The twofold debris disk around HD 113766 A - Warm and cold dust as seen with VLTI/Midi and Herschel/Pacs

0.49 MB·English

by  Johan Olofsson

Astronomy&Astrophysicsmanuscriptno. HD113766 (cid:13)cESO2013
January30,2013
The twofold debris disk around HD113766A(cid:63)
Warm and cold dust as seen with VLTI/Midi and Herschel/Pacs
J.Olofsson1,Th. Henning1,M.Nielbock1,J.-C.Augereau2,A.Juhàsz3,I.Oliveira4,O.Absil5,andA.Tamanai6
1 MaxPlanckInstitutfürAstronomie,Königstuhl17,69117Heidelberg,Germany
e-mail:[email protected]
2 UJF-Grenoble1/CNRS-INSU,InstitutdePlanétologieetd’AstrophysiquedeGrenoble(IPAG)UMR5274,Grenoble,France
3 LeidenObservatory,LeidenUniversity,POBox9513,NL-2300RALeiden,TheNetherlands
4 AstronomyDepartment,UniversityofTexasatAustin,1UniversityStationC1400,Austin,TX78712-0259,USA
5 Départementd’Astrophysique,GéophysiqueetOcéanographie,UniversitédeLiège,17AlléeduSixAoût,B-4000SartTilman,
3 Belgium
1 6 UniversityHeidelberg,Kirchhoff-InstitutfürPhysik,69120Heidelberg,Germany
0
ReceivedJanuary30,2013;accepted
2

n
a ABSTRACT
J
Context.Warmdebrisdisksareasub-sampleofthelargepopulationofdebrisdisks,anddisplayexcessemissioninthemid-infrared.
9
Aroundsolar-typestars,veryfewobjects(∼2%ofalldebrisdisks)showemissionfeaturesinmid-IRspectroscopicobservations,that
2
areattributedtosmall,warmsilicatedustgrains.Theoriginofthiswarmdustcanpossiblybeexplainedeitherbyarecentcatastrophic

collisionbetweenseveralbodiesorbytransportfromanouterbeltsimilartotheKuiperbeltintheSolarSystem.
]
R Aims. Wepresentandanalysenewfar-IRHerschel/Pacsphotometricobservations,supplementedbynewandarchivalground-based
datainthemid-IR(VLTI/MidiandVLT/Visir),foroneoftheseraresystems: the10–16MyrolddebrisdiskaroundHD113766A.
S
Weimproveanexistingmodeltoaccountforthesenewobservations.
h. Methods.WeimplementedthecontributionofanouterplanetesimalbeltintheDebracode,andsuccessfullyusedittomodeltheSED
p aswellascomplementaryobservations,notablyMididata.Webetterconstrainthespatialdistributionofthedustanditscomposition.
- Results.WeunderlinethelimitationsofSEDmodellingandtheneedforspatiallyresolvedobservations.Weimproveexistingmodels
o andpushfurtherourunderstandingofthediskaroundHD113766A.Wefindthatthesystemisbestdescribedbyaninnerdisklocated
r withinthefirstAU,wellconstrainedbytheMididata,andanouterdisklocatedbetween9–13AU.Intheinnerdustbelt,ourprevious
t
s findingofFe-richcrystallineolivinegrainsstillholds. Wedonotobservetimevariabilityoftheemissionfeaturesoveratleasta8
a yearstimespan,inaenvironmentsubjectedtostrongradiationpressure.
[
Conclusions.Thetimestabilityoftheemissionfeaturesindicatesthatµm-sizeddustgrainsareconstantlyreplenishedfromthesame

reservoir,withapossibledepletionofsub-µm-sizedgrains.Wesuggestthattheemissionfeaturesmayarisefrommulti-composition
1
aggregates.WediscusspossiblescenariosconcerningtheoriginofthewarmdustobservedaroundHD113766A.Thecompactnessof
v
theinnermostregionsasprobedbytheMidivisibilities,aswellasthedustcomposition,suggestthatwearewitnessingtheoutcomes
6
of(atleast)onecollisionbetweenpartiallydifferentiatedbodies,inanenvironmentpossiblyrenderedunstablebyterrestrialplanetary
7
formation.
8
6 Keywords. Stars:individual:HD113766A–planetarysystems:Zodiacaldust–circumstellarmatter–Infrared:stars–Techniques:
1. spectroscopic-highangularresolution
0
3
11. Introduction Departure from photospheric emission at these wavelengths is
v: the consequence of thermal emission arising from dust grains
Debris disks are byproducts of the planet formation process,
i heatedbythestellarradiation.Severalhundredsofstarsarenow
Xoncethemassive,gas-richcircumstellardiskhasdissipated. Af-
known to harbor such excess emission (e.g., Chen et al. 2006).
ter the gas disks are lost, planetary systems including asteroid
r Ourunderstandingoftheirnaturehasprogressedduringthepast
aand comets may remain and evolve through gravitational inter-
decades,bothobservationallyandtheoretically(seeWyatt2008
actions. The presence of dust, that originates from collissions
and Krivov 2010 for recent reviews). In most cases, these sys-
or evaporation of these bodies, translates into a mid- to far-
tems compare very well with the Kuiper belt observed in our
infrared(IR)excess,asdemonstratedforthefirsttimeforVega
Solar System (e.g., Carpenter et al. 2009, Liseau et al. 2010,
(Aumannetal.1984),usingtheInfraredAstronomicalSatellite.
Löhneetal.2012),whereexcessemissionisdetectedinthefar-
IR. Several studies made use of high angular resolution instru-
Sendoffprintrequeststo:[email protected]
mentstodetecthotexozodiacaldustaroundwell-knowndebris
(cid:63) Based on Herschel observations, OBSIDs: 1342227026,
disks (e.g., Fomalhaut and Vega, see Absil et al. 2009, Defrère
1342227027, 1342237934, and 1342237935. Herschel is an ESA
spaceobservatorywithscienceinstrumentsprovidedbyEuropean-led etal.2011,respectively).Insuchcases,thenear-IRexcessisrel-
Principal Investigator consortia and with important participation from ativelysmall(∼1%)anddeparturefromphotosphericemission
NASA. Based on Visir observations collected at the VLT (European onlybecomessignificantatmid-orfar-IRwavelengths. Asmall
SouthernObservatory,Paranal,Chile)withprogram089.C-0322(A)
Articlenumber,page1of10
sampleofthedebrisdiskpopulationisreferredtoas“warmde- Table1.FinalproductsofHerschel/Pacsdataprocessing. “HP”corre-
spondstotheHighPassfilterwidthsinunitsofnumberofreadouts.
brisdisks" (e.g., Moór etal.2009) whereemission inexcess is
significantinthemid-IR,suggestingthepresenceofwarmdust
grains,closetothestar. Withinthisclassofwarmdebrisdisks, Filter Wavelength Pixelsize HP Flux
a few objects are of particular interest since they also display [µm] [(cid:48)(cid:48)] [mJy]
emission features in mid-IR spectroscopic observations. These 15 386.5±7.2
emissionfeaturesareassociatedto(sub-)µm-sizeddustgrains, Blue 70 1.1 20 390.1±8.7
that are warm enough to contribute significantly in the mid-IR 35 391.4±9.7
(seeHenning2010forareviewonopticalpropertiesofsilicate 20 223.4±5.6
dust). Green 100 1.4 30 225.4±6.6
45 226.7±7.5
These warm debris disks with emission features around
Solar-typestarsarerare(∼2%ofalldebrisdisks,Brydenetal. 35 94.3±11.1
Red 160 2.1 45 99.3±10.1
2006, Chenetal.2006). AsdiscussedinWyatt(2008), theab-
70 113.6±12.6
normalousbrightnessofthesedebrisdiskscannotbesolelyex-
plainedbyasteadystatecollisionalevolution.Ifitwas,thecolli-
sionallifetimeoftheparentbodiesresponsibleforthedustpro-
duction, and hence the infrared luminosity, would be too short
ThedatawereprocessedusingtheHerschelinteractivepro-
compared to the age of the host star. Consequently, two sce-
cessing environment (HIPE, version 10.0.667), using standard
narios are possible, either (i) a recent catastrophic collision be-
scripts that include bad pixel flagging, detection and correction
tween two planetesimals, or (ii) an outer belt feeding the inner
for glitches and HighPass filtering in order to remove the 1/f
regions, for instance via scattering of bodies by one ore more
noise(seePoglitschetal.2010formoredetails). Wediscarded
planet (Bonsor et al. 2012) or in a similar scenario to the Late
thefirstcolumnofeachdetectorsub-arrayinordertoeliminate
HeavyBombardmentphase, thathappenedintheSolarSystem
electroniccrosstalk.Finally,weprocessedthedatawithdifferent
(Gomes et al. 2005). The overall scarcity of the “warm debris
HighPassfilterwidthsinordertotesttherobustnessofthedata
disk”phenomenonwouldsuggestarelativelylowprobabilityfor
reduction. ThedifferentvaluesaretabulatedinTable1(column
suchevents tohappenand produceenough dusttobe detected.
“HP”).
Thoroughstudiesofsuchobjectscanthereforeimproveourun-
derstandingofthefinalstepsofplanetaryformation, aswellas The aperture photometry in all three filters was extracted
providekeycluesabouttheoriginofzodiacaldustorKuiper-belt within HIPE from the combined maps. The aperture radii are
dustinoursolarsystem. setat10(cid:48)(cid:48)fortheblueandgreenfiltersand20(cid:48)(cid:48)fortheredfilter.
The final flux values do not depend on the aperture sizes since
HD113766isoneoftheserarewarmdebrisdisksthatshow
an aperture correction is performed to compensate for flux loss
prominent emission features in the mid-IR. The system is a bi-
nary of two almost identical stars with spectral types F3/F5, outsideoftheaperture. Thiscorrectionisbasedonacalibrated
with a projected separation of 1.3(cid:48)(cid:48) (∼160AU at d = 123pc, encircled energy fraction parametrisation contained in the cali-
(cid:63) bration data that is included in HIPE. In order to estimate the
van Leeuwen 2007), the dust being located around the primary
HD113766A (L = 4.4L , Lisse et al. 2008). The Spitzer/Irs noiselevelandthusthequalityofthedetectioninallfilters,we
(cid:63) (cid:12) randomlyplacedapertureswithinadistanceof100, 75, and50
spectrum of the 10–16Myr old debris disk was first studied by
pixels (for blue, green, and red filters, respectively) to the cen-
Lisse et al. (2008). In Olofsson et al. (2012) we presented a
firstmodeltoreproducetheSpitzer/Irsspectrumofthissource. tralpixel. Doingso, weavoidedbordersofmapsthataremore
InthefollowingwepresentnewHerschel/Pacsphotometricob- noisy due to a smaller detector coverage, and are therefore not
representativeofthenoiseclosetothesource. Weextractedthe
servations at 70, 100 and 160µm, supplemented by archival
VLTI/Midi interferometric measurements as well as (new and photometryforalloftheseaperturesandthenfittedaGaussian
archival) VLT/Visir observations in the N- and Q-bands. In profile to the flux distribution histogram to estimate the σ un-
certaintyfromthewidthoftheGaussian. Overall, theseuncer-
section2 we describe the observations and data processing, in
taintiesareconsistentwithpredictionsfromtheHerschelSpot1
Sect.3wepresentourmodelandresults. Wediscusstheimpli-
software. Figure1showthePacsimagesat70,100,and160µm
cationsofourfindingsinSect.4beforeconcluding.
(lefttoright, respectively), wherethesourceisclearlydetected
atallthreewavelengths. Table1summarizesthefinalresultsof
the data processing, with pixel sizes for the three filters, aper-
2. Observationsanddataprocessing turecorrectedfluxesforthedifferentHighPassfilterwidths,and
estimated 1σ uncertainties in units of mJy. The fluxes used in
2.1. Herschel/Pacsobservations
theanalysisaretheaveragesofthosereportedinTable1. Forall
WeobservedHD113766AwiththePacsphotometer(Poglitsch three filters, the photometric values are almost identical for the
different HighPass filter widths, within the uncertainties. One
etal.2010)onboardHerschel(Pilbrattetal.2010). Thedataset
consistsof4Pacsmini-scanmaps(program“OT1_jolofsso_1”), shouldnoticethatthemeasured70µmfluxisconsistentwiththe
MipsobservationsreportedbyChenetal.(2011, F =390mJy
two in the blue filter (70µm, OBSIDs 1342227026 and 70
±11mJy).
1342227027), and two in the green filter (100µm, OBSIDs
1342237934 and 1342237935). The observing duration was
220×2 sec in the blue filter and 670×2 sec in the green fil-
2.2. Complementaryobservations
ter. When observing with Pacs with the blue camera (blue or
greenfilters,at70and100µm,respectively),dataisalsosimul- In addition to the far-IR observations, we included the
taneouslyacquiredwiththeredcamera(160µm). Wetherefore Spitzer/IrsspectrumofHD113766A(between5and35µm)in
combined the 4 mini-scan maps in the red filter to obtain the
photometryat160µm. 1 http://herschel.esac.esa.int/Tools.shtmlversion6.2.0
Articlenumber,page2of10
Olofssonetal.:ThedebrisdiskaroundHD113766A
-46°01'20.0"
40.0"
0)
0
0
2 02'00.0"
c (J
e
D
20.0"
40.0"
40s 38s 36s 34s 13h06m32s 40s 38s 36s 34s 13h06m32s 40s 38s 36s 34s 13h06m32s
RA (J2000) RA (J2000) RA (J2000)
Fig.1. Herschel/Pacsobservationsat70,100,and160µm(lefttoright,respectively).
theanalysis(seeOlofssonetal.2012fordetailsonthedatapro- cause of the challenge of calibrating accurately the Visir spec-
cessing). WesupplementedthesedatawithVLT/Visirobserva- tra, we used the Irs spectrum to compute visibilities from the
tions(ourprogramanddatapublishedinSmithetal.2012)and correlated fluxes. The calibrated dataset compares very well
VLTI/Midiinterferometricmeasurements(Smithetal.2012). with the data reduction of Smith et al. (2012), except for one
On the night of the 2012-Apr-21, HD113766A was ob- single run (083.C-0775(B), observed at 05:32:55). When cal-
servedwiththeVLT/Visirinstrument(program089.C-0322(A)) ibrating these observations with their corresponding calibrator
in the N-band. We defined four spectral windows, centered at (HD112213, observed at 05:15:45), we found a correlated flux
8.8µm(8.0≤λ≤9.6µm),9.8µm(9.0≤λ≤10.6µm),11.4µm muchhigherthantheIrsspectrum. Weconstructedthetransfer
(10.4≤λ≤12.5µm),and12.4µm(11.4≤λ≤13.5µm),inlow functionbasedonallthecalibratorsobservedduringthatnight.
resolutionmode(R ∼ 350at10µm). Weadditionallyretrieved At the time of the science observations, we found a variation
thespectroscopicobservationspresentedandanalysedinSmith ofaboutafactor2comparedtoaveragedtransferfunctionover
et al. (2012). These observations were performed on the night the rest of the night. This suggests that the observing condi-
of the 2009-May-11 in low resolution mode as well, for two tionswerestronglyvarying. Wereducedthesescienceobserva-
spectralwindowscenteredat8.8and11.4µm(program083.C- tionswithalaterobservationofthesamecalibrator(06:34:00).
0775(E)). Both datasets were processed using standard scripts OnecanseeinFig.2thatbothvisibilitiesandcorrelatedfluxes
providedwithintheESOGasganosoftware2.RightpanelofFig- are large for this run (orange dashed line) and that there is a
ure2showstheIrsandVisirspectraobtainedin2004,2009,and fairlysignificantdifferencewiththeconsecutiveobservationson
2012(openblackcircles, thickred, andthinbluelines, respec- the same night, with the same baseline (06:09:53, orange dot-
tively). One should note that both Visir spectra, taken almost ted line). As explained in Burtscher et al. (2012) uncertainties
threeyearsapartagreesurprisinglywell. However,thespectrum are given by the quadratic sum of the photon noise error de-
obtainedinourprograminthe9.8µmspectralwindowdisplays livered by the MIA+EWS software and the standard deviation of
anoffsetofabout10%higherinflux. Such“stiching”problems thetransferfunctionoverthenightofobservations. Overall,the
arenotunusual,buttheexactcausesarenotclear. Wechecked final uncertainties are dominated by the latter one (between 5
the atmospheric conditions during our observing program: the and10%).Forthe“problematic”observingrun(083.C-0775(B),
seeingandatmosphericpressuresappeartobeconstantoverthe time 05:32:55), the standard deviation of the transfer function
run. Besides the 9.8µm band offset, both Visir spectra show a overthenightiscloserto25%.
deficitoffluxofabout30%comparedtotheIrsspectrum.Smith
et al. (2012) already discussed in detail these differences (e.g.,
spatiallyextendedemission)andconcludedthesedifferencesto
bewithintheabsolutecalibrationuncertainitesofVisir(seedis-
cussioninGeersetal.2007).
ThearchivalMididatapresentedinSmithetal.(2012)were WedidnotreducetheVLT/VisirimagesanalyzedinSmith
et al. (2012). We instead used the full width at half maximum
processedusingscriptsdedicatedtoobservationsoffainttargets
withMidi(seeBurtscheretal.2012formoredetails)interfaced (FWHM)presentedintheirstudy. InnoneoftheN-andQ-band
with the MIA+EWS software3. Table2 summarizes the observ- images(λc = 11.85and18.72µmwith∆λ = 2.34and0.88µm,
respectively)presentedbytheauthors, thedebrisdiskwasspa-
inglogfortheinterferometricmeasurements. AsinSmithetal.
tiallyresolvedcomparedtoareferencePSF.Theyfittedtheaz-
(2012), we found that the observations performed on the night
imuthally averaged surface brightness distributions with Gaus-
ofthe2007-May-30(program079.C-0259(F))hadalowsignal
sian profiles with FWHM of 0.322(cid:48)(cid:48) and 0.498(cid:48)(cid:48) in the N- and
tonoiseratio,andwedidnotincludetheminouranalysis. The
Q-bands,respectively.Duringtheanalysis,wewillcheckifsyn-
calibratedvisibilitiesandcorrelatedfluxesforHD113766Aare
displayedintheleftandmiddlepanelsofFig.2. TheIrsspec- theticimagesconvolvedwithsuchPSFsareconsistentwiththe
measurementsofSmithetal.(2012). Afterconvolvingthesyn-
trumisalsoshownforcomparisontothecorrelatedfluxes. Be-
thetic images with the reference PSF, we will compare the sur-
2 http://www.eso.org/sci/software/gasgano/version2.4.0 facebrightnessdistributiontocheckforpossibleresolvedemis-
3 http://home.strw.leidenuniv.nl/∼jaffe/ews/version2.0 sioninbothbands.
Articlenumber,page3of10
1.0 3.0 3.0
MIDI MIDI VISIR
2.5 2.5
0.8
2.0 2.0
0.6
y] y]
V0.4 UT1-3, B = 101.8 m F [Jν1.5 F [Jν1.5
UT1-3, B = 101.8 m 1.0 1.0
UT1-3, B = 82.2 m
0.2 UT1-3, B = 72.9 m 0.5 0.5 IRS [2004]
UT2-4, B = 74.9 m VISIR [2009]
UT2-4, B = 69.8 m VISIR [2012]
0.0 8 9 10 11 12 13 0.0 8 9 10 11 12 13 8 9 10 11 12 13
λ [µm] λ [µm] λ [µm]
Fig.2. VLTI/MidivisibilitiesandcorrelatedfluxesforHD113766A(leftandmiddlepanels,respectively).Thebaselinesusedandtheirlengths
arereportedinthecaptionoftheleftpanel. RightpanelshowstheVisirN-bandspectrumatdifferentepochs(2009and2012,inthickredand
thinblue,respectively).TheSpitzer/Irsspectrumisshowninopenblackcirclesinthemiddleandrightpanels.
Table2.LogoftheVLTI/Midiobservationsandaveragedvisibilitiesintherange10.5µm±0.5µm,with1σuncertainties(seetextfordetails,
Sect.3).The(u,v)valuesareinm.
Program Date Time Baseline (u,v) V
079.C-0259(G) 2007-Apr-08 01:11:11 UT1–UT3 (4.9,101.7) 0.48±0.03
079.C-0259(G) 2007-Apr-09 00:33:40 UT1–UT3 (−4.7,101.7) 0.45±0.03
083.C-0775(D) 2009-May-08 05:15:29 UT1–UT3 (65.3,49.9) 0.61±0.03
083.C-0775(D) 2009-May-08 06:11:31 UT1–UT3 (61.8,38.7 0.68±0.04
083.C-0775(B) 2009-May-09 05:32:55 UT2–UT4 (67.2,−33.0) 0.75±0.10
083.C-0775(B) 2009-May-09 06:09:53 UT2–UT4 (57.0,−40.3) 0.67±0.04
3. Multi-technics,multi-wavelengthsmodelling ofα=−1.8). Wecanthereforealreadyanticipatethatfittingthe
SEDandtheMididatasimultaneouslywillbechallenging.
As in Olofsson et al. (2012), a given debris belt is fully de-
InordertocompareourmodeltotheMidiandVisirobser-
scribed by four free parameters plus a dust model: the inner
vationsweupdatedtheDebracodetoproducesyntheticimages
and outer radii of the dust belt (r and r ), a volume density
exponent α (Σ(r) = Σ (r/r )α, αin< 0) aonudt an exponent p for at different wavelengths. To do so, at a given wavelength, we
thegrainsizedistributi0on(d0n(s) ∝ spds, p < 0). TheΣ refer- distributethethermalsurfacebrightnessdistribution(Fν(r),be-
0 tween r and r ), assuming the disk is axisymmetric and in-
encevalueisfoundbyadjustingthetotalmassofthedustM , in out
dust finitely flat. Possible inclinations effects are taken into account
whichinturnisobtainedbyfittingtheSED.Therelativeabun-
dances forthe different dustspecies arealso freeparameters in byprojectingpixelsoftheimageontotheskyplane. However,
because we assumed the disk to be infinitely flat, we chose the
the modelling and are found by fitting the SED and the emis-
sion features detected in the Irs spectrum. For each set of r , inclination i (0◦ being face-on) to be smaller than 60◦. In the
in caseofHD113766A,thischoiceissupportedbytheanalysisof
r , α and p, the fitting of the abundances is performed via a
out Smithetal.(2012)whocouldsuccessfullymodeltheMididata
Levenberg-Marquardt algorithm on a linear combination of the
with a circularly symmetric flux distribution (i.e., small i). As
thermal contributions of all the dust species considered. The
we are solely interested in the variation of the visibilities as a
best-fit disk model is found via a Monte-Carlo Markov Chain
function of the baseline lengths, we averaged the Midi visibil-
(MCMC) exploration of the parameter space (emcee package,
ities in the spectral range 10.5±0.5µm for each Midi baseline.
Foreman-Mackey et al. 2012). All the uncertainties quoted in
We tested different values for i and the position angle (PA) of
thenextsectionsrepresentthestandarddeviationsofalltheval-
thedebrisdisk. Wetestedfourdifferentvaluesfori(0, 20, 40,
uestestedduringthefittingprocess.
and60◦)and30valueslinearlydistributedbetween0and180◦
In thefollowing, we willfirst presenta model ofthe debris
forthePA(0◦meaningthesemi-majoraxisisorientedalongthe
diskassumingthatthedustisdistributedinasinglebeltaround
North-Southdirection).
the central star (referred to as model “1B” for “one belt”). We
willtesttherobustnessofthismodel,comparabletotheonepre-
sented in Olofsson et al. (2012), with respect to the new far-IR 3.1. Onebelt(model“1B”)
photometric observations and mid-IR interferometric measure-
ments. The Herschel data provide new constraints on the cold At first, we tried to model solely the SED, including the new
counterpart of the debris disk, while the mid-IR Midi data are Pacs photometric points, and the Irs spectrum. As mentioned
moresensitivetothewarmerdustandprovidedirectinformation earliertheMididatasuggestthattheregionsemittinginthe N-
onthegeometryofthedustdistribution. Asamatteroffact,the band is compact (r ≤ 1.2AU). Therefore, we first want to test
interferometric observations suggest the region emitting in the howrobusttheassumptionofasingledustbeltis, withrespect
N-bandiscompact((cid:46) 10mas,or1.2AUindiameter,according tothefar-IRmeasurements, andunderlineatthesametimethe
to Smith et al. 2012). On the other side, our previous best-fit limitationsofsuchapproach. Wewillthencomparethebest-fit
model described in Olofsson et al. 2012 was found to be fairly “1B” model to the interferometric data and discuss the subse-
extended(r ∼ 12AU,withadensityprofilewithanexponent quent implications on the spatial distribution of the dust. The
out
Articlenumber,page4of10
Olofssonetal.:ThedebrisdiskaroundHD113766A
1.0 1.0
erg/s/cm/cm] 11110000--11--9810 SED F [Jy]λ20211.....05505 IRS Visibility000...468 MIDI ce brightness000...468 VISIR
F [λλ1100--1132 Diff-22000 0.2 Surfa0.2
1.0 1.0
erg/s/cm/cm] 11110000--11--9810 F [Jy]λ20211.....05505 Visibility000...468 ce brightness000...468
F [λλ1100--1132 Diff-22000 0.2 Surfa0.2
1 10 100 5 10 15 20 25 30 35 60 70 80 90 100 110 0.0 0.1 0.2 0.3 0.4 0.5
λ [µm] λ [µm] Baseline [m] Radius [ ]
00
Fig.3. Upperandlowerpanels: Resultsforthe“1B”and“2B”models. Leftpanel: SEDofHD113766A.TheIrsspectrumisinblack,Pacs
dataareshownasblackopencircles.Forthe“1B”model,thefinalbestfitisinred.Forthe“2B”model,thefinaltotalfitisingreen,theseparate
contributionsoftheinnerandouterbeltsareinredandblue,respectively.Middleleftpanel:blow-upofthestellarsubtractedIrsspectrum.Color
codingisthesameasfortheleftpanel. Residuals(100×[F −F ]/F )aredisplayedbelowthespectra. Middlerightpanel: Observed
ν,obs ν,mod ν,obs
andmodelledMidivisibilitiesasafunctionofthebaseline(openblackcirclesandredsquares,respectively,with1σuncertainties).Rightpanel:
modelledsurfacebrightnessat10and20µm(thinandthickblacklines,respectively)ofsyntheticimagesconvoledwith2DGaussianwithFWHM
of0.322and0.498(cid:48)(cid:48). The2DGaussianwiththeaforementionedFWHM,assumedtobePSFrepresentativeoftheimages,areshowninthinand
thickdashedredlines,forcomparison.
dust properties (absorption efficiences and grains sizes) are the respectively. The third column panel shows the observed and
sameastheonesdescribedinOlofssonetal.(2012).Grainssizes modelledMidivisibilitiesforthebest-fittotheSED(uncertain-
for the crystalline dust grains are 0.1 ≤ s ≤ 1µm and 0.1µm ties are 1 σ). Right panel shows the comparison between the
≤ s ≤ 1mmfortheamorphousdustgrains. Theminimumand PSFs fitted to the Visir observations (Smith et al. 2012), in the
maximumvaluesforthegrainsizesarenotfreeparametersinthe N- and Q-bands and the azimuthally averaged profiles of syn-
modelling. Thischoicemostlyoriginatesfromtheexperimental thetic images for the final best-fit model, convolved with PSFs
setupforthelaboratoryexperimentsthatledtotheextinctionef- with FWHMs of 0.322(cid:48)(cid:48) and 0.498(cid:48)(cid:48), respectively, as in Smith
ficiencies of crystalline olivine grains (see Olofsson et al. 2012 etal.(2012).
formoredetails).
Modelling only the SED with the new far-IR observations, While the best-fit model agrees well with the SED and the
we now find the dust to be located between (0.4 ± 0.2)AU Irs spectrum, the observed Midi visibilities are severely under-
and (51.9±25.2)AU, with a volume density exponent of α = predicted. Asdiscussedabove,thiscanbeexplainedbytherela-
−1.9 ± 0.3, and an exponent for the grain size distribution tivecompactnessoftheemittingregionsprobedbytheinterfero-
p = −3.0 ± 0.1. The total dust mass is 1.3 × 10−2M . The metricdata.TheN-bandemissioninourbest-fitmodelistooex-
⊕
relative abundances for the different dust species are as fol- tendedandthereforethemodelledvisibilitiesdropveryquickly.
lows: (43.1 ± 1.4)% of amorphous silicates (with a predom- This is also true for the Visir observations. The modelled sur-
inance of MgFeSi O and MgFeSiO grains, optical constant face brightness profiles over-predict the observed 2D Gaussian
2 6 4
fromDorschneretal.1995),(17.3±0.7)%ofMg-richcrystalline profilesassumedtoberepresentativeofthePSFs,intheN-band
olivinegrains,(21.0±0.6)%ofFe-richcrystallineolivinegrains and even more in the Q-band (the disk was not resolved in the
(laboratory measurements from Tamanai & Mutschke 2010 for Visirobservations).
theolivinedata),(11.6±0.5)%ofenstatite(Jägeretal.1998a),
(0.0+0.8)%ofβ-cristobalite(Tamanai2010),andof(7.0±0.6)%
As a final test, we tried to include the Midi observations in
amorphouscarbondustgrains(Jägeretal.1998b,mostlyinthe
the fitting process (by summing the reduced χ2 from the fits to
form of cellulose pyrolized at 800◦C, “cel800”). Disk parame- r
theSEDandMididata),butcouldnotachieveanybetterresults.
tersforthebest-fitmodelarereportedinthefirstlineofTable3.
BoththeMidiandthemid-tofar-IRpartoftheSEDareunder-
Fits to the SED, the stellar subtracted Irs spectrum, Midi predicted. Thiscanbeexplainedasatradeoffbetweenthecom-
data,andacomparisontotheVisirimagesaredisplayedonthe pactnessofthedustbeltanditscontributioninthefar-IR.Under
upperfourpanelsofFigure3(lefttoright,respectively). Forthe theassumptionofasingledustbelt, thefitstotheSEDandin-
SED,theIrsspectrumisshowninblack,thePacsdataareshown terferometricdataarediametricallyopposed. Fromamodelling
asblackopencircles,andthebestfitisrepresentedasasolidred point of view, the main conclusion of this exercise is that SED
line.Thecolorcodingisthesamefortheblowupofthefittothe modellingcanbemisleading:asshownintheupperleftpanelof
Irs spectrum (second column), and residuals are shown below Fig.3,wecouldobtainadecentfittotheSED,butthismodelis
thefit. Residualsarecomputedas100×[F −F ]/F , quicklyprovenwrongassoonasspatiallyresolvedobservations
ν,obs ν,mod ν,obs
where F and F are the observed and modelled fluxes, areavailable.
ν,obs ν,mod
Articlenumber,page5of10
0.6 atlargerdistances(Sect.4.1). Wesettheminimumgrainsizeto
Mg-rich s = 4µm for all dust species in the outer belt, as there are
min
Fe-rich no observational evidences, such as emission features at long
0.5
wavelengths, that would require including smaller dust grains.
Because we assumed in our model that crystalline dust grains
0.4 have sizes smaller than 1µm, they are not accounted for in the
y
t outer belt (see Sect.4.3 for further discussion). This time, for
bili each model of the MCMC run we fitted the SED and the Midi
a0.3
b observationssimultaneously.
o
r
P Fortheinnerdustbelt,weobtainedabestfittothedatawith
0.2
dust being located between (0.6±0.1)AU and (0.9±0.2)AU,
a density profile in α = −1.7 ± 0.3, and a grain size distri-
0.1 bution exponent of −3.5 ± 0.1 (consistent with the prediction
for a collisional cascade p = −3.5, Dohnanyi 1969). The to-
tal dust mass is 9.0 × 10−5M⊕. The relative abundances for
10 15 20 25 30 the inner belt are the following: (44.5 ± 1.4)% of amorphous
Abundance [%]
silicate grains (dominated by Mg SiO grains, optical constant
2 4
fromJägeretal.2003),(17.0±0.8)%and(18.8±0.8)%ofMg-
Fig.4. ProbabilitydistributionsfromtheMCMCrunformodel“2B”, rich and Fe-rich crystalline olivine grains, respectively, as well
asafunctionofrelativeabundancesofMg-andFe-richolivinegrains as (7.7±1.2)% of enstatite, (9.4±0.4)% of β-cristobalite, and
(solidblack,anddashedred,respectively). (2.7±0.5)%ofamorphouscarbongrains(cellulosepyrolizedat
800◦C,“cel800”). Figure4showsthemarginalizedprobability
distributions built from the MCMC run, for the relative abun-
dancesofMg-andFe-richcrystallineolivinegrains(solidblack
3.2. Twobelts(model“2B”)
anddashedredhistograms,respectively). EventhoughmoreFe-
Severalrecentstudieshaveshownthatdebrisdisksmaycontain rich crystalline olivine grains are found compared to Mg-rich
several planetesimal belts (e.g., Morales et al. 2011 or the disk olivine, both distributions are overall close to each other. This
around τ Ceti, di Folco et al. 2007; Greaves et al. 2004). We resultdiffersfromtheabundancespreviouslyfoundinOlofsson
have shown in Sect.3.1 that the assumption of one single dust etal.(2012,∼4timesmoreFe-richolivinegrainsthanMg-rich
belt around HD113766A cannot account for all the available crystallinegrains). Thisdifferencecanbeexplainedastheouter
dataset. Wecanthereforestartdiscussingthepossibilityoftwo, planetesimalbeltactsasasourceoffeatureless,continuumemis-
spatially separated dust belts around the central star: an inner sion.Consequently,thismodifiestherequiredcontributionofthe
dustbelt,compact,closetothestarthatcanreproducetheMidi innerdustbeltthatisresponsiblefortheemissionfeatures,and
data and the near- to mid-IR flux, and a second dust belt, far- hencetherelativeabundancesofthedustspecies. Nonetheless,
ther away from the star that will contribute to the emission in it means that Fe-rich crystalline olivine dust grains are neces-
thefar-IR.Toaccountforanadditionalbelt,themodificationto sarytoprovideagoodfittotheemissionfeaturesdetectedinthe
be applied to the Debra code is straightforward as we simply mid-IRspectrum.
duplicated the computations done for one dust belt to the sec- Fortheouterbelt,wefoundr =(9.0±1.4)AUandr =
in out
ondone. Asecondradialgrid,withdifferentr andr values, (13.1±3.4)AU,withα = −1.2±0.3,and p = −3.8±0.2. The
in out
withitsownvolumedensity(α)isgenerated,aswellasthecor- dustmassintheouterplanetesimalbeltisofabout2.4×10−3M .
⊕
responding dust temperatures as the function of the grain sizes Thedustcontentoftheouterplanetesimalbeltconsistsentirely
and compositions. The dust properties in the outer belt can be of amorphous carbon grains (68.7% of “cel600” and 31.3% of
definedseparatelyfromtheonesintheinnerbelt(differentdust “cel1000”). Thiscanbeexplainedbyourchoiceof s =4µm
min
species, s , s ,and p). Finally,thefittingtotheSEDisper- whichdampsmostoftheemissionfeatures, aswellasthelack
min max
formed over the sum of the contributions of the two dust belts of significant emission features in the spectral range where the
(returningthedustmassforeachbelt). outer dust belt contributes most. Similar results were found
AsdiscussedinOlofssonetal.(2012),theradiationpressure by Lebreton et al. (2012) in the process of modelling the SED
forceisstrongaroundHD113766A.Wecomputedtheunitless of HD181327. When considering only silicates and carbona-
β ratio between the radiation pressure and gravitational forces, ceous grains, carbon grains were preferentially used to fit the
for different grain sizes and dust compositions as described in data. The authors pushed their models further by including the
Burnsetal.(1979). Wefindthatgrainswithsizessmallerthan effects of porosity as well as the contribution of icy grains, re-
(4±0.5)µm(dependingontheircompositions)areexpectedto ducing at the same time the relative fraction of carbon grains.
be short-lived around the central star (see Sect.4.1). However, For HD113766A, given the intertwined contributions of both
the detection of strong emission features indicates that warm the inner and outer belt, and the limited spectral coverage at
(sub-)µm-sized grains must be located close to the star to con- wavelengthslongerthan160µm,wedidnotperformsimilarin-
tribute in the Irs spectrum. Therefore, the dust properties (ab- vestigations.Oneshouldalsonotethatourresultforthelocation
sorption efficiencies and grain sizes) of the innermost dust belt oftheouterbeltishighlydependentontheminimumgrainsize
inmodel“2B”(asin“twobelts”)arechosentobesimilartothe s opted in our model. We find typical temperatures for the
min
“1B” model (Sect.3.1), regardless of the radiation pressure is- carbongrainsbetween100and200K.Tohavecomparabletem-
sue. The relative abundances are still free parameters. For the peratures,ifs >4µm,grainswillhavetobelocatedcloserto
min
outer dust belt, we did not include such small grains. The ra- thestar,andvice-versaif s <4µm. Spatiallyresolvedobser-
min
tio β between the radiation pressure and gravitational forces is vationsoftheouterbeltaremandatorytobetterconstrainitslo-
distance-independent (both forces decreasing as r−2, their ratio cationandthedustcontent. Eventhoughthediskisnotdetected
nolongerdependsonr),whilethecollisionaltimescaleislonger intheVisirimages, theyprovideanupperlimitofabout0.14(cid:48)(cid:48)
Articlenumber,page6of10
Olofssonetal.:ThedebrisdiskaroundHD113766A
Table3.Best-fitresultsforthediskparametersforboththe“1B”and“2B”models
Model Belt# r r α p M
in out dust
[AU] [AU] [M ]
⊕
Onebelt(“1B”) 1 0.4±0.2 51.9±25.2 −1.9±0.3 −3.0±0.1 1.3×10−2
1 0.6±0.1 0.9±0.3 −1.7±0.3 −3.5±0.1 9.0×10−5
Twobelts(“2B”)
2 9.0±1.4 13.1±3.4 −1.2±0.3 −3.8±0.2 2.4×10−3
(17AUat123pc)forthemaximalextentofthedisk(Smithetal. AsmentionedinSect.1,thesecondaryofthebinarysystem
2012). Diskparametersforthebest-fitmodelarereportedinthe islocatedat1.3(cid:48)(cid:48) ofHD113766A(160AUat123pc). Without
lasttwolinesofTable3. a fully characterized orbital solution for the binary system it is
challengingtodiscussindetailtheinfluenceofthesecondaryon
ThelowerfourpanelsofFig.3showtheresultsforthe“2B”
the debris disk. However, it is unlikely that it can have a ma-
model,withthesamearrangementasdescribedinSect.3.1. For joreffectontheouterdisclocatedat∼13AUfromtheprimary.
the left and middle left panels (SED and Irs spectrum) the to-
The only possibility would be if the companion star was on a
tal fit is shown as a solid green line, while the two contribu-
very eccentricorbit with aperiastron passing closeto the outer
tionsoftheinnerandouterdustbeltsarerepresentedasredand
belt,butinthiscasethesecondarywouldspendmostofitstime
bluesolidlines,respectively. Comparedtothe“1B”model,the
in remote regions close to its apoastron, leaving much time for
best-fitmodelismuchmorecompactandthereforeboththeMidi
the circumprimary disc to “recover” from the perturbations at
andVisirdataaremuchbetterreproduced(exceptthe“problem-
periastronpassages(Thébaultetal.2010;Thébault2012).
atic”MidirundiscussedinSect.2). AsshownbytheSED,the
contribution of the outer belt in the N-band is almost negligi-
blecomparedtothetotalflux. Therefore,theMidiobservations 4. Discussion
aremostlysensitivetotheinnerbelt,whichismuchmorecom-
Results presented in Sect.3 show that the available dataset for
pact. Wefinditsinclinationtobeof40◦withapositionangleof
HD113766A can be best reproduced under the assumption of
120◦,however,oneshouldnotethattheinclinationissampledby
two spatially separated dust belts. Even though the SED alone
stepsof20◦only. Theoverallcompactnessoftheinnerdustbelt
can be reproduced under the assumption of a single dust belt,
also explains why the modelled N-band Visir synthetic image
the Midi observations proved extremely useful to constrain the
agrees with the observations, as most of the emission at 10µm
spatial extent of the region responsible for the emission in the
arisesfromtheinnerbelt. AsfortheQ-bandimage,Smithetal.
N-band. If the inner dust belt extends farther out than ∼1AU,
(2012) found an upper limit at 0.14(cid:48)(cid:48) (17AU at 123pc) for the
the modelled Midi visibilities are greatly under-predicted at all
maximumextentofthedisk,consistentwithourr =13AU.
out baselines. Our model is in good agreement with the results of
Ourbestfitmodelindicatesthattheinnerandouterbelthas Smith et al. (2012) who modelled the Midi data with either a
tobespatiallyseparated. However,wefurtherpushedourmod- symmetric Gaussian model with a FWHM of 10mas (1.2AU
eling effort and tried to fit the dataset with a single dust belt indiameter)oraringmodelwitharadiusof6mas(∼0.7AU).
that shows a “break” in the density distribution (i.e., a single Our modelling results, supported by the new Pacs observations
butnotcontinuousbeltasopposedtoSect.3.1). Themotivation enabledustogoonestepfurtheranddemonstratethepresence
being to mimic an inward radial drift of dust grains, followed ofanadditionalplanetesimalbeltfartheroutinthesystem.
bytheformationofadustringatthesublimationradius,asde-
scribed in Kobayashi et al. (2009) for instance. We therefore
4.1. Timevariability
run a new set of models, for which we force the inner radius
of the outer belt to be the same as the outer radius of the in- EventhoughtheabsolutecalibrationoftheVisirspectraischal-
ner belt (seven free parameters instead of eight). The best fit lenging,onecanalreadyseefromtherightpanelofFig.2thatthe
resultsfortheSEDandtheMididatacouldnotreachthequal- shape of the emission features has not changed between 2004,
ity of the best-fit “2B” model. We find that the disk extends 2009,and2012. Therelativestrengths,thewidths,andpeakpo-
from0.6AUupto16.5AU,withabreakinthedensityatabout sitionsofthefeaturesseemtoremainthesameoveratimespan
0.7AU. The fit to the Irs spectrum is acceptable except for the ofatleast8years. Suchaphenomenonappearstobealsovalid
redwingofthe10µmemissionfeaturewherethemodeledflux for other debris disks with strong emission features (Olofsson
is larger than the observed flux. Because the disk extends far- etal.2012, Johnsonetal.2012), whilerapiddissipationofcir-
theroutthan∼1AU,theinterferometricmeasurementsarestill cumstellarmaterialhasonlybeenobservedforonesingleobject
under-predicted (V ∼ 0.5−0.6). To compare our results with (Melisetal.2012). If(sub-)µm-sizedcrystallinedustgrainsare
the calculations presented in Kobayashi et al. (2009), we com- solely responsible for the emission features observed at 10 and
pute the number density n as a function of the distance r. For 11µm,itwouldsuggestthatthispopulationofgrainsisreplen-
thebest-fitmodel,thebreakattheintersectionbetweenthetwo ished from the very same reservoir (same composition), on the
adjacentsdustbeltshastobeextremelylarge: thequantityn×r timescale of the evacuation by radiation pressure. To compute
ismorethanfiveordersofmagnitudelargerintheinnermostre- thistimescalet ,wefollowedtheformalismtobepresentedin
rp
gions (r < 0.7) compared to the outermost regions of the disk Lebreton et al. (in prep, private communication). The authors
(r > 0.7). Ifthewarmringwas theconsequenceofradialdrift definet , whichdependsontheβratio, asthetimeneededfor
rp
and dust sublimation, Kobayashi et al. (2009) predicted an en- adustgraintobetransportedfromitsinitialdistancer totwice
hancementfactorsmallerthan10forsilicatedustgrainsaround thisdistance2×r.Asmentionedearlier,wecomputedtheβratio
aF-typestar. Wecanthereforeconfidentlyrulethisscenarioout forgrainsofdifferentcompositionsasafunctionoftheirsizes.
toexplaintheoriginofthetransientdustaroundHD113766A. ForHD113766A,thisratiopeaksatβ = 3forgrainswithsizes
Articlenumber,page7of10
of about 0.5µm. For these dust grains, assuming r = 0.75AU 4.3. Theoriginofthetransientdust
([r −r ]/2),wefindt ∼ 1month. Ontheotherhand,small
out in rp
dustgrainsarecontinuouslyproducedbycollisions. Toestimate InOlofssonetal.(2012)wehypothesizedthatthetransientdust
the collisional lifetime of the smallest grains in the inner dust grainsobservedaroundHD113766Amaybetheoutcomeofa
belt, we used the formula t = (Ωτ)−1, where Ω is the angular disruptive collision of partially differentiated bodies. This con-
c
velocity at a distance r and τ is the geometrical vertical opti- clusion was based on specific mineralogic markers: the com-
cal depth (Thébault & Augereau 2007). Even though the exact positionofcrystallineolivinegrainscontainingaFe/[Fe+Mg]
collisionaltimescalesmaydepartfromthisvalue,itprovidesan fractionofabout20%. Basedonthenewobservationsavailable,
orderofmagnitudeestimate. Atadistancer=0.75AU,wefind werefinedourpreviousbest-fitmodelbyincludinganadditional
acollisionallifetime-sourceofdustreplenishment-ofabout2 planetesimalbeltfartherawayfromthestar. Howdoesthisnew
months,twicethetimescaleofevacuationbyradiationpressure modelfitinthispicture? Onescenariothathasbeensuggested
t . Fortheouterbelt,assumingr=10AU,wefindt ∼5years intheliteraturetoexplaintheexcessemissionobservedaround
rp rp
(for grains with β = 3) and t = 300years, which supports our warm debris disks is that an outer belt is “feeding” the inner-
c
choiceof s =4µmintheouterbelt. Oneshouldnotethatthe mostregions. Comet-likebodiesfromtheouterregionsmaybe
min
timescaleforthePoynting-Robertsondrag,ismuchlongercom- scattered inwards, for instance by a (chain of) planet(s) as de-
paredtotheaforementionedtimescalesandcanbeneglectedin scribed in Bonsor et al. (2012). These bodies will then collide
thecaseofHD113766A(Wyatt2005). witheachotherorsublimateandreleasesmalldustgrainsinthe
immediatevicinityofthestar. Withourfindingofanouterdust
belt, one may wonder if such an event is taking place around
Thesub-µm-sizedgrainsresponsiblefortheobservedemis-
HD113766A.
sion features should be evacuated rapidly from the inner belt.
One may therefore consider dust aggregates of different com- The N-band Midi observations ruled out any significant
emission to be outside of ∼1AU, suggesting that the phe-
positions (e.g., Lindsay et al. 2010) as a possible explana-
tion. Computations of absorption coefficients, making use of nomenonresponsibleforthetransientdusthastobe“local”,as
opposedtoextended. Foricycometsscatteredinwardsfromthe
the discrete-dipole approximation method (Draine 1988) have
outerbelt,sublimationmayreleaseapopulationofdustygrains.
shown that small crystalline monomeres may be incorporated
Themajorityofthesegrainswouldbereleasedinsideof3.3AU,
intolargerdustaggregatesbutstillshowprominentemissionfea-
basedonheatingoflarge,km-sizedcometsbyastellarradiation
turesassociatedwithcrystallinedustgrains(e.g.,Woodenetal.
fieldfroma4.4L star(privatecommunicationwithAmyBon-
2012). Theselargeraggregateswouldthenbelesssubjectedto (cid:12)
sor and Ulysse Marboeuf, Bonsor et al. in prep). The fate of
radiation pressure and stay gravitationally bound to the central
suchgrainsreleasedfromacometisunclear-whilsttheycould
staronlongertimescales. Buteventheseaggregateswilleven-
supplytheobservedexcessemission,releaseofdustgrainsfrom
tually collide, fragment, and be blown out of the system. This
aneccentriccometdoesnotbyitselfexplainthecompactnature
suggeststhatreplenishmentfromareservoirwiththesamebulk
oftheinnerdustbelt. Iftheobservedwarmmaterialoriginated
compositionhastooccur.Anotherwaytoexplaintheoverallsta-
fromtheouterregions,theMididatacouldonlybeexplainedby
bilityoftheemissionfeatureswouldbetointroduceadeficitin
thegrainsizedistributionforgrainsthatareaffectedbyradiation thecaptureofamassivecometthatisout-gasingandevaporat-
ingclosetothestar(alocalproductionofdust,Beichmanetal.
pressure(e.g.,Johnsonetal.2012). However,thismodellingis
2005concerningHD69830).
outofthescopeofthispapergiventhetotalnumberoffreepa-
Our best-fit model can successfully reproduce the emission
rametersandthelackofspatiallyresolvedobservationstobetter
features detected in the Irs spectrum without including crys-
constrainthelocationofthedust.
talline dust grains in the outer belt. This choice, initially mo-
tivatedbythestrengthoftheradiationpressurecanalsobejusti-
fiedbythelackofemissionfeaturesassociatedwithcrystalline
4.2. Theopticaldepthandopeningangle grainsbeyond30µm. Iftheouterbeltwouldcontainsmallcrys-
talline dust grains, they would be cold enough to be respon-
sible for emission features at long wavelengths (especially the
Natively,themodelspaceintheDebracodeis1D:thereisonly 33µmfeaturearisingfromcrystallineolivinegrains). Thether-
aradialdependenceonthedensityandtemperaturesofthedust malemissionoftheouterbeltpeaksatabout25–30µm,andthe
grains. However, an infinitely flat disk would quickly become factthatthe33µmfeatureisnotdetectedsuggeststhatsuchµm-
radiallyopticallythick. Therefore,theverticaldimensionisde- sized crystalline grains are not present, at a detectable level, in
scribedviaanopeningangleθ (0◦ beinganinfinitelyflatdisk). theouterbelt. Theeffectofcrystallinemonomeresinclusionsin
Atconstantvolumedensity,increasingθdecreasesthelocalden- aggregateshasbeenstudiedforthe10µmemissionfeature(e.g.,
sityinalloftheradialcells,andhencetheopticaldepthbecomes Min et al. 2008), but not for other emission features at longer
smalleraswell. Foragivendiskmodel,theDebracodereturns wavelengths. Furtherinvestigationshavetobeperformedtoes-
a threshold value θ , which is the smallest opening angle for timate how much crystalline grains can potentially be included
lim
which the debris disk is still in the optically thin regime. One inaggregatesthatwouldstillnotshowanydetectableemission
should note that for small opening angles, θ is comparable to featurearound30µm.Overall,thissuggeststhatthecrystallinity
twice the aspect ratio of the disk 2H/r (same formalism as in fraction has to be smaller in the outer belt compared to the in-
Thébault2009).Forthebest-fit“2B”model,wefoundaθ ner belt, pointing towards a different composition between the
lim,inner
ofabout3.8◦ fortheinnermostbelt,avalueconsistentwiththe twodustbelts. Wouldthemineralogicaldifferencesbetweenthe
onefoundbyLisseetal.(2008,4◦). Theopeningangleforthe twobeltsbeconfirmed,thiswouldargueagainstthecaptureofa
outer belt is smaller (θ ∼ 0.9◦). The lower limit for the massivecometoriginatingfromtheouterregions.
lim,outer
openingangleθ isingoodagreementwiththeworkpre- Our spectral decomposition results suggest that the crys-
lim,inner
sentedinThébault(2009),whofoundaminimum“natural”ob- tallineolivinegrainsintheinnerbeltcontainsmoreFethanwhat
servedaspectratioof∼2.3◦(definedasθ/2). isexpectedforcomet-likebodies(Wooden2008;Zolenskyetal.
Articlenumber,page8of10
Olofssonetal.:ThedebrisdiskaroundHD113766A
2008;Hanner&Zolensky2010),coldcrystallineolivinegrains the star, a distance well constrained by the Midi observations.
observed in other debris disks (βPic, de Vries et al. 2012), or Thisinnerbeltcontainswarmsub-µm-sizeddustgrains. These
even younger proto-planetary disks (e.g., Olofsson et al. 2010; small grains are responsible for the emission features observed
Sturm et al. 2010; Sturm et al. submitted, and Juhász et al. via mid-IR spectroscopy. An outer planetesimal belt, located
2010 who found an upper limit of 10% for the Fe fraction). between 9–13AU (distances much less constrained by the ob-
As previously discussed in Olofsson et al. (2012), crystalliza- servations)accountforthefar-IRemissionobservedwithPacs.
tion via thermal annealing or gas-phase condensation are un- The new observations support our previous conclusion (Olofs-
likely to produce Fe-bearing crystalline olivine grains in proto- sonetal.2012),wherewesuggestedthattheobservedtransient
planetary disks (Nuth & Johnson 2006; Gail 2004). Observ- dustistheresultofadisruptivecollision.Thiscollisioncouldbe
ingasignificantfractionofFe-richcrystals(morethanhalfthe theconsequenceofon-goingterrestrialplanetformationaround
total abundance of crystalline olivine grains) indicates that the theyoungdebrisdisk(10–16Myr).
dust composition has been altered. Internal metamorphism of
Acknowledgements. Theauthorsthankthereferee,PhilippeThébault,forposi-
theolivinecontent,asdiscussedinNakamuraetal.(2011)con-
tiveandconstructivecommentsthatimprovedthepaper.JCAthankstheFrench
cerningtheasteroidItokawa,canprovideanexplanationforour NationalResearchAgency(ANR)forfinancialsupportthroughcontractANR-
finding. Consequently,withpresent-dayobservationsandlabo- 2010BLAN-0505-01(EXOZODI)
ratorymeasurementsofopticalproperties,wereiterateourpre-
vious conclusion that we may be witnessing the aftermath of
a disruptive collision within the first AU from the central star. References
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