Discovery of two new methanol masers in NGC 7538

0.08 MB·English

by  M. Pestalozzi

1
Wereportontheserendipitousdiscoveryoftwoadditional
6.7GHz methanol masers in the same region, not associated
withIRS1.Interferometrymaserpositionsarecomparedwith
recentsingle-dishandinterferometrycontinuumobservations.
The positions of the masers agree to high accuracy with the
1.2mm continuum peak emission in NGC7538 IRS 9 and
NGC7538S.Thisclearassociationisalsoconfirmedbythepo-
sitionalagreementofthemaserswith existinghighresolution
continuumobservationsatcmand/ormmwavelengths.Making
useoftheestablishedstrongrelationbetweenmethanolmasers
andhigh-masstarformation,weclaimthatwehaveaccurately
positioned the high-mass protostars within the regions where
they are detected. The variety of objects hosting a 6.7GHz
methanol maser in NGC7538 shows that this emission prob-
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0 ablytracesdifferentevolutionarystageswithintheprotostellar
0 phase.
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n Key words. star formation – massive stars – Interstellar
a medium–masers
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Astronomy&Astrophysicsmanuscriptno.ngc˙FG15 February5,2008
(DOI:willbeinsertedbyhandlater)
Discovery of two new methanol masers in NGC7538
Locating of massive protostars
M.R.Pestalozzi1,V.Minier2,3,F.Motte3,andJ.E.Conway4
1 SchoolofPhysics,AstronomyandMathematics,UniversityofHertfordshire,AL109BSHatfield,UK,
e-mail:[email protected]
2 Serviced’Astrophysique,DAPNIA/DSM/CEASaclay,91191Gif-sur-Yvette,France
3 AIM,UMR7158,CEA-CNRS-Universite´ParisVII,CEA/Saclay,91191GifsurYvette,France
4 OnsalaSpaceObservatory,43992Onsala,Sweden
InpreparationforsubmissiontoA&A
Abstract.NGC7538isknowntohosta6.7and12.2GHzmethanolmasercospatialwithaUltraCompact(UC)Hregion,
IRS1.
1. Introduction cursorsofOBassociations(e.g.Motteetal.2003;Minieretal.
2005).Highangularresolutionobservationsofmethanolmaser
TheNGC7538nebulaisafurnaceofintensemassivestarfor-
environmentsrevealthat these masersare associated with hot
mation located at some 2.7 kpc from the Sun. At least 11 in- cores,UCH regionsandveryoftendonotcoincidewith ra-
fraredsourceshavebeenidentifiedembeddedwithinit(Fig.1, dio continuum emission from bright Ultra Compact (UC)H
Ojhaetal. 2004 and referencestherein). These IR sources lie
regions (Walshetal. 1998; Minieretal. 2001). The detection
infourregionshostingalargerangeofdifferenttypesofyoung
ofamethanolmaserinsuchregionsisaclearindicationofthe
stellar objects (YSOs) with signs of decreasing age moving
presenceofadeeplyembeddedhotcorethatischaracterisedby
fromthe North-Westto the South-East.In the far North-West
largemethanolabundanceand100-200Kgasanddusttemper-
thefirststarformationregioncontainstheIRsourcesIRS5,6,
ature. These physical conditions are in agreement with those
7whichareassociatedwithadevelopedHregionof∼3pcin
derivedfrommasermodelling,ase.g.Sobolevetal.1997.The
size.IRS6hasbeenproposedasitsexcitingsource(Ojhaetal.
high astrometric accuracy of VLBI observations of methanol
2004). A further star formation region can be identified with
masersgivesthusthebestestimateofthepositionofthehigh-
the IR sources IRS 1-3, where infrared emission at K-band
massprotostellarcoreatascaleof∼10AU.Oneclearexam-
and long-ward is dominated by IRS 1 (DeBuizer&Minier
pleofthisfactisgivenbythemaincomponentoftheCH OH
2005).IRS1isknowntopoweraUltraCompact(UC)Hre- 3
maser in NGC7538 (component A in Fig. 2): the maser pin-
gion, an early stage of massive star formation.Masers of dif-
points the massive protostellar core within the IRS 1 submm
ferentspecieshavealsobeendetectedtowardthissource(OH,
clump to milliarcsecond accuracy (1mas at ∼3kpc is ∼3AU,
H O,CH OH,see Minieretal.1998andreferencestherein).
2 3 seePestalozzietal.2004).
A third star-forming region is centered on NGC7538 S and
In this letter we report on the serendipitous detection of
alsocontainsIRS11,10′′ toitsnorthwest(Fig.1).Coincident
6.7GHzmethanolmasersinthethirdandfourthstarformation
with NGC7538 S OH and H O masers have been detected
2 regionsin NGC7538, cospatialwith NGC7538 S and IRS 9.
(Kameyaetal. 1990). Finally, furthest to the South-East, the
Supportedbywhatexposedabove,wearguethatwiththisdis-
fourthknownregionofstarformationcontainsthedeeplyem-
covery we have accurately positioned the two massive proto-
beddedIRsourceIRS9, believedto bein a stage priorto the
starsinthoseregions.Thisisthenconfirmedbythecoincidence
formationofaUCHregion.H Omaserswerealsodetected
2 of the new maserswith the peakof the 1.2mm dust emission
towardthissource(Davisetal.1998;Kameyaetal.1990).
comingfromthethickcocoonsurroundingthem.
Methanol masers are indicative of the earliest stages of
massive star formation. They arise from cold, embedded
molecularclumpsandinsomecasesfromdarkinfraredclouds 2. Observationsanddatareduction
(Hilletal. 2005; Purcelletal. 2005). These clumps are often
The data used for this paper was taken in two runs, with the
interpreted to be protoclusters of high mass YSOs, i.e. pre-
principalaimofstudyingthe known6.7GHzmethanolmaser
Sendoffprintrequeststo:M.Pestalozzi,[email protected] emissiontowardNGC7538(i.e.spectralcomponentsA-E,see
Pestalozzietal.:DiscoveryoftwonewmethanolmasersinNGC7538 3
Fig.2).Asexplainedbelow,theresultspresentedherereferto
Fig.1.GeneralviewoftheNGC7538region.Left:Theunder-
theserendipitousdiscoveryoftwonewmaserfeatures,unseen
lyingcolourimageisaRGBimageofJ,H,KNIRbandsmade
untilnowinanyinterferometryexperiment.
fromthe2MASSarchive.Thegreycontours(2,5,10,15,20,
The first data set (run1, November 4th-5th 2004) was ob- 30,40,50,60,70,80,90%ofpeakemission,∼4Jybeam−1)
tainedduringajointexperimentincludingtheEuropeanVLBI show1.2mmdustcontinuummappedwithMAMBO-2/IRAM-
Network1andthethreeantennasoftheMERLIN2arraywhich 30m. White circles indicate the location of the identified in-
were then operational at 5cm wavelength, Cambridge (CM), frared sources. White crosses indicate the 6.7 GHz CH OH
3
Knockin(KN)andDarnhall(DA).Thespectralresolutionwas maser positions. Right: The underlying image is the 1.2 mm
1.9 and 3.9 kHz/chan (or 0.087 and 0.175 kms−1per chan- continuummap.1arcminis0.78pc.
nel,respectively)over512channelsforthejointEVNandthe
MERLINonlydata,respectively.OnlytheMERLINportionof
thisdatasethassofarbeenreducedandispresentedhere.The
seconddataset(run2,December11th-12th2004)wasa13hour
3. Results
MERLIN only experiment, including 5 antennas: CM, KN,
DA,the25-mJodrellBankMk2antenna(JB)andTabley(TA). 3.1.CHOH masersfeaturesF&G
3
The spectral resolutionwas 0.976kHz/chan(0.044kms−1per
channel) over 512 channels. In both runs the telescopes were Inthefollowing,wedenotethetwonewspectralfeaturesfound
pointed,andthedatacorrelated,atthepositionofthebrightest during data reduction as F and G3. Their positions and other
methanolmaserfeatureinIRS1,featureAinFigure2(seealso propertiesaresummarisedinTable1.
Table1). The comparison of the cross correlated spectrum on the
shortest baseline with the integrated spectrum of the maser
Firstordercalibrationandtranslationintofitsformatwas
emission shows discrepancy in the peak flux of feature F &
performedwithtdprocatJodrellBank.Correctionstothepri- Gof∼20%(Fig.2).Unfortunately,theamplitudeerrorsmen-
maryamplitudecalibration,phase calibration(self calibration
tioned in Section 2 and the unreliable baseline subtraction in
onthebrightestemissionchannel)andimageproductionwere
the integratedspectrum introduceerrors in the amplitudethat
performed in AIPS. To locate all the maser emission, large
can be estimatedto 15%. Thesepreventusto invoqueresolu-
10x10 arcsec spectral cubes were made spanning the full ob-
tion as responsiblefor the missing flux. Our bestestimate for
servedvelocityrange.
thesizeofF&G(upperlimit)isabouthalfthefringespacing
Inthecrosscorrelationdatafromrun1(confirmedthenby oftheshortestbaseline,i.e.∼85mas,or260AU,whichistobe
theanalysisofthedatafromrun2)emissionwasdetectedatthe takenasanupperlimit.Notethatinthehigherspectralresolu-
velocitiesoftheknownspectralfeaturesbutalsounexpectedly tiondataofrun2(notshownhere),F&Gshowclearmultiplic-
insomeotherspectralchannels.Thelatterwaslargenonnoise- ity(atleasttwoandthreecomponentsforF&Grespectively).
likeside-lobeemission,indicatingthatmaseremissionatthese Observationspointedatthenewmasersarenecessarytoobtain
velocitiescamefromsourceslocatedoutsideofthemapregion. reliableandaccuratemapsofallfeatures.
Maps were then iteratively re-centereduntilthe new features, AreviewofarchivalEVNVLBIautocorrelationdataforall
werebroughtclosetotheimagecentre. antennasexceptEffelsbergshowsthatbothFandGareclearly
present in all experiments since 1997. These features always
Thelargepositionoffsetsofthenewfeaturesfromthecor-
havesimilarfluxratiostospectralfeatureA(withinavariation
related position cause large variations of the visibility phase
of the latter of some 10%). This indicates that the emission
overtheintegrationtime,andhenceintroduceamplitudeerrors
from F and G is probablynon-variable.The new features are
onthevisibilities.Thesedonotaffectfeaturepositiondetermi-
notdetectedintheautocorrelationsfromtheEffelsberg100m
nation,butpreventusfrommakingreliableimages.FeaturesF
antenna because they fall outside the HPBW of the primary
&Gshowedmaximalphasechangesof0.6and1.8radoverthe
integrationtimeof7s,producingmeanamplitudelossesof∼1 beamat5cm(i.e.1.6arcmin).Notethatdespitethereductionin
amplitudeduetotheprimarybeamattenuationahintoffeature
and8%respectively,onthebaselineinFigure2.
G waspresentin the discoveryspectrumforNGC7538taken
The positions of the masers were compared to those of withthe140footGreenBanktelescope(Menten1991).
1.2mmdustpeakemissionthatwereobtainedinaMAMBO-
2/IRAM-30m map. The image was obtained in January 2004
3.2.CHOHmasersinNGC7538IRS9&S
usingtheMAMBO-2bolometerarray(Kreysaetal.1998)in- 3
stalled at the IRAM 30 m telescope in the on–the–fly mode.
Figure 1 shows a general view of the NGC7538 region. The
TheresultingangularresolutionisHPBW∼11′′andtheabso-
CH OH masersareclearlyassociatedwiththemostembedded
lutepointingisaccuratetowithin∼3′′. 3
sitesofhigh-massstarformation:IRS1,IRS9andS.Masers
in IRS 1 coincide with the 1.2mm continuum peak emission
(withinthepositionaccuracy)tracingcolddustemissionfrom
1 TheEVNisajointfacilityofEuropean,Chinese,SouthAfrican massiveanddeeplyembeddedprotoclusters.
andothersradioastronomyinstitutes
2 The Multi-Element Radio Linked Interferometer Network is a 3 The nomenclature of all features follows the one adopted in
NationalFacilityrunbytheUniversityofManchester,UK Minieretal.(2000),basedonthespatialdistributionofthemasers
4 Pestalozzietal.:DiscoveryoftwonewmethanolmasersinNGC7538
350 newlydetectedmaserGmarksthedrivingsourceoftheoutflow
16 (togetherwiththecontinuum),whiletheothermasersarisein
300 14 theoutflowshocks.Thisissupportedbythegeometryproposed
12
250 Flux [Jy]180 ionutSflaonwdeflrloemtaIlR. S(2090p5o)lfeo-ornth.aTthseouprrcoeto:swteellamriogbhjteocbtsienrvIReSth9e
6
4 isaccuratelylocatedbothwiththeclassIImethanolmaserand
y]200 2 thecm/mmcontinuum.
ux [J 0−48 −49 −50 LO−S5 1veloci−ty5 2[km/s]−53 −54 −55
Fl150
4. Discussion
100
The presence of three methanol maser sources associated to
three different objects in the same star formation region of-
50
GG FF AA BB EE DDCC fers a unique opportunity to study the evolutionary details of
0 methanolmaserbearingsources.
−48 −50 −52 −54 −56 −58 −60 −62 The methanol maser phase is considered to be relatively
LOS velocity [km/s]
short as compared to the lifetime of the massive protostar, as
Fig.2. Cross correlated spectrum on the KN-DA baseline av- shortasafew104yr(vanderWalt2005).Thisimpliesthatthe
eraged over the whole observing run (dashed line) and auto- sourceshostingmethanolmasersat6.7GHzshouldshowvery
correlation spectrum from the DA antenna (solid line), both similarproperties.ThemethanolmasersinNGC7538(features
fromrun1.Minimalfringespacingon the KN-DA baselineis A,G&F)ontheotherhandseemtomarkmassiveyoungstellar
∼190mas.FeaturesA,B,C,DandEarelocatedwithin0.5arcsec objectsatdifferentstagesofevolution:anIR-quietprotostellar
ofIRS1.Theinsetisazoom-inofthevelocityregioninclud- object(NGC7538S), an IR-brightprotostellarobject(IRS 9)
ingfeaturesF&G. ThemultiplicityofF&Gisdiscussedin and an UCH region (IRS 1). The same distinction seems to
thetext. beshownbythefluxdensitiesofthemasers:300,15,5Jyfor
A,G,Frespectively.
We propose 2 scenarios for this apparent sequence. The
Maser feature F coincides with the 1.2mm dust contin- first and most intuitive is the evolutionary sequence. Feature
uum emission peak within ∼ 2′′ (Fig. 1). This is less than F mightmarkthe youngestobjectin theregion,featureA the
the position accuracy of the 1.2mm data (∼ 3′′), and there- most evolved but still within the protostellar phase. This sce-
fore we can state that they are cospatial. The methanolmaser narioisalsosupportedbythepresenceofdiscs/outflowsinthe
alsocoincideswithothersignpostsofhigh-massstarformation masersources:inFalargescalerotatingtorusisdetectedbut
in NGC7538 S as reported in the literature, within the posi- no outflow (Sandell&Sievers 2004); in G a powerful, prob-
tion accuracies (Sandell&Sievers 2004; Sandelletal. 2003; ably very young, outflow is mapped (Sandelletal. 2005); in
Kameyaetal. 1990). In Sandelletal. (2003) this protoclus- A a clear disc/outflow geometry is visible (e.g. Gaumeetal.
ter/cloudisstatedtobeoneofthemostmassiveoftheregion, 1995;Pestalozzietal.2004;DeBuizer&Minier2005).More
with 1000M⊙ estimated within a circle of 20′′(or 50000AU) evidencehastobecollectedonthecapacityofmethanolmasers
in diameter. The authors of that paper also report of a large toprovidesuchaccuracyintheagethesourceshostingthem.
rotating torus of some 14000AU in radius. Single grey–body The second scenario invokes geometry. The difference in
Spectral Energy Distribution (SED) fitting of the cold dust 1.2mm dust emission peak flux among the methanol maser
emission in the FIR/(sub)mm/mm emission with an aperture bearingsourcescouldbeduetodifferentviewinganglestothe
of 60′′ of this source gives a dust temperature of ∼35K and source. IRS 1 is seen edge-on, showing the thick dust in its
anemissivityindexβ = 1.6.Thissourceliessome10′′ tothe disc (Gaumeetal. 1995;Pestalozzietal. 2004). IRS 9 is sug-
southofIRS11,whereaCO outflowisinferredbythedistri- gestedtobeseenclosetopole-on(Sandelletal.2005):thedisc
bution of the magnetic field orientation(Kameyaetal. 1991). wouldbeseenalmostface-on,notcontributingsignificantlyto
On the basis of the above multiple evidence, we consider the the1.2mmflux.NGC7538Sisstilldeeplyembedded,sodust
positionof the methanolmaser to be the mostreliable forthe is close to uniformly distributed around the source. This in-
protostellarobjectinNGC7538S. terpretation seems to be more difficultto accept, because e.g.
Maser feature G is coincident within 1′′ with the peak IRS 1 suffers of significant contamination by the very close
of the 1.2mm dust emission. The position of G also agrees, IRS2andIRS3.
within the position inaccuracies, with the high resolution
positions of the peak emission of the continuum at 4.8
5. SummaryandConclusions
to 107GHz (vanderTaketal. 2000; vanderTak&Menten
2005). Furthermore, one water vapour maser spot as well as From the high spatial resolution MERLIN observations of
oneclassImethanolmaserspotseemalsotocoincide,within 6.7GHz methanol maser emission toward the massive star
thepositionalaccuracies,withfeatureG(Kameyaetal.1990; formingregionNGC7538wehavedetectedandpositionedtwo
Sandelletal.2005),althoughwithanoffsetinvelocityofsome newspectralfeatures.Wenotethatthisachievementwaspossi-
6kms−1. Because of the factthatclass I methanolmasersare bleonlythankstotheintermediateresolutionoftheMERLIN
mainlyfoundinoutflows(Kurtzetal.2004),wearguethatthe withitsshortestbaselines.Weconcludethefollowing:
Pestalozzietal.:DiscoveryoftwonewmethanolmasersinNGC7538 5
Table1.Position,lineofsight(LOS)velocity(v)andoffset(r) Sandell,G.,Wright,M.,&Forster,J.2003,ApJ,590,L45
fromfeatureAofthetwomasercomponentsFandG.Theun- Sobolev,A.,Cragg,D.,&Godfrey,P.1997,MNRAS,288,L39
certaintiesinthepositionsandinthevelocitywidthsare50mas vanderTak,F.&Menten,K.2005,A&A,437,947
(135AU)and0.044kms−1respectively. van der Tak, F., van Dishoeck, E., & Caselli, P. 2000, A&A,
361,327
Masers RA DEC v/FWHM r vanderWalt,D.2005,MNRAS,360,153
J2000 J2000 kms−1 arcmin/pc Walsh, A., Burton, M., Hylard, A., & Robinson, G. 1998,
A 231345.363 612810.55 –56.2/1.0 MNRAS,301,640
F 231344.86 612648.1 –53.2/0.5 1.36/1.06
G 231401.78 612719.7 –48.8/0.9 2.14/1.67
– onthebasisoftheacceptedideathatmethanolmasersare
the most efficient tracers of young and embedded mas-
sive protostars, the accurate location (to within 50mas or
135AU) of F and G is equivalent with the accurate lo-
cation of the massive protostars in NGC7538 IRS 9 and
NGC7538S;
– thevarietyofobjectsmarkedbymethanolmaseremission
could be due to different ages or viewing angle geome-
try.Totestthesetwoscenarios,moreaccurateobservations
(targetedtowardF&G)arerequired.
Acknowledgements. M.P.thanksTomMuxlowforhishelpduringthe
datacalibrationattheJodrellBankObservatory.Wethankallthepar-
ticipantsof the NGC7538 collaboration for the veryfruitful discus-
sionsthatcontributedtotheacheivementofthispaper.
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