The molecular gas around the luminous blue variable star G24.73+0.69

1 MB·English

by  A. Petriella

Astronomy&Astrophysicsmanuscriptno.Petriella˙ax (cid:13)c ESO2012
January6,2012
The molecular gas around the luminous blue
variable star G24.73+0.69
2
1
0 A.Petriella1,2,S.A.Paron1,3,andE.B.Giacani1,3
2

n
a 1 InstitutodeAstronom´ıayF´ısicadelEspacio(CONICET-UBA),CC67,Suc.28,1428Buenos
J
Aires,Argentina

5
e-mail:[email protected]

] 2 CBC-UniversidaddeBuenosAires,Argentina
R
3 FADU-UniversidaddeBuenosAires,Argentina
S
.
h Received¡date¿;Accepted¡date¿
p
-
o ABSTRACT
r
t
s
a Aims.Westudythemolecular environment of theluminous bluevariablestarG24.73+0.69 to
[ investigatetheoriginofthetwoinfraredshellsaroundthismassivestaranddetermineitseffects

2 onthesurroundinginterstellarmedium.
v
Methods.Weanalyzethedistributionofthemoleculargasusingthe13COJ=1–0emissionex-
3
4 tracted from the Galactic Ring Survey. We use near and mid-infrared data from 2MASS and
0
GLIMPSEtoidentifytheyoungstellarobjectsinthefield.
6
. Results.WediscoverthemolecularcounterparttotheouterinfraredshellaroundG24.73+0.69.
1
1 TheCOshellwasprobablyblownbythestellarwindofthestarmainlyduringitsmainsequence
1
phase.Wealsofindmoleculargasthatcorrespondstotheinnerinfraredshell,althoughitsorigin
1
: remainsuncertain.Weidentifysevenyoungstellarobjectswithinthemolecularmaterial,whose
v
i birthmighthavebeentriggeredbythestellarwindoftheluminousbluevariablestar.Wesug-
X
gestthatbothG24.73+0.69andtheprogenitorofthenearbysupernovaremnantG24.7+0.6were
r
a
formedfromthesamenatalcloudandrepresentthemostevolvedmembersofasofarundetected
clusterofmassivestars.
Keywords.stars:individual:G24.73+0.69–stars:winds,outflows–ISM:clouds–stars:for-
mation
1. Introduction
Luminous blue variable (LBV) stars are very massive objects that evolve from O-type main se-
quence(MS)starsburninghydrogenintheircoretobecomeWolf-Rayet(WR)heliumcoreburn-
ingstars(Maeder1989;Humphreysetal.1989).Thistransitionalstageischaracterizedbyahigh
mass-lossrate(typicallybetween10−5and10−4M /yr)sometimesaccompaniedbyso-calledgiant
⊙
eruptions,andsignificantphotometricvariabilityontimescalesfrommonthstoyears(Humphreys
&Davidson1994).TheLBVphaseisshort-livedandasaconsequenceonly12casesand23candi-
dateshavebeenreportedinourGalaxysofar(Clarketal.2005).Asaresultofthemassloss,most
Sendoffprintrequeststo:A.Petriella
A.Petriellaetal.:ThemoleculargasaroundG24.73+0.69
of the LBV stars (either confirmed or candidates) are surroundedby a nebula that expandswith
characteristicvelocitiesofbetween30kms−1and200kms−1(seeClarketal.2005).Thesenebulae
havedustyandgaseouscomponents(ofa fewsolarmasses) andalargevarietyofmorphologies,
fromcirculartobipolar(Notaetal.1995;Weis2001).
SincenebulaearoundLBVstarsarestrongemittersintheinfrared(IR)regime,mostofthem
havebeenobservedinthiswavelengthrange.Atpresent,thereareonlyafewcasesin whichthe
chemicalevolutionandthepresenceofcircumstellarmoleculargasrelatedtothese nebulaehave
beendeterminedbymolecularstudies,themostrepresentativesourcesbeingAGCar(Notaetal.
2002)andG79.29+0.46(Rizzoetal.2008;Jime´nez-Estebanetal.2010).Molecularmaterialwas
alsodiscoveredaroundtheyellowhypergiant(YHG)starsIRC+10420andAFGL2343*(Castro-
Carrizoetal.2007).Theoreticalmodelsofstellarevolutionandobservationsofpost-MSmassive
stars (Meynet & Maeder 2000;Humphreys& Davidson 1994) suggest that low luminousLBVs
may pass through a cooler red supergiant (RSG)/yellow hypergiant phase prior to the LBV pe-
riod.TheRSG/YHG phaseis also characterizedbymass-loss activity(fora review,see de Jager
1998)thatcanleadtothepresenceofmoleculargasaroundthesemassivestars.Byanalyzingthe
molecularmaterial,we can determinethe mass-losshistory ofthe massivestar andits effects on
thesurroundinginterstellarmedium(ISM).
2. PresentationofG24.73+0.69
TheringnebulaaroundtheLBVstarG24.73+0.69(hereafterG24)wasdiscoveredbyClarketal.
(2003) in the mid-IR. Fig. 1 displays the emission in the Spitzer-IRAC 8 µm band toward G24,
which clearly shows the nebula around the central massive LBV star. It has a roughly circular
morphologywithaslightelongationinadirectionparalleltotheGalacticplane.Itsshapecanbe
approximatedbyanellipseof40′′×34′′.Takinganexpansionvelocityof200kms−1,Clarketal.
(2003)estimatedamass-lossrateof∼ 9.5×10−5M /yrfortheLBV starandanageof∼ 4,800
⊙
yrforthenebula.Theyquotedanupperlimitof5.2kpcforthedistancebasedontheconsistency
between the modeled and the observed spectral energydistribution of the star. In addition, there
is a fragmentaryoutershell (OS) of a bipolarmorphologywith lobes(labeled 1 and 2 in Fig. 1)
projectingintooppositedirections.Thisstructureformsanincompleteshellof4.6′×2.3′ oriented
∼ 60◦ with respectto the Galactic plane,whose geometriccenter doesnotmatch the position of
theLBVstar.Fig.1alsoshowsthepositionoftheIRdarkcloud(IRDC)024.789+0.633,whichis
adjacenttothelobe2.AllIRDCsareconsideredastheprecursorsofprotostars,beingasignature
ofactivestarformation(Rathborneetal.2006,2007).
ToexplaintheoriginoftheIRshells,Clarketal.(2003)arguedthattheinnerone(i.e.thering
nebula)originatesinmaterialejectedfromthecentralstarduringtheLBVphase.However,based
onthesizeandageofthebipolarOS,theysuggestthatitformedfromtheinteractionbetweenthe
stellar wind and the ISM and/or the natal molecular cloud (MC) in a period prior to the current
phase.
Inthiswork,westudythemoleculargastowardG24.73+0.69anditsenvironmentwiththeaim
ofunveilingtheoriginoftheseIRshells.
A.Petriellaetal.:ThemoleculargasaroundG24.73+0.69
Fig.1.EmissionintheSpitzer-IRAC8µmbandtowardtheLBVstarG24.73+0.69.Weindicatethepositions
oftheLBVstaranditsnebula,thelobesofthebipolaroutershell,andtheIRDC024.789+0.633.
3. Resultsanddiscussion
3.1.Thedistributionofthemoleculargas
WestudythemoleculargasaroundG24usingdatafromtheGalacticRingSurvey(GRS,Jackson
et al. 2006). The GRS was performed by the Boston University and the Five College Radio
Astronomy Observatory (FCRAO). The survey maps the Galactic ring in the 13CO J=1–0 line
with an angular and spectral resolutions of 46′′ and 0.2 km s−1, respectively. The observations
were performedin both position-switchingand on-the-fly mapping modes, achieving an angular
samplingof22′′.
Fromtheinspectionofthe13COdatacubeinthewholevelocityrange,wefoundmorphological
signatures of a possible association between G24 and the surrounding molecular material in the
velocityrangebetween+39and+44kms−1(allvelocitiesherebeingreferredtothelocalstandard
ofrest).InFig.2,weshowtheintegratedvelocitychannelmapsofthe13COJ=1–0emissionevery
0.7kms−1 withthemid-IRemissionfromtheSpitzer-IRAC8µmband.Fromthisfigure,wecan
discernseveralcondensationsofmoleculargasformingamolecularshellaroundG24(seemainly
panelsat+40.9and+41.5kms−1).ThisshelldelineatestheexteriorborderofthebipolarOS.This
distributionindicatesthatthereis a connectionbetweenthe moleculargasandthe bipolarOS. If
thiswerethecase,bothofthemwouldbeatthesamedistance.UsingtherotationalmodelofFich
etal.(1989)andadoptingavelocityof+42kms−1 asasystemicvelocityforthemolecularshell,
weobtainedkinematicdistancesofeither3.5kpcor12kpc.Lackinganyadditionaldiscriminator
betweenthetwokinematicdistancesandtakingintoaccountthatadistanceof5.2kpcisquotedas
anupperlimitforG24(Clarketal.2003),weadoptedadistanceof3.5kpcasthemostplausible
value for the LBV star and the molecular shell. This new distance led to a substantial reduction
in the calculated intrinsic luminosity of G24. Clark et al. (2003)quoted log(L /L ) = 5.6 for a
⋆ ⊙
A.Petriellaetal.:ThemoleculargasaroundG24.73+0.69
Fig.2. Two-color image of the field around the LBV star. In red: Spitzer-IRAC 8 µm band. In green: the
integrated13COJ=1–0emissionevery0.7kms−1.TheCOcontourslevelsare0.5,0.9,1.8,and2.7Kkms−1.
distance of 5.2 kpc. Taking a distance of 3.5 kpc, the luminosity drops to log(L /L ) ∼ 5.25,
⋆ ⊙
whichwouldmakeitoneofthefaintestLBVsyetidentified(seeforcomparisontheHRdiagram
ofClarketal.2009),thusapossiblepostRSG/YHGobject.
We were able to derive a number of parameters characterizing the CO. By assuming local
thermodynamic equilibrium (LTE), we estimated the H mass of the molecular shell using the
2
equations
T +0.88
N(13CO)=2.42×1014 ex τ dv,
1−e−5.29/Tex Z 13
where T is the excitationtemperatureof the 13CO transition and τ is the opticaldepth of the
ex 13
line.Assumingthatthe13COJ=1–0lineisopticallythin,wecanusetheapproximation
1
τ dv∼ T dv,
Z 13 J(T )−J(T )Z B
ex b
where
5.29
J(T)= ,
e5.29/T −1
T = 2.7 K is the backgroundtemperature,and T is the brightnesstemperatureof the line. We
b B
assumed that T = 10 K and used the relation N(H )/N(13CO)∼ 5×105 (Simon et al. 2001)to
ex 2
estimatethecolumndensityofmolecularhydrogenN(H ).Themassofthemolecularclumpswas
2
calculatedfrom
M=µm D2ΩN(H ) ,
H 2
Xh i
whereΩisthesolidanglesubtendedbythe13COJ=1–0beamsize,m isthehydrogenmass,µis
H
themeanmolecularweightassumedtobe2.8bytakingintoaccountarelativeheliumabundance
of 25 %, and D ∼ 3.5 kpc is the distance. Our summation was performedover the area of each
molecularclump.We summedthe massof the clumpsaroundG24 and foundthat the molecular
shellhasatotalmassof∼ 1,000M .Theoriginalambientdensityn ,animportantparameterto
⊙ 0
studythedynamicsoftheshell,couldbeestimatedbyassumingthatthecalculatedtotalmasswas
originallydistributedinthevolumelimitedbythebipolarOS,whichhasameanradiusof∼ 3.5′.
Atadistanceof3.5kpc,thiscorrespondstoameanradiusr ∼3.5pc.Usingthisvalue,wefound
0
thatn ∼230cm−3.
0
A.Petriellaetal.:ThemoleculargasaroundG24.73+0.69
3.2.Theoriginofthemolecularshell
Thereiscompellingevidenceofshellsofinterstellarmaterialproducedbytheeffectofthestrong
stellarwindsofmassivestarsontheISM.Theseso-calledstellarwindbubbleshavebeenobserved
inneutral(Cappaetal.2005;Cichowolskietal.2008;Giacanietal.2011)andmolecular(Cappa
etal.2009,2010)gas.WeexploredthehypothesisthattheactionofthestellarwindoftheLBVstar
and/oritsprogenitorO-typestarhasblownacavityofinterstellarmaterialpilingupthemolecular
gasintheborderandformingthemolecularshelldiscoveredinthepresentwork.
Totestthisscenario,wecomparedthemechanicalenergyE releasedbythestarintotheISM
ω
duringthelifetimeofthemolecularshell,withthekineticenergyE oftheswept-upmaterial.We
k
firstestimatedthedynamicalage(t ) oftheshellusingthe equationofWhitworthetal.(1994)
dyn
for a stellar wind bubble:t ∼ 0.02r5/3L−1/3n1/3 Myr, where r is the radius of the shell in pc,
dyn 0 37 3 0
L is the mechanical luminosity L of the stellar wind in units of 1037 erg s−1, and n is the
37 ω 3
originalambientdensityinunitsof103 cm−3.We calculated L = 1/2M˙v2 bytakingthetypical
ω ω
parametersofanO-typestarinPrinjaetal.(1990)andMokiemetal.(2007),namelyamass-loss
rate M˙ = 2×10−6 M /yr and a stellar wind velocity v = 2,000 km s−1. Thus, we found that
⊙ ω
L ∼ 2.5×1036 erg s−1. Taking the mean radius of the shell ∼ 3.5 pc and the originalambient
ω
density∼ 230cm−3, we evaluatedthe t to be∼ 0.2Myr. Fromthisresult, we cansee thatthe
dyn
COshellismorethananorderofmagnitudeolderthantheageoftheLBVphase,whichtypically
hasadurationof. 104yr.ThisindicatesthatthedetectedCOshellmaybethematerialswept-up
bythestellarwindofthecentralmassivestarsduringitsMSphase.
The kinetic energy can be estimated as E = 1/2Mv2, where M is the mass of the shell and
k e
v its expansion velocity. We calculated the expansion velocity to be v ∼ 6.2L1/5n−1/5t−2/5 km
e e 37 3 dyn
s−1 (Whitworth et al. 1994), where t = 0.2 Myr. We obtained v ∼ 13 km s−1. Using M =
dyn e
1,000M ,weinferredthatE ∼ 1.7×1048erg.Forthemechanicalenergy,weassumethatE =
⊙ k ω
L t ∼1.6×1049erg.Wecalculatedtheefficiencyoftheenergyconversionofthewindǫ = Ek and
ω dyn Eω
obtaineda valueof ∼ 0.11.This agreeswith the expectedvaluefor an energy-conservingstellar
wind bubble (Mellema & Lundqvist2002), hence we conclude that the central star has blown a
stellarwindstrongenoughtocreatethemolecularshell.Wealsoevaluatedthecontributionofthe
starduringtheLBVphasetotheenergyoutflow.TakingM˙ =9.5×10−5 M /yrandv =200km
⊙ ω
s−1 from Clark et al. (2003)and ∼ 104 yr as a typicaldurationfor the LBV phase, we obtained
ELBV ∼3.8×1047erg,whichrepresentslessthan3%oftheenergyreleasedbythestarduringthe
ω
MSphase.
Thesecalculationslinktheformationofthemolecularshelltothestellarwindofthecentralstar
duringtheMSphase,ingoodagreementwiththeoriginofthebipolarOSsuggestedbyClarketal.
(2003).In this scenario, the elongated morphologyof the bipolar OS (and also of the molecular
shell) may be a consequence of the asymmetric stellar wind from the central star. Evidence of
non-sphericalwinds has been found in other massive stars. For example, Meaburn et al. (2004)
discovereda giantlobeprojectingfromtheringnebulaaroundtheLBVstar PCygni.According
to the authors,thelobe formedfromthe mass-ejectionactivitypriorto the currenteruptionsthat
producedthenebula.Theysuggestedthatthepeculiarmorphologyofthisgiantlobeisduetothe
interactionbetweentheejectedmaterialandanasymmetriccavityintheISMblownbythetoroidal
stellarwindofthecentralstarduringtheMSphase.
A.Petriellaetal.:ThemoleculargasaroundG24.73+0.69
Finally, we note the presence of molecular gas superimposedon the ring nebula (see Fig. 2,
panelsat+42.1and +42.8km s−1). Thismaterialmayi) originatein a mass-ejectioneventfrom
thecentralLBVstar,asobservedintheringnebulaaroundG79.29+0.46(Jime´nez-Estebanetal.
2010);orii)bepartofthemolecularshell.UsingtheequationsofSect.3.1,weestimatedamass
of∼ 20M andadensityof∼120cm−3.Theobtainedmassishigherthanthemassofthenebula
⊙
aroundG24estimatedbyClarketal.(2003)(∼ 0.45M ) andofthenebulaearoundotherLBVs
⊙
(see Smith&Owocki2006andClark etal.2009).Thus,we proposethatthe molecularmaterial
locatedrightupontheringnebulabelongsmainlytothemolecularshell.However,wecannotrule
outthepresenceofsomemoleculargasejectedbytheLBVstar.Weplantoobserveafieldaround
G24withhigherdensitytracersandhigherresolutiontoconfirmthepresenceofthismaterial.
3.3.StarformationactivityaroundG24.73+0.69
The environmentof the LBV star G24 might be one that promotes triggered star formation. As
wehaveshownintheprevioussection,themassivestarhasbeeninteractingwiththeneighboring
moleculargasbymeansofitsstrongstellarwind.Tosearchforprimarytracersofstarformation,
weidentifiedtheyoungstellarobject(YSO)candidatesusingtheGLIMPSEPointSourceCatalog
(GPSC)intheSpitzer-IRACbands.Weconstructeda[5.8]-[8.0]versus[3.6]-[4.5]color-color(CC)
diagramforthesourcesaroundG24.IntheCCdiagram,weusedthephotometriccriteriaofAllen
etal.(2004)toclassifythesourcesaccordingtotheirevolutionarystage:classIareprotostarswith
prominentcircumstellarenvelopes,classIIaredisk-dominatedobjects,andclassIIIarethemost
evolvedYSOswhoseemissioncomesmainlyfromthecentralstar.
Fig.3.Two-color imagetoward G24.73+0.69. Ingreen withcontours: emission of the13COintegratedbe-
tween+39and+44kms−1(contourlevelsare2.7,4.4,and6.2Kkms−1).Inred:8µmband.Themagenta
crossesaretheclassIYSOcandidatesselectedaccordingtothecriteriondiscussedinthetext.
Fig.3displaysthedistributionoftheclassIYSOcandidatesaroundG24.Wecandiscernseven
oftheseYSOcandidatessuperimposedonthemoleculargasswept-upbythemassivestar.Fourof
them(sources1,2,3,and4)spatiallycoincidewiththemaximumofmolecularemission.Wenote
A.Petriellaetal.:ThemoleculargasaroundG24.73+0.69
thatthisregioncoincideswiththeIRDC024.789+0.633(indicativeofactivestar-formingactivity)
andliesnexttothelobe2ofthebipolarOS.
Wefitthespectralenergydistribution(SED)oftheclassIYSOcandidatesusingthefluxesin
the2MASSJ,H,andKsbandsandinthefourSpitzer-IRACbands.Weusedthetooldevelopedby
Robitailleetal.(2007)andavailableonline1.Weassumedthedistancetobebetween3and4kpc.In
Table1,wereporttheclassIYSOcandidatesmagnitudesinthe2MASSandSpitzer-IRACbands.
Robitailleetal.(2006)definedthreeevolutionarystagesbasedonthevaluesofthecentralsource
massM ,thediskmassM ,theenvelopemassM ,andtheenvelopeaccretionrateM˙ .Stage
⋆ disk env env
I YSOsarethose thathave M˙ /M > 10−6 yr−1, i.e.,protostarswithlargeaccretionenvelopes;
env ⋆
stageIIarethosewith M˙ /M > 10−6 yr−1 and M˙ /M < 10−6 yr−1,i.e.,youngobjectswith
disk ⋆ env ⋆
prominentdisks;andstageIIIarethosewithM˙ /M <10−6yr−1andM˙ /M <10−6yr−1,i.e.,
disk ⋆ env ⋆
evolvedsourceswherethefluxisdominatedbythecentralsource.Thegoodnessofthefittingfor
eachmodelwasmeasuredbyaχ2 value.Wedefinedthe“selectedmodels”asthosesatisfyingthe
equationχ2−χ2 < 2N,whereχ2 correspondstothebest-fittingmodelandNisthenumberof
min min
inputdatafluxes(fluxesspecifiedasupperlimitdonotcontributetoN).Thefittingtoolalsofitthe
IRfluxestoastellarphotospheretocheckwhetherthesourcecouldbemodeledbyamain-sequence
starwithinterstellarreddening.Thegoodnessofthefittingisdefinedbyaχ2 value.
star
Table1.2MASSandSpitzer-IRACmagnitudesofclassIYSOcandidates.
Source Source(GPSC) 2MASSQual. J H KS 3.6µm 4.5µm 5.8µm 8.0µm
1 G024.7956+00.6558 N/D 13.047 12.173 11.715 11.321
2 G024.8051+00.6456 UUA 18.131 17.059 13.707 10.548 9.589 8.791 8.059
3 G024.7901+00.6410 N/D 12.874 11.096 9.983 9.639
4 G024.7907+00.6379 N/D 13.439 12.518 11.638 11.059
5 G024.7545+00.6771 AAA 14.818 13.227 12.158 10.967 10.497 10.194 9.859
6 G024.7532+00.6400 UUA 14.643 12.784 12.312 11.792 11.463 11.722 11.386
7 G024.7735+00.6068 AAA 13.898 13.172 12.803 11.907 11.485 10.083 8.577
Notes.2MASS Qual.:Aisthebestphotometric quality, withasnr ≥ 10,and Umeansthat themagnitude
valueisanupperlimit.N/Dindicatesnodetectionofthesourceinthe2MASSsurvey.
InTable2,we reporttheresultsoftheSEDfitting forthesevenclassIcandidateswithinthe
molecularshell.Forallofthem,alltheselectedmodelscorrespondtostageIandII(exceptsource
2 for which we get one stage III model). Moreover, for all of them apart from source 6 we get
χ2 <χ2 .Thus,wecanconfirmthattheyareyoungsourcesattheearlierstagesofevolution.
min star
Aswenoteintheprevioussection,themolecularshellmighthaveformedasaconsequenceof
theinteractionbetweenthestellar windofthe centralmassivestarandtheISM.Thepresenceof
YSOcandidateswithinthemolecularshellandanIRDCaresignsofactivestar-formingactivity.
We suggest that the birth of these new stars might have been triggered by the expanding wind
bubbleactingonthemoleculargas.
1 http://caravan.astro.wisc.edu/protostars/
A.Petriellaetal.:ThemoleculargasaroundG24.73+0.69
Table2.ResultsoftheSEDfittingforclassIYSOcandidates.Inthelastcolumn,wereporttheevolutionary
stageofthesource,basedonthecriterionofRobitailleetal.(2006)fortheselectedmodels.Whenthemodels
correspondtodifferentstages,thelesslikelyispresentedinbrackets.
Source χ2 /N χ2 /N Stage
min star
1 0.4 15 Ior(II)
2 4 77 II
3 0.1 93 I
4 0.02 37 Ior(II)
5 0.9 4 IIor(I)
6 2.3 2 II
7 10 91 II
3.4.G24.73+0.69anditssurrounding:thebigpicture
Massive stars form in giant molecular clouds which, under certain circumstances, fragment into
denseclumpsthatcollapsetoformnewstars.Thesestarsspendashortperiodoftime(∼ 106yrs)
in the MS before evolving to become LBV and WR stars, which eventually end their lifetimes
explodingas supernovae.Theseexplosionsusuallyoccurwhenthe progenitorstar still lies close
toits natalcloudandwhileothercompanionstars maystill bein theMSorLBV/WR phase.As
aconsequence,severalsupernovaremnantshavebeenobservedinteractingwithevolvedmassive
stars(seeVela´zquezetal.2003;Sushchetal.2011)andmoleculargas(seeJiangetal.2010foran
exhaustivelistofSNR/MCassociations).
TheLBVstarG24.73+0.69lies∼7′fromtheSNRG24.7+0.6.Intheradioband,thisremnant
displaysacoupleofincompleteshellsandapolarizedfilledcentralcorewithaflatspectrum(Reich
et al. 1984),which indicates that it consists of a plerionpowered by an undetectedpulsar. Thus,
theSNRformedfromthecollapseofamassivestar. Petriellaetal.(2008)reportedtheexistence
ofmoleculargasinteractingwithG24.7+0.6inthevelocityrangebetween+38and+50kms−1,
whichinterestinglyplacestheSNRatthesamekinematicdistanceasG24.Inaddition,activestar
formation probably triggered by the SNR and/or its progenitor was also discovered around the
remnant(Petriella et al. 2010).In Fig. 4, we show the emission of the 13CO J=1–0 integratedin
thevelocityrangebetween+38and+50kms−1.Weindicatethepositionofthedifferentfeatures
inthefield:theLBVstar(redcross),theSNR(inbluecontours),andtheMCinteractingwiththe
remnant(cloud2inthenomenclatureofPetriellaetal.2008).
Fromthepreviousfigure,weseethatthislargeregion,whichharborsarichvarietyofobjects
(aLBVstaranditsnebula,aSNR,abundantmolecularmaterialhostingdarkclouds,possiblestar-
formingsites),isanideallaboratorytoinvestigatethegeneticconnectionbetweendifferentstellar
populations. On the basis of the morphology and distribution of the molecular gas, it is likely
thattheprogenitorstaroftheSNRG24.7+0.6andtheLBVstarmayhaveformedfromthesame
giant MC that now we observe fragmentedinto smaller features. They may be the most evolved
membersofasofarundetectedcluster/associationofmassivestarsthatalsoformedfromthesame
molecularmaterial.Inaddition,theymayhavetriggeredtheformationofa secondgenerationof
starsthatnowweobserveasyoungobjectsdeeplyembeddedinthemoleculargas.Thisscenario
should be investigated further by searching for the missing intermediate-age stellar population,
A.Petriellaetal.:ThemoleculargasaroundG24.73+0.69
Fig.4.Emissionofthe13COintegratedbetween+38and+50kms−1.Thebluecontoursaretheradiocontin-
uumemissionat20cmoftheSNRG24.7+0.6takenfromtheMAGPIS.Theredcrossindicatestheposition
oftheLBVstarG24.73+0.69.
namelymassivestarsstillintheMSphase,whichformedtogetherwiththeLBVandthesupernova
progenitor.
4. Summary
Wehaveanalyzedthe13COemissioninthesurroundingsoftheLBVstarG24.73+0.69.Wehave
discoveredafragmentedmolecularshellinthevelocityrangebetween+39kms−1and+44kms−1,
whichdelineatestheinfraredbipolaroutershellofG24.Onthebasisofspatial,morphological,and
kinematiccoincidenceoffeatures,wesuggestthatthereisaconnectionbetweenthisIRshelland
themoleculargas.Wearguethatthemolecularshellformedfromtheinterstellarmaterialswept-up
bythe stellar windof the centralstar mainlyduringits MSphase. Theelongatedmorphologyof
the bipolar and molecular shells may be a consequence of the asymmetric stellar wind from the
centralmassivestar.Wehavealsodetectedmolecularemissionprobablyassociatedwiththeinner
infrarednebula,butobservationsof higherangularresolutionare neededto spatially resolvethis
emissionandestablishitsorigin.
WehavestudiedthestarformationactivityaroundG24anddiscoveredsevenYSOswithspec-
tralcharacteristicsofprotostars,projectedonthemolecularcloudthatinterestinglycoincideswith
the IRDC 024.789+0.633.Thus, this region harborsthe typical componentscommonlyfoundin
thevicinityofstarformingregions,namelyanIRDC, severalsolarmassesofmoleculargas,and
embedded YSO candidates. We propose that the birth of these young objects might have been
triggeredbytheexpandingstellarwindbubble.
FromthestudyofthedistributionofthemoleculargasinalargeregiontowardG24,wesuggest
a link betweenthe originofthe LBV star and the progenitorstar of the nearbySNR G24.7+0.6.
ThisSNR isalso interactingwith the neighboringmoleculargasandshowsstar-formingactivity
in its surroundings.We proposea scenario where both massive stars belong to a first generation
ofstarsthatformedfromthe same natalMC, pointingto thepossibilityof findingothermassive
A.Petriellaetal.:ThemoleculargasaroundG24.73+0.69
starswitha commonorigin.Thestrongstellarwindsofthese massivestarsmighthavetriggered
theformationofasecondgenerationofstars,whichnowappearasprotostarsdeeplyembeddedin
themoleculargas.
Acknowledgments
We wish to thank the anonymous referee whose comments and suggestions have helped to im-
provethepaper.A.P.isadoctoralfellowofCONICET,Argentina.S.P.andE.G.aremembersof
theCarreradelinvestigadorcient´ıficoof CONICET, Argentina. This research was partially sup-
ported by Argentina Grants awarded by CONICET, ANPCYT and University of Buenos Aires
(UBACYT).
References
Allen,L.E.,Calvet,N.,D’Alessio,P.,etal.2004,ApJS,154,363
Cappa,C.,Niemela,V.S.,Mart´ın,M.C.,&McClure-Griffiths,N.M.2005,A&A,436,155
Cappa,C.E.,Rubio,M.,Mart´ın,M.C.,&Romero,G.A.2009,A&A,508,759
Cappa,C.E.,Vasquez,J.,Pineault,S.,&Cichowolski,S.2010,MNRAS,403,387
Castro-Carrizo,A.,Quintana-Lacaci,G.,Bujarrabal,V.,Neri,R.,&Alcolea,J.2007,A&A,465,457
Cichowolski,S.,Pineault,S.,Arnal,E.M.,&Cappa,C.E.2008,A&A,478,443
Clark,J.S.,Crowther,P.A.,Larionov,V.M.,etal.2009,A&A,507,1555
Clark,J.S.,Egan,M.P.,Crowther,P.A.,etal.2003,A&A,412,185
Clark,J.S.,Larionov,V.M.,&Arkharov,A.2005,A&A,435,239
deJager,C.1998,A&ARev.,8,145
Fich,M.,Blitz,L.,&Stark,A.A.1989,ApJ,342,272
Giacani,E.,Smith,M.J.S.,Dubner,G.,&Loiseau,N.2011,A&A,531,A138+
Humphreys,R.M.&Davidson,K.1994,PASP,106,1025
Humphreys,R.M.,Lamers,H.J.G.L.M.,Hoekzema,N.,&Cassatella,A.1989,A&A,218,L17
Jackson,J.M.,Rathborne,J.M.,Shah,R.Y.,etal.2006,ApJS,163,145
Jiang,B.,Chen,Y.,Wang,J.,etal.2010,ApJ,712,1147
Jime´nez-Esteban,F.M.,Rizzo,J.R.,&Palau,A.2010,ApJ,713,429
Maeder, A. 1989, in Astrophysics and Space Science Library, Vol. 157, IAU Colloq. 113: Physics of Luminous Blue
Variables,ed.K.Davidson,A.F.J.Moffat,&H.J.G.L.M.Lamers,15–23
Meaburn,J.,Boumis,P.,Redman,M.P.,Lo´pez,J.A.,&Mavromatakis,F.2004,A&A,422,603
Mellema,G.&Lundqvist,P.2002,A&A,394,901
Meynet,G.&Maeder,A.2000,A&A,361,101
Mokiem,M.R.,deKoter,A.,Vink,J.S.,etal.2007,A&A,473,603
Nota,A.,Livio,M.,Clampin,M.,&Schulte-Ladbeck,R.1995,ApJ,448,788
Nota,A.,Pasquali,A.,Marston,A.P.,etal.2002,AJ,124,2920
Petriella,A.,Paron,S.,&Giacani,E.2008,BoletindelaAsociacionArgentinadeAstronomia,51,209
Petriella,A.,Paron,S.,&Giacani,E.2010,BoletindelaAsociacionArgentinadeAstronomia,53,221
Prinja,R.K.,Barlow,M.J.,&Howarth,I.D.1990,ApJ,361,607
Rathborne,J.M.,Jackson,J.M.,&Simon,R.2006,ApJ,641,389
Rathborne,J.M.,Simon,R.,&Jackson,J.M.2007,ApJ,662,1082
Reich,W.,Furst,E.,&Sofue,Y.1984,A&A,133,L4
Rizzo,J.R.,Jime´nez-Esteban,F.M.,&Ortiz,E.2008,ApJ,681,355
Robitaille,T.P.,Whitney,B.A.,Indebetouw,R.,&Wood,K.2007,ApJS,169,328
Robitaille,T.P.,Whitney,B.A.,Indebetouw,R.,Wood,K.,&Denzmore,P.2006,ApJS,167,256
Simon,R.,Jackson,J.M.,Clemens,D.P.,Bania,T.M.,&Heyer,M.H.2001,ApJ,551,747
Smith,N.&Owocki,S.P.2006,ApJ,645,L45