee &Myers
1991). We sele tedonly the andidates whose FWH-
Minoftheintensities anmakea losed ontoursothat
anellipse ttingofthe ontour anbepossible. By t-
tingtheFWHMin ontourofea hIRDC orewithan
ellipse, weassigned the oordinates of theIRDC ore
andidates as the entral position of the ellipse and
derived thegeometri alparametersof theIRDC ore,
su h as theangularsizesof themajorandminor axes
(aandb),PositionAngles(PA),andEquivalentDiam-
eter(ED). Thelatteris de nedas 2
q
A
, whereA is
theareawithin theFWHMin ontouroftheIRDC.
Fromthiswork,atotalof789IRDC ore andidates
wereidenti ed {
. Mostofthem(79%)havetypi alan-
gularsizesbetween0 0
.4and1 0
.3withamean(s.e.m.:
standard error of the mean) of 0 0
.880 0
.02 and have
typi al aspe t ratios between 1 and 2.2 with a mean
(s.e.m.) of 1.820.02. Whether these IRDC ore
andidates are real louds an be tested with mole -
ular line observations made toward them. We used
13
CO(1-0) BU-FCRAO Gala ti Ring Survey (GRS)
on the Gala ti plane of 18 Æ
<l<55 Æ
and jbj <1 Æ
ob-
tained by Ja ksonet al. (2006) for this purpose and
sele ted 17 IRDC ore andidates in the GRS region
whoseangularsizeislargerthan2 0
(Notethatangular
resolution of the GRS surveyis 46 00
). We found that
the extin tionshapesof 11 amongthe tested samples
well oin idewiththeshapeof 13
CO ontourmap(Fig-
ure 3), implying that a good fra tion (about65%) of
theIRDC ore andidatessele tedinthisstudy anbe
real louds.
{
About half of these targets are found to be also listedin the
new atalog of SpitzerDarkClouds byPeretto &Fuller2009,
meaningthattheotherhalfoftheIRDC oresthatwefoundare
Fig. 3.|Comparison oftheextin ted imageofanIRDC ore with 13
CO(1-0)mole ular line emissions. The 13
CO(1-0)
pro leontheleft panelisfromtheBU-FCRAOGala ti RingSurveybyJa ksonetal. (2006). Therightpanel ompares
theextin tedimageoftheIRDCwiththeCOemission,indi atingthatthedistributionoftheCOemissionat34kms 1
oin ideswiththeshapeoftheIRDC ore. Thewhite ir leattheleftedgeoftherightpanelindi atesthebeamsize(46 00
)
oftheteles opefor 13
CO(1-0)observation.
Fig. 4.| A histogram of the slope ( ) of the Spe -
tralEnergyDistributionfromtheGLIMPSEpointsour es.
They were lassi ed asClass I( 0.3), Flatspe trum (-
0.3 <0.3),andClassII(-1.6 <-0.3)followingGreene
etal. (1994). 198,045pointsour esmaybeEYSOs(Class
I obje ts)or PMSs(Flat spe trum obje ts orClass IIob-
je ts).
Fig. 5.|Anumberdistributionof13,650IRDC oresa -
ordingtothenumberofYSO andidates. TheIRDC ores
withoutYSO andidate o upy about 70%of this sample
whiletheIRDC oreswithYSO andidatesareabout30%.
Itisinterestingtonotethatabout49%oftheIRDC ores
withYSO andidateshave asingleYSO andidateswhile
others(51%)havemultipleYSOs.
Table 2
Newlysele ted789IRDC ores.
Num. Sour e R.A. De . a b ED PA DC YSO
a
(2000) (2000) ( 0
) ( 0
) ( 0
) ( Æ
)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
1 MSXIRDC0.02+0.08 174521.7 -28 52 45.9 0.92 0.58 07.11 071 0.18
2 MSXIRDC0.05-0.19 174629.6 -28 59 20.6 1.82 0.82 10.43 354 0.19 S
I(2);II(17)
3 MSXIRDC0.10-0.08 174609.8 -28 53 23.7 1.10 0.95 10.09 332 0.35 S II(3)
,I E(1)
4 MSXIRDC0.15-0.09 174619.6 -28 51 15.4 1.35 0.85 10.49 034 0.26 S II(5)
5 MSXIRDC0.17-0.10 174625.8 -28 50 36.8 1.16 0.64 09.19 085 0.24
6 MSXIRDC0.32-0.13 174653.4 -28 43 47.6 0.53 0.50 06.05 091 0.39 S II(1)
7 MSXIRDC0.34+0.10 174601.8 -28 35 53.5 1.01 0.63 07.74 343 0.27 S
F(1);II(3)
8 MSXIRDC0.38-0.69 174913.5 -28 58 12.0 0.84 0.73 07.33 125 0.24
9 MSXIRDC0.39-0.83 174948.1 -29 01 59.7 0.90 0.45 06.08 326 0.22
10 MSXIRDC0.40-0.15 174708.7 -28 40 08.4 0.77 0.75 07.04 094 0.33 S II(1)
11 MSXIRDC0.42-0.17 174716.3 -28 39 53.8 1.03 0.57 07.51 031 0.30 S I(1);II(3)
12 MSXIRDC0.42-0.26 174736.1 -28 42 48.1 0.64 0.59 06.06 050 0.24 S II(3)
13 MSXIRDC0.46-0.66 174917.2 -28 53 03.3 1.98 1.02 13.94 332 0.43 S F(1)
14 MSXIRDC0.48-0.69 174927.4 -28 53 09.0 0.58 0.36 04.47 328 0.22 S I(2)
15 MSXIRDC0.53-0.59 174911.0 -28 47 38.1 1.11 0.66 08.13 012 0.24 S II(2)
16 MSXIRDC0.55-0.66 174930.1 -28 48 39.8 1.21 0.48 07.37 000 0.25 S II(1)
17 MSXIRDC0.58-0.53 174904.0 -28 43 09.9 1.63 1.12 12.10 113 0.29 S I(1);II(4)
,M F(1)
18 MSXIRDC0.59-0.58 174917.6 -28 44 00.6 1.29 0.69 09.36 357 0.25
Thisisa ut-indata
771 MSXIRDC359.33-0.09 174421.8 -29 33 22.4 0.72 0.48 05.74 111 0.23 S II(2)
772 MSXIRDC359.36-0.13 174435.0 -29 32 57.1 1.49 0.75 10.35 030 0.40 S I(1);II(8)
773 MSXIRDC359.39-0.11 174433.9 -29 30 49.2 0.57 0.40 04.75 017 0.18 S II(1)
774 MSXIRDC359.40-0.59 174630.8 -29 45 22.6 1.75 1.34 12.72 057 0.26 S II(1)
775 MSXIRDC359.41-0.20 174500.7 -29 32 37.0 0.66 0.41 05.08 338 0.17
776 MSXIRDC359.46-0.05 174431.4 -29 25 33.9 1.65 0.45 07.89 347 0.21 S
F(1);II(3)
777 MSXIRDC359.46-0.11 174446.1 -29 27 36.4 0.67 0.40 05.05 016 0.31 S II(2)
778 MSXIRDC359.53+0.05 174418.2 -29 19 00.8 1.29 0.73 08.93 351 0.18 S
F(1);II(3)
779 MSXIRDC359.55+0.03 174424.6 -29 18 23.0 1.29 0.50 07.48 106 0.14 S II(1)
780 MSXIRDC359.55+0.00 174433.0 -29 18 55.6 2.37 0.54 11.16 351 0.26 S
F(1);II(16)
781 MSXIRDC359.56-0.21 174522.7 -29 24 57.8 1.50 1.24 12.24 043 0.23 S II(3)
782 MSXIRDC359.67+0.03 174441.4 -29 12 14.2 2.42 0.34 08.33 020 0.10 S I(1);II(5)
783 MSXIRDC359.73+0.04 174449.4 -29 08 51.8 2.44 0.62 11.75 017 0.19 S
F(1);II(9)
784 MSXIRDC359.75+0.07 174443.7 -29 06 44.4 0.88 0.70 06.77 076 0.25 S II(2)
785 MSXIRDC359.80+0.05 174456.9 -29 04 51.5 0.79 0.23 03.88 358 0.06
786 MSXIRDC359.81+0.04 174500.6 -29 04 27.8 0.75 0.52 05.92 039 0.14
787 MSXIRDC359.91+0.11 174457.9 -28 57 24.3 0.81 0.29 04.47 090 0.10
788 MSXIRDC359.94+0.11 174503.0 -28 55 45.3 1.07 0.76 08.56 082 0.22 S
F(2);II(12)
789 MSXIRDC0.00+0.11 174511.7 -28 52 41.8 1.31 0.81 09.98 024 0.32 S II(5)
Note.|Thefull versionofthistableisavailableinele troni form. This atalog ontains
789IRDC oresnewlysele tedfrom thisstudy. Inthe tablesequentialnumbersofthe IRDC ores
are givenin olumn1,the sour enamein olumn2,the oordinatesoftheIRDC oresin olumn3
and4astheR.A.andDe . [J2000℄,thediameterofthemajorandminoraxesoftheIRDC oresin
olumn5 and6,the Equivalent Diameter(ED)ofthe IRDC ores in olumn7,thePosition Angle
(PA)in olumn8,theDarknessContrast (DC)of theIRDC oresin olumn9,and possibleYSOs
asso iation in olumn10.
a
H(#): watermaser, Me(#): Methanol maser, I E(#)
: IRAS point sour e(EYSO), I P(#)
: IRAS
point sour e(PMS),M I(#)
: MSXpointsour e( lassI), M F(#)
: MSXpointsour e lass(FlatSED
sour e), M II(#)
: MSXpoint sour e( lass II), S I(#)
: SST GLIMPSEpoint sour e ( lass I),S F(#)
:
SSTGLIMPSEpointsour e lass(FlatSEDsour e),andS II(#)
: SSTGLIMPSEpointsour e( lass
Table 3
87IRDC oressele tedfromliteratures.
Num. Sour e R.A. De . a b ED PA DC YSO
a
Referen e b
(2000) (2000) ( 0
) ( 0
) ( 0
) ( Æ
)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
1 G79.34+0.33 203227.0 +40 21 06.5 4.23 4.01 38.29 088 0.70 I E(1)
,M I(1);F(1)
(1)
2 DF+25.90-0.17 183909.4 -06 20 07.3 0.79 0.43 05.64 108 0.19 (2)
3 DF+30.31-0.28 184738.7 -02 27 57.6 0.93 0.58 07.34 004 0.25 S F(2)
(2)
4 DF+30.36+0.11 184620.5 -02 14 06.8 0.67 0.20 03.47 039 0.10 (2)
5 DF+36.95+0.22 185759.1 +03 41 46.6 4.45 4.27 29.45 109 0.29 S I(1)
(2)
6 DF+51.47+0.00(B) 192612.1 +16 25 49.0 2.29 1.81 14.39 091 0.38 S F(1);II(5)
(2)
7 G305.136+0.068 131040.7 -62 43 12.6 0.64 0.42 05.19 060 0.36 S I(1)
(3)
8 G333.125-0.562 162134.5 -50 40 59.7 0.44 0.30 02.95 107 0.05 H(1) (3)
9 18090-1832-2 181200.9 -18 31 19.9 0.48 0.13 02.00 069 0.06 (4)
10 18151-1208-2 181750.2 -12 07 54.1 0.58 0.43 04.84 011 0.16 S I(2);II(2)
(4)
11 18247-1147-3 182733.8 -11 44 23.2 1.30 0.36 06.09 038 0.13 (4)
12 18308-0841-2 183334.3 -08 38 38.7 0.40 0.29 03.33 324 0.15 (4)
13 18308-0841-3 183329.2 -08 38 14.7 0.58 0.27 03.97 074 0.16 (4)
14 18308-0841-5 183334.4 -08 37 37.5 0.36 0.14 02.01 342 0.04 (4)
15 18348-0616-2 183727.9 -06 14 08.4 0.45 0.33 03.79 029 0.24 (4)
16 18385-0512-3 184117.3 -05 10 04.3 0.68 0.53 05.95 009 0.38 (4)
17 18431-0312-3 184545.1 -03 08 41.0 0.56 0.24 03.25 110 0.12 (4)
18 18431-0312-4 184553.3 -03 08 48.8 1.08 0.75 08.64 346 0.12 (4)
Thisisa ut-indata
71 G33.36-0.01 185214.4 +00 22 36.1 1.95 0.97 12.32 355 0.29 S I(1);II(4)
(6)
72 G33.42+0.13 185152.9 +00 29 33.0 0.57 0.31 04.15 004 0.29 (6)
73 G34.26+0.19 185312.7 +01 15 53.2 1.67 0.58 10.06 079 0.27 S I(1);II(4)
(6)
74 G34.74+0.01 185444.2 +01 36 58.5 0.87 0.54 06.36 028 0.27 (6)
75 G35.04-0.47 185658.8 +01 39 50.5 0.59 0.44 05.39 326 0.29 S F(1);II(2)
(6)
76 G34.63-1.03 185818.8 +01 01 04.5 3.02 1.79 12.39 086 0.59 S F(1)
(6)
77 G35.02-1.50 190034.7 +01 10 39.3 1.41 1.07 11.09 128 0.46 (6)
78 G43.19-0.16 191053.2 +09 02 30.1 0.42 0.21 02.93 004 0.31 (6)
79 G43.32-0.20 191117.5 +09 08 19.7 1.01 1.00 06.55 099 0.33 S I(1)
(6)
80 G43.78+0.05 191113.6 +09 39 39.9 1.32 0.96 08.55 011 0.32 (6)
81 G43.64-0.82 191408.2 +09 05 57.3 2.85 2.51 08.62 002 0.53 (6)
82 G48.84+0.15 192029.9 +14 10 55.3 1.22 0.61 06.22 001 0.31 S F(2)
(6)
83 G48.84+0.14 192034.6 +14 11 36.2 1.05 0.66 07.04 005 0.27 S I(1)
(6)
84 G50.07+0.06 192314.0 +15 13 49.0 0.81 0.47 06.08 022 0.47 S I(2)
(6)
85 G51.00-0.18 192254.1 +15 56 11.1 2.57 1.91 15.13 353 0.40 I E(1);P(1)
(6)
86 G53.88-0.18 193142.6 +18 28 00.5 0.74 0.43 04.21 344 0.40 (6)
87 G61.52+0.02 194708.1 +25 12 38.7 1.82 1.80 09.32 319 0.52 S II(1)
(6)
Note.|Thefull version of this table is available in ele troni form. This atalog ontains87
IRDC oressele tedfrommanyliteratures. Alltheinformationin olumnsisthesameasthoseinthetable
2ex eptthatthishasonemore olumnasthelastonewherereferen esforthesour esaregiven.
b
Referen es; (1)Careyetal. (1998),(2)Teyssieretal. (2002), (3)Garayetal. (2004),(4)Sridharanet
al. (2005),(5)Rathborneetal. (2006),and(6)Raganetal. (2006).
Table 4
TheSJRC ores.
Num. Sour e R.A. De . a b ED PA DC YSO
a
(2000) (2000) ( 0
) ( 0
) ( 0
) ( Æ
)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
1 G000.00+00.65 174304.5 -28 35 41.2 0.9 0.8 00.54 125 0.08
2 G000.00-00.59 174758.4 -29 14 57.9 1.1 0.7 00.62 125 0.06
3 G000.00-00.74 174832.2 -29 19 15.6 1.3 0.6 00.61 095 0.07
4 G000.00-00.33 174653.2 -29 05 42.7 1.3 1.0 01.00 099 0.13
5 G000.00-00.47 174728.8 -29 11 03.3 1.6 0.7 00.87 146 0.15 S
F(4);II(6)
6 G000.00-00.98 174931.7 -29 27 02.7 0.8 0.4 00.28 215 0.04
7 G000.00-00.78 174842.8 -29 20 18.2 0.9 0.8 00.53 218 0.07
8 G000.01+00.63 174311.9 -28 35 20.9 1.2 1.1 01.09 123 0.10
9 G000.02+00.57 174325.6 -28 36 30.4 2.2 0.8 01.38 104 0.16
10 G000.02-00.98 174931.5 -29 25 28.8 1.3 0.7 00.70 235 0.11
11 G000.03-00.35 174704.5 -29 05 27.3 1.1 1.1 00.94 245 0.09 S II(1)
12 G000.04-00.64 174816.5 -29 13 33.2 1.3 0.6 00.63 117 0.06 S
F(1);II(1)
13 G000.05-00.55 174755.7 -29 10 36.6 1.3 0.8 00.87 241 0.09
14 G000.05-00.68 174828.7 -29 14 56.9 1.1 0.8 00.71 125 0.15 S II(2)
15 G000.05-00.68 174832.7 -29 14 04.5 1.9 0.9 01.38 095 0.21 S I(1);II(3)
16 G000.06+00.21 174455.4 -28 46 38.2 1.1 0.8 00.70 246 0.11 S II(3)
17 G000.06-00.77 174855.3 -29 15 17.6 1.5 0.9 01.08 217 0.18
18 G000.06-00.77 174849.1 -29 16 22.4 1.5 1.0 01.17 260 0.17
Thisisa ut-indata
12756 G359.91+00.62 174259.7 -28 41 23.1 0.9 0.7 00.49 215 0.05
12757 G359.91+00.73 174235.1 -28 37 33.3 1.1 0.7 00.63 116 0.07
12758 G359.91+00.17 174447.7 -28 53 53.8 1.6 1.3 01.59 251 0.64 S
I(5);F(6);II(23)
12759 G359.91+00.17 174443.6 -28 55 27.8 1.3 1.1 01.20 131 0.37 S
I(3);F(3);II(25)
12760 G359.91+00.39 174352.4 -28 47 48.2 1.6 0.8 00.96 125 0.11 S II(1)
12761 G359.92-00.86 174850.3 -29 26 53.3 1.2 0.9 00.86 145 0.10
12762 G359.93+00.61 174303.5 -28 40 08.4 1.1 1.0 00.86 162 0.07
12763 G359.94-00.34 174652.3 -29 09 44.0 0.9 0.7 00.53 117 0.06 S II(2)
12764 G359.94+01.20 174047.4 -28 20 51.4 0.9 0.8 00.56 251 0.09
12765 G359.95+00.59 174312.3 -28 39 46.4 1.2 0.9 00.82 126 0.08 S II(1)
12766 G359.95-00.94 174913.9 -29 27 39.8 1.1 0.8 00.67 125 0.09 S II(1)
12767 G359.96-00.76 174832.4 -29 22 18.4 1.0 0.7 00.59 091 0.06
12768 G359.96+01.15 174103.8 -28 21 32.9 0.7 0.6 00.34 125 0.07
12769 G359.96-00.78 174837.1 -29 22 08.6 1.2 1.0 00.92 099 0.09
12770 G359.98-00.81 174847.5 -29 22 11.2 0.9 0.6 00.45 143 0.04
12771 G359.98-01.01 174932.0 -29 28 41.4 0.9 0.9 00.66 191 0.10
12772 G359.98+00.27 174431.7 -28 47 40.1 0.9 0.7 00.50 260 0.05 S II(2)
12773 G359.99+00.50 174339.4 -28 40 58.7 1.3 0.7 00.72 215 0.06
12774 G359.99-01.21 175023.3 -29 34 19.3 1.2 0.6 00.63 260 0.14
Note.|Thefullversionofthistableisavailableinele troni form. This atalog
ontainsthe12,774SJRCIRDC ores. Alltheinformationin olumnsis thesameas those
inthetable2.
In total, 13,650 IRDC ores were olle ted for this
study. These onsist of12,774IRDC oresfrom SJRC
atalog,87IRDC oresfromvariousstudiesmentioned
above, and 789 IRDC ores newly sele ted from this
study. All these IRDC oresare listed in Table 2for
789IRDC ores,Table3for87IRDC ores,andTable4
fortheSJRC atalogwithsomeusefulinformationsu h
as their geometri propertiesand YSOsasso iationof
theIRDC oreswhi hwillbedis ussedinnextse tion
k
.
In the tables the sequential numbers of the IRDC
ores are given in olumn 1, the sour e name in ol-
umn2,theR.A.andDe . [J2000℄oftheIRDC oresin
olumn 3and4,thediameter ofthemajorand minor
axesoftheIRDC oresin olumn5and6,theEquiva-
lentDiameter(ED)oftheIRDC oresin olumn7,the
Position Angle (PA) in olumn 8, the DarknessCon-
trast (DC) of the IRDC ores in olumn 9, possible
YSOs asso iationin olumn 10. Notethat table 3has
olumn 11wherereferen esaregiven.
III. ASSOCIATION OF THE IRDC CORES
WITHYSOS
WhetherIRDC oresareasso iatedwithnewlyborn
\young"starsisan importantissue forunderstanding
howstars form in theIRDC ores. In thisse tion we
sear h for YSO andidates using various riteria, ex-
plore the asso iation possibility of IRDC ores with
YSOs and dis ussimpli ation ofthestatisti sof star-
lessorstarredIRDC oresinhigh-massstarformation.
HereYSOsrefertoprotostarswhi h orrespondto lass
0orI(sometimes alledasembeddedYSO-EYSO)and
PMSstars(Flatspe trum, lassIIorIII).
(a) Identi ation ofYSO Candidates
The IRAS point sour es meeting the riteria sug-
gestedbyLee&Myers(1999),mainlyrising ux den-
sitiesatthelongerIRASbands,wereusedtosear hfor
EYSO andidateslying within theIRDC oresin the
rangeof0 Æ
<l<360 Æ
andjbj<3 Æ
. However,theIRAS
sensitivityrestri tsdete tion to sour esstrongerthan
0.1 L
at the Taurus distan e (140 p ) (Myerset
al. 1987);EYSOswouldnotbedete tedin themostly
distant(overafewkp )IRDC oresunlesstheyareat
leastasbrightas80L
at4kp . Wefoundonly172
IRDC ores from the sear h of entire IRDC ores to
ontainIRAS point sour es. Wealso used olor- olor
diagram riteria in 12, 25, and 60 m IRAS bands of
Weintraub et al. (1990) to nd a possible PMS star
within anIRDC ore; 2:00<
log(12F12=25F25)
log(
12
=
25 )
<1:35
and 1:75 <
log(
25 F
25
=
60 F
60 )
log(25=60)
< 2:20. Fifty eight
k
Here we present the part of the tables only. The full
version of the tables is available in ele troni form at
http://gjkim.kasi.re.kr/IRDC.
The MSX survey also provides a atalog of nearly
0.45 million point sour es (MSX point sour e atalog
v2.3from NASA/IRSA Ar hive) withdete tion sensi-
tivity of 100 - 10,000 mJy at 4.29, 4.35, 8.28, 12.13,
14.65,and21.34m withintheregionofjlj<180 Æ
and
jbj <5 Æ
(Egan et al. 1999). A total of 18,220 point
sour esarepossiblyYSOsa ordingtotheslopeofthe
SEDfrom4.29to21.34m. SixtynineIRDC oresare
found to oin ide with the positions of some of these
YSO andidates.
Re ent SST missions explore very faint embedded
sour es in dense mole ular ores (e.g., Young et al.
2004; Bourke et al. 2006; Dunham et al. 2006; Lee
et al. 2009). Inparti ular, theGLIMPSElega y pro-
gram ondu ted a survey of the inner galaxy at 3.6,
4.5, 5.8, and 8.0 m , overing the region of jlj <65 Æ
and jbj <0.75 Æ
4.2 Æ
and released a point sour e
atalog
of nearly 70million from GLIMPSE survey.
The GLIMPSE point sour e atalog lists highly reli-
ablepointsour esthathavedete tionatleasttwi ein
oneIRACbandandatleaston einanadja entband,
using thehighangular resolution(2 00
)andhigh sen-
sitivity (1 0:2 0:4 mJy) IRAC (InfraRed Array
Camera; Fazioet al. 2004). GLIMPSE point sour es
were identi ed as EYSO or PMS andidates, and as-
signedto one offour lassesbasedon theslope( ) of
theSpe tralEnergyDistribution(SED)from3.6to8.0
m( = dlog f
dlog
[Wm 2
℄). Followingthede nitionby
Greene et al. (1994), wemake lassi ationsas lass
I ( 0.3), Flat spe trum (-0.3 <0.3), lass II (-
1.6 <-0.3),and lass III( <-1.6). A histogramof
theobservedvaluesof isshowninFigure4.
For lassIIIobje tsitisnoteasytodistinguishthem
frommainsequen estarswithout omplementarydata
su hasX-ray uxes. Sotheidenti ationof lassIIIis
notspe i allygiveninthispaper. Inthe lassi ation
weminimized ontaminationfromunrelatedsour esby
droppingsour esfainter thanm
3:6
=14 m
to eliminate
possiblefainthighred-shiftAGNs (Allenet al. 2007).
The GLIMPSE survey region ontains about 198,045
pointsour eswhi hmaybeEYSOs( lassIobje ts)or
PMSs(Flatspe trumobje tsor lassIIobje ts). The
mean surfa e density of the GLIMPSE point sour es
is about 0.13sour es perar min 2
. The typi al an-
gular sizeof theIRDC oreswithinGLIMPSEsurvey
regionisabout1 0
. Hen e,thelikelihoodthataline-of-
sight oin iden e of the point sour es with the IRDC
ores is due to han e alignment of a foreground or
ba kgroundsour e would beabout 0.13point sour es
perIRDC ore,implyingthat thealignmentofafore-
groundorba kgroundsour ewithanIRDC oreisrel-
ativelyunlikely. Thus,theassumptionthatGLIMPSE
pointsour essatisfyingtheredness riterionandalign-
This point sour e atalogs onsist of GLIMPSE I spring '07,
GLIMPSE II spring '08 and GLIMPSE 3D (2007-2009). See
http:==data:spitzer: alte h:edu=popular=GLIMPSE=
ing with the IRDC ore are asso iated to the IRDC
oreisreasonable. Atotalof12,200IRDC oreswithin
theGLIMPSEsurveyareawereexaminedtoseeifthe
GLIMPSEpointsour es(i.e.,EYSOorPMSstar an-
didates) werelo atedwithintheFWHMin ontoursof
theIRDC ores. Ofthe12,200IRDC ores,4,028were
foundto haveatleastone GLIMPSEpointsour e.
Anasso iationofmasersour eswiththeIRDC ores
wasalso he ked. Masersareknowntobeagoodindi-
atoroftheearlystagesofstarformationandmayalso
beagoodtra erofstar-forminga tivities. Wesear hed
forthepresen eofmaserswithintheIRDC oresusing
theAr etri Catalogofwatermasers(Valdettaroet al.
2001) and the General Catalog of 6.7 GHz methanol
masers (Pestalozzi &Minier 2003) ndingthat 8and
33IRDC oresin ludewaterandmethanolmasers,re-
spe tively.
(b) Asso iationChe koftheIRDCCoreswith
YSO Candidates
We assumed that IRDC ores might have asso ia-
tion with the YSO andidates if the YSOs are posi-
tionedwithintheFWHMin ontouroftheIRDC ore.
However, it is pra ti allyvery diÆ ultto he k every
asso iationofYSOsforallIRDC oresbyeye. There-
forewemadeanellipse ttingtotheFWHMin ontour
of ea h IRDCand ompared the position of the YSO
withdimensionofthe ttedellipse. IfYSOsarelo ated
within theradiusoftheminoraxisoftheellipse,these
areassumedtobeasso iatedwithIRDC ores. Onthe
other hand if YSOs are lo ated outside the radius of
themajoraxisoftheellipse,theseareregardednotto
beasso iatedwiththeIRDC ore. TheYSOsthatare
lo atedbetweentheradiioftheminorandmajoraxes
of the ellipseweredire tly he kedwith eyesto see if
theyarelo atedwithintheareaof theellipseor not.
( ) Statisti softheIRDCCoreswithorwith-
out (embedded) YSO Candidates, and its
Impli ationand Limitation
Inthiswaywefound4,036,69,172,8,and33IRDC
ores ontainingGLIMPSE,MSX,IRASpointsour es,
water maser and methanol maser sour es within the
ellipse of the IRDC ore, respe tively (Table 1). We
notethat33IRDC oreswiththemethanolmaserhave
theasso iationbyotherYSO andidates,too.
Therefore 9,540 out of 13,650 IRDC ores haveno
YSO andidates while 4,110 IRDC ores are possi-
bly asso iated with YSO andidates. Dealing with
GLIMPSE point sour es and IRDC ores within the
GLIMPSEarea,wefoundthat 8,102IRDC oreshave
noYSOasso iationwhile4,028IRDC oreshaveYSOs.
Outof13,650IRDC ores,1,077 oresare foundto
haveEYSO andidatesandthusthisratioofN(IRDC
ore with protostars)/N(starless IRDC ore) is about
0.11 (1,077/9,540). Within the GLIMPSE survey
area, there are 1,072 IRDC oreswith protostars and
thustheratioofN(IRDC orewithprotostars)/N(starless
IRDC ore) is about 0.13 (1,072/8,102), whi h is
mu hlessthanthatoflow-mass ores(about0.31;Lee
& Myers 1999). Re ently Peretto and Fuller (2009)
haveexamined theasso iation ofIRDC oreswith 24
m sour es from Multiband Imaging Photometer for
Spitzer GALa ti plane survey (MIPSGAL), suggest-
ing that about20 - 68% of the IRDC ores show the
asso iation with24m sour es. Parsonsetal. (2009)
alsosuggestedthattwo-thirdsofthe oresdete tedat
850m are asso iated with24 m sour es. Ourfra -
tionalratiooftheIRDC oreswithembeddedYSOsis
thereforesmallerthanthese values. However,M.Kim
et al. (in prep)have found that asigni antfra tion
(over30%)of24msour esareeithergalaxiesorAGB
starsinsteadofbeingEYSOs. Thus itispossiblethat
theirestimationofthefra tionoftheIRDC oreswith
EYSOsisoverestimatedinthesensitivitylimitedsam-
ple. The lifetime range of embedded high-mass stars
is suggested to be between 10 3
10 4
years (Wood
& Chur hwell 1989a,b; Van der walt 2005; Motte et
al. 2007; Pestalozzi et al. 2007). Therefore, ifall the
IRDC ores are in a pathway to the high-mass star
formation,thefra tionalratiooftheIRDC oreswith
EYSOs may imply that theIRDC ores spend about
10 4
10 5
yearsto formhigh-massstars.
However,it should be notedthat this ratio for the
IRDC ores an bemu h moreun ertainthanthera-
tio for the low-mass ores. The Spitzer lega y Cores
to Disks ( 2d) program (Evans et al. 2003) has sur-
veyed67 low-mass starless ores andfound VeryLow
Luminosity Obje ts(VeLLOs) in20% ofthe sample
(Dunhametal. 2008),whi hwouldonlyslightlya e t
theratioofthestarred orestostarless ores. However,
theratiofortheIRDC ore anbesigni antlya e ted
be ausetheIRDC oresareusuallymu hmoredistant
thanthenearbylow-mass ores,andallthesurveysare
sensitivity-limited.
We al ulatedthe luminosity of thefaintest proto-
starin theIRDC oresthat GLIMPSEsurveyhasde-
te ted. In the al ulation we assumed that the pro-
tostar has the same Spe tral Energy Distribution as
assumedbyMyersetal. (1987). Wealsoassumedthat
theobserved peak ux densityof theprotostar would
bethemaximumovertheentirespe trumandthepro-
tostarhadthespe trumofabla kbodyforwavelengths
greater than or equal to the wavelength of the peak
ux density. The minimum ux density (0.36 mJy)
at8.0m amongthesour edete tedat all4bandsin
theGLIMPSEsurveywas omparedwiththeminimum
uxdensitythatIRASwoulddete tatthesamewave-
lengthas inferred from anextrapolation of themodel
ux density to 8m . The minimumdete table lumi-
nosityofGLIMPSEpointsour eswas al ulatedtobe
about 1:17(
4
dkp )
2
L
, where d is a distan e of the
IRDC orefromus. Thislimitismu hhigherthanthe
minimum dete table luminosity (0.1 L
) hara ter-
isti ofthe VeLLOs inthelow-mass ores. Hen e,the
better.
IV. STATISTICS OFTHE IRDCCORES
Using geometri properties of all olle ted 13,650
IRDC oresand their asso iationwith YSOs, we dis-
uss the statisti s of various parametersof theIRDC
ores.
(a) Multipli ity ofYSOs in the IRDC Cores
Figure 5 shows a number distribution of 13,650
IRDC ores a ording to the number of YSO andi-
dates. The IRDC ores without YSO andidate o -
upy about 70% of the entire IRDC ores while the
IRDC ores with YSO andidates do about 30%. It
is interesting to note that about 49% of the starred
IRDC oreshaveasingleYSO andidate whileothers
(51%) have multiple YSOs. Considering that even
a GLIMPSE point sour efound as asingle with SST
having poor spatial resolution an bepossibly double
or multiple stars,thissigni antfra tionoftheIRDC
oreswithmultipleYSOsmaysuggestthatmanyIRDC
oresarelikelymultiple starformationsites.
(b) p
ab and a/b of the IRDC Cores
The numberdistribution of theapparentgeometri
angular size ( p
ab) of the IRDC ore is presented in
Figure6. Mostof13,650IRDC ores(76%)havean-
gularsizesbetween0 0
.7and1 0
.2withamean(s.e.m.)
of1 0
.0120 0
.002. Figure6alsoshowsthattheangular
size of the IRDC ores is somewhat dependent upon
howtheIRDC ores ontainYSOs. ThestarlessIRDC
ores havea meanangular size of0 0
.9650 0
.002while
thestarred IRDC oreshavethat of1 0
.120 0
.01. The
mean angular size of IRDC ores with a single YSO
and twoor moreYSO andidates are1 0
.050 0
.01 and
1 0
.190 0
.01, respe tively. This indi ates that the an-
gular size of IRDC ores has a slight tenden y to be
larger as the IRDC ores have more YSOs, although
the di eren eamong them is small ompared to their
angularsizes.
Figure7displaysthenumberdistributionoftheas-
pe tratio(a/b)ofallIRDC ores,andtheIRDC ores
withand withoutYSOs. Theaspe tratioofallIRDC
ores is mostly (80%) between 1.0 and 1.9 with a
mean of 1.610.01. The aspe t ratios for the IRDC
oreswithandwithoutYSO andidatesare1.820.01
and 1.510.01,respe tively. Themean aspe tratioof
IRDC oreswithasingleYSO andidateis1.680.01
whilethatofIRDC oreswithtwoormoreYSO andi-
dates is1.960.02. These statisti sare also ompared
with the mean aspe t ratios of low-mass ores with-
out and with YSO andidates, 2.40.1 and 2.20.2,
respe tively(Lee &Myers1999). ThereforetheIRDC
oreswithmoreYSOstendtobemoreelongated,but
lesselongatedthanthelow-mass ores.
Fig. 6.|A numberdistributionofthe apparentangular
size of the IRDCwithits geometri p
ab. Most of13,650
IRDC ores(76%)haveangularsizesbetween0 0
.7and1 0
.2
andameanangularsizeof1 0
.0120 0
.002. Thepositionsof
meanangularsizesfortheIRDC oresinseveralgroupsare
marked with verti al bars inea h panel. The gure also
showsthat the angularsize of theIRDC ores withYSO,
areslightlylargerthanthestarlessIRDC ores.
( ) DarknessContrastoftheIRDCCoreswith
and Without YSO
The darkness ontrast may represent the olumn
densitytowardIRDC oresandthusitmaybeinterest-
ingtoinvestigatethedistribution ofdarkness ontrast
fortwosub-groupsofIRDCs withand withoutYSOs.
A natural predi tion would be that the IRDC ores
having higher olumn density will have more han e
to have YSOs and thus the IRDC ores with YSOs
have better darkness ontrastthan IRDC ores with-
outYSOs.
Figure8showsthenumberdistributionofthedark-
ness ontrastfortwosub-groupsofstarredIRDCsand
starless IRDCs for two samples of SJRC IRDCs and
other (789and 87)IRDC oresthat wesele ted. (We
investigated the distribution of the darkness ontrast
for two samples be ause the de nition of the ba k-
groundlevelofthebrightnessissomewhatdi erentin
twosamples). InthesampleofSJRCIRDC ores,the
starlessIRDC oreshaveameandarkness ontrast of
0.2210.001whilethestarredIRDC oreshavethatof
Fig. 7.|Anumberdistributionoftheaspe tratio(a/b)
ofIRDC ores. Thepositionsofmeanaspe tratiosforthe
IRDC oresinseveralgroupsaremarkedwithverti albars
inea hpanel. Theaspe tratioofallIRDC oresismostly
(80%)between1.0and1.9withameanof1.610.01. The
gure also shows that the aspe t ratio of the IRDC ores
withYSO,areslightlylargerthanthestarlessIRDC ores.
have a mean darkness ontrast of 0.2650.005 while
the starred IRDC ores have that of 0.2920.005. It
is interestingtonote that inbothsamplesthestarred
IRDC oreshavehigherdarkness ontrastthanstarless
IRDC ores, indi ating that the starred IRDC ores
havehigher olumn densitythanstarlessIRDC ores.
V. SUMMARY
Constru tingafullypossiblesetofIRDC oresand
examininganasso iationoftheIRDC oreswithYSOs
is the rststepfor asystemati study oftheir rolein
high-massstarformation. Thispaperexaminestheas-
so iation of InfraRed Dark Cloud (IRDC) ores with
YSOsanddis ussespropertiesoftheIRDC ores. For
this studyatotalof13,650IRDC oreswere olle ted
mainly from the atalogsoftheIRDC orespublished
fromotherstudiesandpartiallyfromoursubsetofnew
789IRDC ore andidates. Ournew oresaremostly
dark regions surrounded by bright PAH mid-infrared
emissionsoftheba kgroundwhi haremissedinprevi-
ous other works of onstru ting atalogsof theIRDC
Fig. 8.|Anumberdistributionofthedarkness ontrast
ofstarredIRDC oresandstarlessIRDC ores. Uppertwo
panels(a)and(b)arefor thesampleofSJRCIRDC ores
and lowertwopanels( ) and (d)for IRDC ores that we
olle tedforthisstudy. Inbothsamplesthedarkness on-
trastofstarredIRDC orestendstobehigherthanthatof
starlessIRDC ores.
than 2 0
are found to oin ide withthe distribution of
13
CO mole ular emissions,implying that manyof the
sele tedIRDC oresarelikelythereal louds.
TheYSO andidateswereidenti edwiththeslopes
of the SED of the GLIMPSE, MSX, and IRAS point
sour es, and the existen eof the water and methanol
masers. Theasso iationoftheIRDC oreswiththese
YSOs was he ked by their line-of-sight oin iden e
within the dimension of the IRDC ore. A ounting
forthedupli atedasso iationofmultiple YSO indi a-
torsinthesameIRDC ore,atotalof4,110IRDC ores
arefoundtohaveYSO andidates.
Consideringthe12,200IRDC oreswithin theGLI-
MPSEsurveyregion,forwhi htheasso iationofYSO
andidates with IRDC ores is rather well examined,
we found that 4,098 IRDC ores (34%) have at least
oneYSO andidateand1,072IRDC oresamongthem
haveembeddedYSOs,whiletherest8,102(66%)have
no YSO andidate. The ratio of [N(IRDC ore with
protostars)℄/[N(IRDC ore without YSO)℄ for 12,200
IRDC oresisabout0.13.
Taking into a ount this ratio and typi al lifetime
of high-mass embedded YSOs, we suggest that the
10 4
10 5
years. However, we also found that the
GLIMPSE point sour es have a minimum dete table
luminosityofabout1.2L
atatypi alIRDCdistan e
of4kp . Therefore,thefra tionalratiooftheIRDC
ores with EYSOs given hereshould bea lowerlimit
and the estimated lifetime of starlessIRDC ores an
beanupperlimit.
It wasalso found that thegeometri parametersof
the IRDC ores vary depending on how many YSO
andidates the IRDC ores ontain. The IRDC ores
withmoreYSOstendtobelarger,moreelongated,and
havebetterdarkness ontrastthantheIRDC oreswith
feweror noYSOs.
This resear hwassupported bytheBasi Resear h
Program (KOSEF R01-2003-000-10513-0) of the Ko-
reaS ien e. J.B.Pa knowledgessupportfromUNAM-
PAPIIT grant 110606. This resear h has made use
of the NASA/IPAC InfraRed S ien e Ar hive (MSX,
IRAS, and Spitzer), whi h is operated by the Jet
PropulsionLaboratory,CaliforniaInstituteofTe hnol-
ogy,under ontra twiththeNationalAeronauti sand
Spa eAdministration. Thisresear halsohasmadeuse
of Ar etri Catalogofwatermasers (Valadettaroet al.
2001) and the General Catalog of 6.7 GHz methanol
masers (Pestalozzi&Minnier2003).
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