Submillimeter/millimeter observations of the high-mass star forming region IRAS 22506+5944

0.28 MB·English

by  Jin-Long Xu

ResearchinAstron.Astrophys. 2010Vol.10No.2,151–158 R
esearchin
http://www.raa-journal.org http://www.iop.org/journals/raa A
stronomyand
A
strophysics
Submillimeter/millimeter observations of the high-mass star
forming region IRAS 22506+5944
0 ∗
1
0
Jin-LongXuandJun-JieWang
2

n NationalAstronomicalObservatories,ChineseAcademyofSciences,Beijing100012,China;
a
[email protected]
J
NAOC-TUJointCenterforAstrophysics,Lhasa850000,China
8
2
Received2009May5;accepted2009October24

]
R
S Abstract ThemappingobservationsofCOJ =2 1,COJ =3 2,13COJ =2 1
. and 13CO J = 3 2 lines in the directionof IRA−S 22506+5944−have been made.T−he
h −
resultsshowthatthe coresinthe J = 2 1 transitionlineshavea similar morphology
p −
to those in the J = 3 2 transitionlines. Bipolar molecularoutflowsare verified.The
-
−
o prior IRAS 22506+5944 observations indicated that two IRAS sources and three H O
2
r maserswerelocatedclosetothepeakpositionofthecore.OneoftheIRASsourcesmay
t
s bethedrivingsourceoftheoutflows.Inaddition,theH Omasersmayoccurinrelatively
2
a
warmenvironments.Theparametersofthedensecoreandoutflow,obtainedbytheLTE
[
method,indicatethatIRAS22506+5944isahigh-massstarformationregion.

1
v
Keywords: ISM:jetsandoutflows—ISM:kinematicsanddynamics—ISM:molecules
2
—stars:formation
9
0
5
1 INTRODUCTION
.
1
0 Low-mass stars in molecular clouds are formed throughthe processes of collapse, accretion and out-
0
flow(Shuetal.1987).However,theprocessesforhigh-massstarsremainunclearbecauseoftheshort
1
evolutiontimescalesandlargedistancesofhigh-massstars.Observationalevidencesuggeststhatyoung
:
v high-massstars usually form in a cluster environment(Lada et al. 1993).Bipolar molecular outflows
i arecommonlyfoundaroundyounghigh-massstars(Shepherd&Churchwell1996;Zhangetal.2001).
X
Theseoutflowsaregenerallymuchmoremassiveandenergeticthanthosefromlow-massstars.Recent
r
a surveyswithsingle-dishtelescopesshowthatmassiveoutflowsarecommonlyassociatedwithUCHII
regions(Shepherd& Churchwell1996;Zhangetal. 2001)and H O masers(Elitzur etal. 1989).The
2
H Omasersmayoriginateinhotcores,andbeexcitedbytheshocksassociatedwithoutflows.However,
2
howtheoutflowsaredrivenispoorlyunderstood.
IRAS22506+5944hasbeenproposedasaprecursorofaUCHIIregion(Molinarietal.1998).Two
ofthethreeH OmasersinIRAS22506+5944arefoundclosetotheoutflowpeakposition(Jennessetal.
2
1995).Wang(1997)imagedthissourceinthenear-infraredJ,H andK bandsandfoundastarcluster
within the core.Ithasmagnitudesof 17.7,14.5and 11.7,respectively.Moreover,Zhanget al. (2001)
detectedbipolaroutflowsfromanobservationofCO J = 2 1inthisregion.Thebipolarmolecular
−
outflowsareconfirmedfromobservationoftheCOJ = 2 1line(Wuetal.2005).Inaddition,Suet
al.(2004)reportedinterferometricobservationsinCOJ =−1 0,13COJ = 1 0,C18OJ = 1 0
− − −
∗SupportedbytheNationalNaturalScienceFoundationofChina.
152 J.L.Xu&J.J.Wang
andthe3mmcontinuum,aswellassingle-dishobservationsinCO J = 1 0and13COJ = 1 0.
− −
TheIRAS22506+5944sourcewasidentifiedastheoutflowdrivingsource.
Inthispaper,wereportthefirstmappingobservationsofIRAS22506+5944intheCOJ = 3 2,
13COJ = 2 1and13COJ = 3 2lines,aswellasthemappingobservationintheCOJ = 2− 1
− − −
line.Duetotheobservedmolecularlinesathigherfrequencies,wecanattainhigherangularresolution,
whichiscriticalforreducingthebeamdilutionandtoidentifytherelativelycompactcoreandoutflows.
Also,higherJ transitionsarerelativelymoresensitivetohotgases.Suchhotgasesaremorelikelyto
bephysicallyassociatedwithhigh-massyoungstellarobjects(Wuetal.2005).
2 OBSERVATIONS
The mapping observations of IRAS 22506+5944 (R.A.(B1950) = 22h50m38.7s, Dec(B1950) =
59◦44′58′′)weremadeintheCOJ =2 1,COJ =3 2,13COJ =2 1and13COJ =3 2lines
− − − −
usingtheKOSMA3mtelescopeatGornergrat,SwitzerlandinApril2004.Thehalf-powerbeamwidths
ofthetelescopeatobservingfrequenciesof230.538GHz,345.789GHz,220.399GHzand330.588GHz
are130′′,80′′,130′′ and80′′,respectively.Thepointingandtrackingaccuracyisbetterthan10′′.The
DSBreceivernoisetemperaturewasabout120K.Themediumandvariableresolutionacousto-optical
spectrometers have 1501 and 1601 channels, with total bandwidths of 248MHz and 544MHz, and
equivalentvelocityresolutionsof0.21kms−1and0.29kms−1,respectively.ThebeamefficiencyB
eff
is0.68at230GHzand220GHz.ThebeamefficiencyB is0.72at330GHzand345GHz.Thefor-
eff
ward efficiency F is 0.93. The mapping was done using on-the-fly mode with a 1′ 1′ grid. The
eff
×
datawerereducedusingtheCLASS(ContinuumandLineAnalysisSingle-DiskSoftware)andGREG
(GrenobleGraphic)software.The80′′ resolutionoftheCOJ = 3 2and13COJ = 3 2datawas
convolved to 130′′ with an effective beam size of √1302 802 =−102′′. The correction−for the line
intensitiestothemainbeamtemperaturescalewasmadeus−ingtheformulaT =(F /B T∗).
mb eff eff × A
3 RESULTSANDANALYSIS
3.1 TheMolecularLineSpectra
Figure 1 gives the spectra of the different transition lines of CO isotopes at the IRAS 22506+5944
position. Each spectrum shows broad line wings. The blue and red wings are asymmetrical and the
spectral profiles are not Gaussian shaped. The observed parameters of the IRAS 22506+5944source
aresummarizedinTable1.InTable1,thelargerfullwidths(FW)appeartoindicatehigh-velocitygas
motioninthisregion.TherangesoffullwidthsaredeterminedfromthePVdiagram.
Table1 ObservationalParametersoftheIRAS22506+5944
Source
Name Tmb FWHM FW VLSR
(K) (kms−1) (kms−1) (kms−1)
COJ =2−1 12.8±0.1 3.98±0.03 11.11 51.43±0.01
COJ =3−2 11.9±0.1 4.15±0.06 13.63 51.59±0.02
13COJ =2−1 4.1±0.1 2.37±0.04 6.69 51.52±0.02
13COJ =3−2 4.0±0.3 2.32±0.08 7.22 51.54±0.01
3.2 DenseMolecularCore
InFigure2,allthestructuresofthemolecularcloudcorearepresentedandIRAS22506+5944hasan
isolated core. The cores in the J = 2 1 transition lines have a similar morphologyto those of the
−
J =3 2transitionlines.Basedontheresultsobservedbyotherauthors,thereisanear-infraredsource
−
S4whichwasdetectedbyWang(1997)aswellasthreeH Omasersinthisregion.TheIRASsources
2
Submillimeter/MillimeterObservationsofIRAS22506+5944 153
Fig.1 Spectra at the central position of the region (0, 0) of the mapping
observations.
andtheH OmasersarelocatedclosertothecorepeakpositiontracedbythetransitionJ =2 1,but
2
−
alittledeviationfromthatisalsotracedbythetransitionJ = 3 2.TheIRASsourcesareassociated
−
with the H O masers. This indicates thatthey are still undergoingactivity in an early evolution stage
2
(Fellietal.1997).TheH Omasersoccurinrelativelywarmenvironments.UsingtheIRASpoint-source
2
catalog,infraredluminosity(Casoli etal. 1986)and dusttemperature(Henninget al. 1990)are given
by,respectively:
L =(20.653 F +7.358 F +4.578 F +1.762 F ) D2 0.30, (1)
IR 12 25 60 100
× × × × × ×
96
TD = , (2)
(3+β)ln(100/60) ln(F /F )
60 100
−
whereDisthedistancefromthesuninkpc;F ,F ,F ,andF aretheinfraredfluxesatfourIRAS
12 25 60 100
bands(12µm,25µm,60µmand100µm),respectively.Theemissivityindexofthedustparticles(β)is
assumedtobe2.ThecalculatedresultsarepresentedinTable2.
ThecoreparametersarecalculatedfollowingtheprocedureofLeeetal.(1990).AssumingLTE,we
writethecolumndensityas
exp[hBJ(J +1)/kT ] (T +hB/3K) τ ∆V
N =2.4 1014 ex ex × × cm−2, (3)
× J +1 × 1 exp( hν/kT )
ex
− −
where B is the rotational constant of the molecule, J is the rotational quantum number and ν is the
frequencyofthespectralline.∆V isahalffull-widthofspeed,T istheexcitationtemperatureandτ
ex
istheopticaldepth.HereweassumeN(H )/N(CO)=104(Dickman1978).
2
TheCOemissionisalwaysconsideredopticallythickwhilethe13COemissionisusuallyoptically
thin.Therefore,theexcitationtemperatureT (CO)isgivenby(Gardenetal.1991):
ex
−1 −1
hv hv T hv hv
T (CO)= ln 1+ mb + exp 1 , (4)
ex
k ( k (cid:20) f k (cid:18) (cid:16)kTbg(cid:17)− (cid:19)(cid:21) )!
154 J.L.Xu&J.J.Wang
Fig.2 Integrated intensity maps of the core emission. In each map, the integrated range is
from–53.26to–49.68kms−1.Thecontourlevelsare30%,40%,...,90%ofthepeakvalue.
The redstar representsthe IRAS22506+5944sourceposition,the greenstar representsthe
near-infraredsourceS4,andthefilledtrianglesindicatethepositionsofthethreeH Omasers.
2
Thedotsymbolsmarkthemappingpoints.
Table 2 IRAS Fluxes and Derived
Parameters
Source Distance F12 F25 F60 F100 lg(F25/F12) lg(F60/F12) TD LIR
(kpc) (Jν) (Jν) (Jν) (Jν) (K) (×104L⊙)
IRAS22506+5944 5.1 6.37 34.74 187.50 295.10 0.74 1.47 31.90 1.38
where T =2.732K is the temperature of the cosmic backgroundradiation, and f is the beam filling
bg
factor.Inaddition,τ(13CO)isgivenby(Gardenetal.1991):
−1
kT 1 1
τ(13CO)= ln 1 mb . (5)
− ( − hv (cid:20)exp(hv/kTex)−1 − exp(hv/kTbg)−1(cid:21) )
Wealsouseanothermethodtocalculateτ(13CO):
T (CO) 1 exp[ τ(CO)]
mb
− − . (6)
T (13CO) ≈ 1 exp[ τ(13CO)]
mb
− −
Submillimeter/MillimeterObservationsofIRAS22506+5944 155
We assume the solar abundance ratio [CO]/[13CO]=τ(CO)/τ(13CO)=89 (Lang 1980). The calculated
resultsarelistedinTable3.Fromthetable,wecanseethattheopticaldepthscalculatedbytheabove
two methodsare almost equal. Thus, we suggest that the abundance ratio [CO]/[13CO] in this region
isnearlythesameasthatinoursolarsystem.Usingthecolumndensity,themassesofoutflowgasare
obtainedby:
M =µN S/(2.0 1033), (7)
H2 ×
wherethemeanatomicweightofthe gasµ is1.36,andS isthe size ofthecoreregion.Thephysical
parametersofthecorearesummarizedinTable4.
Table 3 Calculated Results of the Optical
Depth
Name τ1 τ2
COJ =2−1 33.55 34.00
COJ =3−2 52.33 53.59
13COJ =2−1 0.38 0.38
13COJ =3−2 0.59 0.60
The optical depth (τ1) is derived based on the first
method.(τ2)isderivedbasedonthesecondmethod.
Table 4 Physical Parameters of the
Core
Name Tex τ N(COJ =2−1) N(H2) M
(K) (×1017cm−2) (×1021cm−2) (×103M⊙)
IRAS22506+5944 20.5 33.55 7.07 7.07 2.03
3.3 TheBipolarOutflows
FromFigure3,bipolaroutflowsareclearlyrevealedintheIRAS22506+5944region.Theredwingand
blue wing lobesmostly overlap,whichmay be attributed to the axis of outflowsbeingnearlyparallel
with the line of sightdirection.The position-velocity(PV)diagramsin Figure 4,with a cutalongthe
north-southdirection,alsoclearlyshowhigh-velocityoutflows.TheoutflowsoftheCOJ = 3 2are
−
muchclearerthanthoseoftheCOJ = 2 1.TwoIRASsourcesarelocatedclosetothecenterofthe
−
outflows,sotheymaybetheoutflowsdrivingthesource,butwecannotdistinguishwhichsourceisthe
drivingsource.
Under the LTE assumption, the averaged column density of the outflows can be obtained by
(Scovilleetal.1986):
exp[hBJ(J +1)/kT ] (T +hB/3K) τdv
N =2.4 1014 ex ex T cm−2. (8)
mb
× J +1 × exp( hv/kT ) × [1 exp( τ)]
ex
− Z − −
Weconsiderthattheexcitationtemperatureisuniformintheobservedregion.Theexcitationtempera-
ture(T )is20.5Kfromthecalculatedresultsofthecores.Theτ canbedeterminedbyEquation(6).
ex
The integral is carried out in the blue and red wing regionsand the integrated ranges are determined
fromthePVdiagram.
Inaddition,wecanderivetheoutflows’mass(M)fromEquation(7).ThemomentumP andenergy
E arecalculatedbyP = MV andE = MV2,whereV isthemeanvelocityofthegasrelativetothe
cloud’ssystemicvelocity.Adynamictimescalecanbedeterminedbyt =R/V,whereRisthemean
d
sizeoftheoutflows.ThedrivingforceisdeterminedbyF =P/t .Themechanicalluminosityandthe
d
masslossrateoftheoutflowsarecalculatedusingL = E/t andM˙ =P/(t v ),wherethefinal
mech d d w
156 J.L.Xu&J.J.Wang
Fig.3 Leftpanel:outflowcontoursoftheCOJ =2 1aresuperimposedontheintegrated
intensitymapofthecore;theintegratedrangeisfrom−–57.07to–53.26kms−1 fortheblue
wing(blacksolidline)andfrom–49.68to–45.96kms−1fortheredwing(reddashedline).
Right panel: outflow contours of the CO J = 3 2 are superimposed on the integrated
intensitymapofthecore;theintegratedrangeisfro−m–58.96to–53.26kms−1 fortheblue
wing(blacksolidline)andfrom–49.68to–45.33kms−1fortheredwing(reddashedline).
Thecontourlevelsforcoreandoutflowsare30%,40%,...,90%ofthepeakvalues.
Fig.4 Leftpanel:P-VdiagramconstructedfromtheCOJ =2 1transition.Contourlevels
are1,2,3,4.5,6,6.5,7.5,9,10.5,12,14,...,25K.Rightpanel:P-−VdiagramforCOJ =3 2.
Contourlevelsarethesameasintheleftpanel.Theleftandrightverticallinesindicate−the
beginningoftheblueandredwings,respectively.
windvelocityis takento be v = 500kms−1 (Martietal. 1986).The calculatedresultsarelisted in
w
Table5.
Table 5 Physical Parameters of the
Outflows
Name Wing N(H2) M td M˙ F P E LMech
IRAS (×1020cm−2)(M⊙)(×105yr)(M⊙yr−1)(M⊙kms−1yr−1)(M⊙kms−1)(×1046erg) (L⊙)
22506+5944 Blue 1.35 18.7 4.14 8.0×10−7 4.0×10−4 164.6 1.08 0.22
Red 1.35 18.6 4.38 8.2×10−7 4.1×10−4 179.6 1.30 0.24
Submillimeter/MillimeterObservationsofIRAS22506+5944 157
4 DISCUSSION
FromFigure1andTable1,thefullwidthoftheCOJ =3 2lineislargerthanthatoftheCOJ =2 1
− −
lineandthelinewingsinCOJ =2 1arebroaderthanthoseinCOJ =1 0(Wuetal.2005).These
− −
resultssuggestthattheoutflowsmaybecausedbystellarwindsweepingupthesurroundingmaterials
indifferentintensitylayersandtheoutflowstracedbythehigherJ levelCOlinesarisefromthewarm
layerclosertothecentralexcitingstar.
Althoughthe color indexes of the source satisfy the criteria of Wood & Churchwell(1989),they
arenotdetectedin thecentimeteror millimetercontinuumemission(Wuetal. 2005).Whetherornot
itisaUCHIIregionstillneedstobedeterminedbyfurtherhighersensitivitycontinuumobservations.
Sofar,themajorityofauthorshaveusedconstantvaluesofτ andT toestimatetotalmass.Because
ex
ourobservationsweresimultaneouslymadeinCOand13CO,wecanobtainrelativelyaccuratevalues
of τ and Tex, as well as the total mass. From Table 4, the total mass of the core is 2.03 103M⊙;
×
the value indicates that IRAS 22506+5944is a high-mass star formation region. Wang (1997) found
clustersofstellarobjectsandaninfraredjetinthisregion.Thus,weconcludethatIRAS22506+5944
is associated with molecular cloud complexes.The IRAS 22506+5944source appearsto be a deeply
embeddedprotostar.
Inaddition,thecontourmapsandPVdiagramoftheIRAS22506+5944sourceclearlyshowbipolar
outflows in this region.The total mass of the outflows is 37.3M⊙, and the total amountof momenta
is 344.2M⊙km s−1. Both parameters are much larger than the typical values in low-mass outflows.
The larger outflow’s mass suggests that the outflows’ mass is not likely to originate from the stellar
surface, but could be caused by the entrainmentof ambientgas (Shepherd& Churchwell1996). The
IRAS22506+5944sourceisconsideredastheoutflowdrivingsourcebySuetal.(2004),whileWuet
al.(2005)considerS4tobetheoutflowdrivingsource.TwoIRASsourcesarelocatedatthegeometric
position of the driving source of the outflow, and we also cannot identify which one is the driving
source.We needtoapplyforobservationswithamuchhigherangularresolutiontelescopetoconfirm
it.L /L 1,andL /CF <1,whereL isthetotalmechanicalluminosity,suggestingthat
IR Mech IR Mech
≫
despite the strong radiation from the centralsource, the radiation pressure still cannotsupply enough
forcetodrivesuchmassiveoutflows(Wuetal.1998).
5 CONCLUSIONS
We observed the CO J = 2 1, CO J = 3 2, 13CO J = 2 1 and 13CO J = 3 2 lines in
− − − −
thedirectionofIRAS22506+5944.IRAS22506+5944hasanisolatedcore.Theinfrareddataindicate
that the IRAS 22506 +5944 source appears to be a deeply embedded protostar. Bipolar outflows are
furtheridentifiedinthisregion.IRAS22506+5944orNIRsourceS4maybetheoutflowdrivingsource.
The two IRAS sources are associated with H O masers. The H O masers occur in relatively warm
2 2
environments.Theparametersofthecoreandtheoutflowsarederived;thederivedvaluessuggestthat
IRAS22506+5944isahigh-massstarformationregionandthetotalmassoftheoutflowsislargerthan
thatinlow-massstarformationregions.ComparedwiththeextensionoflinewingsinCOJ = 2 1,
−
COJ =3 2linewingsextendfurtherinvelocity,suggestingthattheoutflowstracedbythehigherJ
level12CO−linesarisefromthewarmlayerclosertothecentralexcitingstar.
Acknowledgements We wouldliketothankDrs.Sheng-LiQinandMartinMillerfortheirhelpwith
dataacquisitionanddiscussion.We arealso gratefulto therefereeforhis/herhelpfulcomments.This
workwassupportedbytheNationalNaturalScienceFoundationofChina(GrantNo.10473014).
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