SMA observations of young disks: separating envelope, disk, and stellar masses in class I YSOs

0.32 MB·English

by  Dave Lommen

Astronomy&Astrophysicsmanuscriptno.i63e29final (cid:13)c ESO2008
February3,2008
SMA observations of young disks: separating envelope, disk,
and stellar masses in class I YSOs
DaveLommen1,JesK.Jørgensen2,3,EwineF.vanDishoeck1,andAntonioCrapsi1,4
1 LeidenObservatory,LeidenUniversity,P.O.Box9513,2300RALeiden,TheNetherlands
2 Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138,USA
3 Argelander-Institutfu¨rAstronomie,UniversityofBonn,AufdemHu¨gel71,53121Bonn,Germany
4 ObservatorioAstrono´micoNacional(IGN),AlfonsoXII,3,28014Madrid,Spain
Received...;accepted...
ABSTRACT
8 Context.Youngstarsarebornwithenvelopes,whichintheearlystagesobscurethecentral(proto)starandcircumstellardisk.Inthe
0 ClassIstage,thedisksarestillyoung,buttheenvelopesarelargelydispersed.ThismakestheClassIsourcesidealtargetsforstudies
0 oftheearlystagesofdisks.
2 Aims.Weaimtodeterminethemassesoftheenvelopes,disks,andcentralstarsofyoungstellarobjects(YSOs)intheClassIstage.
n Methods. We observed the embedded Class I objects IRS 63 and Elias 29 in the ρ Ophiuchi star-forming region with the
a SubmillimeterArray(SMA)at1.1mm.
J Results.IRS63andElias29arebothclearlydetectedinthecontinuum,withpeakfluxesof459and47mJy/beam,respectively.
ThecontinuumemissiontowardElias29isclearlyresolved,whereasIRS63isconsistentwithapointsourcedowntoascaleof3(cid:48)(cid:48)
3
(400AU).TheSMAdataarecombinedwithsingle-dishdata,andbothdiskmassesof0.055and≤ 0.007M andenvelopemasses
2 (cid:12)
of0.058and≤0.058M areempiricallydeterminedforIRS63andElias29,respectively.Thedisk+envelopesystemsaremodelled
(cid:12)
withtheaxisymmetricradiative-transfercodeRADMC,yieldingdiskandenvelopemassesthatdifferfromtheempiricalresultsby
]
h factors of a few. HCO+ J = 3–2 is detected toward both sources, HCN J = 3–2 is not. The HCO+ position-velocity diagrams are
p indicativeofKeplerianrotationandallowanestimateofthemassofthecentralstars.Forafiducialinclinationof30◦,wefindstellar
- massesof0.37±0.13forIRS63and2.5±0.6M(cid:12)forElias29.
o Conclusions.ThesensitivityandspatialresolutionoftheSMAat1.1mmallowagoodseparationofthedisksaroundClassIYSOs
r fromtheircircumstellarenvelopesandenvironments,andthespectralresolutionmakesitpossibletoresolvetheirdynamicalstructure
t
s andestimatethemassesofthecentralstars.TheratiosoftheenvelopeanddiskmassesM /M arefoundtobe0.2forIRS63and
env disk
a 6forElias29.ThisislowerthanthevaluesforClass0sources,whichhaveM /M ≥10,suggestingthatthisratioisatracerof
env disk
[
theevolutionarystageofaYSO.

1 Keywords.circumstellarmatter–planetarysystems:protoplanetarydisks–stars:formation–stars:individual(IRS63,Elias29)
v
9
6
5 1. Introduction the central star evolve over time, and how long does it take for
3 thecircumstellarmattertobeaccretedontothestar?
. Low- and intermediate-mass stars are formed from the gravia-
1 Youngstellarobjects(YSOs)areusuallyclassifiedaccording
tional collapse of molecular cloud cores. In the earliest stages,
0 totheirslopesintheinfrared(IR)wavelengthregime.Originally,
the newly-formed protostar remains embedded in the remnants
8 the LW classification (Lada & Wilking 1984; Lada 1987) ran
ofthiscore,acoldenvelopeofdustandgas,whichisgradually
0 fromtheembeddedClassI,viatheopticallyvisibleClassIIor
: accreted by the young star (e.g., Shu 1977). Due to the angu- classical T Tauri stars, to the Class III spectral energy distribu-
v lar momentum initially present in the core, most of the enve-
i tionsofpost-TTauristars.Later,theClass0stagewasaddedto
X lopematerialdoesnotfalldirectlyontothecentralprotostarbut
thisclassification(see,e.g.,Andre´etal.1993),wheretheClass0
is piled-up in a circumstellar disk (e.g., Terebey et al. 1984).
r sourcesaredistinguishedfromtheClassIsourcesthroughtheir
a Understandingthisinterplaybetweenstar,disk,andenvelopeis
highrelativeluminosityatsubmillimetre(submm)wavelengths.
crucialinordertobeabletorelatetheinitialconditionsofstar
Thisclassificationroughlyreflectstheevolutionarystageofthe
formation such as the mass of the protostellar core to the end-
YSOsunderconsideration.ThemostdeeplyembeddedClass0
product–namelythepropertiesoftheyoungstar,andthemass
sourcesarethoughttoevolvethroughtheClassIstagewhiledis-
and thus potential of the disk for forming planets. Some of the
sipating their circumstellar envelopes. Eventually they become
keyquestionsinclude:Wheredoesmostofthemassresideata
opticallyvisibleaspre-mainsequenceTTauristarswithcircum-
given time? Will all the mass that is seen in prestellar cores or
stellardisks.
intheenvelopesarounddeeplyembeddedsourcesendupinthe
star,orwillalargefractionbedispersedfromthesystem?How TheClass0andClassIIYSOshavebeenstudiedquiteex-
dothemassesofthecircumstellarenvelope,thedisk,andthatof tensively with high-resolution (sub)millimetre interferometers
(e.g.,Jørgensenetal.2007;Andrews&Williams2007,andref-
erencestherein).StudiesofdeeplyembeddedClass0YSOshave
Sendoffprintrequeststo:DaveLommen, shown that circumstellar disks are formed early (Harvey et al.
e-mail:[email protected] 2003;Jørgensenetal.2004),butinthesesystems,itisdifficult
2 DaveLommenetal.:SeparatingthemassesinClassIYSOs
toseparatetheemissionfromthediskfromthatoftheenvelope. by the Caltech Submillimeter Observatory). Conditions were
TheClassIsources,inwhichalargepartoftheoriginalcircum- slightlyworseon17May,withτ startingat0.16,fallingto0.1
225
stellar envelopes has been dissipated, are ideal to study young duringthefirsthalfofthenight.Physicalbaselinesrangedfrom
disksinYSOs.InterferometricstudiesofClassIobjectshaveso 11.6to62.0metres,andtheresultingprojectedbaselinesranged
farbeenlargelylimitedtostudiesataround3mm(e.g.,Ohashi from 5.9 to 63.8 kλ, yielding a resolution of about 4.0×2.3(cid:48)(cid:48)
etal.1997;Hogerheijdeetal.1997,1998;Looneyetal.2000). (natural weighting). The correlator was configured to observe
The Submillimeter Array (SMA) allows observations around thelinesofHCO+ J =3–2andHCN J =3–2togetherwiththe
1 mm, where the thermal dust continuum emission is an order continuum emission at 268 and 278 GHz. The line data were
ofmagnitudestrongerthanat3mm.Alsoattheseshorterwave- taken with 512 channels of 0.2 MHz width each, resulting in a
lengths it is possible to detect the higher rotational transitions velocityresolutionof0.23kms−1.
of the molecules that trace the dense gas in the disks and inner Calibration was done using the MIR package (Qi 2005),
envelopes of the young systems, rather than lower-density ex- andimagingwasdoneusingtheMIRIADpackage(Saultetal.
tendedenvelopeemission. 1995). The quasars QSO B1622-297 and QSO B1514-241
We here present SMA observations of two Class I objects served as gain calibrators, and the absolute fluxes were cali-
thatappeartobeinanevolutionarystagewherethekinematics brated on Uranus. The absolute flux calibration is estimated to
ofthecircumstellarmaterialarenolongerdominatedbyinfall. haveanuncertaintyof∼20%.Thepassbandswerecalibratedon
IRS 63 (WLY 2-63, GWAYL 4) and Elias 29 (Elia 2-29, WLY CallistoandthequasarQSOB1253-055(3C279).
1-7) are located in the ρ Ophiuchi cloud, taken to be at a dis-
tanceD=125±25pc(deGeusetal.1989).Spitzerphotometry
from the “Cores to Disks” Legacy program (Evans et al. 2003) 3. Results
was used to determine the source’s infrared colours and their
The basic results of the observations are shown in Fig. 1 and
bolometric luminosities and temperatures. IRS 63 and Elias 29
summarisedinTable1.
havesimilarbolometrictemperatures,T = 351Kand391K,
bol
respectively, which places them in the LW Class I regime. The
bolometric luminosities were calculated to be L = 0.79 L 3.1. Continuumdata
bol (cid:12)
forIRS63and13.6L forElias29.Accordingtotheirinfrared
(cid:12)
slopes of α = 0.15 and 0.42, IRS 63 is the slightly more Both sources are clearly detected in the continuum at 1.1 mm.
2−24µm Point-source and Gaussian fits were done in the (u,v) plane to
evolved,fallingintothe“flat-spectrum”classthatseparatesthe
determine the peak and integrated fluxes, respectively. A con-
ClassIfromtheClassIIsources(Greeneetal.1994).Theval-
tinuum peak flux of 459 mJy bm−1 and an integrated flux of
ues for α quoted here are from un-dereddened observations.
IR
474mJywerefoundforIRS63,atapositionwhichagreeswith
CorrectionfortheextinctiontowardtheρOphiuchistar-forming
the 2MASS K -band position within 0(cid:48).(cid:48)1. IRS 63 was also ob-
region(e.g.,Flahertyetal.2007)wouldresultinlowervaluesfor S
servedwiththeSMAbyAndrews&Williams(2007)athigher
α ; in other words, the sources may be slightly more evolved
IR
resolution at 1.3 mm. Their interferometer flux for IRS 63, to
than the raw values of α suggest. In terms of environment,
IR whichtheyreferasL1709B2,isconsistentwiththevaluehere
the two sources are quite each other’s opposites. The SCUBA
andammslopeα = 2.0±1.0.ThedataofElias29arebest
850µmmap(Johnstoneetal.2004)fromtheCOMPLETEsur- mm
fitted by two sources. The brightest of the two agrees with the
vey(Ridgeetal.2006)showsthatIRS63isanisolatedcompact
2MASS K -band position of Elias 29 within 0(cid:48).(cid:48)2, whereas the
source. Elias 29, on the other hand, is located in a dense ridge S
othersourceisoffsetby3(cid:48)(cid:48).Thesecondpeakisattributedtoan
ofmolecularmaterial,whichcontainsseveralmoreYSOs.Itis
enhancement in the ridge, from which Elias 29 likely formed,
likelythattheseYSOswereformedfromcondensationsinthis
andisdesignatedby“Ridge”inTable1.Elias29andtheridge
molecularridge.
componentyieldcontinuumpeakfluxesof47and31mJybm−1,
We present the masses of all main components of Class I
andintegratedfluxesof72and28mJy,respectively.
YSOs,i.e.,thecentralstar,thedisk,andtheenvelope,forthefirst
Plots of the visibility amplitudes as functions of projected
time.IRS63andElias29arethefirsttwosourcesinalargersur-
baseline for IRS 63 and Elias 29 are shown in Fig. 2. Elias 29
vey of Class I objects studied with the SMA at 1.1 mm. These
showsasteeplyrisingfluxtowardtheshortestbaselines,indicat-
observations complement the survey of Class 0 sources in the
ingthatanenvelopeispresentaroundthissource.Theemission
PROSAC programme (Jørgensen et al. 2007) and will allow us
towardIRS63doesnotchangeappreciablywithbaselinelength,
totracethesimilaritiesanddifferencesoftheseevolvingproto-
indicatingthatthissourceisunresolveduptobaselinesof60kλ,
stars.Theresultsofthecompletecampaignwillbepresentedin
ordowntophysicalscalesofabout400AU.
afuturepaper.In§2,theobservationsarepresented,andthere-
Fig. 2 also shows the estimated single-dish flux at 1.1 mm.
sultsandimplicationsarediscussedin§§3and4.Wesummarise
Forthis,the“mapped”fluxesfromAndre´ &Montmerle(1994)
themainconclusionsin§5.
at 1.25 mm were combined with newly determined 850 µm
fluxes (SCUBA maps from the COMPLETE survey, Johnstone
2. Observations et al. 2004; Ridge et al. 2006). The SEDs are assumed to have
a F ∝ να dependence in this wavelength range, and are in-
ν
IRS 63 and Elias 29 were observed with the SMA1 (Ho et al. terpolated to the 1.1 mm wavelength at which the SMA ob-
2004)on15and17May2006,respectively.Weatherconditions servations were conducted. Fluxes of 355 and 780 mJy are
were good on 15 May, with zenith optical depths at 225 GHz found for Elias 29 and IRS 63, respectively, with an estimated
τ =0.04–0.06(asmeasuredbyatippingradiometeroperated
225
2 IRS63andL1709Bareoftenreferredtoasthesamesource(e.g.,
1 TheSubmillimeterArrayisajointprojectbetweentheSmithsonian Andre´ & Montmerle 1994). However, L1709 B would appear to lie
Astrophysical Observatory and the Academia Sinica Institute of about 4 arcmin north of IRS 63 (e.g., Benson & Myers 1989). The
Astronomy and Astrophysics and is funded by the Smithsonian sourcethatAndrews&Williams(2007)refertoasL1709B,isinfact
InstitutionandtheAcademiaSinica. IRS63.
DaveLommenetal.:SeparatingthemassesinClassIYSOs 3
Fig.3.First-momentmapofIRS63.Contoursaredrawnat2,4,
and8timestheRMSof67mJybm−1.Thepositionaloffsetsare
withrespecttothecoordinatesoftheobservationalphasecentre,
andthecrossindicatesthecontinuumpositionofIRS63froma
point-sourcefitinthe(u,v)plane,seeTable1.Acolourversion
ofthisfigurecanbeobtainedfromtheelectronicversionofthe
Fig.1.SMAimagesofthe1.1mmcontinuum(toppanels)and paper.
integrated HCO+ 3–2 (bottom panels) emission. Contours are
drawnat2,4,8,16,32,and64timestherespectiveRMSnoise
3.2. Linedata
levels,withtheRMSbeingabout6mJybm−1forthecontinuum
andabout65mJybm−1 forthelinedata.Negativecontoursare HCO+ J=3–2isdetectedtowardbothsources,whereasHCNJ
dashed.Thepositionaloffsetsarewithrespecttothecoordinates =3–2isnot.Elias29appearstobeabouttentimesasstrongin
oftheobservationalphasecentre.Thedashedlinesindicatehow HCO+ J=3–2integratedemissionasIRS63(seeTable1).The
thepositionvelocitydiagrams(Fig.7)aredirected. emission toward IRS 63 is compact, coincident with the con-
tinuum peak, and shows a velocity gradient in the north-south
direction, see Fig. 3. The integrated emission in the direction
of Elias 29 is more extended and can be fitted with two el-
liptical Gaussians, as is shown in the left panel of Fig. 4. The
centrepositionofthesmallerGaussiancoincideswiththecon-
tinuum peak, as well as with the infrared position, and is at-
tributedtoElias29itself.ThesecondGaussianhasaveryelon-
gated shape, offset from the infrared source, and is attributed
to a density enhancement in the nearby ridge. The right panel
of Fig. 4 shows the HCO+ J = 3–2 spectra toward the contin-
uumpositionsofElias29andIRS63,binnedto0.9kms−1.The
HCO+ J =3–2emissiontowardIRS63appearstobecentredat
Fig.2.Continuumamplitudeasafunctionofprojectedbaseline V =3.3kms−1.
LSR
forIRS63(leftpanel)andElias29(rightpanel).Thedatapoints
givethe amplitudeper bin,where thedataare binnedin annuli
according to (u,v) distance. The error bars show the statistical 4. Discussion and interpretations
1σerrors,mostoftensmallerthanthedatapoints,andthedot-
4.1. Envelopeanddiskmasses
ted lines give the expected amplitude for zero signal, i.e., the
anticipated amplitude in the absence of source emission. The The basic data as presented in the previous section are used
half-open circles at zero (u,v) distance give the zero-spacing to determine the masses of the disks and the envelopes in the
1.1mmflux,interpolatedbetween850µmand1.25mmsingle- systems, both empirically (Sect. 4.1.1) and through detailed
dish fluxes (Andre´ & Montmerle 1994; Johnstone et al. 2004; radiative-transer modelling (Sect. 4.1.2). The differences be-
Ridgeetal.2006,seetext). tween the two methods and the over-all conclusions are dis-
cussedinSect.4.1.3.
uncertainty of 25%. These fluxes are significantly larger than
4.1.1. Empiricalresults
the interferometer fluxes and indicate that extended emission
is present around the compact components picked up with the A first-order estimate of the disk and envelope masses can be
SMA.Thisextendedemissionmaybeduetoacircumstellaren- obtained by assuming that the continuum flux of the envelope
velope,ortosurroundingorinterveninginterstellarclouds,since is resolved out by the interferometer on the longest baselines
the“mapped”regionsofAndre´ &Montmerle(1994)arerather (Fig. 2). Hence, the emission on the longest baselines is solely
large,40(cid:48)(cid:48)forElias29and60(cid:48)(cid:48)forIRS63. duetothedisk,whereasthesingle-dishfluxcomesfromthedisk
4 DaveLommenetal.:SeparatingthemassesinClassIYSOs
a dust opacity at 273 GHz of κ = 2.73 cm2 g−1 is found, and
ν
thediskmassesdecreaseto0.024M forIRS63and0.003M
(cid:12) (cid:12)
forElias29.3 Notethatthismethodofestimatingthediskmass
is only valid when the disk is unresolved by the observations.
If the disk were clearly resolved, the flux on the longest base-
lines would not include the emission from the outer disk, but
onlythatfromsmallerradii,unresolvedbythosebaselines.Due
toitscloseness,theenhancementintheridgenearElias29may
contributetoitsflux,evenonthelongestbaselinesof≥50kλor
scales< 4(cid:48)(cid:48).However,intheestimateofthefluxonthelongest
baselines (Table 1), the ridge component was fitted simultane-
ously with Elias 29, and hence its contribution to the derived
Fig.4. Left panel: HCO+ J = 3–2 integrated emission toward diskmassisexpectedtobeminimal.
Elias 29. The grayscale is linear from 0.2 to 1.2 Jy bm−1. The In a similar fashion the envelope masses in the systems
canbefound.First,thecontributionsfromtheenvelopestothe
emissionisfittedwithtwoGaussians,theoneinthenorth-south
1.1 mm single-dish fluxes are determined by subtracting the
direction tracing the emission due to Elias 29, and the other,
fluxes at baselines ≥ 16 kλ (see Table 1) from the single-dish
larger one tracing emission from a density enhancement in the
ridge.Rightpanel:spectrallines,integratedover2×2(cid:48)(cid:48)regions, fluxes asfound inSect. 3. Theenvelope massesare then found
byusingEq.(1),withthetemperaturesoftheenvelopestakento
centred on the continuum positions of Elias 29, of the ridge
(shiftedby+4Jybm−1),andofIRS63(shiftedby-4Jybm−1), be20K.Foranopticallythinenvelopearoundstarswithlumi-
andbinnedto0.9kms−1.TheHCO+ J =3–2emissiontoward nositiesof0.79and13.6L(cid:12) applicabletoIRS63andElias29,
IRS63appearstobecentredatV =3.3kms−1. a temperature of 17–29 K is expected at a radius of 940 AU
LSR
correspondingtothesizeoftheJCMTsingle-dishbeam(from,
e.g.,Eq.(2)of Chandler&Richer2000,takingtheopacityin-
Table1.ResultsofSMAobservationsat1.1mm.
dexβ = 1).Thisyieldsanenvelopemass M = 0.058M for
env (cid:12)
IRS63withina30(cid:48)(cid:48) radius,wherethecontributionfromtheen-
IRS63 Elias29 Ridge
F (P)a[mJybm−1] 459 47 31 velopetothetotal1.1mmfluxis38%.Coincidentally,alsofor
RνAa[J2000] 16:31:35.65 16:27:09.42 16:27:09.48 Elias 29 an envelope mass Menv = 0.058 M(cid:12), within a 20(cid:48)(cid:48) ra-
Deca[J2000] -24:01:29.56 -24:37:18.91 -24:37:22.48 dius,isfound.However,inthissystemthecontributionfromthe
RMSb[mJybm−1] 6 5 5 envelopetothetotal1.1mmfluxis84%.Asforthediskmass,
CircularGaussianfitstoalldata alsotheenvelopemassfoundforElias29shouldbetakenasan
F (G)c[mJy] 474 72 28 upper limit because of the closeness of the ridge enhancement.
ν
FWHMc[arcsec] 0.55±0.08 2.2±0.5 – Note,however,thatitisunlikelythatthebulkofthecontinuum
CircularGaussianfitstodata≥16kλ(scales<13arcsec) emissionisduetothisenhancement,becauseofthecoincidence
Fν(G)c[mJy] 480 57 27 of the continuum peak with the infrared source and also with
FWHMc[arcsec] 0.62±0.08 1.7±0.7 – the single-dish submillimetre positions. Using the opacities of
Linedata
Beckwithetal.(1990)willlowerallmassesbyafactorofabout
HCO+d[Kkms−1] 3.1±0.3 23.7±2.5 23.5±2.5
2.
HCNd[Kkms−1] <1.8 <3.8 <3.8
a Pointsourcefitinthe(u,v)plane. 4.1.2. Modelling
b Calculatedfromthecleanedimage.
c CircularGaussianfitinthe(u,v)plane. To provide more physical grounding to the values for M
disk
d Integratedintensitiesarefroma4×4(cid:48)(cid:48) squarearoundthecontin- and M estimated in the previous section, a radiative transfer
env
uumemission.Thesynthesisedbeamis∼ 4×2.5(cid:48)(cid:48).HCNwasnot code was used to fit simultaneously the millimetre interferom-
detected;quotedvaluesare3σupperlimits.
etry and the spectral energy distributions. IRS 63 and Elias 29
were modelled with a new version of the programme RADMC
(e.g.,Dullemond&Dominik2004).RADMCisanaxisymmet-
andenvelopecombined.Assumingthatthediskandtheenvelope ric Monte-Carlo code for dust continuum radiative transfer in
are isothermal and the dust emission is optically thin, the disk circumstellar disks and envelopes in which the stellar photons
massisgivenby aretracedinthreedimensions.Thecodeisbasedonthemethod
ofBjorkman&Wood(2001).
M = FνΨD2 , (1) Thedensitystructureofthediskisgivenby(seeCrapsietal.
disk κ B (T ) 2007)
ν ν dust
wishtehreegFasν-itso-tdhuesflturaxtiaot,fκrνeqisuethnecyduνstonoptahceitlyonagteνs,tabnadseBliνn(Tesd,usΨt) ρdisk(r,θ)= Σ√0((2rπ/R)H0)(−r1)exp−12(cid:34)rcHo(sr()θ)(cid:35)2, (2)
is the emission from a black body at T . Using fiducial val-
dust
ues for all these parameters – Ψ = 100, T = 30 K, and
dust where r is the radial distance from the central star and θ is the
κ = 1.18 cm2 g−1 (“OH5” coagulated dust with icy mantles,
ν anglefromtheaxisofsymmetry.Theinnerradiuswasfixedto
found in the fifth column of Table 1 of Ossenkopf & Henning
0.1AUforIRS63,andto0.25AUforElias29,toaccountfor
1994, interpolated to the observed frequency ν = 273 GHz) –
disk masses of 0.055 M(cid:12) and 0.007 M(cid:12) are found for IRS 63 3 The opacity law quoted by Beckwith et al. (1990) is κν =
and Elias 29, respectively. Using the opacity law as quoted by (cid:16) (cid:17)β
0.1 ν/1012Hz forthegasanddustcombined,implicitlyassuminga
Beckwithetal.(1990),κν = 10(cid:16)ν/1012Hz(cid:17)β,andtakingβ = 1, gas-to-dustratioΨ=100,consistentwithourwork.
DaveLommenetal.:SeparatingthemassesinClassIYSOs 5
Table2.ModelparametersforIRS63andElias29.
IRS63 Elias29
Modellingparametersandresults
Stellarluminosity(fixed) 0.79L 13.6L
(cid:12) (cid:12)
Diskinnerradius(fixed) 0.1AU 0.25AU
Diskouterradius,R 100AU 200AU
0
Diskheightatouterradius,H /R (fixed) 0.17 0.17
0 0
Envelopeouterradius(fixed) 10,000AU 10,000AU
Centrifugalradius,R 100AU 300AU
rot
Inclination 30◦ 30◦
Diskmass,Mdisk 0.13M(cid:12) 0.004M(cid:12) Fig.5.Continuumamplitudeasafunctionofprojectedbaseline
Envelopemass,Ma 0.022M 0.025M
env (cid:12) (cid:12) for IRS 63 (left panel) and Elias 29 (right panel), with models
Empiricalresults
overplotted.Thedashedlinesshowmodels,consistingofanen-
Ma [M ] 0.058 ≤0.058
env (cid:12) velopeonly.Theyfitthesingle-dish(sub)mmdatawell,butfall
M [M ] 0.055 ≤0.007
disk (cid:12) offtoofasttolongerprojectedbaselines.Thedash-dottedlines
Mb [M ] 0.37±0.13 2.5±0.6
star (cid:12) arefromdisk-onlymodels.Theyfittheinterferometricdatawell,
a The envelope mass is defined as the mass, not contained in the butdonothaveenoughfluxontheshortestbaselines.Thesolid
disk,withinasingle-dishradius,i.e.,30(cid:48)(cid:48)/3750AUforIRS63,and linesareforthemodelsthatprovideagoodfittotheSEDandto
20(cid:48)(cid:48)/2500AUforElias29. thespatialinformationprovidedbytheinterferometricobserva-
b Assuminganinclinationi=30◦(§4.2fordiscussion). tions.Thedottedlinesindicatetheexpectedamplitudeforzero
signal.
thehigherluminosityofthestar.Attheseradii,thetemperature
and 13.6 L for Elias 29, which were taken to be equal to
isoftheorderofthedustsublimationtemperature.Notethatthe (cid:12)
thebolometricluminositiesofthesources,andthetemperature
exactsublimationtemperature,andhencethediskinnerradius,
structures in the disks and envelopes were then calculated by
dependsontheexactdustspeciesandlocaldensity.Thevertical
scaleheightofthediskisgivenbyH(r)=r·H /R ·(r/R )0.17, propagatingthestellarphotonsthroughthesystems.
0 0 0
The results of the modelling are shown in Fig. 5. A model
whereR wasleftfreetovary,andH /R wasarbitrarilyfixedto
0 0 0
0.17.Chiang&Goldreich(1997)find H(r)/r ∝ r2/7.However, withonlya0.055M(cid:12) diskreproducestheamplitudeasafunc-
tionofbaselinequitewellforIRS63,butfailstoproduceenough
that estimate is based on grey opacities and constant surface
single-dish flux. A 0.35 M envelope-only model, on the other
density, and somewhat smaller flaring is usually more realistic. (cid:12)
Hence,wechoseH(r)∝r·r0.17.Σ wasscaledtothediskmass, hand,producesenoughsingle-dishfluxinthe(sub)mmregime,
0
but falls off too quickly toward longer baselines. To allow for
which was also left as a free parameter. The envelope density
a comparison with the empirical model (Sect. 4.1.1) the enve-
followstheequationforarotating,infallingmodel(Ulrich1976;
lope mass was defined as the mass, present within the single-
Crapsietal.2007)
dishaperture(i.e.,a30(cid:48)(cid:48) radiusforIRS63anda20(cid:48)(cid:48) radiusfor
(cid:32) r (cid:33)−1.5(cid:32) cosθ (cid:33)0.5 Elias 29), and not contained in the disk. A model with both a
ρenvelope(r,θ)=ρ0 R 1+ cosθ × diskandanenvelopeisnecessarytoexplaintheobservationsof
rot 0 IRS63,andthebestmodelconsistsofadiskof0.13M andan
(cid:32) cosθ R (cid:33)−1 envelopeof0.022M .Thismodelalsofitstheobserved(cid:12)IRS63
× 2cosθ0 + rrot cos2θ0 , (3) SEDwell,ifaforegro(cid:12)undextinctionofAV =7magisassumed,
seeFig.6.TheregioninwhichIRS63islocatedhasaveryhigh
where θ0 is the solution of the parabolic motion of an infalling AV of about 24 mag as found in extinction maps produced by
particle, Rrot is the centrifugal radius of the envelope, and ρ0 is theSpitzer“CorestoDisks”Legacyprogram(Evansetal.2003,
the density in the equatorial plane at Rrot. ρ0 was scaled to ac- 2007).Thesemapsmeasurethelargescale(∼ 5(cid:48))cloudextinc-
comodate the total envelope mass, and Rrot, which can have a tiononthebasisofbackgroundstars,andthereforeincludesan
significant influence on the amplitude as a function of baseline extracontributionbesidesenvelopeextinction.
length, was left free to vary. The outer radius of the envelope The amplitude as a function of baseline for Elias 29 is best
wasfixedto10,000AU,wherethetemperatureissimilartothat explainedbya0.004M diskanda0.025M envelope.Forthis
(cid:12) (cid:12)
oftheambientinterstellarcloud.Theparameters,assummarised source,nofurtherattempttomodeltheSEDwasmade,because
in Table 2, give the best-fit models by eye; a full χ2 minimiza- theclosepresenceoftheridgeenhancementmakesamodelthat
tion is not warranted by the relatively low resolution and S/N isaxisymmetriconscaleslargerthanseveral100AUunapplica-
of the data. Our models do not have outflow cavities in the en- ble.
velopes. Note that the actual physical structure of the envelope
(centrifugal radius, outflow cavities, etc.) may considerably af-
fect the mid-IR SED and thus the derived inclination from the 4.1.3. Discussion
models(e.g.,Whitneyetal.2003;Jørgensenetal.2005).
Eventhoughthediskandenvelopemassesderivedfromtheem-
Two different dust species are used in these models, “hot”
pirical and detailed model results differ by up to a factor of a
dust where the temperature is higher than 90 K, and “cold”
few,weareconfidentthattheyareaccuratewithinthisrange.For
dust where the temperature is below 90 K. The opacities used
example, a significantly larger disk in the IRS 63 model would
are “OH2” (coagulated dust without ice mantles, Ossenkopf &
showaverysteepfall-offintheamplitudeasafunctionofbase-
Henning 1994) for the hot dust, and “OH5” (coagulated dust
with thin ice mantles) for the cold dust. The central stars were 4 Delivery of data from the c2d Legacy Project: IRAC and MIPS
representedbysourceswithluminositiesof0.79 L(cid:12) forIRS63 (Pasadena,SSC,Evansetal.2007).
6 DaveLommenetal.:SeparatingthemassesinClassIYSOs
4.2. Keplerianrotationandstellarmasses
BothIRS63andElias29showavelocitygradientintheHCO+
emission, which is interpreted as the rotation of a circumstel-
lar disk. The mass of the central object can be estimated from
position-velocity diagrams along the major axis of the disk, as
indicated in Fig. 1. An elliptical Gaussian was fitted to each
channelintheHCO+datainthe(u,v)plane,usingtheMIRIAD
task uvfit. This yielded a best-fit position per channel with cor-
respondinguncertaintiesinrightascensionanddeclination,and
consequently a declination offset from the phase centre in the
physicalplane.Notethatthefittingwascarriedoutinthe(u,v)
plane,soasnottobehinderedbyartefactsthatmayariseinthe
deconvolutionprocess.Position-velocitydiagramswerecreated
byplottingthedeclinationoffsetasafunctionofvelocitychan-
nel, where only coordinates with an uncertainty in the declina-
tionoflessthan0(cid:48).(cid:48)5weretakenintoaccount.Subsequently,the
signaturesofKeplerianrotationaroundapointsourcewerefitted
Fig.6. Spectral energy distribution of IRS 63 using the model
totheposition-velocitydiagrams(Fig.7).Inthefittingprocess,
parameters of Table 2 extincted by AV=7 mag (solid line). The the values for M (the mass of the central object), the central
star
dashed line shows the model star, which has a luminosity of
velocity of the system with respect to the local rest frame, and
0.79L ,andisrepresentedbyablack-bodywithatemperature
(cid:12) the declination offset from the source to the phase centre were
of 3 000 K. Overplotted are JHKL photometry (Haisch et al. varied,andaglobalminimumχ2wasdetermined.
2002), MIPS 24 and 70 µm fluxes (c2d4), JCMT 850 µm 60(cid:48)(cid:48) AssumingaKepleriandiskseenedge-on(i=90◦),acentral
integrated flux (COMPLETE), SMA 1.10 mm peak flux (this massof M = 0.09±0.03M isfoundforIRS63.Theexact
work; square), and IRAM 1.25 mm 60(cid:48)(cid:48) integrated flux (Andre´ star (cid:12)
inclination of IRS 63 is hard to determine due to the high ex-
&Montmerle1994). tinctionintheregion.However,aninclinationlargerthan∼45◦
can be ruled out because of the source’s brightness at 3–5 µm
(Pontoppidanetal.2003).Assuminganinclinationof30◦,and
taking into account the sin2i dependence, the mass of the cen-
line length, which is not observed (Fig. 5). Likewise, an enve-
tralobjectincreasestoM =0.37±0.13M .ForElias29,the
lopesignificantlymoremassivethan0.022M(cid:12)foundforIRS63 star (cid:12)
points attributed to the emission from the dense ridge (Fig. 7)
would obscure the central star so much that the near- and mid-
were disregarded in the fitting procedure. A lower limit to the
IRfluxwouldbeseverlyunderestimated(Fig.6).Inthefurther
central mass of M = 0.62±0.14 M is found, under the as-
discussion, we will use the disk and envelope masses as found star (cid:12)
sumptioni = 90◦.TheflatnessoftheSEDlimitstheinclination
bythe detailedmodelling,butnote thattheoverall conclusions
of this source to < 60◦ (Boogert et al. 2002). A fully face-on
donotchangeiftheempiricalresultsareused.
orientation,ontheotherhand,isunlikely,giventhepresenceof
TheenvelopemassesofIRS63andElias29areinthesame
low surface brightness scattered K-band light (Zinnecker et al.
rangeasthosepresentedby,e.g.,Hogerheijdeetal.(1997)fora
1988).Aninclinationi=30◦yieldsamassforthecentralsource
sample of embedded Class I sources in the Taurus-Auriga star-
of2.5±0.6M .Withsuchahighmass,thecentralstarislikely
forming region. However, the envelope masses of the deeply (cid:12)
toemergeasaHerbigAe/Bestar,oncethesurroundingenvelope
embedded Class 0 objects are found to be considerably higher,
(cid:38) 1M (Jørgensenetal.2002;Shirleyetal.2002;Youngetal. isdissipated.
(cid:12)
It is interesting to compare the inferred masses to those in-
2003;Hatchelletal.2007).Whilesomefractionofsourcesmay
ferredforopticallyvisiblepre-mainsequencestars.Withastel-
simplyoriginatefromlowermasscores,itisclearthatallClass
larmassof0.37M andastellarluminosityof0.79L ,IRS63
0objectsmustpassthroughastagewithM ≈0.1M ontheir (cid:12) (cid:12)
env (cid:12) wouldbefoundtohaveanageofabout5×105 yr,whencom-
waytothepre-mainsequencestage.Alargersampleisneededto
paredtoevolutionarytracksofclassicalandweak-linedTTauri
addressthequestionwhetherthemassthatwasinitiallypresent
stars in the ρ Ophiuchi cloud (Wilking et al. 2005; D’Antona
in the envelope will first accumulate in the disk, or whether it
& Mazzitelli 1997). A similar age would be found for Elias 29
willpassthroughthediskandontothestardirectly,leavingthe
(e.g.,Palla&Stahler1993).
diskinasteadystatethroughtheentireClassIstage.
An interesting quantity for these YSOs is the ratio
M /M ,asthismaybeadirectmeasurefortheirevolutionary
env disk 5. Conclusions
stage.Thevaluesforthetwoobjectsunderconsiderationhere–
0.2forIRS63and6forElias29–arequitedifferent,although We used the SMA to observe the Class I YSOs Elias 29 and
the value for Elias 29 is rather uncertain due to the contribu- IRS63at1.1mm.Themainresultsareasfollows.
tionoftheridgeenhancementtothecontinuumemission.From
thesesimplearguments,Elias29isthelessevolved,ratherlike – BothsourcesaredetectedinthecontinuumandintheHCO+
the deeply embedded Class 0 sources in the PROSAC sample J = 3–2 line, but not in the HCN J = 3–2 line. The HCO+
(Jørgensen et al. 2007), which show M /M (cid:38) 10. IRS 63, emissiontowardIRS63iscompactandassociatedwiththe
env disk
ontheotherhand,iswellonitswaytowardtheopticallyvisible YSO, whereas a significant part of the molecular emission
ClassIIorTTauristage,whichhaveM ≈0.Thisconfirmsthe inthedirectionofElias29isduetoanenhancementinthe
env
notionthattheLadaclassificationisprimarilyaphenomenolog- ridgefromwhichElias29formed.
ical one, not necessarily representing the actual physical stage – Assumingthatthecontinuumemissiontowardthesourcesis
theYSOisin(see,e.g.,Crapsietal.2007). opticallythin,andthatanycircumstellarenvelopeisresolved
DaveLommenetal.:SeparatingthemassesinClassIYSOs 7
– Velocity gradients in the HCO+ J = 3–2 emission are in-
terpretedassignsofKeplerianrotation,andindicatecentral
massesof2.5±0.6M forElias29and0.37±0.13M for
(cid:12) (cid:12)
IRS63,forinclinationsof30◦.Thesemassescorrespondto
agesofafew105yr.
With a larger set of sources treated in a similar manner as
is presented for IRS 63 and Elias 29 here, it will be possible
toconstrainmodelsfortheevolutionoftheenvelope,disk,and
stellarmassesthroughthisimportantstageofYSOs.
Acknowledgements. Wewouldliketothanktheanonymousrefereeforavery
constructive report, which greatly improved the paper. Demerese Salter and
DougJohnstonearethankedforprovidinguswithdatafromJCMTSCUBAob-
servations.ThediscussionswiththemembersoftheLeiden“AstroChem”group
wereveryuseful.WearegratefultoKeesDullemondforprovidinguswiththe
mostrecentversionoftheRADMCpackage.Partialsupportforthisworkwas
providedbyaNetherlandsResearchSchoolForAstronomynetwork2grant,and
byaNetherlandsOrganisationforScientificResearchSpinozagrant.
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