ALMA 690 GHz observations of IRAS 16293-2422B: Infall in a highly optically-thick disk

0.19 MB·English

by  Luis A. Zapata

ToappearinApJL
PreprinttypesetusingLATEXstyleemulateapjv.08/22/09
ALMA690GHZOBSERVATIONSOFIRAS16293-2422B:INFALLINAHIGHLYOPTICALLY-THICKDISK
LuisA.Zapata1,LaurentLoinard1,2,LuisF.Rodr´ıguez1,3,VicenteHerna´ndez1,
SatokoTakahashi4,AlfonsoTrejo4,andBe´renge`reParise2
ToappearinApJL
ABSTRACT
Wepresentsensitive,highangularresolution(∼0.2arcsec)submillimetercontinuumandlineobservations
3 ofIRAS16293-2422BmadewiththeAtacamaLargeMillimeter/SubmillimeterArray(ALMA).The0.45mm
1
continuum observations reveal a single and very compact source associated with IRAS 16293-2422B. This
0
submillimeter source has a deconvolved angular size of about 400 milli-arcseconds (50 AU), and does not
2
showanyinnerstructureinsideof thisdiameter. TheH13CN, HC15N, andCH OH line emissionregionsare
3
n about twice as large as the continuum emission and reveal a pronounced inner depression or ”hole” with a
a sizecomparabletothatestimatedforthesubmillimetercontinuum. Wesuggestthatthepresenceofthisinner
J
depression and the fact that we do not see inner structure (or a flat structure) in the continuum is produced

4 by very optically thick dust located in the innermostparts of IRAS 16293-2422B.All three lines also show
1 pronouncedinverse P-Cygni profiles with infall and dispersion velocities largerthan those recently reported
fromobservationsatlowerfrequencies,suggestingthatwearedetectingfaster,andmoreturbulentgaslocated
] closertothecentralobject. Finally,wereportasmalleast-westvelocitygradientinIRAS16293-2422Bthat
A
suggeststhatitsdiskplaneislikelylocatedveryclosetotheplaneofthesky.
G
Subjectheadings: stars: pre-mainsequence–ISM:jetsandoutflows–ISM:individual:(IRAS16293−2422)
. –techniques:spectroscopic
h
p
-
o 1. INTRODUCTION and lacking of a jet-like feature along its symmetry axis. In
r addition,itsdynamicalageisonlyabout200years.
Locatedatadistanceof120pc(Loinardetal.2008)intheρ
t
s Ophiuchistarformingregion,IRAS16293−2422B,isawell- One of the first evidences of the detection of infall mo-
a tionsassociatedwith IRAS16293−2422BcamefromChan-
studiedlow-massveryyoungstar. Togetherwithitsclose-by
[ companionseparatedbyonly600AU(IRAS16293−2422A), dler et al. (2005) using Submillimeter Array (SMA) obser-
1 theentireregion(IRAS16293−2422)hasabolometriclumi- vations at 300 GHz. More recently, Pineda et al. (2012)
using ALMA Science Verification observations with high-
v nosityof25L⊙,andisembeddedina2M⊙ envelopeofsize
5 ∼ 2000AU (Correia et al. 2004). Both sourcesshow a very spectral resolution studied the gas kinematics with detail in
0 rich and complex chemistry, with hot-core-like (hot-corino) IRAS 16293−2422Bat 220GHz, and reportedclear inverse
1 properties at scales of ∼ 100 AU and temperatures of about P-Cygni profiles toward this source in their three brightest
3 100K(Ceccarellietal.1998;Cazauxetal.2003;Bottinelliet linesandderivedfromasimpletwo-layermodelaninfallrate
1. al.2004;Chandleretal.2005;Cauxetal.2011). of4.5×10−5 M⊙ yr−1,whichisatypicalvalueforlow-mass
0 Sensitiveandhighangularresolutionobservationsat7mm protostars.
3 revealed a compact, possibly isolated disk associated with In this Letter, we report ∼0.2 arcsec resolution 690 GHz
1 IRAS 16293−2422B with a Gaussian half-power radius of observations obtained with the Atacama Large Millime-
: only8 AU (Rodr´ıguezet al. 2005). However, the search for ter/Submillimeter Array (ALMA) from the object IRAS
v an outflow associated with this source has been a hard task. 16293-2422B. The continuum observations reveal a very
Xi Jørgensenet al. (2011)using submillimeter(SMA) observa- compactsourcewithadeconvolvedangularsizeofabout400
tionsofIRAS16293−2422witharelativelyhighangularres- milli-arcsecondsor a spatial size of about 50 AU, while the
r
a olution resolved the A and B components, and did not find line emission shows a clear pronouncedinner depression or
”hole” in the middle of IRAS 16293−2422B. All the three
anystrongindicationsforhighvelocitygastowardB.Yehet
al.(2008)reportedthedetectionofacompactblue-shiftedCO linesmappedinthisstudyshowpronouncedinverseP-Cygni
profiles.
structuretothesouth-eastofsourceB,andmentionedthatit
might correspond to a compact outflow ejected from source 2. OBSERVATIONS
B. Loinardetal. (2012)usingALMAobservationsrevealed
The observations were made with fifteen antennas of
indeed that the source B is driving a south-east blueshifted
ALMAonApril2012,duringtheALMAscienceverification
compactoutflow. However,the flowhaspeculiarproperties:
itishighlyasymmetric,bubble-like,fairlyslow(10kms−1), dataprogram. Thearrayatthatpointonlyincludedantennas
withdiametersof12meters. The105independentbaselines
ranged in projected length from 26 to 403 m. The observa-
1Centro deRadioastronom´ıa yAstrof´ısica, UNAM,Apdo. Postal3-72
tionsweremadeinmosaicingmodeusingahalf-powerpoint
(Xangari),58089Morelia,Michoaca´n,Me´xico
2Max-Planck-Institut fu¨r Radioastronomie, Auf dem Hu¨gel 69, 53121, spacingbetweenfieldcentersandthuscoveringbothsources
Bonn,Germany IRAS 16293−2422Aand B. However, in this study we will
3AstronomyDepartment,FacultyofScience,KingAbdulazizUniversity, focusonlyonthemolecularandcontinuumemissionarising
P.O.Box80203,Jeddah21589,SaudiArabia from IRAS 16293−2422B. The primary beam of ALMA at
4AcademiaSinicaInstituteofAstronomyandAstrophysics,P.O.Box23-
690GHzhasaFWHMof∼8arcsec.
141,Taipei10617,Taiwan
TheALMA digitalcorrelatorwas configuredin 4 spectral
2 Zapata,etal.
TABLE1
Observationalandphysicalparametersofthesubmillimeterlines
Restfrequencya Elower RangeofVelocities Linewidth LSRVelocityb LinePeakflux
Lines [GHz] [K] [kms−1] [kms−1] [kms−1] JyBeam−1
H13CN[J=8−7]ν=0 690.55207 80.6 −1,+7 4.0 −3.0 1.00
HC15N[J=8−7]ν=0 688.27379 80.3 −1,+7 3.8 −3.0 1.00
CH3OH[9(3,6)-8(2,7)]νt=0 687.22456 84.3 −1,+7 3.8 −3.0 1.02
aThe rest frequencies were obtained from the Splatalogue:
http://splatalogue.net
windowsof1875MHzand3840channelseach.Thisprovides
achannelwidthof0.488MHz(∼0.2kms−1),butthespectral
resolution is a factor of two lower (0.4 km/s) due to online
Hanningsmoothing.
Observations of Juno provided the absolute scale for the
flux density calibration while observations of the quasars
J1625−254andNRAO530(withfluxdensitiesof0.4and0.6
Jy, respectively) provided the gain phase calibration. The
quasars3C279andJ1924-292wereusedforthebandpasscal-
ibration.
The data were calibrated, imaged, and analyzed using the
CommonAstronomySoftwareApplications(CASA).Toan-
alyze the data, we also used the KARMA software (Gooch
1996).Theresultingr.m.s.noiseforthelineimageswasabout
50mJybeam−1inavelocitywidthof0.4kms−1 and20mJy
beam−1forthecontinuumemissionatanangularresolutionof
0′.′31×0′.′18withaP.A.=−69.3◦. Weusedarobustparam-
eterof0.5intheCLEANtask. Thespectraandthephysical
parameters of the observed lines are shown in Figure 1 and
Table1,respectively.Manymorespectrallinesfromdifferent
molecularspecieswerefoundacrosstheentirespectralband-
width, however, this study will concentrate on the analysis
ofthecontinuumemissionandthelinespresentedinTable1 IRFAigS.116.—293H−1234C2N2B(b.lTueh)e,HblCac1k5Nda(mshaegdelnitnae),manadrkCsHth3eOsHys(tgermeeicn)LsSuRmmveeldocsiptyecotrfaIfRroAmS
thatareassociatedwithIRAS16293−2422B.Theseselected 16293−2422B(VLSR∼3kms−1).
linesshowagoodcontrastbetweentheabsorptionandemis-
sionfeaturesascomparedwiththerest. Wegivethelinepeak
whenourbeamsizeisabouthalfofthesource’ssizeatthese
emissionofeverylineinTable1.
wavelengths. Thissuggeststhatweareseeingveryoptically
thickdustemissionatthesewavelengths.
3. RESULTSANDDISCUSSION
The flux density of IRAS 16293−2422B at these wave-
3.1. 0.45mmcontinuumemission lengths is 12.5 ± 0.5 Jy and has a peak flux of 3.2 ± 0.2 Jy
In Figure 2, we show color and contour maps of the line Beam−1. Using the full Planck equation, we can obtain the
and continuum emission as mapped by ALMA from IRAS brightnesstemperature:
16293−2422Batthesewavelengths. InthisFigure,we have
hν
overlaidthe resultingcontinuummapwith the integratedin- T = ,
b
tensity (moment zero) maps of the spectral molecular lines.  
Istuirsroculenadrslythoebsceornvteidnuiunmallelminiesssi,otnhaatnthdehmasolaecsutrloarngemciesnstiroanl kln2Shννc32Ω +1
depressionor”hole”inthemiddle.
where c is the speed of light, S is the flux density, ν is
Thepeakofthecontinuumshowsasmalloffsettothewest the frequency, k is Boltzmann conνstant, and Ω is the solid
withrespecttothecentralpositionofthe”hole”. Thissmall
angle. Following this relation, and using a Gaussian beam,
deviation might be explained by opacity effects of the dust
we estimated a brigthness temperature at these wavelengths
emission at these wavelengths. However, this shift effect forIRAS16293−2422Bof160K.
is also observed at longer wavelengths by Rodr´ıguez et al.
Withthesefluxvaluesonecanestimatealowerlimitforthe
(2005).
massofthedisk. Assumingthatthedustisopticallythinand
The dust compact source has a deconvolvedsize of about
isothermal, the dust mass (M ) will be directly proportional
400 ± 55 milli-arcseconds or a spatial size of 50 AU at the d
tothefluxdensity(S )as:
distanceofIRAS16293−2422.Thissizeisquitelarge(afac- ν
torofaboutsix)comparedtothatfoundat7mmbyRodr´ıguez d2S
etal.(2005).Thisdifferenceinapparentangularsizesismost Md = κ B (Tν ),
probably the result of the increasing optical depth with fre- ν ν d
quencyofthedust. Moreover,insideofthe50AUdiameter, wherethedisthedistancetotheobject,κ thedustmassopac-
ν
the source B does not show any more inner structure even ity, and B (T ) the Planck function for the dust temperature
ν d
3
13
H CN
Fig.3.—Integratedintensityoftheweightedvelocity(moment1)colormapofthe
H13CNemissionfromthesourceIRAS16293−2422Boverlaidincontourswiththe
15 integratedintensitycontourmap(white).Thewhitecontoursarefrom15%to90%with
HC N stepsof5%ofthepeakoftheintegratedlineemission;thepeakoftheintegratedline
emissionis4.1×103mJyBeam−1kms−1.Thecolor-scalebarontherightindicatesthe
LSRvelocitiesinkms−1.
T . Assuming a dust mass opacity (κ ) of 2.2 cm2 g−1, ob-
d ν
tained extrapolatingat these wavelenghts the value obtained
byOssenkopf&Henning(1994)forcoagulateddustparticles
with no ice mantles, and at a density of 108 cm−3. Assum-
ing also an opacity power-law index β = 0.6 (Rodr´ıguez et
al.2005),aswellasacharacteristicdusttemperature(T )of
d
160K,weestimatedalowerlimitforthemassforthediskof
about0.03M⊙. Pleasenotethatthelevelofuncertaintyinthe
masslowerlimitis a factoroffive, giventherangeof 0.435
mmopacitiesinTable1ofOssenkopf&Henning(1994).
3.2. Molecularlineemission
InFigure1and2,asmentionedearlier,wepresenttheinte-
gratedintensitymaps(moment0) ofthe molecularemission
reportedonthiswork. Thespectrumfromalllineswereob-
CH OH tained averaging in an area (box) similar to the size of the
3 molecular ring like structure (∼ 1.0′′). The spectrum of all
lines is found to be well centered at an LSR velocity of +3
kms−1,whichisapproximatelythesystemicvelocityforthis
source (Pinedaet al. 2012;Jørgensenet al. 2011). All three
linesalsoshowmarkedinverseP-Cygniprofilesbutwiththat
oftheCH OHshowingthemorepronouncedabsorptionfea-
3
ture. Thisis probablyduetothisline frequentlybeingmore
optically thick. The H13CN and HC15N show line profiles
very similar with the emission components being stronger
compared with the CH OH spectra, see Figure 1. This lat-
3
ter line mostly shows two faint condensations surrounding
thecontinuumsourceanddoesnotpresentamarkedringlike
structureascomparedwiththerestofthelines.
The morphology of the line emission in general forms a
welldefinedringlikestructurearoundthecontinuum. How-
ever, the ring like structure is not completely closed, there
is a smallcavity towardsits southeast. This cavity is proba-
Fig.2.—H13CN(blue),HC15N(magenta),andCH3OH(yellow)colorscalemoment bly created by the southeast monopolaroutflow reported by
0mapsfromIRAS16293−2422Boverlaidingreycontourswithitsownlineemission Loinardetal. (2012). Themolecularringlikestructurehas a
andinredcontourswiththe0.45mmcontinuumemission. Theredcontoursarefrom diameterof850±50milli-arcsecondsandaninnerdiameter
15%to90%withstepsof7%ofthepeakofthelineemission;thepeakofthe0.45 of300±50milli-arcseconds,whichiscomparabletothede-
mmcontinuumemissionis3.2JyBeam−1.Forthelineemissionthegreycontoursare
from15%to90%withstepsof5%ofthepeakoftheintegratedlineemission. The convolvedsizeofthe0.45mmcontinuumsource(about400
synthesizedbeamofthecontinuumimageisshowninthebottomleftcornerofeach milli-arcseconds).
image.
4 Zapata,etal.
the ratio of solid angles between the region in absorptionto
theregionin emission. Fromthefits ofPinedaetal. (2012)
to lines at lower frequencies (220 GHz) they estimate T =
x
40K.Thelinessampledbyusareprobablyoriginatingingas
closer to the star (see below). Assuming that the excitation
temperatureofthemoleculesdecreasesasthesquarerootof
thedistanceandthatthegassampledbyPinedaetal. (2012)
is50%moredistantthanthatsampledbyus, weadoptT =
x
50K.Thefittingwasminimizedwithagridsearch,obtaining
thefollowingparameters: V =0.7kms−1,σ =0.6kms−1,
in v
andτ =0.17.Finally,thesystemicvelocityobtainedfromthe
0
fitwasV =3.0kms−1.
LSR
Severaloftheparametersarequitesimilartothosederived
byPinedaetal. (2012)fromlinesat220GHz. However,oth-
ers are not and we discuss them here. First, the brightness
temperatureof the optically thick continuumsource is 20 K
in the case of Pineda et al. (2012) and 160 K in this paper.
This is as expected since the optical depth of dust increases
sharplywithfrequency. Thelargebrightnesstemperaturede-
rivedbyusandconsistentwith ourprofilemodelingimplies
thatat690GHzweareobservingatrulyoptically-thickdisk
sincethebrightnesstemperatureiscomparablewiththether-
modynamictemperaturesexpectedintheinnerpartsofaYSO
accretiondisk. Theopticaldepthusedbyusisaboutonehalf
ofthatusedbyPinedaetal. (2012).Finally,theinfallvelocity
andvelocitydispersionrequiredbyourmodeling(0.7and0.6
Fig.4.—Observed(solidline)andmodeled(dashedline)spectraoftheCH3OH. kms−1, respectively)arelargerthanthoseusedbyPinedaet
al. (about0.5and0.3kms−1,respectively),implyingthatwe
maybedetectingfaster, moreturbulentgaslocatedcloserto
In Figure 3, we show the integrated intensity of the
thecentralobject.Thisisconsistentwiththestandardpicture
weightedvelocity(moment1)colormapoftheH13CN.This
of infall, where highervelocitiesoccuratsmaller radii. The
figure reveals a clear east-west velocity gradient of approx-
velocities reported here are supersoinic as those reported in
imately 1.3 km s−1 over 60 AU. This small gradient might
Pinedaetal. (2012).
suggestthattheorientationofthe diskplaneisverycloseto
the plane of the sky. If we assume that the rotation is Kep- 4. SUMMARY
lerianandattributeittoadiskinrotationthenthedynamical
In this paper, we have reportedline and continuumobser-
massassociatedtothisvelocitygradientcorrespondstoonly
vations obtained with ALMA at 690 GHz with a very high
0.1 M⊙, which is indeed very small and is comparable with angularresolution(∼0.2arcsec)ofIRAS16293-2422B.The
thedustdiskmass. However,ifwecorrectbytheinclination
angledividingthismassbythesinθwithθsay5◦,weobtaina mainconclusionsareasfollows:
valueof1.2M⊙,amorereasonablevalueforthecentralobject • The 0.45 mm continuum emission revealed a very
andthediskassociatedwithasolartypeyoungstar,seeCor-
compact object with a deconvolved angular size of
reia et al. (2004). We thereforeconcludethatthe disk plane
ofIRAS16293−2422Bmustbelocatedalmostontheplaneof about400milli-arcsecondsthatisassociatedwithIRAS
16293-2422B.This size is very large comparedto the
thesky,asalreadysuggestedbyRodr´ıguezetal.(2005).
onereportedat7mm(about8AU)anddoesnotshow
anystructureinsideofthisdiameter(oraflatstructure).
3.3. Modeling
InFigure4,weshowtheresultofthemodeledspectrathat • The H13CN, HC15N, and CH OH images revealed a
3
we made in this study. We fitted the spectral profile using a pronouncedinnerdepressionor”hole”withasizecom-
modifiedtwo-layermodel(Myerset al. 1996; Di Francesco parable to that estimated forthe submillimetercontin-
etal2001;Kristensenetal. 2012),asdescribedbyPinedaet uum.
al. (2012).Thismodelconsistsoftwolayersofgas,frontand
rear, thatare infalling towardsthe centralsource with an in- • We suggest that the presence of this inner depression
fallvelocity,velocitydispersion,excitationtemperature,and with an angularsize comparable with that of the con-
opacityatthecenterofthelineofVin,σv,Tx,andτ0,respec- tinuum source and the fact that we do not see inner
tively. In between the two layers there is an optically thick structure in the continuum is produced by very opti-
continuumsourceemittingasablackbodyoftemperatureTc, callythickdustlocatedintheinnermostpartsofIRAS
andfillingafractionofthebeam,Φ.Thebackgroundtemper- 16293-2422B.
aturethatilluminatestherearlayeristakentobethecosmic
background,T =3K. • AllthreelinesalsoshowinverseP-Cygniprofileswith
f
Thebrightnesstemperatureoftheopticallythickcontinuum infall and dispersion velocities larger than those re-
sourceistakentobesuchthatitmatchesthepeakcontinuum centlyreportedatsmallerwavelengths,suggestingthat
flux density of the image, T = 160 K. The adopted filling wearerevealingfaster,andmoreturbulentgaslocated
c
fractionofthecontinuumsource, Φ =0.37isconsistentwith closertothecentralobject.
5
• We reporta smalleast-west velocitygradientin IRAS indebted to the Alexandervon HumboldtStiftung for finan-
16293-2422Bobservedinalllinesthatsuggeststhatthe cial support. This papermakes use of the followingALMA
disk planeof thisobjectis likely locatedverycloseto data: ADS/JAO.ALMA#2011.0.00007.SV.ALMA is a part-
theplaneofthesky. nershipofESO(representingitsmemberstates),NSF(USA)
and NINS (Japan), together with NRC (Canada) and NSC
and ASIAA (Taiwan), in cooperation with the Republic of
Chile. The Joint ALMA Observatory is operated by ESO,
L.A.Z, L. L. and L. F. R. acknowledge the financial sup-
AUI/NRAOandNAOJ.
portfromDGAPA,UNAM,andCONACyT,Me´xico.L.L.is
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