The Morphology of M17-UC1 - A Disk Candidate Surrounding a Hypercompact HII Region

0.17 MB·English

by  M. Nielbock

DRAFTVERSIONFEBRUARY5,2008
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
THEMORPHOLOGYOFM17-UC1–
ADISKCANDIDATESURROUNDINGAHYPER-COMPACTHIIREGION1
M.NIELBOCK,R.CHINI,V.H.HOFFMEISTER,ANDC.M.SCHEYDA
AstronomischesInstitut,Ruhr–UniversitätBochum,Universitätsstraße150,44780Bochum,Germany
J.STEINACKER
Max–Planck–InstitutfürAstronomie,Königstuhl17,69117Heidelberg,Germany
AstronomischesRechen-InstitutamZentrumfürAstronomieHeidelberg,Mönchhofstraße12-14,69120Heidelberg,Germany
7 D.NÜRNBERGER
0 EuropeanSouthernObservatory,AlonsodeCordova3107,Vitacura,Santiago,Chile
0
2
R.SIEBENMORGEN
n
EuropeanSouthernObservatory,Karl-Schwarzschild-Straße2,85748Garching,Germany
a
DraftversionFebruary5,2008
J

6 ABSTRACT
2
Weinvestigatethemorphologyandtheevolutionarystageofthehyper-compactHIIregionM17-UC1using

observationsatinfraredwavelengthsandNIRradiativetransfermodelling. Itisforthefirsttimeresolvedinto
6
twoemissionareasseparatedbyadarklanereminiscentofanobscuringsilhouettecausedbyacircumstellar
v
9 disk. The observationaldata as well as model calculationsso far suggest that M17-UC1 is surroundedby a
5 diskofcooldust. Thisdirectdetectionofacircumstellardiskcandidatearoundahyper-compactHIIregionis
5 inagreementwiththeexpectationsofthediskaccretionmodelforhigh-massstarformation.
0 Subjectheadings:stars: formation,circumstellarmatter,pre-mainsequence–openclustersandassociations:
1 general,individual(M17)
6
0
h/ 1. INTRODUCTION Inthepresentstudy,weinvestigatedthehyper-compactHII
region(HCHII)M17-UC1(seeSect.1.2fordetails). HCHII
p 1.1. Generalmotivation
- arecommonlyassociatedwiththeearlieststagesofhigh-mass
o The hypothesis that high-mass stars form by accretion is starformation(Kurtz2000;DePreeetal.2005).Theycanbe
r supported by a number of indirect indicators like rotating explainedasaphaseinearlystellarevolution,whenthehigh-
t
s molecularenvelopescombinedwithbipolaroutflows,gasin- mass star begins to ionise its molecular accretion flow. In
a fallormaseremissiontowardscandidatehigh-massprotostel- suchastage,ionisationandaccretioncanco-exist(Keto2003,
v: lar objects (HMPOs) (e. g. Ho&Haschick 1986; Ketoetal. 2006). Weusednear-IRobservationsinordertoi)overcome
i 1987, 1988; Cesaronietal. 1997, 2005; Zhang&Ho 1997; thestrongextinctiontowardsthesource,andii)utiliseadap-
X
Zhangetal. 1998; Beltránetal. 2004, 2005; Keto&Wood tiveopticstechniquesallowingforhighspatialresolution.As
r 2006). Although it seems plausible that such rotating and aresult,wefindabipolarstructurereminiscentofasilhouette
a
collapsing configurations will eventually form a disk, the diskagainstabrightbackgroundascommonlyfoundforlow-
involved observations usually lack the spatial resolution to mass protostars. Starting from this hypothesis, we perform
firmly verifysuch a claim. Presently, there is noconvincing radiativetransfermodellingtoverifythisclaim.
exampleofadirectdetectionofacircumstellardiskarounda
HMPOthatcanbestudiedtoinferthedetailsoftheaccretion 1.2. Theobject:M17-UC1
process including the ionisation effects. Even the currently
M17-UC1 was initially discovered as a cometary ultra-
mostpromisingcandidate,IRAS20126+4104,mustbeques-
compact H II region by Fellietal. (1980, 1984), who sug-
tioned:Sridharanetal.(2005)reportabipolarstructure,sim-
gested a B0 - B0.5 ZAMS star as the ionising source pos-
ilartowhatwepresentinthisletter. However,Cesaronietal.
sessing a modelled Lyman continuum photon flux of 2
(2005)recentlyreviseditsmasstoavaluethatliesjustbelow 1047s- 1. It was later re-classified as a HCHII with≃broa×d
what is commonly accepted as “high mass”. Furthermore, ( 35kms- 1) radiorecombinationlinesanda risingspectral
Hofneretal. (1999) explained their detection of radio con- in≥dex of +1 between 1.4 and 43GHz (Johnsonetal. 1998;
tinuumemission asshockedgascausedby a protostellarjet,
Sewiloetal. 2004). A number of class II methanol masers
althoughtheyalsodiscussedfree-freeemissionfromionised
(Menten 1991) and hydroxyl masers (e.g. Churchwelletal.
gasasapossiblesource.
1990; Caswell 1997; Walshetal. 1998; Forster&Caswell
1999) are located in the vicinity of M17-UC1 being typical
Electronicaddress:[nielbock,chini,vhoff,scheyda]@astro.rub.de
Electronicaddress:[email protected] forthesurroundingsofhigh-massprotostars.
Electronicaddress:[dnuernbe,rsiebenm]@eso.org Withinthepositionalaccuracyof5.′′7,Harperetal.(1976)
1Based on observations made with ESO telescopes at the La Silla and
detecteda strongcompact10.6µm source(IRc2)coinciding
Paranal observatories under programme IDs 71.C-0185(A), 71.C-0353(A),
73.C-0170(A),73.C-0407(A),75.C-0418(A),77.C-0174(A) with M17-UC1 with an SED suggesting the presence of hot
dust. The IR flux was assumed to originate entirely from
2
FIG. 1.—M17-UC1asseenbyNACO(KsandL′)andVISIR(SiC/N). Thecentralpositionis18:20:24.82,- 16:11:34.9(J2000),thefieldsizeis10′′×10′′.
The3σcontouroftheKsbandfluxissuperimposedonallimages.ThelocationsofassociatedOH(×)andclassIImethanolmasers(+)areindicatedintheKs
bandimage. Anenlargement(1.′′1×1.′′1)ofthesourceisshownintheupperrightcorner. TheinsetintheVISIRimageillustratestheintensitydistribution
acrossthebrightinneremission.Blackcontoursatarbitraryintensityvaluesareshownforenhancement.ThelowerrightpanelisaRGBcodedcompositeofthe
threeimages(blue:Ks,green:L′,red:N).
M17-UC1 (Fellietal. 1984). When Felli&Stanga (1987) sourceswith the detectionsof the ISAAC data that were as-
observed M17-UC1 from 1.25 to 18.1µm, they also found trometricallycalibratedusingthe2MASSdatabase.Withthis
anunresolvedIRsourcewhichtheyattributedtotheHCHII. procedure,weestimatearelativeastrometricaccuracyofbet-
However,they used the strongestK band peakas their posi- terthan 0.′′1.
±
tionalreference,whichinfactisnotM17-UC1butthenearby TheTIMMI2(Reimannetal.1998)NIR/MIRimagingwas
IR-bright star IRS5 (Chini&Wargau 1998). It is dominat- carriedoutduring3observingrunsinJuly2003,April2004,
ingtheemissionatJHK. AdditionalIRimagingwascarried andJuly 2005at the ESO 3.6m telescopeatLa Silla, Chile.
outbyGiardetal.(1994),Chinietal.(2000)andJiangetal. TheobservationscoveredtheL,M,N1,N10.4,N11.9andQ1
(2002). Kassisetal. (2002) characterised the source as a bands. Astandardchoppingandnoddingtechniquewasused
ZAMS B0 type surroundedby a shell of 0.6- 3.4M and a withanamplitudeof10′′. Alldataarelimitedbydiffraction
⊙
radiusoflessthan6600AU. withaFWHMof0.′′7.
M17-UC1wasimagedwithVISIR(Lagageetal.2004)in
2. OBSERVATIONSANDCALIBRATIONS May2006throughtheSiC filter. Thepixelscale was0.′′127
The JHKsL′ adaptive optics imaging was carried out resultinginafield-of-viewof32.′′5 32.′′5. Astandardchop-
×
in June 2003 using NAOS/CONICA (Lenzenetal. 2003; pingandnoddingtechniquein perpendiculardirectionswith
Roussetetal. 2003) at the ESO VLT. The pixel resolution throw amplitudes of 15′′ was applied. The measured image
was 0.′′027, the limiting magnitudes are J =20.6, H =19.3, quality(FWHM)of 0.32′′islimitedbydiffraction.Theas-
≃
Ks = 18.4 and L′ = 16.2. The photometric calibration was trometric calibration was achieved by fitting transformation
obtained from our ISAAC observations in September 2002 equations between the NACO L′ data and the VISIR data
(Chinietal. 2004; Hoffmeisteretal. 2006). We adjusted the after identifying several point-sources visible in both wave-
astrometry in the JHKsL′ images by referencing the NACO bands. Image restoration of the TIMMI2 and VISIR data
ThemorphologyofM17-UC1 3
wasachievedwiththesoftwareMOPSI(maintainedbyR.W. 106 10 Wavelength [m m] 1
Zylka,IRAM,Grenoble,France).Theoverallastrometricac-
105
curacy is demonstrated by the RGB coded KLN composite
imagepresentedinFig.1. 104
y]
TheKbandspectroscopywasobtainedinAugust2004with mJ 103
aIaSbsAsloiAtrpwCtii(odMnthofeooafrwt0uo.r′′eo3d.w1aT9sh9ep7e)srpafoetcramtrsopesdeccowtrpiaytlhroeTfsIotMhlueMtiNoIn2boaafnt1dth5se0il0EicaSanOtde Flux density [ 110021 TNIAMOMSI/C2 OspNeIcCtrAum (2 (020030)3) 12 W11avelen1g0th [mm9] 8
3.6m telescope at La Silla, Chile in July 2003. The seeing 100 TIMMI2 (2003)
was0.′′7;theslitwidthwas1.′′2. 10-1 TTKIIaMMssMMis II(221 9((22900900)45))
VISIR (2006)
3. RESULTS 10-12013 1014 1015
3.1. Morphology Frequency [Hz]
FIG. 2.—SEDofM17-UC1. Thedatawereobtainedbetween1999and
Fig. 1 displays the morphology of M17-UC1 at NIR and
2006.TheredsolidlineistheMIRspectrumofthesilicateabsorptionfeature
MIR wavelengths. On our images, M17-UC1 is barely vis- at9.7µm. Theinsetisazoomintothespectrum. Acontinuum fitoftwo
ible at H with a brightness of 17.83 0.25mag. Surpass- black-bodycomponentswithformalcolourtemperatures ofT =425Kand
± 225Kisshownasadottedline.
ing previousNIR studies, we resolve the source for the first
time into two K band emission blobs, separated by 0.′′46
at a position angle of 126◦ measured clockwise from north
4. DISCUSSION
to south. A dark lane separates the two K band nebu-
4.1. Observationalevidenceforadisk
lae (Fig. 1). The south-western emission has an elliptical
shape of 0.′′9 0.′′5 (3σ contour) and a pronounced peak at The symmetric absorption pattern of M17-UC1 in the
18:20:24.83,-×16:11:35.0(J2000)with a FWHM (fullwidth K band along with the perpendicularly scattered light
at half maximum)of 0.′′19 0.′′13. The north-easternemis- nebulosities on both sides very much resembles the ap-
sionat18:20:24.85,- 16:11×:34.7(J2000)ismorediffuseand pearance of young low-mass stars with circumstellar
has a size of 0.′′8 0.′′5. The integrated brightness of both disks (e.g McCaughrean&O’dell 1996; Padgettetal. 1999;
×
blobswithin a radius of 3600AU is Ks=13.1mag, their in- Brandneretal. 2000; Grossoetal. 2003). This view is sup-
tensityratiois10:1(SW:NE).AtLband,thesourceattainsa ported by the morphology of theoretically derived synthetic
sphericalshapewitharadiusof 1.′′0(3σcontour)andanL′ imagesof protostellardisks presentedby Starketal. (2006).
∼
brightnessof6.1magwithin3600AU. Theseimagesarebasedonvaryingdiskradiusandaccretion
On previous MIR images (Chinietal. 2000; Kassisetal. rate,amongwhichonecanfindsuitingrepresentationsofthe
2002),M17-UC1appearedasapoint-likesourcewithspheri- Ks image of M17-UC1. In addition, the class II methanol
callysymmetriccircumstellaremission.OurnewNbanddata masers that are located in the general direction of the sup-
resolve M17-UC1(see inset in Fig. 1). The peak brightness poseddisksupportourconjecture.
is centred on the south-western K band peak, and shows an Apart from the almost uniformly extended L and N band
elongationwithaFWHMof0.′′7 0.′′5towardsnortheastbe- emission one would expect to see the supposed disk to be
×
ingcompatiblewithwithnorth-easternK bandpeak. Wede- emittingstronglyintheMIR,too.Nevertheless,theremustbe
riveafluxdensityof29.87 0.13Jyforthecentralcompact cooldustinside thedisk aswitnessed bythe silicate absorp-
source. Thetotalcircumste±llaremission( 5′′ 3′′) isrela- tion feature. We interpretthe elongatedshape of the central
∼ ×
tively complex with a noticeable extent to the northwest. It MIR emission perpendicular to the disk orientation as scat-
addsanother3.52Jytothetotalfluxdensity. teredlight fromthe centralsource. Due to the lower optical
depth,however,theMIRdiskemissionisveiledbythesilicate
3.2. Spectralappearance absorption and scattered MIR photons. At larger distances
from the HCHII, we see a warmer and less dense envelope
Our K band spectrum (not shown) of M17-UC1 includes
stronglyemittingintheMIR.
both nebulae and shows an extremelyred continuumcaused
Interestingly, stellar MIR emission is not necessarily at-
by hot dust. Apart from the Brγ line emission, which most
tributedtodisks,butcanalsooriginatefromthewallsofout-
likely originates from scattered photons emitted by the gas
flowcavitieswheretheambientinterstellarmediuminteracts
insidetheHCHII,thespectrumappearsfeatureless.
with the gas stream (DeBuizer 2006, 2007). In fact, the K
In the N band, M17-UC1 displays a deep silicate feature
band nebulae could also be dominated by emission from an
(Fig.2);itsexistencewasalreadysuggestedbyearlierphoto-
outflow with a wide openingangle. In this case, the narrow
metricdata(Harperetal. 1976;Kassisetal. 2002). Thisab-
waist would indicate a small disk that restricts the outflow
sorptionindicatesthepresenceofcooldustalongthelineof
inthese directions. However,ourK bandspectrumdoesnot
sighttowardsM17-UC1. Usingthestandardrelationforcon-
showanyhintsforshocksoroutflows,sowediscardthisin-
vertingopticaldepthsofτ 17 τ (Krügel2003),weinfer
V ∼ · 9.7 terpretation.
avisualextinctionofabout40mag. Theluminositybetween
1.6and20µm correctedforextinctionis1.1 104L being
⊙
× 4.2. Modelcalculations
consistentwithanearlyB-typestar. Allourbroad-bandpho-
tometric data from previous epochs that were obtained with We have modelled the 2.2µm radiation with a star-disk
differentfilterstracethe shapeofthe silicate absorptionfea- system using a 3D radiative transfer code described by
ture fairly well. This rules out possible MIR flux variations Steinackeretal.(2006). Thestar wasassumedtohaveT =
eff
thatweresuggestedbyNielbocketal. (2001). Althoughour 30000K.Thediskhasaradialpower-lawdensityprofilewith
broadN bandimagingfilteralsoincludesthesilicatefeature, an exponential atmosphere above and below the disk mid-
weonlyseetheextendedwarmemittingdustinFig.1. planelike
4
(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1) flif(inaaFgvitinhegontt.usgnr3ietot)ynh.rteerhraTd-ateihiloslaeyksrbteemeexfrtoontwerdenepe,edelnstaoh5kftv.ehaecreCrrtahaOstndhteNrnrieoCaoonfitLtiugUvlbateSsemaIoOlteurorNeantnhpntg-rsmwoftehdoereudsctecfieelalr.dlancmuspaleeatnaitstik,ofaaancnnstddoarttlihhsloyee
DFisIkG.m3o.d—eliMncoldineeldfitatof30M◦1a7s-UdiCsc1u.sLseedft:inOtrhieginteaxlt.KsRbigahntd:iSmcaagttee.reCdenlitgrhe:t We have presentednew NIR and MIR observationsof the
imageat2.2µmfromafilamentinfrontofaB0star. Thefilamenthasa hyper-compact HII region M17-UC1. As a prominent fea-
diameterofabout3000AUandadistanceof5000AUtothestar,thedust ture, the Ks band image shows a dark lane in scattered light
propertiesareidenticaltothoseofthediskmodel. producingasubstantialsilicateabsorptionfeatureobservedin
theMIR.Analysingtheimagewithradiativetransfermodels,
α
r z 2 wefindthebestagreementbyassumingadisk-likestructure
n(r,z)=n exp - (1)
0 around the central source instead of a foreground filament.
(cid:18)r0(cid:19) (cid:20) (cid:16)hr(cid:17) (cid:21)
Forthisreasonandbecauseofthe"highmass" natureofthe
with r = √x2+z2, where n is the number density normal- embeddedstarasdeterminedbymanyindependentmeasure-
0
isation, h the scale height, r the inner radius, and r the ments,wesuggestthatM17-UC1mightbethefirstdoubtless
0 1
outer radius where the disk vanishes. The K band image candidateof a HCHII where at least parts of a circumstellar
withthetriangularabsorptioniswellrepresentedbyparame- diskarestillpresent.
tersn =104m- 3, α=1.3,h=0.2,r =20AU,r =1000AU
d 0 1
(Fig.3)andstandard0.12µm-sizedsilicatedustparticleswith
opacitiestakenfromDraine&Lee(1984). ThisworkwaspartlyfundedbytheNordrhein-Westfälische
Theinclinationofthediskisabout30◦(edge-on=0◦). We Akademie der Wissenschaften. M. N. acknowledges the
obtain a lower mass limit of 4 10- 4M from the scattered support by the Deutsche Forschungsgemeinschaft, project
⊙
×
lightthatonlyprobesthesurfaceofthedisk. Asimilarmass SFB 591 and the Ministerium für Innovation, Wissenschaft,
limitisderivedfromtheextinctionthatblocksthestellarlight Forschung und Technik (MIWFT) des Landes Nordrhein-
at 2.2µm. Of course, more mass can be hidden in the inte- Westfalen. WethankE.Churchwell,H.BeutherandM.Haas
riorofthediskwithoutaffectingtheappearanceoftheobject. for helpful discussions. Likewise, we thank the referee
Likewise,foramildlyedge-ondisk,additionaldustcouldre- E.Keto forhis exemplaryandvaluablesuggestionsthatsig-
sideoutsidethescatteredlightpattern. nificantly improvedthe manuscript. This publication makes
We havealsocarriedoutradiativetransfercalculationsfor use of data products from the Two Micron All Sky Survey,
the alternative scenario of a dusty filament that is unrelated whichisajointprojectoftheUniversityofMassachusettsand
to M17-UC1. We modelled dust configurations of differing theInfraredProcessingandAnalysisCenter/CaliforniaInsti-
sizes and masses located at various distances in front of the tute of Technology,fundedby the NationalAeronauticsand
star. In contrast to the observed morphology, the scattered SpaceAdministrationandtheNationalScienceFoundation.
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