A rotating disk around the very young massive star AFGL 490

0.23 MB·English

by  K. Schreyer

BASEDONOBSERVATIONSCARRIEDOUTWITHTHEIRAMPLATEAUDEBUREINTERFEROMETER.IRAMISSUPPORTEDBYINSU/CNRS(FRANCE),
MPG(GERMANY)ANDIGN(SPAIN).
PreprinttypesetusingLATEXstyleemulateapjv.6/22/04
AROTATINGDISKAROUNDTHEVERYYOUNGMASSIVESTARAFGL490
K.SCHREYER
AstrophysikalischesInstitutundUniversitätssternwarte,Schillergäßchen2–3,D–07745Jena,Germany
D.SEMENOV,TH. HENNING
Max–Planck–InstitutfürAstronomie,Königstuhl17,D–69117Heidelberg,Germany
AND
J.FORBRICH
6 Max–Planck–InstitutfürRadioastronomieBonn,AufdemHügel69,D-53121Bonn,Germany
0 (Accepted28.12.05atApJL)
0 BasedonobservationscarriedoutwiththeIRAMPlateaudeBureInterferometer.IRAMissupportedbyINSU/CNRS(France),MPG(Germany)andIGN(Spain).
2
ABSTRACT

n
Weobservedtheembedded,young8–10M starAFGL490atsubarcsecondresolutionwiththePlateaude
a ⊙
BureInterferometerintheC17O(2–1)transitionandfoundconvincingevidencethatAFGL490issurrounded
J
by a rotating disk. Using two-dimensionalmodeling of the physical and chemical disk structure coupled to
2
lineradiativetransfer,weconstrainitsbasicparameters. Weobtainarelativelyhighdiskmassof1M anda
1 ⊙
radiusof∼1500AU.Aplausibleexplanationfortheapparentasymmetryofthediskmorphologyisgiven.

1 Subjectheadings:circumstellarmatter—line:profiles—radiativetransfer—planetarysystems: protoplane-
v tarydisks—stars: formation—stars: individual:AFGL490
0
7
2 1. INTRODUCTION diativetransfermodeling.
1 The study of very young massive stars is of partic-
0 ular importance for star formation since it is not yet 2. OBSERVATIONSWITHTHEPLATEAUDEBURE
6 firmly established by what process, disk accretion or stel- INTERFEROMETER
0
larmerging,suchstarsform(e.g.,Yorke&Sonnhalter2002; WeobservedAFGL490inC17O(2–1)at224.714389GHz
/
h Dobbs,Bonnell,&Clark 2005). Previous authors have usingthePdBIinitsBconfigurationinDecember2003(base-
p demonstratedthatmassivestarscanbeformedviaaccretion, lines61.5m–330.5m).Thephasereferencecenterofourmea-
- leadingtotheformationoflarge(.10000AU)andmassive surementswasα =03h27m38s.55,δ =+58◦46′59′′.8,
ro (.1 M⊙) circumstellar disks. However, most of these stars andthesourceve2l0o0c0itywassettoVlsr=-201030.4kms- 1. Forthe
t are located more than 2 kpc away, which makesit challeng- line measurements, we used one correlator unit with a total
s
a ingtoconfirmunambiguouslythepresenceofaccretiondisks bandwidthof 20MHz and512channels(velocityresolution
: aroundsingleobjectswithradiointerferometryduetothelack of0.052kms- 1).Twoothercorrelatorunitswithabandwidth
v
ofspatialresolution. of320MHzeachwereutilizedtomeasurethecontinuum.
i
X We focus on AFGL 490 – one of the best-studied young The bandpass and phase calibration were performed on
stars that are in a transition stage to Herbig Be stars. This the objects 3C454.3 and 2145+067. The additional calibra-
r
a very young star is located nearby (1±0.3 kpc, Snelletal. tions of the phase and amplitude were obtained by observ-
1984), has a mass of 8–10 M , a bolometric luminos- ing the objects 0355+508 and 0224+671 every 20 minutes.
⊙
ity of ∼ 2×103 L (B2–3 star, Panagia 1973), is still ForthefinalphasecalibrationanddatareductiontheGreno-
⊙
embedded in its parent molecular cloud (A ∼ 40 mag, bleSoftwareenvironmentGILDASwasapplied.Themapsof
V
Alonso-Costa&Kwan1989),anddrivesahigh-velocityout- 512×512squarepixelswith0.1′′pixelsizewereproducedby
flow(Mitchelletal.1995). Interferometricmeasurementsby the Fourier transform of the calibrated visibilities using nat-
e.g. Schreyeretal. (2002, hereafter Paper I) as well as IR uralweighting. The synthesized beam size is 0.89′′×0.77′′
observations of Campbell,Persson,&McGregor (1986) and (=890AU×770AUat1kpc)withapositionalangleof73◦.
Bunn,Hoare,&Drew(1995)haverevealedthatthestarislo- Forreductionofthecontinuum,weexcludedthelineemission
catedinsideanionized∼100AUcavityclearedupbyafast fromthedata.Finally,thecontinuumwassubtractedfromthe
stellar wind and surroundedby a .500 AU disk-like struc- original data in the uv-plane, as it was detected only at the
ture, which is enshrouded in a 22000 AU × 6000 AU en- sourceposition.
velope. Allthesestructuresarefurtherembeddedinaneven
moreextendedenvelope. 3. RESULTS
InthisLetter,wepresentsubarcsecond-resolutionPdBIob-
3.1. Continuummeasurements
servationsofAFGL490thatprovetheexistenceofarotating
diskaroundAFGL490anddetermineitsorientation,size,and Within the 22′′ primaryPdBI beam at 1.3mm, AFGL 490
massbyusingcomprehensivephysical,chemical,andlinera- was detected point source with a total flux of 1.41 Jy (peak
valueis0.68Jybeam- 1). Thecontinuumisdominatedbythe
Electronicaddress:[email protected] thermaldust emission because at 1.3mm the free-freeradia-
Electronicaddress:semenov,[email protected] tioncontributeslessthan5%tothetotalflux(≈69±3mJy,
Electronicaddress:[email protected]
seeCampbelletal.1986).
2 Schreyeretal.
3.2. C17O(2–1)linemeasurements istheH abundancerelativetotheamountofC17Omolecules
2
TheintegratedC17O(2–1)lineintensitymapoverlaidwith (H2/CO=104, 16O/17O=2500),andD=1kpcisthe distance
tothesource.Themassesofthered-andblue-shiftedgasare
the 1.3mm continuum is shown in Fig. 1a. The continuum
isbarelyresolvedatthe5σ levelwithinthe.2000AUarea 1.0and0.6M⊙,respectively,givingatotalvalueof≈1.5M⊙
whichmatchestheestimateobtainedabove.
aroundthe star, and has a beam-like circular shape. In con-
trast,thelineintensitymaprevealsa∼4000AUarcofemis-
sionthatiscenteredonthecontinuumpeakandhasaPAof∼ 3.2.2. Diskvelocityfield
15◦.Furthermore,theC17Oemissionabovethe50%intensity The gas velocity distribution across the red- and blue-
level appears as a ≈ 700 AU × 2000 AU bar-like structure shiftedC17Oemission(solidlineinFig.1b)ispresentedinthe
with the main axis shifted by 400 AU to the northeast from position-velocity(PV)diagraminFig.2. ThePVmapshows
thecenter. anumberofsinglegasclumps,indicatinganinhomogeneous
The red- and blue-shifted parts of the integrated C17O diskstructure.Thisdistributionisnotcausedbyopticaldepth
(2–1) line intensity overlaid with an H-band speckle image effectsasdemonstratedbyradiativetransfercalculations(see
(Alvarezetal. 2004) and the 1.3mm continuum is presented Sect. 3.3). To fit the PV diagram, we adopt a simple model
inFig.1b.Bothlobeshavetheirmaximum700AUawayfrom of a Keplerian disk with a mass that linearly increases with
the 1.3mm continuum point source, and the line connecting radius(Vogeletal.1985)andastellarmassof8±1M . We
⊙
them is shifted 260 AU to the northeast(PA of the axis per- assume a disk radius of 1600 AU based on the spatial ex-
pendiculartotheconnectinglineis≈26◦). Duetothesimul- tent of the C17O emission and a disk mass between 0.5 and
taneousmeasurementofbothlineandcontinuuminthesame 2M⊙. Withthismodel,wefittheinnerclumps(R<±0′′.7)
bandpass,thepositionaloffsetiscertainlyreal. Remarkably, of the the PV map and obtain the best-fit inclination angle
the position of the red- and blue-shifted C17O (2–1) peaks of35◦±5◦ (Fig. 2, solid curve). The fit to the most intense
perfectlycoincideswiththepositionoftheCS(2–1)peaksre- clumps in the PV map at R=±0′′.7 is not as good. It gives
portedinPaperI.ThePAoftheC17Oemissionissmallerthan i=25◦±5◦ and super-Keplerian gas velocities in the inner
the∼45◦positionalangleofthe20000AUbar-likestructure disk. In contrastto massive disks aroundactive galactic nu-
(see Paper I), but the emission structure is orthogonalto the cleithatshowamodifiedKeplerianlawwithanexponentof∼
large-scale CO outflow (Mitchelletal. 1995) and coincides - 0.35 (e.g., Kondratkoetal. 2005), the correspondingcurve
withthePAobtainedfromtheinfraredpolarizedmapandthe forAFGL490(Fig.2,dottedcurve)wouldonlyfittotheouter
2cmradioemission(Campbelletal.1986;Haasetal.1992). diskpart. However,itdoesnotfittheinnerclumps. Further-
NotethattheC17Oemissionpattern(Fig.1b)ischaracteristic more, the four most intense clumps in the PV map around
ofoutflowsorrotatingdisk-likeconfigurations.Todistinguish - 12.6kms- 1 can be best fitted by a rotating gas ring with a
betweenthesetwopossibilities,weanalyzethecorresponding radiusof700AUandsimilari=35◦±5◦(dashedline). Such
position-velocitydiagrambelow. a small inclination angle is in contrast to the apparentC17O
morphologyimplyinganinclinationanglelargerthan60◦for
3.2.1. Diskmass any circularly-symmetric configuration, e.g. a disk (see el-
We estimate the mass of the AFGL 490 “inner disk” lipseinFig.1a).Howeverrotationcurvesforanydiskorring
from the 1.3mm continuum emission, using Eq. 2 from modelwithlargerinclinationangles,i>40◦,canonlybewell
Henningetal. (2000). We assume the standard gas-to-dust fittedassumingalow-masscentralstar,M⋆≤2M⊙ whichis
mass ratio of 100, a mean disk temperatureof 100 K, and a definitelynotthecaseforAFGL490. Thus,weproposethat
dust mass absorption coefficient κd at 1.3mm between 1.99 thereisamassive,clumpydiskaroundAFGL490withnearly
and 5.86 cm2g- 1 (grains without imcy mantles, gas densities face-onorientation(i∼30◦)andKeplerianrotation.
106 and 108 cm- 3, respectively, see Ossenkopf&Henning
1994). The resulting H gas mass is between 0.8 and 3.3. Diskmodeling
2
2.3M⊙ and,thus,thesource-averagedH2 columndensityis ThePdBI C17O (2–1)spectralmapof the AFGL 490disk
N(H2)(1.3mm)=1÷20·1023cm- 2. is presented in Fig. 3. We fit these spectra using the “step-
ThegasmasscanbealsoderivedfromtheC17Odata. Due by-step”modelingapproach(Semenovetal.2005,fordetails
to missingflux of ∼30%in the vicinityof AFGL 490, even see Semenov et al. 2006, in preparation). Briefly, we apply
in the reverse map side of the red- and the blue-shifted line the2Dflared-diskmodelofDullemond&Dominik(2004)to
emission, the total integrated flux value is smaller than the simulatethediskphysicalstructure,agas-grainchemicalnet-
sumofindividualcontributionsfromthered-andblue-shifted workto calculatetime-dependentabundances,anda 2Dline
emission.Thesidelobesofthesynthesizedbeamarenotcon- radiativetransfercodeofPavlyuchenkov&Shustov(2004)to
tributingtothelowintegratedfluxesduetotheirlowintensi- synthesize the C17O spectra. We assume that the disk has
ties(<20%). Thus,wefocusonthesumofthered-andthe the power-law surface density Σ(r)=Σ (r /r)p and the ve-
0 0
blue-shiftedemission. locity profile V(r)=V (r /r)s. The dust grains have MRN-
0 0
Assuming opticallythin C17O line emission, the gasmass like size distribution and the gas-to-dust mass ratio is 100.
canbederivedby Moreover,weassumeanageof∼0.1Myrduetothelackof
reliable estimates. Churchwell (1999) reported the dynami-
τ (T +0.9)exp(16.18/T )
M(H2)[M⊙]=1.5810- 10 ex (1- exp(- τ)) ex (1) Hcaelnonuintflgoewtatli.m2e00o0f).1.T8h1e0b4esytrfiwtthoicthheisobasnerlvoewdeCr 1li7mOit(2(–se1e)
linesisobtainedwiththefollowingdiskparameters:(1)incli-
×X(C17O)(D[kpc])2Z Sν∆v[Jykms- 1], nationandpositionalanglesof≈30◦,(2)innerandouterradii
line
of.400AUand1400AU,respectively,(3)asurfacedensity
where τ =0.01 is the optical depth of the C17O (2–1) line, gradient p∼- 1.5, and (4) a Keplerian-like velocity profile,
T =100Kistheexcitationtemperature,X(C17O)=2.5·107 s≈- 0.5. The best-fit disk mass ranges between ∼0.2 and
ex
ArotatingdiskaroundAFGL490 3
1 M . Note that the modeled line profiles are indeed opti- nerthinKepleriandiskfedbyathickerself-gravitatingtorus
⊙
callythin,τ .0.01.Allthesevaluesareinagoodagreement with nearly uniform rotation. Finally, a clumpy disk struc-
withtheobservationalresults,andthevaluesderivedfromthe turecouldbearesultofarecentencounterwithanearbystar
PVdiagram. or due to the gravitationalinteraction with a wide low-mass
companion. The complex multiple outflow systems seen in
4. DISCUSSIONANDCONCLUSIONS theclosevicinityofAFGL490stronglysupportthisidea(see
Why do we see such a clumpy disk structure? This ob- Fig.4binPaperI).
ject seems to be relativelyyoungand could still be in a per- InthisLetter,wepresentclearevidencethat∼10M⊙stars
turbedstatethatremainsfromanearlier,non-steadyaccretion can be surrounded by Keplerian disks in their earliest evo-
phase. The viscous dissipation timescale for the AFGL 490 lutionallyphase(seealsoShepherd,Claussen&Kurtz2001).
diskisestimatedtobe∼1Myr(Pringle1981),whichfarex- Incontrasttothepreviouslyreportedhugedisksaroundsuch
ceedsitsage. Thishypothesisisfurthersupportedbythefact stars with radii of .10000 AU (e.g., Chini et al. 2004, but
that the ∼104 years old, large-scale CO outflow consists of see Sako et al. 2005) the AFGL 490 disk is much smaller,
singlemovinggasclumps,andthusaccretionisindeednon- R∼1500AU.Usingadvancedtheoreticalmodeling,wecon-
steady (Mitchelletal. 1995). Another explanation could be strainbasicdiskparameters:(1)theinclinationandpositional
that the C17O (2–1) emission traces the densest parts of the anglesare∼30◦,(2)thesurfacedensityprofileis∼- 1.5,(3)
spiral arms that are easily excited in massive disks by grav- themassis∼1.5M⊙ (≈50%uncertainty),and(4)thedisk
itational instabilities. Indeed, the morphology of the C17O rotationisclosetoKepler’slaw.
emissionresemblesthedensitystructureofaself-gravitating
diskwithdoublearms(uppercornerinFig.1a)calculatedby
Fromangetal.(2004a). Moreover,thenumericalsimulations
by Fromang,Balbus,&DeVilliers (2004b) imply that such We acknowledge the help of the IRAM staff both of the
disksshouldalsodevelopadualstructurecomposedofanin- PlateaudeBureandGrenoble.
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FIG. 1.— (a)MapofthetotalintegratedC17O(2–1)intensity(thinlines)overlaidwiththe1.3mmcontinuum(grayimagewithdottedlines). Thecontours
oftheintegratedC17Oemissioncoverintensitylevelsfrom20%(1.5σrms)to90%ofthepeakvalue(0.25Jykms- 1beam- 1)in10%steps;the50%contouris
shownbyathickline. Thecontourlinesofthecontinuumemissioncorrespondto20%(3σrms),50%,and90%ofthepeakvalue(0.6Jybeam- 1). (Topleft)
Thischartshowsthemidplanedensitydistributionofaself-gravitating massivedisk(Fromangetal.2004a,Fig.9)inclinedto35◦. Theellipseindicatesthe
skyareacoveredbysuchaninclineddisk. (b)Integratedred-andblue-shiftedC17Olineemissionoverlaidwiththelowestcontourasshownintheleftpanel.
Fortheblue-shiftedlobe,solidthickcontourscorrespondtogaswithvelocitiesbetween- 15.5and- 13.4kms- 1,whileforthered-shiftedlobe,dashed-thick
contoursindicategaswithvelocitiesbetween- 12.5and- 9.5kms- 1. Levelsare30,50,70,&90%oftheemissionpeaks(blue: 0.09Jybeam- 1kms- 1;red:
0.12Jybeam- 1kms- 1).ThegrayareaistheH-bandspeckleimagefromAlvarezetal.(2004).Thestraightsolidlineindicatesthecutfortheposition-velocity
diagramshowninFig.2.Thedashedlinesshowthemajorandtheminoraxesoftheemissiondistributionsintheskyplane,wherebytheminoraxiswithdifferent
positionangles(PA)isassumedtobeinlinewiththeoutflowaxis.
ArotatingdiskaroundAFGL490 5
FIG.2.—Theposition-velocitydiagramoftheC17O(2–1)emissionalongthestraightlineshowninFig.1b.ThethicksolidlinerepresentstheKepleriandisk
modelwithMdisk=1M⊙andi=35◦,whilethedashedlinecorrespondstoamodelofa700AUgasringwiththesameorientation. Thethickdottedcurve
indicatesthediskmodelwithnon-Keplerianrotation,V(r)∝r- 0.35.
6 Schreyeretal.
FIG.3.—TheobservedPdBIC17Ospectra(thinline)arecomparedtothesyntheticones(thickline)obtainedwiththebest-fitmodeloftheAFGL490disk.