Dust properties across the CO snowline in the HD 163296 disk from ALMA and VLA observations

0.83 MB·

by  G. Guidi

Astronomy&Astrophysicsmanuscriptno.guidihd163296 (cid:13)c ESO2016
January28,2016
Dust properties across the CO snowline in the HD 163296 disk from
ALMA and VLA observations
G.Guidi1,2,M.Tazzari3,4,L.Testi1,3,4,I.deGregorio-Monsalvo3,5,C.J.Chandler6,L.Pe´rez6,7,A.Isella8,A.Natta1,9,
S.Ortolani10,11,Th.Henning12,S.Corder5,H.Linz12,S.Andrews13,D.Wilner13,L.Ricci13,J.Carpenter14,
A.Sargent14,L.Mundy15,S.Storm15,N.Calvet16,C.Dullemond17,J.Greaves18,J.Lazio19,A.Deller20,and
W.Kwon21
1 INAF-OsservatorioAstrofisicodiArcetri,LargoE.Fermi5,I-50125Firenze,Italy
2 DipartimentodiFisicaeAstronomia,Universita`degliStudidiFirenze,Italy
3 ESO,KarlSchwarzschildstr.2,D-85748GarchingbeiMu¨nchen,Germany
6 4 ExcellenceCluster‘Universe’,Boltzmannstr.2,D-85748GarchingbeiMuenchen,Germany
1 5 JointALMAObservatory(JAO),AlonsodeCordova3107Vitacura-SantiagodeChile
0 6 NationalRadioAstronomyObservatory,SocorroNM87801,USA
2 7 JanskyFellow
8 DepartmentofPhysicsandAstronomy,RiceUniversity,6100MainStreet,Houston,TX,77005,USA
n
9 SchoolofCosmicPhysics,DublinInstituteforAdvancedStudies,31FitzwilliamsPlace,2Dublin,Ireland
a
J 10 DipartimentodiFisicaeAstronomia,Universita`diPadova,Italy
11 INAF-OsservatorioAstronomicodiPadova,VicolodellOsservatorio5,I-35122Padova,Italy
7
12 MaxPlanckInstitutfu¨rAstronomie,Ko¨nigstuhl17,D-69117Heidelberg,Germany
2
13 Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138,USA

14 DepartmentofAstronomy,CaliforniaInstituteofTechnology,MC249-17,Pasadena,CA91125,USA
]
R 15 DepartmentofAstronomy,UniversityofMaryland,CollegePark,MD20742
16 DepartmentofAstronomy,UniversityofMichigan,830DennisonBuilding,500ChurchStreet,AnnArbor,MI48109,USA
S
17 InstituteforTheoreticalAstrophysics,HeidelbergUniversity,Albert-Ueberle-Strasse2,D-69120Heidelberg,Germany
h. 18 SUPA,SchoolofPhysicsandAstronomy,UniversityofStAndrews,NorthHaugh,StAndrews,KY169SSScotland,UK
p 19 JetPropulsionLaboratory,CaliforniaInstituteofTechnologyPasadena,CA91109,USA
- 20 TheNetherlandsInstituteforRadioAstronomy(ASTRON),Dwingeloo,TheNetherlands
o 21 KoreaAstronomyandSpaceScienceInstitute,776Daedeok-daero,Yuseong-gu,Daejeon34055,RepublicofKorea
r
t
s Preprintonlineversion:January28,2016
a
[ ABSTRACT

1
Context.Tocharacterizethemechanismsofplanetformationitiscrucialtoinvestigatethepropertiesandevolutionofprotoplanetary
v
disksaroundyoungstars,wheretheinitialconditionsforthegrowthofplanetsareset.ThehighspatialresolutionofAtacamaLarge
2 Millimeter/submillimeterArray(ALMA)andKarlG.JanskyVeryLargeArray(VLA)observationsallowsnowthestudyofradial
4
variationsofdustpropertiesinnearbyresolveddisksandtheinvestigatationoftheearlystagesofgraingrowthindiskmidplanes.
5
Aims.Ourgoalistostudygraingrowthinthewell-studieddiskoftheyoung,intermediatemassstarHD163296wheredustprocessing
7
hasalreadybeenobserved,andtolookforevidenceofgrowthbyicecondensationacrosstheCOsnowline,alreadyidentifiedinthis
0
diskwithALMA.
1. Methods. Under the hypothesis of optically thin emission we compare images at different wavelengths from ALMA and VLA to
0 measuretheopacityspectralindexacrossthediskandthusthemaximumgrainsize.WealsouseaBayesiantoolbasedonatwo-layer
6 diskmodeltofittheobservationsandconstrainthedustsurfacedensity.
1 Results. The measurements of the opacity spectral index indicate the presence of large grains and pebbles (≥1 cm) in the inner
: regionsofthedisk(inside∼50AU)andsmallergrains,consistentwithISMsizes,intheouterdisk(beyond150AU).Re-analysing
v
ALMABand7ScienceVerificationdatawefind(radially)unresolvedexcesscontinuumemissioncenterednearthelocationofthe
Xi COsnowlineat∼90AU.
Conclusions.OuranalysissuggestsagrainsizedistributionconsistentwithanenhancedproductionoflargegrainsattheCOsnowline
r andconsequenttransporttotheinnerregions.OurresultscombinedwiththeexcessininfraredscatteredlightfoundbyGarufietal.
a
(2014)suggeststhepresenceofastructureat90AUinvolvingthewholeverticalextentofthedisk.Thiscouldbeevidenceforsmall
scaleprocessingofdustattheCOsnowline.
Keywords.protoplanetarydisks–stars:circumstellarmatter–stars:formation–radiocontinuum:planetarysystems
1. Introduction cessesarethoughttooccurincircumstellardisksaroundyoung
stars on rather short timescales: infrared surveys show that the
Understandingthemechanismsthatcausesmalldustparticlesto fractionoflowmassstarswithdisksdropsdramaticallyforstars
grow into larger bodies and eventually form planets is still an olderthan10Myr(Mamajek2009;Herna´ndezetal.2007),sug-
openissueinspiteofthelargelaboratory,theoreticalandobser- gesting that in order to form large solids before the disk dis-
vationalefforts(seethereviewbyTestietal.2014).Thesepro-
1
Guidietal.:DustpropertiesacrosstheCOsnowlineinHD163296
Fig.1.Continuummapsatdifferentwavelengths,fromtheleft:850µm,1.3mmand9mm,thislatterobtainedfromthecombination
ofVLA8.0mmand9.8mmbands.Thecontourlevelscorrespondto-3(dashed),3,6,12,24,48,100,200,400,800,1600σ.The
bottomrightofeverypanelshowsthesynthesizedbeam(seeTable1).
λ ν F F rms CLEANbeam BeamP.A.
int peak
[mm] [GHz] [Jy] [Jy/beam] [mJy/beam] [FWHM] [◦]
ALMAB7 0.85 352.9 2.13±0.21 0.44 0.18 0.57(cid:48)(cid:48)×0.37(cid:48)(cid:48) 86.5
ALMAB6 1.33 225.3 0.60±0.06 0.22 0.37 0.74(cid:48)(cid:48)×0.60(cid:48)(cid:48) 77.3
VLAKa 9.00 34.0 (1.83±0.02)·10−3 0.847·10−3 0.013 0.18(cid:48)(cid:48)×0.11(cid:48)(cid:48) -49.4
VLAKa 8.00 37.5 (2.02±0.2)·10−3 0.928·10−3 0.024 0.16(cid:48)(cid:48)×0.10(cid:48)(cid:48) −56.1
VLAKa 9.83 30.5 (1.65±0.2)·10−3 0.806·10−3 0.015 0.19(cid:48)(cid:48)×0.12(cid:48)(cid:48) 131.5
Table1.ParametersforthedeconvolvedimagesdisplayedinFig.1andfortheimagesobtainedfromthesingleVLAfrequencies
of30and37GHz.
perses the growth process has to take place within a few mil- condensation across the snowline or by changing the sticking
lionyears.Analysisofdustpropertiesinprotostellarenvelopes propertiesofice-coatedgrainsand,inaddition,thelocalrelease
and young circumstellar disks suggests the formation of large ofvolatilesfromtheicesmayinducealocalpressurebumpthat
grains (even up to few millimeter size) during the disk forma- could trap large grains (e.g. Supulver & Lin 2000; Wada et al.
tion stage in infalling envelopes (Kwon et al. 2009; Miotello 2009;Ros&Johansen2013;Gundlach&Blum2015).
etal.2014);still,substantialgrowthofdustgrainsoccursinthe
The dense regions of disk midplanes can be investigated
cold midplane of protoplanetary disks, where most of the solid
at sub-millimeter and millimeter wavelengths, where the dust
mass is confined and planetesimals and planets are thought to
emission is more optically thin and can now be spatially
form. The recent ALMA results for the young protoplanetary
resolved with present-day facilities, like the Atacama Large
disksurroundingHLTausuggestthatplanetformationmayin-
Millimeter/submillimeterArray(ALMA)andtheKarlG.Jansky
deed occur very early in the disk lifetime (ALMA Partnership
VeryLargeArray(VLA),amongothers.Thesefacilitiesallowus
etal.2015;Dipierroetal.2015;Tamayoetal.2015).
to obtain sensitive and high angular resolution observations of
The life of a growing dust grain in a protoplanetary disk
the dust continuum and gas emission lines from protoplanetary
is not an easy one. It was realized already four decades ago
disksandthereforestudytheradialvariationsofthephysicalpa-
thataerodynamicalfrictionmayeffectivelypreventgraingrowth
rametersofthedisk.Recentresultsincludethestudyofthedust
(Weidenschilling 1977). The process of rapid radial migration
propertiesasafunctionofradiusindisks(e.g.,Guilloteauetal.
and fragmentation effectively sets an upper limit to the grain
2011;Banzattietal.2011;Pe´rezetal.2012,2015;Menuetal.
sizes as a function of radius and a very rapid evolutionary
2014;Tazzarietal.2015),andtheclearidentificationoftheCO
timescalefordustparticlesindisks(Braueretal.2008;Birnstiel
snowline in nearby disks (Mathews et al. 2013; Qi et al. 2013,
etal.2012).Thesetheoreticalexpectationsareatoddswithdi- 2015), through its effects on DCO+ and N H+ abundances. It
2
rectobservationsofdustpropertiesintheouterdisksfrommil-
isthusbecomingpossibletodirectlyinvestigateobservationally
limeter observations (Ricci et al. 2010a; Birnstiel et al. 2010). the effect of the CO snowline on grain growth. This snowline
This is a general result, although some authors have shown
is thought to be one of the most important after that of water,
that disks with peculiar growth processes (e.g., Laibe 2014;
as the CO abundance in protostellar ices is found to be about
Dra¸z˙kowska et al. 2014) or specific dust properties (Okuzumi 30-40% of H O (O¨berg et al. 2011). In this work we focus on
etal.2012)mayretainlargeparticlesmoreefficiently.Toover- 2
the Herbig Ae star HD 163296, a bright and isolated object at
come the general inconsistency between models and observa- a distance of 122+17 parsec (van den Ancker et al. 1998) with
tions,themostcommonlyacceptedscenariosinvolvelocalgrain −13
a relatively massive disk (∼ 0.1M : Qi et al. 2011; Isella et al.
(cid:12)
growthandtrappinginsmallregions,withsizesoftheorderor
2007) and an excellent prototype for gas- and dust-rich proto-
smallerthanthelocaldiskscaleheight(Klahr&Henning1997;
planetary disks. An estimated age of 5 Myr was obtained from
Pinillaetal.2012;Testietal.2014;Johansenetal.2014).
the comparison between Hipparcos astrometric measurements
The regions in the disk midplane that correspond to andpre-mainsequenceevolutionarymodelsbyvandenAncker
the snowlines of major volatiles are particularly interesting. et al. (1998). The stellar parameters computed by Natta et al.
These may promote locally efficient grain growth through re- (2004) are M = 2.3M , L = 36L , T = 9500K. To study
∗ (cid:12) ∗ (cid:12) eff
2
Guidietal.:DustpropertiesacrosstheCOsnowlineinHD163296
thedustpropertiesintheprotoplanetarydiskaroundHD163296, Rosenfeldetal.(2013).Inthisworkwefocusontheimagingof
were-analysetheALMAScienceVerificationobservations(de thecontinuumemission,obtainedwiththetaskCLEANapply-
Gregorio-Monsalvoetal.2013;Mathewsetal.2013;Rosenfeld ingarobustweightingwithBriggsparameter0.5andachieving
et al. 2013) discussing for the first time the continuum emis- asynthesizedbeamof0.57”×0.37”andarmsof0.18mJy/beam
sion in Band 6, combined with new VLA observations from (seeTable1).
[email protected]§2wedescribetheobserva-
tionaldata,in§3wereportthemainnewresultsofouranalysis,
2.2. VLAObservations
in §4 we present the result of our disk modeling, and in §5 we
discussthemainimplicationsforgrainproperties. ObservationsofHD163296weremadeusingtheKarlG.Jansky
Very Large Array (VLA) of the National Radio Astronomy
Observatory2 as part of the Disks@EVLA project (AC982) in
2. Observations 2011MayandJune,intheBnAandAconfigurations.TheKa-
band (λ ∼1cm) receivers were used with two 1GHz basebands
2.1. ALMAObservations
centeredat30.5and37.5GHz,providingprojecteduv-spacings
The ALMA observations of HD 163296 (also known as from25to3,800kλ.Thecomplexgainwastrackedviafrequent
MWC 275) were part of the ALMA Science Verification observationsofJ1755−2232,andthespectralshapeofthecom-
Program2011.0.000010.SV1. plex bandpass was determined through observations of 3C279.
Band 6 observations were performed on 2012 June 9, June The absolute flux density scale was derived from observations
23, and July 7 using a set of configurations comprising 20, 21 of3C286(e.g.,Perley&Butler2013),anditsoverallaccuracy
and 19 antennas, respectively. The total integration time was is estimated to be 10%. The data were calibrated, flagged,
3.14 hours (1.4 hours on the science source), the field of view and imaged using a modified version of the VLA Calibration
was ∼20 arcseconds and the baselines ranged from 20 to 400 Pipeline (see https://science.nrao.edu/facilities/vla/data-
meters, corresponding to spatial scales of 1600 AU to 80 AU processing/pipeline/scripted-pipeline) with CASA. The as-
at the distance of the object. The flux density calibrator for the trometry reported here corresponds to that derived from the A
three execution blocks were Juno, Neptune and Mars respec- configurationdata.
tively, while the phase calibrator was J1733-130 and the band- In addition, HD 163296 was observed with the C-band
passcalibratorwasJ1924-292.Thecorrelatorwassetwithfour (λ∼6cm)receiversintheDnCconfigurationinSeptember2010,
spectral windows in dual polarization mode, two spectral win- inordertoevaluateanypotentialcontaminationfromionizedgas
dows in the upper side band and two in the lower side band: at shorter wavelengths. Two 1GHz basebands were centered at
two spectral windows, #0 (216.2 - 218 GHz) and #3 (233.1 - 5.3and6.3GHz.Complexgainvariationsweretrackedthrough
234.9GHz),wereusedtoobservetheline-freecontinuumwith observationsofJ1820−2528,andthebandpassandabsoluteflux
channel widths of 488 kHz; while at higher resolution (244 densityscalewasobtainedthroughobservationsof3C286.The
kHz) spectral window #1 (219.5 - 220.4 GHz) included the datawerecalibrated,flagged,andimagedusingtheCASAdata
C18O(2 − 1) line at 219.560 GHz and the 13CO(2 − 1) line at reductionpackage.HD163296wasdetectedwithintegratedflux
220.398GHz,andspectralwindow#2(230.5-231.5GHz)cov- densityF5.2cm =(410±57)µJy.
ered the CO(2 − 1) line at 230.539 GHz. Imaging of the con-
tinuum emission of HD 163296 was performed excluding the
above-mentionedlines(seeRosenfeldetal.2013;Klaassenetal. 3. Observationalresults
2013).Datacalibrationwasperformedusingversion4.1.0ofthe
3.1. Continuummaps
Common Astronomy Software Application (CASA), self cali-
brationwasappliedmakinguseoftheline-freechannels,andus- InFigure1weshowthecontinuumintensitymapsobtainedfrom
ingrobustweightingduringtheCLEANdeconvolutionwewere the line-free channels at three different wavelengths: ALMA
able to reach a resolution of 0.74”×0.60” with a rms of 0.37 Band 7, ALMA Band 6 and the combination of the two VLA
mJy/beam(seeTable1). frequency ranges at 30.5 and 37.5 GHz. The parameters of the
Band7observationsweremadeon2012June9,June11and imagesarelistedinTable1.
June 22, with the same antenna configurations as for Band 6. In agreement with previous work (de Gregorio-Monsalvo
Collectively,thefivedatasetscoveredanintegrationtimeof3.9 etal.2013;Nattaetal.2004)weobservemorecompactemission
hours,with2.3hoursonthesciencetarget.Thefluxdensitycal- atlongerwavelengths,obtainingat850µmaprojectedradiusof
ibrators were Juno and Neptune, while the bandpass and phase ∼2.4(cid:48)(cid:48) ata3sigmalevel,correspondingtoabout290AU,while
calibrators were the same as those used for Band 6. The two at 1.3 mm the outer radius of the emission is ∼260 AU, and is
spectral windows in the lower side band were #2 (345.56 − ∼40AUat8-10mm.Wenotethatthelowsignal-to-noiseofthe
346.03GHz)and#3(346.52−347.47)withchannelwidthsof VLA data at 8 and 10 mm can lead to an underestimate of the
122 and 244 kHz respectively; the ones in the upper side band extent of the emission at these wavelengths, and it is critical to
were#1(356.50−356.97GHz)with122kHzchannelwidthand considerthevisibilityfunctionforaproperanalysisofthedisk
#0(360.11−360.23GHz)atahighspectralresolutionof30.5 structure. We use 44◦ for the disk inclination and 133◦ for the
kHz; these included the emission lines: CO(3 − 2) at 345.796 diskpositionangle(fromQietal.2011)andplotthenormalized
GHz (#2), HCO+(4 − 3) at 356.734 GHz (#1), H13CO+(4 − 3) realandimaginarypartofthevisibilitiesinFigure2.Theseplots
at346.998GHz(#3)andDCO+(5−4)at360.160GHz(#0).A show that the real part of the visibilities decline more steeply
detailed analysis of these spectral lines has been published by at the shorter wavelengths (ALMA 850 µm and 1.3 mm) than
Mathewsetal.(2013),deGregorio-Monsalvoetal.(2013),and atlongerwavelengths(VLA8mmand9.8mm),demonstrating
1 The ALMA Science Verification data can be found at: 2 The National Radio Astronomy Observatory is a facility of the
https://almascience.eso.org/alma-data/science-verifica NationalScienceFoundationoperatedundercooperativeagreementby
tion AssociatedUniversities,Inc.
3
Guidietal.:DustpropertiesacrosstheCOsnowlineinHD163296
Hipparcos J1991.25
-21.57.21.53
1.0 VLA 9.8 mm
VLA - 8.0 mm
ALMA 1.3 mm
0.8 ALMA 0.85 mm
al
e
R
d 0.6 -21.57.21.72
e
z
ali
m 0.4
or
N
Hipparcos J2000.0
0.2
-21.57.21.9
"]
0.0 .'.
d Imag 00..12 DEC [◦
ze 0.0 -21.57.22.09
ali
m 0.1
or
N
0.2
0 100 200 300 400 500 600
uv distance [kλ]
-21.57.22.28
Fig.2. Real and imaginary part of the measured visibilities VLA 2011.5 σ 2σ 3σ
as a function of uv-distance, deprojected assuming PA=133 ◦,
ALMA B7 2012.5 σ 2σ 3σ
i=44◦and bin-averaged every 40 kλ. Visibilities at each wave-
lengthhavebeennormalizedbytheaveragevalueat40kλand
errorbarsdisplaythestandarderrorofthemean,negligiblefor
17:56:21.292 17:56:21.288 17:56:21.283 17:56:21.279
ALMAobservations. RA [hh.mm.ss]
Fig.3. Position of HD 163296 at the different observing
epochs. The dashed line represents the proper motions from
thatthemillimeterwavelengthemissionisintrinsicallyconsider- the Hipparcos measurement (J1991.25) to 2012, with respec-
ablymoreextendedthanthecentimeterwavelengthemission(a tive proper motion error ellipses at 1, 2 and 3 σ from the pre-
pointsourcewouldbeaconstant1.0asafunctionofuv-distance dicted positions. The black dot indicates our position estimate
in this plot). The integrated flux density above the 3σ level at for the star at epoch 2012.5 based on the peak of the image
850µmisF850µm =2.13±0.02Jy,similartothevaluefoundby in Band 7, while the red dot is the peak of the VLA image at
Isellaetal.(2007)anddeGregorio-Monsalvoetal.(2013),while 9mmin2011.5.Theerrorbarsaregivenbytheastrometricac-
inBand6wefindF1.3mm = 0.59±0.06Jy.Atthelongerwave- curacy of 0.1 arcsec for ALMA and 0.02 arcsec for VLA. The
lengthsthefluxdensitydecreasesbyalmost3ordersofmagni- blue solid line represents the proper motions calculated from a
tude,withF8mm =2.0±0.2mJyandF10mm =1.6±0.2mJy.We least squares regression between Hipparcos measurements and
include a calibration error of 10% in the measurements of the theobservations.
fluxdensities.Wefindatallwavelengthsasmoothlydecreasing
intensity profile, consistent with the disk temperature and sur-
for the VLA in A configuration it is expected to be ∼ 0.02(cid:48)(cid:48)4.
facedensitydecreasingwithradius.
The proper motions derived from a least squares interpolation
betweenourpeaksatthetwodifferentepochsandtheJ1991.25
3.2. Propermotions Hipparcospositionareconsistentwiththelatestreductionofthe
Hipparcos data (van Leeuwen 2007) within the errors and are
Wecheckedwhetherthepositionofthestaratthetwodifferent listed in Table 2. The main difference we find is in the Right
epochs of our observations was consistent with the proper mo-
Ascension,whereourbestfitwouldimplyasmallerpropermo-
tionsreportedintheliterature:applyingagaussianfitwithtothe tion. Nevertheless, the difference is still well within the uncer-
imageswefindthatthepositionsofthepeaksfallwithinthe3σ
tainties.
errorellipsefromthepredictedpositionbasedontheHipparcos
astrometric mission measurements (J1991.25). For the ALMA
observations (2012) we used Band 7 data because of its better 3.3. SEDandfree-freecontribution
signal to noise, with the peak position of the images obtained
Theintegratedfluxdensitiesmeasuredwithina3sigmalevelin
from the calibrated dataset before the self-calibration was ap-
our observations and the spectral energy distribution predicted
plied. t In Figure 3 are displayed the estimated positions of the
byourmodel(seeSection4)areplottedinFigure4.Ourmodel
central star; the astrometric error for interferometric observa-
is consistent with the observed flux densities in the literature
tions is affected, among other things, by the phase calibration
(Nattaetal.2004;Isellaetal.2007).Atsub-millimeterandmil-
anddependsonseveralfactors(weatherconditions,thesepara-
limeter wavelengths the continuum emission is due to the dust
tionbetweenthetargetandthecalibrator,etc.).Weassumehere
in the colder regions of the disk midplane, while at centimeter
that the absolute astrometry of the ALMA data is 0.1(cid:48)(cid:48)3; while
wavelengths the emission may also arise from free electrons in
thestellarwind.Sincethiscontributionisthoughttocomefrom
3 https://help.almascience.org/index.php?/Knowledgebase
/Article/View/153/6/what-is-the-astrometric-position- 4 https://science.nrao.edu/facilities/vla/docs/manuals/
accuracy-of-an-alma-observation oss/performance/positional-accuracy
4
Guidietal.:DustpropertiesacrosstheCOsnowlineinHD163296
RA[hh:mm:ss] DEC[◦.(cid:48).(cid:48)(cid:48)] 3.4. Excessemissionat850µm
HipparcosJ1991.25 17:56:21.293 -21.57.21.527
VLA2011.5 17:56:21.283 -21.57.22.30 InFigure5weshowtheintensityprofileoftheimageat850µm:
ALMA2012.5 17:56:21.285 -21.57.22.36 asthediskisinclinedby44◦fromthelineofsight,thebestangu-
pm-ra pm-dec errorellipse larresolutionisreachedusingonlythedataalongtheprojected
[mas/yr] [mas/yr] [masmas] diskmajoraxis.Weconsideredthepixelsinsideonebeamacross
themajoraxis,everypointcorrespondingtoapixelof0.1arcsec
Hipparcos -7.98 -39.21 [0.940.51]
thispaper -6.8 -38.5 [1.01.0] intheimage(thepointsarethereforenotallindependent).The
verticalspreadisduetotheshapeandpositionangleofthesyn-
thesizedbeam,andtoestimatetheerrorwhenaveragingonbins
Table 2. Top table: right ascension and declination of HD
(Fig.5,secondpanel)weweightedthepointsforthenumberof
163296 at the different epochs from Hipparcos. Bottom table: (cid:113)
proper motions with associated error ellipse from Hipparcos correlatedpixels,i.e.,σ = M1Σi(xi−µ)2,whereMisthenumber
N/M−1
measurements and from the least squares interpolation per- of correlated points and N is the number of averaged pixels. A
formedinthispaper. simple analysis of the profiles reveals a bump in the emission
between80and150AU:fittingasimplepolynomialtooutlinea
smoothprofiledoesnotproduceanaccuratefit(seeFig.6).The
degree of the polynomial was chosen as the lowest degree that
wouldprovideareasonablefittotheintensityprofile.Tocharac-
10
terizethepropertiesofthisbumpwefittedacombinationofa3rd
model
degreepolynomialplusagaussiancurvetoourfluxdensitypro-
free free
11 file(seeFig.6,rightpanel):subtractingthepolynomialfromthe
total
dataleavesagaussian-shapedresidual(seeFig.5,secondpanel)
12 centeredatabout(106±4)AU,withafullwidthathalfmaxi-
]
1s− mum(FWHM)of(71±18)AUandapeakat(67±29)%ofthe

2− 13 smoothpolynomialprofile.Anestimateofthemaximumspatial
m
c extentofthefeaturecanbederivedfromdeconvolvingourbest
g
er 14 fit gaussian with the synthesized beam (∼0.5(cid:48)(cid:48) in Band 7), re-
F[ν sultinginanupperlimitof∼40AUinFWHM.Notethatthese
ν
g 15 values are dependent on the choice of the pixel size of the im-
o
l age and the tolerance we use for the points on the major axis,
16 andthusareusefulonlyforgivingaroughestimateofthespa-
tial scale of this unresolved emission excess. An independent
17 analysis of this excess, obtained from modeling the visibilities
directly,isshowninSection4.
3.0 3.5 4.0 4.5 5.0
log λ [µm]
The feature cannot be clearly identified in Band 6 intensity
profiles:wefindanindicationofafaintexcessintheradialpro-
Fig.4. HD 163296 spectral energy distribution: our measure-
filesalongthediskprojectedmajoraxis,butitsdetectionisde-
ments are represented in red filled squares, the white empty
pendentonthesmallvariationofthepositionangleandinclina-
squares are taken from the literature (Isella et al. 2007; Natta
tion parameters, making its characterization unreliable. This is
et al. 2004), and the longer wavelength measurements used
consistent with the lower angular resolution of the Band 6 SV
to evaluate the free free contribution are shown in full black
data:ifweimagetheBand7datasetwitharestoringbeamequal
circle marks (Natta et al. 2004) and blue full circle mark
toBand6resolution,thefeatureisdilutedandcannotbereliably
(Disks@EVLA collaboration). Empty circles show the value
separatedfromthesmoothdiskemission(seeFigure7).
of 0.3 mJy for the free free emission estimated in this paper.
Thedashedcurveshowsthebestfitmodelfromthispaper(see
Section4),thedottedlineistheestimatedfree-freeemissionand
thesolidlinethesumofthetwo.
a0 a1 a2 a3
Polynomial (4.2±0.1)e-01 (-5.6±0.3)e-03 (2.7±0.2)e-05 (-4.8±0.6)e-08
a0 a1 a2 a3
a region in the inner part of the disk, we examined the longest Polynomial (4.6±0.2)e-01 (-7.3±0.9)e-03 (4.0±0.7)e-05 (-7.4±1.6)e-8
baselines(≥1500kλ)intheVLA8.0and9.8mmobservations +Gaussian α[Jy/beam] µ[AU] σ[AU]
(3.4±1.5)e-02 (1.06±0.04)e+02 (3.0±0.8)e+01
andweestimateanupperlimitof0.3mJyforthiswindemission,
correspondingtotheasymptoticvaluereachedatthehigheruv-
Table3.Bestfitparameters,withassociatedstandarddeviations,
distances by the real part of the visibilities. We fit a power law
obtainedfromtheleast-squareinterpolationofthedataalongthe
forthefree-freeemissionbyperformingaleastsquaresinterpo-
majoraxisusinga3rddegreepolynomial(y=a +a x+a x2+
lation between our estimates at 8 and 10 mm and other VLA 0 1 2
a x3) and a polynomial plus a gaussian curve (y = a +a x+
measurements at 3.6 cm (Natta et al. 2004) and 5.2 cm (see 3 0 1
a x2+a x3+αexp(−(x−µ)2/2σ2),where xisinAUandyis
Section 2). The resulting power law is F ∝ ν−0.19±0.11, and is 2 3
ν inJy/beam.
shownwithadottedlineinFigure4(notethatinfigureweplot
νF asafunctionofλ).Accordingtothisestimatethefree-free
ν
componentatwavelengthsshorterthan7mmisnegligible(see
alsoNattaetal.2004).
5
Guidietal.:DustpropertiesacrosstheCOsnowlineinHD163296
d (arcsec)
-1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.45
0.40 DCO+ emission 0.14n s)
m) 00..3305 00..1102ssiokm/
I (Jy/bea 00000.....1201200555 0000....00002468+ emiCOy/beam ·
0.00 0.00D(J)
) 1
am 0.05 Ks band polarized contrast 4 ht 00
ss (Jy/be 0000....00004312 123 arized ligast (x0.0
Exce 00..0001 01 Polcontr
200 100 0 100 200
d (AU)
Fig.5. Top panel: flux density at 850 µm along the disk major axis from SE (left) to NW (right). The solid line represents the fit
(polynomial profile + a gaussian for the excess) performed excluding the inner 0.2 arcsec of the disk (grey shadowed area). The
verticaldashedlinecorrespondstotheCOsnowlineat90±10AU(fromQietal.(2015)).DCO+ emissioninbluesquaresbinned
by0.15arcsec,thebluetrianglemarkersshowtheupperlimitofDCO+ emissionintheinner0.2arcsecregionofthedisk,where
wearelimitedbyresolution.Bottompanel:residualsobtainedbysubtractingthepolynomialfitfromthedataareshownwithblack
dotsbinnedby0.15arcseconds.Thebluesolidlinerepresentsthegaussianthatbestfitstheexcess(seeTable3),withtheshaded
areashowingthe1σfituncertainty.TheredtrianglemarkersarethepolarizedlightcontrastintheKsband(fromGarufietal.2014,
thescaleisontherightsideverticalaxis).
r (arcsec) r (arcsec)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.45 0.45
0.40 0.40
0.35 0.35
m) 0.30 m) 0.30
ea 0.25 ea 0.25
b b
y/ 0.20 y/ 0.20
I (J 0.15 I (J 0.15
0.10 0.10
0.05 0.05
als m) 0.02 als m) 0.02
ua ua
de 0.00 de 0.00
esiy/b 0.02 esiy/b 0.02
R(J R(J
s ()% 231000 s ()% 231000
ual 0 ual 0
d 10 d 10
esi 20 esi 20
R 30 R 30
0 50 100 150 200 0 50 100 150 200
r (AU) r (AU)
Fig.6.Left:fluxdensityacrossthediskmajoraxiswithapolynomialfitofdegreee3(dashedlineintheupperpanel).Inthesecond
and third panel the absolute residuals and the percentage residuals respectively. Right: the same intensity profile fitted with a 3rd
degreeepolynomialplusagaussian(seeTable3forthebestfitparameters).
3.5. DCO+ emission acentralradiusof∼110AUandatotalextentoftheDCO+emis-
sion (detected at greater than 3σ) of 200 AU in radius. In Fig.
WeextractedtheDCO+(J=5−4)emissionlinesat360.160GHz 5(toppanel)weshowtheradialprofileoftheintegratedDCO+
emission along the disk projected major axis: we note a sym-
from the ALMA Band 7 observations in order to compare the
metrybetweenthesouth-eastandthenorth-westdirection,both
dustcontinuumradialprofilewithapotentialmoleculartracerof
displaying a double peak at a distance of ∼60 AU and 140AU
theCOsnowline(seeMathewsetal.2013).WeusedtheCASA
from the central star. We also point out that the minimum be-
task“clean”withnaturalweightingtoproduceanintegratedmap
oftheDCO+ emissioninthevelocityrange0.8-10km/s,there- tweenthetwopeaksonbothsidesappearstofallattheposition
sulting synthesized beam is 0.62(cid:48)(cid:48) ×0.42(cid:48)(cid:48). We find a ring-like oftheexcessinthecontinuumemissionat850µm(∼110AU).
ThesignaltonoiseratiooftheDCO+ imageisverylow,sothis
structure,similartothatreportedbyMathewsetal.(2013),with
6
Guidietal.:DustpropertiesacrosstheCOsnowlineinHD163296
r (arcsec) r (arcsec)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.20 0.6
m) 0.15 m) 0.5
a a 0.4
e e
b 0.10 b 0.3
y/ y/
I (J 0.05 I (J 00..12
30 30
)% 20 )% 20
s ( 10 s ( 10
al 0 al 0
u u
sid 10 sid 10
e e
R 20 R 20
30 30
0 50 100 150 200 0 50 100 150 200
r (AU) r (AU)
Fig.7.Left:radialprofileat1.3mmacrossthediskmajoraxis,withthedashedcurveresultingfromthepolynomialfitofthedata.
Bottompanel:absoluteresidualswithrespecttothepolynomialinterpolation.Theexcessemissionfoundat850µmisnotvisiblein
Band6.Right:radialprofileoftheimageat850µmrestoredwiththesamebeamasBand6(0.74(cid:48)(cid:48)×0.60(cid:48)(cid:48),PA77.3◦).
Testietal.2001,2003;Rodmannetal.2006;Riccietal.2010b;
100 0.7 Kwonetal.2015).
T brightness
850 µm T model Previous measurements of the dust opacity power-law be-
0.6
optical depth tween 0.87 and 7 mm (Isella et al. 2007) and between 1.3 and
80
7mm(Nattaetal.2004)havealreadyshownthatgraingrowth
0.5
occurs in the disk around HD163296. With the high resolution
60 0.4 of ALMA it is now possible to extend these studies and con-
)
K
( τ strain the radial behavior of the opacity spectral index β (see,
T 40 0.3 e.g.,Pe´rezetal.2012,2015),notjustitsaveragevalueacrossthe
disk.Werecallherethattheemissionfromthediskmidplaneis
0.2
generally optically thin and at these wavelengths the Rayleigh-
20
Jeansregimeisagoodapproximation.Toverifytheseassump-
0.1
tionswecomparedthebrightnesstemperaturederivedfromour
0 observations with the temperature profile of our best-fit model
50 100 150 200 250
r (AU) (seeSect.4),inordertoestimatetheopticaldepthτoftheemis-
sionasτ(cid:39)−ln(1−T /T ).Wefoundτincreasingtowardsthe
Fig.8. Temperature profile of the best fit model (see Sect.4) in b model
central regions, as expected, with values <0.6 for the emission
black dots and brightness temperature from the observations at
at850µm,and<0.5fortheemissionat1.3mmoutsideainner
850µm(redsolidline)plottedinfunctionofthedistance,start-
regionof∼30AU(seeFig.8forthe850µmopticaldepthpro-
ingfrom30AUtohaveareliableestimateconsideringtheres-
file). At longer wavelengths we estimate a lower optical depth,
olution of the observations. On the right axis the optical depth withvaluesτ < 10−1 atboth8and10mm.Hencetheassump-
from the comparison of the two temperatures, plotted as the
tionofopticallythincontinuumemissionisconsistentwithour
dashedline.
data, and we expect a linear relation between flux density and
dustopacity.
Thefluxdensityemittedfromaringdratagivenradiuscan
resultisverytentativeandshouldbeverifiedwithhighersensi- bewrittenas
tivityobservations.
F (r)∝Σ(r)·B (T(r))·νβ(r), (1)
ν ν
3.6. Spectralindexprofiles
where T(r) and Σ(r) are the midplane temperature and surface
Thedustopacityatmillimetreandsubmillimetrewavelengthsis density at the distance r from the star, and B is the Planck
ν
usuallyapproximatedbyapowerlawκ ∝ νβ (e.g.,Hildebrand function. So, in the Rayleigh-Jeans regime, we have: F (r) ∝
ν ν
1983).Theemissionpropertiesdependonthedetailsofthecom- Σ(r)·T(r)·ν2 ·νβ(r). Producing matched images (same beam,
position, geometry and size distribution of dust grains, most of pixelsizeandcenteredonthepeakoftheemission)atdifferent
whichareverydifficulttoconstrain.Ageneralconclusion,how- wavelengths and measuring the ratio of the flux densities as a
ever, is that in the conditions expected for dust in the dens- function of distance from the star allows the spectral index, α
est regions of cores and disks, larger grain sizes correspond to (whereF ∝να),tobedetermined.Then,giventheassumptions
ν
lower values of the β index (e.g., Miyake & Nakagawa 1993; noted above, the power-law dependence of the dust opacity, β,
Stognienkoetal.1995;Natta&Testi2004;Draine2006).Even can be derived as β = α−2, and does not depend on tempera-
thoughconnectingdirectlyavalueofβtothedetailedproperties tureorsurfacedensity.AsdiscussedinSec.3.3theobservation
of the dust population is not possible, measurements of β have at8and10mmincludesgasemissionfromthestellarwindthat
been successfully used to infer the growth of dust in disks for needs to be subtracted to study the dust emissivity. The VLA
manyyears(e.g.Beckwith&Sargent1991;Wilneretal.2000; maps used to compute the spectral index were produced sub-
7
Guidietal.:DustpropertiesacrosstheCOsnowlineinHD163296
r (arcsec) r (arcsec)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0 0
850 µm 850 µm
1 9 mm 1 1.3 mm
) )
m 2 m 2
a a
e e
b b
y/ 3 y/ 3
(J (J
g I 4 g I 4
o o
l l
5 5
6 6
50 100 150 200 250 50 100 150 200 250
r (AU) r (AU)
r (arcsec) r (arcsec)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
5.5 5.5
5.0 5.0
)
m) 4.5 mm 4.5
9m 4.0 3 4.0
- 1.
m µ 3.5 m - 3.5
50 3.0 0µ 3.0
8 5
(α 2.5 (8 2.5
α
2.0 2.0
1.5 1.5
50 100 150 200 250 50 100 150 200 250
r (AU) r (AU)
Fig.9.Left:Fluxdensitymaps(toppanel)at850µmand9mm,withcontourlevelsat-3(dashed),3,6,12,24,48,100,200,400,
800,1600σ.Disksurfacebrightnessprofiles(middlepanel)usedtocomputethespectralindex,plottedonalogarithmicscale.The
shaded region shows the dispersion of the individual data points in the images, while the error bars show the uncertainty on the
meanforeachbin.Fluxdensityspectralindex(bottompanel)calculatedbetween850µmand9mmasafunctionofradialdistance.
Theprofilesareaveragedonellipsesofsemimajoraxisr (showninarcsecandAUinthetopandbottomaxis,respectively).The
shadedregionindicatestheuncertaintyofthemeasurement,resultingfromthenoiseofthedeconvolvedimagesandthesystematic
calibrationuncertainty;thelowsignaltonoiseratiointheVLAmapdominatestheuncertaintyonthespectralindexdetermination
beyond∼100AU.Right:asabove,butforthetwoALMAdatasetsat850µmand1.3mm.
tracting a point source at the centre of the system with a flux regions to ∼ 3.5 at 150 AU. The profiles are displayed starting
densityof0.3mJyfromthecalibratedvisibilities(seeSec.3.3). fromandsamplingeveryhalfresolutionelementoftheimages,
corresponding to ∼30 AU; the large error associated with this
InFigure9,leftpanel,weshowtheintensitymapsat850µm spectralindexprofileisdominatedbythelimitedsignaltonoise
and 9 mm with a circular beam of 0.5(cid:48)(cid:48), the corresponding av- ratiooftheVLAimages.Withthesameprocedurewecomputed
eraged radial intensity profiles and the derived α profile. The the spectral index between the ALMA Band 6 and 7 images,
intensityaveragedvalues(andconsequentlythespectralindex) withamatchingcircularbeamof0.6(cid:48)(cid:48),andobtainedtheprofile
are plotted as long as they stay above the 1σ level. We see an shown in Figure 9, right panel. In this case the profile is sam-
increasingtrendofthespectralindexαfrom∼ 2.5intheinner
8
Guidietal.:DustpropertiesacrosstheCOsnowlineinHD163296
pledevery0.3(cid:48)(cid:48)(∼40AU).Thelargeuncertainties,inspiteofthe In panel (a) of Figure 10 we show the comparison be-
highsignaltonoiseoftheALMAimages,iscausedbythesmall tween the observations at 850 µm and the best-fit model
wavelengthleveragebetweentheALMABand6and7observa- (obtained running a MCMC with 500 chains) that corre-
tions. In propagating the uncertainty on α we used a 10% cali- sponds to the following median values γ=0.882±0.002,
brationerrorforeachflux,thatinthecaseofα(850µm−1.3mm) Σ =(13.40±0.03)g/cm2,R =(118.7±0.2)AU.Wenotethat
0 c
represents a pessimistic estimate, as the two ALMA observa- themodelwithmedianvaluesalsogivestheminimumχ2,with
tions were carried out with the same phase calibrator and us- χ2 =1.853).Thering-shapedresidualsareclearlyvisibleinthe
red
ing Neptune as flux calibrator. Within the uncertainties the α rightplotofpanel(a)andemphasizethefactthatasimpletwo
profile seems to be consistent with the measurements between layer disk model with a monotonically decreasing surface den-
850µmand9mm.Thespectralindexremainsbelowthevalue sity(andthussurfacebrightness)isnotsufficienttocompletely
of 3 beyond 50 AU, corresponding to a β¡1 under the assump- accountfortheobservedfluxdensityprofile.
tionsmentionedabove,andindicatingthepresenceofgrainsthat
havegrowntoatleast1mminsize(usingthedustopacitycurves
computedbyTestietal.2014).Wefindnofeaturesintheα(and -21°57'19.0" 850µm Model Two Layer Residual (a)
byimplication,β)profilesacrosstheCOsnowline,aswouldbe 20.0"
expectedforlocalizedgraingrowth,butitshouldbenotedthat 0) 21.0"
0
theresolutionofthespectralindexmapsislimitedbythelower 20 22.0"
resolutionofBand6observations(∼0.6(cid:48)(cid:48)),whichwouldnotbe ec (J 23.0"
D
sufficient to detect small scale variations of the emission (see 24.0"
Section3.4).Wereturntotheestimateofβ(r)inSect.5. 25.0"
21.48s R2A1 (.J3200s0107)h56m21.12sModel With Peak Residual (b)
4. Modelingresults
In Section 3.4 we have shown that the surface brightness ra-
dialprofileforthecontinuumemissionat850µmiscompatible
withhavinganexcesspeakedaround110AU,withdeconvolved
FWHM≤ 40AU.Inordertoassesstherobustnessofthisresult
wehaveperformedadirectfitoftheinterferometricdata(i.e.,the
visibilities) using the fitting scheme described by Tazzari et al.
Fig.10. Continuum maps at 850µm showing the results of the
(2015)towhichwereferfortheanalysisdetails.Inordertoes-
fitsofthevisibilities.Thetopleftpanelshowstheobservations,
timate the disk thermal emission at 850 µm, we use a classical
thetwopanelsinthecenterthebest-fitmodel,andthetwopan-
two layer disk model (Chiang & Goldreich 1997) with refine-
els on the right the residuals. The red solid curve represents
mentsby(Dullemondetal.2001)andareduceddiskflaringthat
the CO snowline at 90 AU. Panel (a): we use a classical two
adequatelydescribestheobservedfar-infraredflux(fromTilling
layerdiskmodelwhichimplementsamonotonicallydecreasing
et al. 2012). The resulting vertical scale height for the surface
surface brightness. Ring-shaped residuals are clearly visible. .
layeratR > 50AUish/R ∼ 0.08.Moreover,weassumeacon-
stantdusttogasmassratioζ = 0.01andthefollowinggassur- Panel (b): we use the two layer disk model with an additional
Gaussianpeak.Wefitthepeaklocation(R =96AU)andwidth
facedensityprofile: p
FWHM = 58AU.Theresidualsaresubstantiallyreduced.The
p
Σg(R)=Σ0(cid:32)RR0(cid:33)−γexp−(cid:32)RRc(cid:33)2−γ, (2) pSFaeigrca.tmi1oenatne2drsaFnuidgs.etd9h.efocronthtoeuCrLleEvAelNs aarreeththeessaammeeaassthdoissceuussseedd iinn
where R =10AU is a fixed scale length and Σ , R and γ
0 0 c
Inordertoassesswhethertheresidualscanbeexplainedby
are free parameters to be fitted. In order to compute the disk
addingasimplering-likepeakorwhethertheyneedamorecom-
emission, the dust opacity is calculated using Mie theory (see
plicatedtreatment,weperformedanotherfitwithamodifiedver-
Trotta et al. 2013, for details of the computation) assuming
sionofthetwolayermodel.Thismodifiedtwolayerdiskmodel
the same dust composition throughout the disk, given by the
implementsanadditionalring-likestructureinthe850µmemis-
following fractional abundances adapted from Pollack et al.
sion,withadisksurfacebrightnessI(cid:48)(R)asfollows:
(1994): 5.4% astronomical silicates, 20.6% carbonaceaous ma-
terial, 44% water ice and 30% vacuum. Furthermore, we as-  
saum≤eaapo,wwerh-elarewagriasinthseizgeradiinstrriabduituios.nInn(ao)rd∝erat−oq mfoordaemlinth≤e I(cid:48)(R)= I2L(R)+IpI2L(Rp)exp−(R2−σR2p)2 , (3)
max p
fact that in the disk midplane we expect larger grains than on
the surface (Testi et al. 2014), we use different parametrization where I2L(R) is the brightness computed by the classical two
for the grain size distribution in these two regions: q = 3.5, layermodel,Rpisthepeakcenter,σpisthepeakwidthandIpis
a = 10nm, a = 100µm in the surface and q = 3, thepeakintensity(inunitsofthebrightnessinthevicinityofthe
min max
amin = 10nm and amax = 0.8 ∗ (R/10 AU)−1.025mm in the peak,namelyI2L(Rp)).Thisnewmodelhasthereforesixfreepa-
midplane, where this variable maximum grain size is chosen rameters:threeofthemforthetwolayermodel(γ, Σ0, Rc)and
toreproducetheβ(R)profilesfoundinSect.3.6.Themodeling another three to define the peak (Rp, σp, Ip). We perform the
methodology is based on a Bayesian approach and employs an fitwiththeTazzarietal.(2015)modelingtooldiscussedabove,
affine-invariantMarkovChainMonteCarlo(MCMC)ensemble and we obtain the results shown in panel (b) of Figure 10. The
sampler(Foreman-Mackeyetal.2013)toexploretheparameter best-fit model (χ2 =1.830) is described by γ=1.32±0.01,
red
spaceandfindthebest-fitmodels(Tazzarietal.2015). Σ =(24.6±0.3) g/cm2, R =(111.3±0.8) AU, which corre-
0 c
9
Guidietal.:DustpropertiesacrosstheCOsnowlineinHD163296
spond to a radial profile that is slightly steeper than the sim-
ple power-law model but has a similar cut-off radius. For the r (arcsec)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
gaussian peak we find that is described by R =(96±1) AU,
p 3.5
σ =(24.9±0.5) AU and I =128±20%. This modified disk
p p
model is able to reproduce the observations with an extremely 3.0
goodagreement,asconfirmedbytheconsiderablysmallerresid- )
m
2.5
uals(rightplotofpanel(b),Figure10).Themidplanetempera- m
ture is computed at every radius according to the two-layer ap- 9
- 2.0
proximation;inFigure8(Section3.6)weshowthetemperature m ISM dust
profileofthisbest-fitmodel. 0µ 1.5
Inconclusion,thepeakinferredfromthedirectfitofthevis- 5
8
ibilities supports the evidence of a√ring-like structure centered (β 1.0
around 96AU with a FWHM= 2 2ln2 · σ ≈ (58 ± 3)AU,
p 0.5
compatiblewiththeupperlimitresultingfromthesimplepoly-
nomial+gaussian fitting of the continuum surface brightness in 0.0
50 100 150 200 250
Section3.4.
r (AU)
r (arcsec)
5. Discussion
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
3.5
InSection3wederivedspatiallyresolvedspectralindexprofiles
for the dust emission from the HD 163296 protoplanetary disk
3.0
andweidentifiedandcharacterizedanunresolvedexcess850µm )
m
emissioncenteredat∼110AU.Thepresenceofthisfeaturehas m 2.5
alsobeenconfirmedthroughadetailedmodelingofthevisibili- 3
tpioesssiinblSeeicmt.p4li.cIantiothnissfsoerctthioengwroewatnhaolyfzgeratihnesseinrtehseulHtsDan1d63th2e9i6r m - 1. 2.0 ISM dust
µ 1.5
disk. 0
5
8 1.0
(
β
5.1. β(r)profilesandgraingrowth 0.5
In Section 3.6 we derived the radial distribution of the spectral
0.0
index as measured combining the ALMA 850µm image with 50 100 150 200 250
the ALMA 1.3 mm or the VLA 10 mm images. Under the as- r (AU)
sumptionsofopticallythinemissionandRayleigh-Jeansregime,
thespectralindexprofilescandirectlybeconvertedintoopacity Fig.11. Dustopacitypowerlawindex(β)profilesderivedusing
powerlawindexprofilesbysubtractingaconstantvalueof2.0. Eq. 4. β(r) is computed between 850 µm and 9 mm in the top
Our modeling of the disk (Sect. 4) and the comparison of the panelandbetween850µmand1.3mminthebottompanel.The
measuredbrightnesstemperaturewiththeexpectedtemperature uncertaintiesarepropagatedfromthesurfacebrightnessprofiles
profile from our model (Sect. 3.6) confirm that the emission is presented in Fig. 9. The derived values of β(r) in the top panel
opticallythinthroughoutthedisk,withtheexceptionofthevery aredominatedbythelowsignaltonoiseintheVLAimagefrom
innerregionthatisnotresolvedbytheALMAandVLAobser- ∼100 AU and are very uncertain beyond this radius. The CO
vations. On the other hand, the Rayleigh-Jeans approximation snowline at 90±10 AU from Qi et al. (2015) is pictured as the
is not fully justified in the outer regions of the disk, especially dashedverticalline.
fortheALMABand7data.Toestimatethevalueoftheopacity
powerlawindexasafunctionofradiuswethusused(seeEq.1):
(cid:34) (cid:32)ν (cid:33)(cid:35)−1(cid:34) (cid:32)F (r)(cid:33) (cid:32)B (T(r))(cid:33)(cid:35) (Guilloteau et al. 2011; Banzatti et al. 2011; Pe´rez et al. 2012,
β(r)= log 1 log ν1 −log ν1 (4) 2015;Trottaetal.2013;Menuetal.2014;Tazzarietal.2015),
ν F (r) B (T(r))
2 ν2 ν2 withasignificantvariationofβthroughoutthedisk(∆β>1),in-
whereT(r)arethetemperatureprofilesderivedfromourmodels. dicatingamaximumgrainsizeamaxdecreasingwiththedistance
InFig.11weshowtheprofilesofβ(r)obtainedfromEq.4using from the star. The ALMA and VLA data provide for the first
theintensityprofilesshowninFig.9. timethecombinationofsignaltonoiseratio,angularresolution
Ouranalysisofthecontinuumemissionatthreedifferentfre- andimagefidelitytosuccessfullyperformananalysisintheim-
quenciesshowsinthefirstplaceadecreasingspatialextentwith ageplane.Unfortunately,theALMABand6scienceverification
increasingwavelength,confirmingthepresenceofdustprocess- data do not have the angular resolution to probe the dust prop-
ing and radial transport in this disk, as already shown by the erties in a localized region across the CO snowline. Similarly,
comparison between the size of the dust and gas disk by de the VLA data do not have a high enough signal to noise ratio
Gregorio-Monsalvoetal.(2013)andconfirmandextendthere- at 100 AU and beyond to derive strong constraints in the outer
sults of Natta et al. (2004, 2007), who showed the presence of disk.
largegrainsintheHD163296diskfromintegratedspectralin- The conclusion that we can draw from the β(r) profiles is
dices and suggested a possible spectral index variation within thatthereisconvincingevidenceforlargegrainsinsidetheCO
thedisk. snowline, but at ∼70 AU resolution the profile appears to be
Theβ(r)profilesarequalitativelyconsistentwiththeresults smooth with no features. The data are consistent with signifi-
of similar analyses performed in other classical smooth disks cantgraingrowththroughouttheinner150-200AUofthedisk.
10