On the physical structure of IRC+10216. Ground-based and Herschel observations of CO and CCH

0.77 MB·English

by  E. De Beck

Astronomy&Astrophysicsmanuscriptno.17635 c ESO2012
(cid:13)
January10,2012
On the physical structure of IRC+10216
⋆
Ground-based and Herschel observations of CO and C H
2
E.DeBeck1,R.Lombaert1,M.Agu´ndez2,3,F.Daniel2,L.Decin1,4,J.Cernicharo2,H.S.P.Mu¨ller5,M.Min6,P.
Royer1,B.Vandenbussche1,A.deKoter4,6,L.B.F.M.Waters7,4,M.A.T.Groenewegen8,M.J.Barlow9,M.
Gue´lin10,13,C.Kahane11,J.C.Pearson12,P.Encrenaz13,R.Szczerba14,andM.R.Schmidt14
1 InstituteforAstronomy,DepartmentofPhysicsandAstronomy,KULeuven,Celestijnenlaan200D,3001Heverlee,Belgium
e-mail:[email protected]
2 2 CAB.INTA-CSIC.CrtaTorrejo´nkm4.28850Torrejo´ndeArdoz.Madrid.Spain
1 3 LUTH,ObservatoiredeParis-Meudon,5PlaceJulesJanssen,92190Meudon,France
0 4 AstronomicalInstitute“AntonPannekoek”,UniversityofAmsterdam,SciencePark904,1098XHAmsterdam,TheNetherlands
2 5 I.PhysikalischesInstitut,Universita¨tzuKo¨ln,Zu¨lpicherStr.77,50937Ko¨ln,Germany
6 AstronomicalInstituteUtrecht,UniversityofUtrecht,POBox8000,NL-3508TAUtrecht,TheNetherlands
n 7 SRONNetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584CAUtrecht,TheNetherlands
a 8 RoyalObservatoryofBelgium,Ringlaan3,1180Brussels,Belgium
J
9 DepartmentofPhysicsandAstronomy,UniversityCollegeLondon,GowerStreet,LondonWC1E6BT,UK

9 10 InstitutdeRadioastronomieMillime´trique(IRAM),300ruedelaPiscine,38406Saint-Martin-dHe`res,France
11 LAOG,ObservatoiredeGrenoble,UMR5571-CNRS,Universite´JosephFourier,Grenoble,France

] 12 JetPropulsionLaboratory,Caltech,Pasadena,CA91109,USA
R
13 LERMA,CNRSUMR8112,ObservatoiredeParisandE´coleNormaleSupe´rieure,24RueLhomond,75231ParisCedex05,France
S 14 N.CopernicusAstronomicalCenter,Rabianska8,87-100Torun,Poland
.
h
Received—;accepted—
p
-
o Abstract
r
t Context. Thecarbon-richasymptoticgiantbranchstarIRC+10216undergoesstrongmassloss,andquasi-periodicenhancements
s
a ofthedensityofthecircumstellarmatterhavepreviouslybeenreported.Thestar’scircumstellarenvironmentisawell-studied,and
[ complexastrochemicallaboratory,withmanymolecularspeciesprovedtobepresent.COisubiquitousinthecircumstellarenvelope,
whileemissionfromtheethynyl(C2H)radicalisdetectedinaspatiallyconfinedshellaroundIRC+10216.Asreportedinthisarticle,
1 werecentlydetectedunexpectedlystrongemissionfromthe N = 4 3, 6 5, 7 6, 8 7,and9 8transitionsofC Hwiththe
v IRAM30mtelescopeandwithHerschel/HIFI,challengingtheavaila−bleche−micala−ndphy−sicalmode−ls. 2
0 Aims. WeaimtoconstrainthephysicalpropertiesofthecircumstellarenvelopeofIRC+10216,includingtheeffectofepisodicmass
5 lossontheobservedemissionlines.Inparticular,weaimtodeterminetheexcitationregionandconditionsofC H,inordertoexplain
2
8 therecentdetections,andtoreconcilethesewithinterferometricmapsoftheN =1 0transitionofC H.
1 Methods. Usingradiative-transfermodelling,weprovideaphysicaldescriptionof−thecircumstellare2nvelopeofIRC+10216,con-
. strainedbythespectral-energydistributionandasampleof20high-resolutionand29low-resolutionCOlines—todate,thelargest
1
modelledrangeofCOlinestowardsanevolvedstar.Wefurtherpresentthemostdetailedradiative-transferanalysisofC Hthathas
0 2
beendonesofar.
2
1 Results.Assuming a distance of 150pc to IRC+10216, the spectral-energy distribution is modelled with a stellar luminosity of
11300L andadust-mass-lossrateof4.0 10 8M yr 1.Basedontheanalysisofthe20high-frequency-resolution COobserva-
: − −
v tions,an⊙averagegas-mass-lossrateforthel×ast1000y⊙earsof1.5 10 5M yr 1isderived.Thisresultsinagas-to-dust-massratioof
− −
i 375,typicalforthistypeofstar.Thekinetictemperaturethrough×outthec⊙ircumstellarenvelopeischaracterisedbythreepowerlaws:
X
T (r) r 0.58forradiir 9stellarradii,T (r) r 0.40forradii9 r 65stellarradii,andT (r) r 1.20forradiir 65stellar
r rakdinii.T∝his−modelsuccessf≤ullydescribesallk4in9ob∝serv−edCOlines.W≤eal≤soshowtheeffectofdekinnsity∝enh−ancementsint≥hewindof
a
IRC+10216ontheC H-abundanceprofile,andthecloseagreementwefindofthemodelpredictionswithinterferometricmapsof
2
theC HN =1 0transitionandwiththerotationallinesobservedwiththeIRAM30mtelescopeandHerschel/HIFI.Wereporton
theim2portanceo−fradiativepumpingtothevibrationallyexcitedlevelsofC Handthesignificanteffectthispumpingmechanismhas
2
ontheexcitationofalllevelsoftheC H-molecule.
2
Keywords.Stars:individual:IRC+10216—stars:massloss—stars:carbon—astrochemistry—stars:AGBandpost-AGB—
radiativetransfer
1. Introduction
Thecarbon-richMira-typestarIRC+10216(CWLeo)islocated
⋆ Herschel is an ESA space observatory with science instruments at the tip of the asymptoticgiantbranch(AGB), whereit loses
providedbyEuropean-ledPrincipalInvestigatorconsortiaandwithim- massat a highrate ( 1 4 10−5M yr−1; Crosas&Menten,
portantparticipationfromNASA. 1997; Groenewegen∼et−al.,×1998; ⊙Cernicharoetal., 2000;
1
E.DeBecketal.:COandCCHaroundIRC+10216
DeBecketal., 2010). Located at a distance of 120 250pc scan coversthe frequencyranges 480 1250GHz and 1410
− − −
(Loupetal.,1993;Crosas&Menten,1997;Groenewegenetal., 1910GHz, with a spectral resolution of 1.1MHz. All spectra
1998; Cernicharoetal., 2000), it is the most nearby C-type were measured in dual beam switch (DBS; deGraauwetal.,
AGB star. Additionaly, since its very dense circumstellar 2010)modewitha3 chopthrow.Thistechniqueallowsoneto
′
envelope (CSE) harbours a rich molecular chemistry, it has correctforanyoff-sourcesignalsinthespectrum,andtoobtain
been deemed the prime carbon-rich AGB astrochemical astablebaseline.
laboratory. More than 70 molecular species have already Since HIFI is a double sideband (DSB) heterodyne instru-
been detected (e.g. Cernicharoetal., 2000; Heetal., 2008; ment, the measured spectra contain lines pertaining to both
Tenenbaumetal., 2010), of which many are carbon chains, the upper and the lower sideband. The observation and data-
e.g. cyanopolyynesHC N (n=1, 3, 5, 7, 9, 11) and C N (n=1, reductionstrategiesdisentanglethese sidebandswith veryhigh
n n
3, 5). Furthermore, several anions have been identified, e.g. accuracy, producing a final single-sideband (SSB) spectrum
C H (n= 4,6,8;Cernicharoetal.,2007;Remijanetal.,2007; without ripples, ghost features, and any other instrumental ef-
n −
Kawaguchietal., 2007), and C N (Thaddeusetal., 2008), fects,coveringthespectralrangesmentionedabove.
3 −
C5N− (Cernicharoetal., 2008), and CN− (Agu´ndezetal., ThetwoorthogonalreceiversofHIFI(horizontalH,andver-
2010). Detections towards IRC+10216 of acetylenic chain ticalV)wereusedsimultaneouslytoacquiredataforthewhole
radicals (CnH), for n=2 up to n=8, have been reported by e.g. spectralscan,exceptforband2a2.Sincewedonotaimtostudy
Gue´linetal. (1978, C4H), Cernicharoetal. (1986a,b, 1987b, polarisationoftheemission,weaveragedthespectrafromboth
C5H), Cernicharoetal. (1987a) and Gue´linetal. (1987, C6H), polarisations, reducing the noise in the final product. This ap-
Gue´linetal. (1997, C7H), and Cernicharo&Gue´lin (1996, proachisjustifiedsincenosignificantdifferencesbetweentheH
C8H). andVspectraareseenforthelinesunderstudy.
The smallest CnH radical, ethynyl (CCH, or C2H), was Adetaileddescriptionoftheobservations,andofthedatare-
first detected by Tuckeretal. (1974) in its N = 1 0 tran- ductionofthislargesurveyisgivenbyCernicharoetal.(2010b)
−
sition in the interstellar medium (ISM) and in the envelope and Cernicharoetal. (2011b, in prep.). All C H data in this
2
aroundIRC+10216.It was shownto be one ofthe mostabun- paper are presented in the antenna temperature (T ) scale3.
A∗
dant ISM molecules. The formation of C2H in the envelopeof Roelfsemaetal.(2012,submitted)describethecalibrationofthe
IRC+10216 is attributed mainly to photodissociation of C2H2 instrumentandmentionuncertaintiesintheintensityoftheorder
(acetylene),oneofthemostprominentmoleculesincarbon-rich of10%.
AGBstars.Fonfr´ıaetal.(2008)modelledC2H2 emissioninthe ForthecurrentstudyweconcentrateonobservationsofCO
mid-infrared,whichsamplesthedust-formationregioninthein- and C H. The ten CO transitions covered in the survey range
2
ner CSE. The lack of a permanent dipole moment in the lin- from J = 5 4upto J = 11 10andfrom J = 14 13upto
earlysymmetricC2H2-moleculeimpliestheabsenceofpurero- J =16 15.−TheC2Hrotation−altransitionsN =6 5,−7 6,and
tational transitions which typically trace the outer parts of the 8 7ar−ecoveredbythesurveysinbands1a(480− 56−0GHz),
envelope.C2H,ontheotherhand,hasprominentrotationallines 1b−(560 640GHz),and2a(640 720GHz),respe−ctively.The
thatprobethechemicalandphysicalconditionslinkedtoC2H2 N = 9 −8transitionisdetectedin−band2b(720 800GHz)of
inthesecoldouterlayersoftheCSE.Ithasbeenestablishedthat thesurv−ey,butwithverylowS/N;higher-Ntransi−tionswerenot
C2H emission arises from a shell of radicals situated at ∼15′′ detected.AsummaryofthepresentedHIFIdataofC2Hisgiven
fromthecentralstar(Gue´linetal.,1993).Observationswiththe inTable1,thespectraareshowninFig.1.
IRAM30mtelescopeandHerschel/HIFIshowstrongemission
inseveralhigh-NrotationaltransitionsofC H,somethingwhich
2
isunexpectedandchallengesourunderstandingofthismolecule 2.2.Herschel/SPIRE
inIRC+10216.
In the framework of the Herschel guaranteed time key
We presentanddiscussthe high-sensitivity,high-resolution
programme “Mass loss of Evolved StarS” (MESS;
data obtained with the instruments on board Herschel
Groenewegenetal., 2011) the SPIRE Fourier Transform
(Pilbrattetal., 2010): HIFI (Heterodyne Instrument for the
Spectrometer (FTS; Griffinetal., 2010) was used to obtain
Far Infrared; deGraauwetal., 2010), SPIRE (Spectral and
IRC+10216’s spectrum on 19 November 2009 (OD 189).
Photometric Imaging Receiver; Griffinetal., 2010), and PACS
The SPIRE FTS measures the Fourier transform of the source
(Photodetector Array Camera & Spectrometer;Poglitschetal.,
spectrum across two wavelength bands simultaneously. The
2010)andwiththeIRAM30mTelescope1inSect.2.Thephys-
short wavelength band (SSW) covers the range 194 313µm,
icalmodelforIRC+10216’scircumstellarenvelopeispresented −
while the long wavelength band (SLW) covers the range
and discussed in Section 3. The C H molecule, and our treat-
2 303 671µm. The total spectrum covers the 446 1575GHz
mentofitisdescribedinSection4.Asummaryofourfindings − −
frequency range, with a final spectral resolution of 2.1GHz.
isprovidedinSection5.
Thequalityoftheacquireddatapermitsthedetectionoflinesas
weakas1 2Jy.Forthetechnicalbackgroundandadescription
−
2. Observations of the data-reduction process we refer to the discussions by
Cernicharoetal. (2010a) and Decinetal. (2010b). These
2.1.Herschel/HIFI:SpectralScan
authors report uncertainties on the SPIRE FTS absolute fluxes
The HIFI data of IRC+10216 presented in this paper are part oftheorderof15 20%forSSWdata,20 30%forSLWdata
− −
ofaspectrallinesurveycarriedoutwithHIFI’swidebandspec- below 500µm, and up to 50% for SLW data beyond 500µm.
trometer(WBS; deGraauwetal.,2010)inMay2010,on3con-
secutive operational days (ODs) of the Herschel mission. The 2 Because of an incompleteness, only the horizontal receiver was
usedduringobservationsinband2a.Supplementaryobservationswill
1 BasedonobservationscarriedoutwiththeIRAM30mTelescope. beexecutedlaterinthemission.
IRAM is supported by INSU/CNRS (France), MPG (Germany) and 3 Theintensityinmain-beamtemperatureT isobtainedviaT =
MB MB
IGN(Spain). T /η ,whereη isthemain-beamefficiency,listedinTable1.
A∗ MB MB
2
E.DeBecketal.:COandCCHaroundIRC+10216
CCH(N = 1−0) IRAM 30m CCH(N = 2−1) IRAM 30m
2.0
J=3/2−1/2 J=1/2−1/2 5 J=5/2−3/2 J=3/2−1/2 J=3/2−3/2
1.5 4
CH, v=1 AlCl
4 7
*T (K)A 1.0 C6H AlCl *T (K)A 23
0.5
1
0.0 0
87.25 87.30 87.35 87.40 87.45 87.50 174.65 174.70 174.75 174.80 174.85
ν (GHz) ν (GHz)
CCH(N = 3−2) IRAM 30m CCH(N = 4−3) IRAM 30m
4
J=7/2−5/2 J=5/2−3/2 J=5/2−5/2 J=9/2−7/2 J=7/2−5/2 J=7/2−7/2
6
3
*T (K)A 4 SiN *T (K)A 2
2 1
0 0
261.9 262.0 262.1 262.2 349.3 349.4 349.5 349.6 349.7
ν (GHz) ν (GHz)
CCH(N = 6−5) HIFI 1a CCH(N = 7−6) HIFI 1b
0.6 J=13/2−11/2 J=11/2−9/2 J=15/2−13/2 J=13/2−11/2
0.3
0.5
0.4 0.2
K) K)
*T (A 00..23 SiC2 *T (A 0.1
0.1
0.0
0.0
523.95 524.00 524.05 611.25 611.30 611.35 611.40
ν (GHz) ν (GHz)
CCH(N = 8−7) HIFI 2a CCH(N = 9−8) HIFI 2b
0.10
J=17/2−15/2 J=15/2−13/2 J=19/2−17/2 J=17/2−15/2
0.10 0.08
0.06
K) 0.05 K) 0.04
*T (A *T (A 0.02
0.00
0.00
SiC2 −0.02 30SiS SiC2
−0.04
−0.05
698.5 698.6 698.7 785.6 785.7 785.8 785.9 786.0
ν (GHz) ν (GHz)
Figure1.C HN =1 0,2 1,3 2,4 3,6 5,7 6,8 7,and9 8rotationaltransitionsinIRC+10216’senvelopeasobserved
2
− − − − − − − −
withtheIRAM30mtelescopeandHerschel/HIFI.Thefinestructurecomponentsarelabelledwiththerespective J-transitionsin
thetopofeachpanel,whilethehyperfinecomponentsareindicatedwithverticaldottedlines.Forthesakeofclarity,weomittedthe
componentsthataretooweaktobedetected.SeeSect.4fordetailsonthespectroscopicstructureofC H.Additionallyobserved
2
featuresrelatedtoothermoleculesarealsoidentified.
We refer to Table 2 for a summary of the presented data, and 2.3.Herschel/PACS
show an instructive comparison between the HIFI and SPIRE
data of the C H lines N = 6 5 up to N = 9 8 in Fig. 2. It
2 − − Decinetal. (2010b) presented PACS data of IRC+10216,
isclearfromtheseplotsthatthelowerresolutionoftheSPIRE
also obtained in the framework of the MESS programme
spectrumcausesmanylineblendsinthespectraofAGBstars.
(Groenewegenetal., 2011). The full data set consists of SED
scans in the wavelength range 52 220µm, obtained at differ-
−
ent spatial pointings on November 12, 2009 (OD182). We re-
reduced this data set, taking into account not only the central
spaxelsofthedetector,butallspaxelscontainingacontribution
3
E.DeBecketal.:COandCCHaroundIRC+10216
80
2 C
C
S N
60 Si N Si H
x (Jy) 40 HCN CS, SiS 13HC CCH, SiS HCN CS SiS, 13CO
u
Fl
20
N
C
H
0
480 500 520 540
80 O S
17C 34C
ux (Jy) 4600 HO2 S, SiO, 29SiS NH3 CS SiS 13HCN SiO CCH SiS HCN , HCN,
Fl Si Cl
20 H
N
O C
C H
0
560 580 600 620
0.3
CCH

HIFI 1b U SiS,ν=2 30SiO SiC SiS,ν=1
0.2 2
* (K)TA 0.1 SiC2 AlCl HCN U U U

0.0
609 610 611 612 613 614
80 C N
N O C
60 H Si H
ux (Jy) 40 SiS, SiS, 13CO SiS CS , CCH, HCN SiS HCN
Fl C2
20 N Si N
C C
H H
0
640 660 680 700
80 2 N H S
C C2 C O C Si
60 S, Si S S S, Si S CO 13, H O S, Si S, C CN,
Flux (Jy) 2400 Si C Si 30HO, Si2 Si 13 SiS Si C 1713CO, C H
0
720 740 760 780
ν (GHz)
Figure2. SPIRE data in the range 475 795GHz, containingthe C H-linesdetected with HIFI. The shaded strips in the panels
2
−
indicatethespectralrangesoftheHIFIdatashowninthepanelsofFig.1.Thelabelsmarktheprincipalcontributorstothemain
spectralfeatures.Acomparisonofemissionintherange609 614GHz,asobtainedwithSPIRE andwith HIFI,isshowninthe
−
insetpanel.
4
E.DeBecketal.:COandCCHaroundIRC+10216
Table1.SummaryoftheIRAM30mandHIFIdetectionsofC H.Columnsare,respectively,theinstrumentorbandwithwhichwe
2
detectedtheC Hemission,themain-beamefficiencyη ,thehalf-powerbeamwidthHPBW,theintegrationtime,thenoiselevel
2 MB
inthespectra,thefrequencyresolution∆ν,thevelocityresolution∆3,thedetectedC Hrotationaltransitions,thefrequencyrange
2
inwhichweobservedthelines,andthefrequency-integratedintensityI = T dν.
ν∗,A A∗
R
Instrument η HPBW Int.Time Noise ∆ν ∆3 CCH Freq.Range I
MB ν∗,A
orBand ( ) (min) (mK) (MHz) (kms 1) transition (MHz) (KMHz)
′′ −
IRAM30m
A100/B100 0.82 28.2 614 2.5 1.0 3.4 N=1 0
J=3/2−1/2 87278.2 87339.6 15.90
J=1/2−1/2 87392.8−87453.6 7.58
C150/D150 0.68 14.1 215 10 1.0 1.7 N=2−1 −
J=5/2−3/2 174621.1 174682.5 58.73
J=3/2−1/2 174708.2−174744.9 38.09
J=3/2−3/2 174796.2−174863.7 5.90
E3 0.57 9.4 161 10 1.0 1.1 N=3−2 −
J=7/2−5/2 261974.3 262023.4 110.74
J=5/2−3/2 262048.8−262084.4 60.63
J=5/2−5/2 262191.3−262270.0 6.33
E3 0.35 7.0 79 5.3 2.0 1.7 N=4−3 −
J=9/2−7/2 349311.5 349368.3 66.98
J=7/2−5/2 349368.3−349421.6 60.15
J=7/2−7/2 349575.1−349671.8 2.35
− −
Herschel/HIFI
1a 0.75 40.5 28 9.9 1.5 0.86 N=6 5
J=13/2 −11/2 523941.9 524000.4 18.60
J=11/2− 9/2 524000.4−524061.9 18.74
J=11/2 −11/2 −
1b 0.75 34.7 13 7.4 0.50 0.25 N=7−6 − −
J=15/2 −13/2 611237.1 611301.6 10.09
J=13/2−11/2 611301.6−611364.1 9.44
J=13/2−13/2 −
2a( ) 0.75 30.4 12 13.7 4.5 1.8 N=8−7 − −
†
J=17/2 −15/2 698487.5 698619.0 4.21
J=15/2−13/2 698619.0−698738.0 4.47
J=15/2−15/2 −
2b 0.75 27.0 15 10 6.0 2.2 N=9−8 − −
J=19/2 −17/2 785750.0 785875.0( ) 0.20( )
‡ ‡
J=17/2−15/2 785875.0−785900.0( ) 0.20( )
‡ ‡
J=17/2−17/2 −
− − −
Notes. ( )Onlythehorizontalpolarisationwasobtained.( )Duetothelowsignal-to-noiseratioofthisdataset,thesenumbersaretobeinterpreted
† ‡
withcaution.
totheflux,i.e.20outof25spaxelsintotal.Theherepresented sitions are shown in Fig. 1, and details on the observationsare
dataset,therefore,reflectsthetotalfluxemittedbytheobserved listedinTable1.
regions,withinthe approximationthatthere isnoloss between
the spaxels. The estimated uncertainty on the line fluxes is of
Table 2.SummarisedinformationontheSPIRE spectrumcon-
the order of 30%. Due to instrumental effects, only the range
tainingtheN =7 6transitionofC H.∆νisthefrequencyreso-
5126C−O1tr9a0nµsimtioonfstJhe=P1A4CS1s3peucptrtuomJi=su4s2abl4e1.,Tchoivserrainnggeenheorlgdys lution,∆3istheve−locityresolution,2andσisthermsnoise.Fν,A∗
levelsfrom 350cm 1 u−pto 3450cm 1.−Asisthecaseforthe isthefrequency-integratedfluxinthegivenfrequencyrange.
− −
∼ ∼
SPIREdata,lineblendsarepresentinthePACSspectrumdueto
thelowspectralresolutionof0.08 0.7GHz. Frequencyrange 446 1575GHz Integrationtime 2664s
− ∆ν − 2.1GHz ∆3 201kms 1
−
F 16915JyMHz σ 850mJy
ν,A∗
2.4.IRAM30m:linesurveys
We combine the Herschel data with data obtained with the
IRAM30mtelescopeatPicoVeleta.TheC H N = 1 0tran-
2 3. Theenvelopemodel:dust&CO
−
sition was observed by Kahaneetal. (1988); N = 2 1 was
−
observedbyCernicharoetal.(2000).N = 3 2andN = 4 3 Weassumedadistanced = 150pctoIRC+10216,ingoodcor-
− −
wereobservedbetweenJanuaryandApril2010,usingtheEMIR respondence with literature values (Groenewegenetal., 1998;
receiversasdescribedindetailby(Kahaneetal.,2011,inprep.). Men’shchikovetal., 2001; Scho¨ieretal., 2007, and references
The data-reduction process of these different data sets is de- therein).Thesecondassumptioninourmodelsisthattheeffec-
scribed in detail in the listed papers. The observed C H tran- tive temperatureT = 2330K, followingthe modelof a large
2 eff
5
E.DeBecketal.:COandCCHaroundIRC+10216
setofmid-IRlinesofC H andHCN,presentedbyFonfr´ıaetal. Table 3. Dust composition in the CSE of IRC+10216, used
2 2
(2008). We determined the luminosity L , the dust-mass-loss to produce the fit to the SED in Fig. 3. The columns list the
⋆
rate M˙ , and the dust composition from a fit to the spec- dustspecies,theassumedshapes,therespectivemassfractions,
dust
tral energy distribution (SED; Sect. 3.1). The kinetic tempera- and the references to the optical constants of the differentdust
tureprofileandthegas-mass-lossrate M˙ weredeterminedfrom species.
a CO-line emission model (Sect. 3.2). The obtained envelope
model will serve as the basis for the C H-modelling presented
2 Dustspecies Shape Massfraction References
in Sect. 4. We point out that all modelling is performedin the (%)
radialdimensiononly,i.e.in1D,assumingsphericalsymmetry
Amorphouscarbon DHS 53 Preibischetal.(1993)
throughouttheCSE. Siliconcarbide CDE 25 Pitmanetal.(2008)
Magnesiumsulfide CDE 22 Begemannetal.(1994)
3.1.Dust
6
The dust modelling was performed using MCMax (Minetal., ϕ=0.00
2009),aMonteCarlodustradiativetransfercode.ThebestSED 5 ϕϕ==00..2540 (ISO) 104
fittotheISOSWSandLWSdata,showninFig.3,isbasedona 102
swteitlhlatrhleumreisnuolstsityofLM⋆=e1n1’s3h0c0hLik⊙o.vTehtiasli.s(i2n0g0o1o)4d,ccoornrseisdpeornindgenthcee Jy) 4 (Jy)Fν 1100−20
gdiivffeesreancseteilnlaardroapdtieudsdRi⋆staonfce2.0Tmhiellci-oamrcbsiencaotniodns,oifnLv⋆erayndgoToeffd 4 (10Fν 23 1100−−64 1 10 102 103
agreement with the values reported by e.g. Ridgway&Keady λ (µm)
(1988,19mas)andMonnieretal.(2000,22mas).
1
IRC+10216 is a Mira-type pulsator, with a pe-
riod of 649days (LeBertre, 1992). Following Eq. 1 0
of Men’shchikovetal., we find that L varies between 10 100 1000
L 15800L at maximum light (at⋆phase ϕ=0), and λ (µm)
⋆,ϕ=0
L ≈ 6250L⊙ at minimum light (ϕ=0.5). Fig. 3 shows the
SfiEg⋆,uDϕr=-e0v,.5ai≈triiasbviliistiyb⊙lceotrhraetspthoendspinrgeatdootnhethLe⋆p-hvoatroiambeiltirtiyc.pForionmtscthane 1F0ig−u8rMe3.ySr−E1D, amndodtehlesdfourstIRcoCm+p1o0s2it1io6n, uasnidngprMo˙pduesrtti=es 4li.s0te×d
beaccountedforbythemodelscoveringthefullL -range. in Tabl⊙e 3 and spatial distribution and properties shown in
⋆
Fig. 4. The SED model is constrained by the ISO SWS and
TheadopteddustcompositionisgiveninTable3.Themain
LWS data (full black); photometric points (black diamonds)
constituents are amorphouscarbon (aC), silicon carbide (SiC),
are taken from Ramstedtetal. (2008) and Ladjaletal. (2010).
andmagnesiumsulfide(MgS),withmassfractionsof53%,25%,
Dash-dotted grey: model at maximum light, with 15800L ,
and 22%, respectively. The aC grains are assumed to follow a
dashed grey: best fit to the ISO data, at phase ϕ=0.24, u⊙s-
distributionofhollowspheres(DHS; Minetal.,2003),withsize
ing11300L ,dash-triple-dottedgrey:modelatminimumlight,
0.01µmandafillingfactorof0.8.ThepopulationoftheSiCand
with 6250L⊙. The inset is identical, but plotted in log log
MgSdustgrainsisrepresentedbyacontinuousdistributionofel-
scale. ⊙ −
lipsoids(CDE; Bohren&Huffman,1983),wheretheellipsoids
allhavethevolumeofaspherewithradiusa =0.1µm.CDEand
d
DHSarebelievedtogivea morerealistic approximationofthe
3.2.CO
characteristicsofcircumstellardustgrainsthana populationof
spherical grains (Mie-particles). Assuming DHS for the domi-
To constrain the gas kinetic temperature T (r) and the gas
kin
nant aC grains was found to provide the best general shape of densityρ (r)throughouttheenvelope,wemodelledtheemis-
gas
theSED.Alldustspeciesareassumedtobeinthermalcontact.
sion of 20 rotational transitions of 12CO (Sect. 3.2), measured
The absorption and emission between 7µm and 10µm with ground-based telescopes and Herschel/HIFI. The gas ra-
and around 14µm that is not fitted in our SED model diative transfer is treated with the non-local thermal equilib-
can be explained by molecular bands of e.g. HCN and rium(NLTE)codeGASTRoNOoM(Decinetal.,2006,2010c).To
C2H2(Gonza´lez-Alfonso&Cernicharo,1999;Cernicharoetal., ensure consistency between the gas and dust radiative transfer
1999). models,wecombineMCMaxandGASTRoNOoM,bypassingonthe
Based on an average of the specific densities of the dust dustproperties(e.g.densityandopacities)fromthemodelpre-
components ρs = 2.41gcm−3, we find a dust mass-loss rate sentedinSect.3.1andFig.4tothegasmodelling.Thegeneral
of 4.0 10−8M yr−1, with the dust density ρdust(r), dusttem- methodbehindthiswillbedescribedindetailbyLombaertetal.
peratur×eTdust(r)⊙and Qext/ad, with Qext the totalextinctioneffi- (2012,inprep.).
ciency,showninFig.4.Theinnerradiusofthedustyenvelope, Thelargedatasetofhigh-spectral-resolutionrotationaltran-
i.e. the dust condensation radius of the first dust species to be sitions of CO consists of 10 lines observed from the ground
formed, is determined at Rinner=2.7R⋆ by taking into account and 10 lines observed with HIFI, which are listed in Table 4.
pressure-dependentcondensationtemperaturesfor the different Sincethecalibrationofground-baseddataisattimesuncertain
dustspecies(Kamaetal.,2009).Thesedustquantitiesareused (Skinneretal., 1999), large data sets of lines that are observed
asinputforthegasradiativetransfer,modelledinSect.3.2. simultaneously and with the same telescope and/or instrument
are of great value. Also, the observational uncertainties of the
4 The extensive modelling of Men’shchikovetal. was based on a HIFIdataaresignificantlylowerthanthoseofthedataobtained
light-curveanalysisandSEDmodelling,andholdsaquoteduncertainty with ground-basedtelescopes (20 40%). Since the HIFI data
−
ontheluminosityof20%. ofCOmakeuphalfoftheavailablehigh-resolutionlinesinour
6
E.DeBecketal.:COandCCHaroundIRC+10216
To reproduce the CO lines within the observational un-
R R
inner outer,dust certainties, we used L =11300L , and a temperature profile5
2.8 R 5000 R ⋆
* * that is a combination of three p⊙ower laws: T (r) r 0.58
kin −
−3m) 10−1 8 ((aa)) Tfor (rr)≤ r9R1.⋆2,atTlkainrg(re)rr∝adiri,−b0.a4sefodro1n0thRe⋆w≤orkrby≤Fo6n5f∝Rr´ı⋆a,etanald.
g c 10−20 (2k0in08)∝and−Decinetal. (2010a). The minimum temperature in
(st 10−2 2 theenvelopeissetto5K.TheradialprofilesofTkin(r)and3(r)
ρdu 10−2 4 a6re s1h0ow4nininthFeigin.n5e.rUwsiinndg,awefraficntidonthaaltaabugnadsa-nmcaessC-lOo/sHs2raotef
−
K) 1000 ((bb)) CM˙o×mofbi1n.i5ng×t1h0e−r5eMsu⊙ltsyrf−ro1mreSpreocdt.u3ce.1s wthiethCtOhoslienefrsovmertyhewCelOl.
(
ust 100 model,we find a gas-to-dust-massratio of 375,in the rangeof
Td typical values for AGB stars (102 103; e.g. Ramstedtetal.,
−
10 2008).
The predicted CO line profiles are shown and compared
1 101 102 103 104
to the observations in Fig. 6. All lines are reproduced very
r (R)
* well in terms of integrated intensity, considering the re-
) 108 spective observational uncertainties. The fraction of the pre-
−1m 106 ((cc)) dicted and observed velocity-integrated main-beam intensities
c
(d 104 IMB,model/IMB,data variesintherange74−145%fortheground-
a based data set, and in the range 89 109% for the HIFI data
/ext 102 set. The shapes of all observed lines−are also well reproduced,
Q 100 withtheexceptionoftheIRAMlines(possiblyduetoexcitation
0.1 1 10 100 1000 and/orresolutioneffects).
λ (µm) Fig.7showsthecomparisonbetweentheobservedCOtran-
sitions J = 14 13 up to J = 42 41 in the PACS spectrum
− −
and the modelled line profiles, convolved to the PACS resolu-
Figure4. Spatialdistributionand propertiesof the dustfor our
best SED fit to the ISO data at ϕ=0.24 in Fig. 3: (a) dust den- tion. Considering the uncertainty on the absolute data calibra-
sity ρ (r),(b)dusttemperatureT (r),and(c) Q /a .The tionandthepresenceoflineblendswithe.g.HCN,C2H2,SiO,
dust dust ext d
SiS,orH Oinseveraloftheshownspectralranges(Decinetal.,
dashedlinesindicatetheinnerandouterradiiofthedustyenve- 2
2010b),theoverallfitoftheCOlinesisverygood,withexcep-
lope.
tionofthelinesJ =14 13, 15 14,and16 15.Thissignificant
− − −
differencebetweenthemodelpredictionsandthedatacould,to
date,notbeexplained.BoththeHIFIdataandthehigher-JPACS
1000
(K)n 100 −0.58 −0.40 lineSskairnenreerperotdaul.c(e1d9v9e9r)ynwoteeldlbthyethpeosmsiobdleelp.resenceofagradi-
Tki 10 −1.20 entintheturbulentvelocity3turb,withdecreasingvaluesforin-
creasingradialdistancefromthestar.Wefindnoclearevidence
−1s) 1250 10 100 1000 10000 isnhoowunr dinataFisge.t6thuastetsh3is de=cr1e.a5skemins31tu,rbanisdprerepsreondtu.cTehsethmeolidneel
m turb −
k 10 shapesandintensitiestoahighdegree.InFig.8wecomparethe
(exp 5 predictionsofasetofCO-linesusingdifferentvaluesfor3turbin
v 0 ordertoassesstheinfluenceofthisparameter.
10 100 1000 10000 Assuming a lower value of 0.5kms−1 or 1.0kms−1 yields
r (R) an incomplete reproduction of the emission profile between
*
44kms 1 and 41kms 1 for all lines observed with HIFI.
− −
−Assuming a high−er value of 2kms 1 leads to the overpredic-
Figure5. Overview of the gas kinetic temperature and expan- −
tion of the emission in this 3-rangefor all observedlines, with
sionvelocityusedintheradiativetransfermodelcalculatedwith
GASTRoNOoM. In the top panel, we indicated the exponents α the strongest effect for the lowest-J lines. The influence of the
from the T (r) rα-power laws used to describe the kinetic differentvaluesofthe turbulentvelocityonthe emissionin the
kin ∝ 3-rangeof 41to 12kms 1isnegligible.
temperature,andtheradialrangestheyapplyto.SeeSect.3.2. −
− −
4. C H
2
sample, these will serve as the starting point for the gas mod- 4.1.Radicalspectroscopy
elling.
C H( C C H)isalinearmoleculewithanopenshellcon-
2
The adopted CO laboratory data are based on the work of figurat•ion,i.e.itisaradical,withanelectronic2Σ+ground-state
Goorvitch&Chackerian (1994) and Winnewisseretal. (1997), configuration (Mu¨lleretal., 2000). Spin-orbit coupling causes
and are summarised in the Cologne Database for Molecular fine structure (FS), while electron-nucleus interaction results
Spectroscopy (CDMS; Mu¨lleretal., 2005). Rotational levels in hyperfine structure (HFS). Therefore, to fully describe C H
2
J = 0 up to J = 60are taken intoaccountforboththe ground in its vibrationalground-state,we need the following coupling
vibrational state and the first excited vibrational state. CO-H
2
collisionalrateswereadoptedfromLarssonetal.(2002). 5 ThecentralstarisassumedtobeablackbodyatatemperatureT .
eff
7
E.DeBecketal.:COandCCHaroundIRC+10216
12 NRAO 0.74 1124 SEST 1.00 20 IRAM 1.01 25 IRAM 0.76 60 IRAM 0.96
10 1−0 1−0 1−0 20 1−0 2−1
10
K) 8 8 15 15 40
(TMB 46 46 10 10 20
2 2 5 5
0 0 0 0 0
80
40 CSO 50 JCM T IRAM 40 CSO CSO
0.92 1.24 100 1.11 1.45 1.29
30 3−2 40 3−2 80 3−2 30 4−3 60 6−5
(K)B 20 30 60 20 40
TM 20 40 20
10 10
10 20
0 0 0 0 0
15 HIFI HIFI HIFI 20 HIFI 20 HIFI
5−4 0.89 15 6−5 0.90 15 7−6 0.98 8−7 0.96 9−8 1.00
15 15
(K)B 10 10 10 10 10
M
T 5 5 5 5 5
0 0 0 0 0
25 25
20 1H0IF−I9 1. 00 20 1H1IF−I1 0 1. 09 20 1H4IF−I1 3 1. 04 20 1H5IF−I1 4 1. 12 20 1H6IF−I1 5 1. 07
K) 15 15 15 15 15
(MB 10 10 10 10 10
T
5 5 5 5 5
0 0 0 0 0
−60 −40 −20 0 −60 −40 −20 0 −60 −40 −20 0 −60 −40 −20 0 −60 −40 −20 0
v (km s−1) v (km s−1) v (km s−1) v (km s−1) v (km s−1)
Figure6.Comparisonoftheobserved12COemissionlines(blackhistogram),andthelineprofilespredictedbytheGASTRoNOoM-
model(red dashedline)with parametersas listed in Table 5. Theline transitions J (J 1) andthe telescopeswith which they
− −
wereobservedareindicatedintheupperleftcornerofeverypanel.ThefactorI /I isgivenintheupperrightcornerof
MB,model MB,data
everypanel.
scheme: C H has one bending mode ν , and two stretching modes,
2 2
ν andν .Therotationalground-stateofthebendingmode(ν )
1 3 2
J = N+S (1) is situated 530K above the ground state. The C C stretch
∼
F = J +I, (2) (ν3)at 2650Kisstrong.TheC Hstretch(ν1)at 4700Kis
weak,a∼ndisresonantwiththefirstexcited2Πelectro∼nicstateat
where N istherotationalangularmomentum,notincludingthe 5750KofC2H.Foreachofthesevibrationalstates—ground-
∼
electron or quadrupolar angular momentum (S and I, respec- state,ν2,ν3,andν1—weconsider20rotationallevels,i.e.rang-
tively), J isthetotalrotationalangularmomentum,andFisthe ingfromN =0uptoN =19.
nuclearspinangularmomentum. The equilibrium dipole moments of the ground and
The strong ∆J = ∆N FS components have been detected first excited electronic states have been calculated as µ =
upto N = 9 8.Relativetothestrongcomponents,theweaker 0.769 and 3.004Debye, respectively (Woon, 1995). The
−
∆J =0componentsdecreaserapidlyinintensitywithincreasing dipole moments of the fundamental vibrational states in the
N, andtheyhavebeendetectedupto N = 4 3.TheHFS has ground electronic state are also assumed to be 0.769Debye.
−
alsobeen(partially)resolvedfortransitionsuptoN =4 3. This is usually a good assumption as shown in the
−
Spectroscopic properties of the ground vibrational state of case of HCN by Deleon&Muenter (1984). The band
C H have most recently been determined by Padovanietal. dipole moments for rovibrational transitions were calcu-
2
(2009)andarethebasisfortheentryintheCDMS. lated from infrared intensities published by Tarroni&Carter
Our treatment of the radiative transfer (Sect. 4.3) does not (2004): µ(ν2=1 0)=0.110Debye, µ(ν3=1 0)=0.178Debye,
→ →
dealwiththeFS andHFSofC2H,andislimitedtothepredic- and µ(ν1=1→0)=0.050Debye. The influence of the vibra-
tionofrotationallinesN N with∆N = 1,accordingtoa1Σ tionally excited states on transitions in the vibrational ground
′
approximation. Therefore→, all levels treated are described with statewillbediscussedinSect.4.3.
only one quantum number N. The final line profiles are calcu- A more completetreatmentof C H will likely haveto take
2
lated by splitting the total predicted intensity of the line over into account, e.g., overtones of the ν -state. These have non-
2
thedifferent(hyper)finecomponents,dependingonthe relative negligible intrinsic strengths (Tarroni&Carter, 2004) because
strengthofthesecomponents(CDMS),andpreservingthetotal of anharmonicity and vibronic coupling with the first excited
intensity.Notethat,strictlyspeaking,thissplittingisonlyvalid electronic state. Spectroscopic data for several of these are al-
underLTEconditions,butthat,duetoourapproachofC Hasa ready available in the CDMS. A multitude of high-lyingstates
2
1Σ-molecule,wewillapplythisschemethroughoutthispaper. mayalsohavetobeconsideredastheyhaveratherlargeintrin-
8
E.DeBecketal.:COandCCHaroundIRC+10216
800
600
400
200
0
−200 1000 800 700 650 550
−400 14−13 15−14 16−15 17−16 18−17
102000300040005000600000
475 375 325 300 250
800
600 −50 −50 −50 −50 −50
400
1607 1612 1617 1721.0 1726.5 1732.0 1835.0 1841.5 1848.0 1949 1956 1963 2063 2071 2079
200
0
−200 450 500 350 350 350
−400 19−18 20−19 21−20 22−21 23−22
102000300040005000600000
200 225 150 150 150
800
600 −50 −50 −50 −50 −50
400
2176 2185 2194 2290.0 2299.5 2309.0 2404 2414 2424 2517 2528 2539 2631 2642 2654
200
0
−200 350 350 750 750 650
−400 24−23 25−24 26−25 27−26 28−27
102000300040005000600000
150 150 350 350 300
)
800 y
600 (J −50 −50 −50 −50 −50
400
x 2744 2756 2769 2857 2870 2883 2980 2984 2988 3093 3098 3103 3206.0 3211.5 3217.0
200 u
0 Fl
−200 650 500 450 300 300
−400 29−28 30−29 31−30 32−31 33−32
102000300040005000600000
300 225 200 125 125
800
600 −50 −50 −50 −50 −50
400
3319 3325 3331 3432.0 3438.5 3445.0 3545 3552 3559 3657.0 3664.5 3672.0 3769.0 3777.5 3786.0
200
0
−200 250 250 200 150 200
−400 34−33 35−34 36−35 37−36 38−37
102000300040005000600000
100 100 75 50 75
800
600 −50 −50 −50 −50 −50
400
3882.0 3890.5 3899.0 3994 4003 4012 4106.0 4115.5 4125.0 4218 4228 4238 4329 4340 4351
200
0
−200 250 100 100 100
−400 39−38 40−39 41−40 42−41
102000300040005000600000
100 25 25 25
−50 −50 −50 −50
4441 4452 4464 4552 4564 4576 4663 4676 4688 4774 4787 4800
ν (GHz)
Figure7.ComparisonofCO-emissionlinesmeasuredwithPACS(blackhistogram)andthepredictedlineprofiles(reddottedline).
ThetransitionsJ (J 1)arelabelledinthetopleftcornerofeachsubpanel.
− −
sic intensities because of the vibronic interaction between the 4.2.Observationaldiagnostics
groundandthefirstexcitedelectronicstates.
The C H-emission doublets due to the molecule’s fine struc-
2
ture(Sect.4.1)areclearlypresentinallhigh-resolutionspectra
showninFig1.ForN =1 0,2 1,and3 2,thehyperfinestruc-
− − −
9
E.DeBecketal.:COandCCHaroundIRC+10216
20
SEST 10 CSO 10 CSO CSO
4
1−0 8 3−2 8 4−3 15 6−5
3 6 10
K) 6
(MB 2 4 4 5
T 1 2
2 0 0
0
0 −2 −5
−1
4 HI FI 5 HI FI 5 HI FI 5 HI FI
5−4 4 8−7 4 11−10 4 16−15
3
3 3 3
K)
(B 2 2 2 2
M
T
1 1 1 1
0 0
0 0
−1
−55 −50 −45 −40 −35 −55 −50 −45 −40 −35 −55 −50 −45 −40 −35 −55 −50 −45 −40 −35
v (km s−1) v (km s−1) v (km s−1) v (km s−1)
Figure8.ZoomonthebluewingofaselectionoftheCOlineprofiles,withthetransitionsJ (J 1)labelledinthetopleftcorner
of each panel.We show the comparisonbetween the observedlines and predictedlines wit−h ass−umed valuesof 3 =0.5kms 1
turb −
(fullgrey),1.0kms 1(dottedgrey),theadoptedvalueof1.5kms 1(dashedgrey),and2.0kms 1(dash-dottedgrey).
− − −
ture is clearlydetected in our observations.Thedouble-peaked
lineprofilesaretypicalforspatiallyresolvedopticallythinemis- 15
sion (Olofsson,in Habing&Olofsson, 2003), andindicate that µν)2 N = 3.84 (0.09) x 10+15 cm− 2
the emitting material has reached the full expansion velocity6, πS3 14 Trot = 23.4 (1.3) K
ip.oe.si3ti∞on=of14th.5ekemmsit−t1in.gThshiselilssi(n∼a1c6c′′o;rdGanuce´elinweitthalt.h,e19o9b9se)ravnedd /8I,corrv 13
thevTeolodceitryivperoifinfleordmeraitvioendfoonrththiseernevgeilmopeei(nSewcth.i3ch.2)t.he lines 3610 12
k
a1r9e83e)x,cuitseidn,gwe construct a rotational diagram (Schloerbetal., og(3 11 1−0 2−1 3−2 4−3 6−5 7−6 8−7
l
0 50 100 150
N exp Eu = 3k×1036 I3,corr, (3) Eu/k (K)
Z(T ) × −kT ! 8π3 νSµ2
rot rot
whereNisthecolumndensity,Zthetemperaturedependentpar- Figure9. Rotational diagram of the measured C2H transitions,
titionfunction,E theenergyoftheupperlevelofthetransition based on Eq. 3. Transitions — in the 1Σ approximation— are
u
incm 1,ktheBoltzmannconstantinunitsofcm 1K 1,T the labelled in grey. The numbersgiven in parenthesesare the un-
− − − rot
rotationaltemperatureinK,µthedipolemomentinDebye,and certaintiesonthederivedcolumndensityandrotationaltemper-
ν the frequency of the transitions in Hz. I3,corr is the velocity- ature.
integratedantennatemperature,
1 3LSR+3 The unique rotational temperature points to excitation of the
I3,corr = fbffηMB Z3LSR−3∞∞TA∗d3 (4) liinnaescoinnfionneedsianrgelaethreagticmane,biencahcacroarcdtaenricseedtowthitehaobnuengdaasntceemppeeark-
correctedforthebeamfillingfactor(Kramer,1997) ature(Gue´linetal.,1999).
2
1 exp √ln2 θ /θ , θ <θ
fbff =1,− (cid:18)−h × source beami (cid:19) θssoouurrccee ≥θbbeeaamm (5) 44..33..1R.aCdoiallitsivioentaralnrastfeesrmodelling
andforthebeamefficiencyη (seeTable1).Wehaveassumed
MB Forrotationaltransitionswithinthevibrationalgroundstateand
auniformemissionsourcewithasizeof32 indiameterforall
′′ within the ν = 1 bending state and the ν = 1 and ν = 1
C Htransitions,basedontheinterferometricobservationsofthe 2 1 3
2 stretching states we adopted the recently published collisional
N =1 0emissionbyGue´linetal.(1999).
Fro−m Fig. 9 we see that the C2H emission is characterised rsaiotensaolfraHteCsNfo−rHC2Hby,aDnudmthoeucshimelileatraml.o(2le0c1u0l)a.rTmhaeslsaacnkdofsiczoelloif-
by a unique rotational temperature T = 23.3( 1.3)K, and a 2
source-averagedcolumndensity N =ro3t.84( 0.09±) 1015cm 2. HCNandC2Hinspirethisassumption.Thecollisionalratesare
± × − givenfor25differenttemperaturesbetween5and500K,hence
6 We assume that the half width at zero level of the CO emission coveringtherangeoftemperaturesrelevantfortheC2Haround
lines,i.e.14.5kms−1,isindicativeforthehighestvelocitiesreachedby IRC+10216.Since ν3 and ν1 are at ∼2650K and ∼4700K, re-
thegasparticlesintheCSE. spectively, collisional pumping to these vibrationally excited
10