Detection of `parent' molecules from the inner wind of AGB stars as tracers of non-equilibrium chemistry

0.7 MB·English

by  L. Decin

Astronomy&Astrophysicsmanuscriptno.8892 (cid:13)c ESO2008
February2,2008
Detection of ‘parent’ molecules from the inner wind of AGB
stars as tracers of non-equilibrium chemistry
L.Decin1,2⋆,I.Cherchneff3,S.Hony4,1,S.Dehaes1⋆⋆,C.DeBreuck5,andK.M.Menten6
1 DepartmentofPhysicsandAstronomy,InstituteforAstronomy,K.U.Leuven,Celestijnenlaan200B,B-3001Leuven,Belgium
e-mail:[email protected]
8
2 SterrenkundigInstituutAntonPannekoek,UniversityofAmsterdam,Kruislaan4031098Amsterdam,TheNetherlands
0
3 Institutfu¨rAstronomie,ETHHo¨nggerberg,Wolfgang-Pauli-Strasse16,8093Zu¨rich,Switzerland
0
e-mail:[email protected]
2 4 LaboratoireAIM,CEA/DSM-CNRS-UniversitParisDiderot,DAPNIA/SAp,91191GifsurYvette,France

n e-mail:[email protected]
a 5 EuropeanSouthernObservatory,Karl-SchwarschildStrasse,D-85748GarchingbeiMu¨nchen,Germany
J e-mail:[email protected]
7 6 MPIfu¨rRadioastronomie,AufdemHu¨gel69,D-53121Bonn,Germany
e-mail:[email protected]

]
h ReceivedSeptember15,2007;acceptedOctober15,2007
p
- Abstract
o
r
Context.Asymptotic Giant Branch (AGB) stars are typified by strong dust-driven, molecular outflows. For long, it was believed
t
s thatthemolecularsetupofthecircumstellarenvelopeofAGBstarsisprimarilydeterminedbytheatmosphericC/Oratio.However,
a
recentobservationsofmoleculessuchasHCN,SiO,andSOrevealgas-phaseabundanceshigherthanpredictedbythermodynamic
[
equilibrium(TE)models.UV-photoninitiateddissociationintheouterenvelopeornon-equilibriumformationbytheeffectofshocks
1 intheinnerenvelopemaybetheoriginoftheanomolousabundances.
v Aims. We aim at detecting (i) a group of ‘parent’ molecules (CO, SiO, HCN, CS), predicted by the non-equilibrium study of
8 Cherchneff (2006)toformwithalmostconstant abundancesindependent oftheC/Oratioandthestellarevolutionarystageonthe
1 AsymptoticGiantBranch(AGB),and(ii)fewmolecules,suchasSiSandSO,whicharesensitivetotheO-orC-richnatureofthe
1 star.
1 Methods.Severallowandhighexcitationrotationaltransitionsofkeymoleculesareobservedatmmandsub-mmwavelengthswith
. JCMTandAPEXinfourAGBstars:theoxygen-richMiraWXPsc,theSstarWAql,andthetwocarbonstarsVCygandIILup.A
1 criticaldensityanalysisisperformedtodeterminetheformationregionofthehigh-excitationmolecularlines.
0 Results.Wedetectthefour‘parent’moleculesinallfourobjects,implyingthat,indeed,thesechemicalspeciesformwhateverthe
8 stageofevolutionontheAGB.High-excitationlinesofSiSarealsodetectedinthreestarswithAPEX,whereasSOisonlydetected
0 intheoxygen-richstarWXPsc.
: Conclusions. Thisisthefirstmulti-molecularobservationalproofthatperiodicallyshockedlayersabovethephotosphereofAGB
v
starsshowsomechemicalhomogeneity,whateverthephotosphericC/Oratioandstageofevolutionofthestar.
i
X
Key words. Astrochemistry, Molecular processes, Stars: AGB and post-AGB, (Stars): circumstellar matter, Stars: mass loss,
r Submillimeter
a
1. Introduction Foralongtime,thegaschemicalcompositionwasbelieved
tobedominatedentirelybytheC/Oratioofthephotosphere.A
Circumstellar envelopes of Asymptotic Giant Branch stars C/O ratiogreaterthan oneimpliedthatallthe oxygenwas tied
(AGBs)havelongbeenknownto beefficientsitesofmolecule inCO,leadingtoanoxygen-freechemistry,whereasaC/Oratio
formation.While theouterlayersofsuchenvelopesexperience lessthanonemeantthatnocarbonbearingmoleculesapartfrom
penetrationofinterstellarUVphotonsandcosmicraysresulting CO could ever form in an oxygen-rich (O-rich) environment.
in a fast ion-molecule chemistry, the deepest layers are dom- This picture, based essentially on thermal equilibrium consid-
inated by a non-equilibrium chemistry due to the passage of erations applied to the gas, has been first disproved by the de-
shocksgeneratedbystellarpulsation.Dustformsinthoseinner tectionofSiOatmillimeter(mm)wavelengthincarbon-rich(C-
gas layers, still bound to the star, and grains couple to the gas rich)AGBs (Bujarrabaletal. 1994). As forO-richAGBs, CO2
to accelerate it, thereby generating stellar wind and mass loss infrared (IR) transition lines were detected in various objects
phenomena.The described processes greatly modify the abun- with the Short-Wavelength Spectrometer (SWS) onboard the
dances established by the equilibrium chemistry in the dense, Infrared Space Observatory (ISO) (e.g., Justtanontetal. 1996;
hotphotosphere(Tsuji1973). Rydeetal.1998).
Theoretical modeling describing the chemistry in the in-
⋆ PostdoctoralFellowoftheFundforScientificResearch,Flanders ner wind of the extreme carbon star IRC+10216 showed that
⋆⋆ ScientificResearcheroftheFundforScientificResearch,Flanders the formation of SiO was due primarily to hydroxyl OH re-
2 L.Decinetal.:Non-equilibriumchemistryinAGBstellarwinds
action with atomic silicon close to the photosphere as a re- determined by shock propagation and not by the photospheric
sult of shock activity and therefore non-equilibrium chem- C/Oratioandthestellarevolutionarystage.
istry(Willacy&Cherchneff1998).Lateron,Duarietal.(1999) SiS is also detected in all stars and we were able to detect
showed that CO2 formation in the O-rich Mira IK Tau results withAPEXthehigh-excitationSiS(19-18)lineintheO-richWX
fromthereactionofOHradicalswithCOintheshockedregions, Psc, the C-rich II Lup and the S-rich W Aql. This is in good
implying again that non-equilibirumchemistry was paramount agreementwithrecentSiSOSO,JCMT,andAPEXobservations
to the formation of C-bearing species in O-rich Miras. It was of Scho¨ieretal. (2007) in a largesample of M and C stars, in-
then recently proposed that the inner wind of AGBs shows cluding WX Psc and V Cyg. This implies that SiS also forms
a striking homogeneity in chemical composition, despite their closetothestar,whateverthestageofstellarevolution.
photosphericC/Oratioandstageofstellarevolution(Cherchneff Both the SO(65 – 54) and the high-excitation SO(1011 −
2006).Inparticular,Cherchneff(2006)showedthatwhentaking 1010) line were detected in O-rich WX Psc. SO was neither
shockchemistryintoaccount,moleculessuchasSiO,HCNand foundintheS-typeWAql,norinthetwocarbonstarsIILupand
CS are presentin comparableamountin the innerlayersof M, VCyg.SOappearstobetypicalforO-richAGBsonly,support-
S,andCAGBs,whereasspecificmolecules(e.g.SOandHSfor ingthenon-detectionofSOinC-starsbyWoodsetal.(2003).
O-rich Mirasand C2H2 for carbonstars) are typicalfor O-rich Both optically thin and optically thick lines occur (e.g.,
orC-richchemistries. 13CO(2-1)versusthe12CO(2-1)lineinWAql,seee.g.Fig.2).
In this letter, we present observations carried out with the Thelineparameters,i.e.,themain-beambrightnesstemperature
JCMTandtheAPEXtelescopeoffourAGBs:oneO-rich,WX at the line centre (T ), the line centre velocity (v ), and half
mb ∗
Psc, one S star (≡ C/O ≈ 1), W Aql, and two carbon stars, II thefulllinewidth(v ),areobtainedbyfittingthe‘softparabola’
e
Lup and V Cyg. We focus on the detection of (sub)mmtransi- lineprofilefunctiontothedata(Olofssonetal.1993)
tionsofCO,SiO, HCN,CS,SiSandSO inordertoconfirmor
2 β/2
disprovetheabovehypothesisandtocheckforhomogeneityin v−v
∗
AGBwinds. T(v)=Tmb 1− , (1)
" (cid:18) ve (cid:19) #
whereβ describestheshapeoftheline.Thevelocity-integrated
2. Observationsandline profiles
intensities are obtained by integrating the emission between
TheobservationswereperformedinOctober2006withthe15m v∗ ± ve and are listed in Table 1. A value of β = 2 repre-
JCMT1 forVCyg,WXPsc,andWAql,andintheperiodfrom sentsaparaboliclineshape,expectedforopticallythickexpand-
SeptembertillOctober2006withtheAPEX212mtelescopefor ingsphericalenvelopesincasethesourceismuchsmallerthan
IILup,WXPscandWAql.Duetotechnicalproblemswiththe the radio telescope beam size; β < 0 results in a profile with
RxB3JCMTreceiver,onlylowfrequencylineswithintheRxA3 horns at the extreme velocity, expected for optically thin lines
receiver(211–276GHz)wereobtained.FortheAPEXobserva- whenthesourceisresolved(Morris1975).In oursample,sev-
tions, both the APEX-2A receiver(279–381GHz) and FLASH erallineshaveaβ-valuedistinctlylargerthan2,indicatingthat
receiver (460–495GHz and 780–887GHz) were used. The ob- thelineformationoccurspartiallyintheinnerregion,wherethe
servations were carried out using a position-switching mode. stellar wind has not yet reached its full terminal velocity. This
The JCMT data reduction was performed with the SPLAT de- isnotablythecasefortheCS,SiO,andHCNlinesinoursam-
voted routinesof STARLINK, the APEX-data with CLASS. A ple,corroboratingtheirformationclosetothestar.Theestimated
polynomialwas fitted to an emision free region of the spectral terminalvelocitiesforWX Psc, W Aql, IILup,and V Cyg are
baseline and subtracted.The velocityresolution for the JCMT- 19.2km/s, 18.8km/s, 20.1km/s, and 12.4km/s, respectively.A
data equals to 0.0305MHz, for the APEX to 0.1221MHz. For maximumof5%differenceisfoundinthederivedterminalve-
WXPsc,WAql,andIILupthedatawererebinnedtoaresolu- locity values for each target, being within the velocity binsize,
tion of 1km/s, forV Cyg to 0.75km/sin orderto haveat least indicatingthatallofthedetectedlinesatleastpartlysurvivethe
40independentresolutionelementsperlineprofile.Theantenna dustformationandwindaccelerationprocesses.Anexceptionis
temperature, T∗, was convertedto the main-beam temperature SiS(19-18)in WX Psc (16km/s),butparticularlythe SO(1011-
A
(Tmb = TA∗/ηmb), using a main-beam efficiency ηmb of 0.69 1010),withanestimatedterminalvelocityofonly10km/s,traces
fortheJCMTRxA3receiver,of0.73fortheAPEX-2Areceiver, amuchsmallergeometricalregion.
and of 0.60 and 0.43 for the 460–495GHz and 780–887GHz
FLASHchannelsrespectively(Gu¨stenetal.2006).
3. Excitationanalysis
TheobservedmolecularemissionlinesoffourAGBstarsin
oursamplearedisplayedintheFigs.1–4. In case many transitions of an individual molecule can be ob-
InallstarsmolecularemissionlinesofCO,SiO,HCN,and served, it is possible to assess whether collisional or radiative
CS are detected. This confirms the prediction of homogeneity excitation mechanisms can produce the observed line intensi-
byCherchneff(2006)asthesespeciesbeing‘parent’molecules ties. While it is well known that CO is formed in both M, C
which form in the inner layers of the CSE. This can be under- andS-typestarsandsurvivesdustcondensation,itisofinterest
stood in terms of the chemistry of these four molecules being tostudytheexcitationrequirementsforthethreeother‘parent’
moleculesSiO,HCNandCSpredictedbyCherchneff(2006)to
1 The James Clerk Maxwell Telescope (JCMT) is operated by The
beabundantintheinnerwindsofM,SandCAGBs.
Joint Astronomy Centre on behalf of the Science and Technology
For all lines, except for those from the CO molecule, the
FacilitiesCounciloftheUnitedKingdom,theNetherlandsOrganisation
emission distribution is expected to be much smaller than the
forScientificResearch,andtheNationalResearchCouncilofCanada.
FWHM beam size of the used telescope. To calculate column
ProgramIDism06bn03(34h).
2 APEX, the Atacama Pathfinder Experiment, is a collaboration densitiesweneedtocorrectforthedifferentbeamfillingfactor,
between the Max-Planck-Institut fur Radioastronomie, the European f = θS2/(θS2 +θB2), with θB and θS being the FWHM of the
SouthernObservatory,andtheOnsalaSpaceObservatory.ProgramID beamandthesourcerespectively.Todothis,wedefineabeam-
isESO.078.D-0534(44h). averaged brightness temperature, T , scaled by correcting our
b
L.Decinetal.:Non-equilibriumchemistryinAGBstellarwinds 3
WX Psc WX Psc WX Psc WX Psc WX Psc
2.5 2.0 0.4 0.6 0.4
12CO3-2 12CO4-3 13CO3-2 SiO7-6 HCN4-3
APEX APEX APEX APEX APEX
2.0 0.3 0.3
1.5
0.4
1.5 0.2 0.2
1.0
K] K] K] K] K]
[B 1.0 [B [B 0.1 [B 0.2 [B 0.1
M M M M M
T T 0.5 T T T
0.5 0.0 0.0
0.0
0.0
0.0 -0.1 -0.1
-0.5 -0.5 -0.2 -0.2 -0.2
-100-50 0 50 100 -100-50 0 50 100 -100-50 0 50 100 -100-50 0 50 100 -100-50 0 50 100
v [km/s] v [km/s] v [km/s] v [km/s] v [km/s]
WX Psc WX Psc WX Psc WX Psc WX Psc
0.3 0.3 0.3 0.20 0.10
H13CN4-3 SiS16-15 SiS19-18 12CS7-6 12CS6-5
APEX APEX APEX 29SiO(8-7) APEX APEX
0.2 0.15
0.2
0.2 0.05
0.1 0.10
0.1
K] K] K] K] K]
[B [B 0.1 [B 0.0 [B 0.05 [B 0.00
M M M M M
T 0.0 T T T T
-0.1 0.00
0.0 -0.05
-0.1
-0.2 -0.05
-0.2 -0.1 -0.3 -0.10 -0.10
-100-50 0 50 100 -100-50 0 50 100 -100-50 0 50 100 -100-50 0 50 100 -100-50 0 50 100
v [km/s] v [km/s] v [km/s] v [km/s] v [km/s]
WX Psc WX Psc WX Psc WX Psc WX Psc
0.20 2.5 0.4 0.6 0.6
SO10_11-10_10 CO2-1 13CO2-1 SiO5-4 SiO6-5
APEX JCMT JCMT JCMT JCMT
0.15 2.0 0.3
0.4 0.4
0.10 1.5 0.2
K] K] K] K] K]
[B 0.05 [B 1.0 [B 0.1 [B 0.2 [B 0.2
M M M M M
T T T T T
0.00 0.5 0.0
0.0 0.0
-0.05 0.0 -0.1
-0.10 -0.5 -0.2 -0.2 -0.2
-100-50 0 50 100 -100-50 0 50 100 -100-50 0 50 100 -100-50 0 50 100 -100-50 0 50 100
v [km/s] v [km/s] v [km/s] v [km/s] v [km/s]
WX Psc WX Psc WX Psc
0.6 0.15 0.15
HCN3-2 SiS12-11 SO5.6-4.5
JCMT JCMT JCMT
0.4 0.10 0.10
K] K] K]
[B 0.2 [B 0.05 [B 0.05
M M M
T T T
0.0 0.00 0.00
-0.2 -0.05 -0.05
-100-50 0 50 100 -100-50 0 50 100 -100-50 0 50 100
v [km/s] v [km/s] v [km/s]
Figure1.MolecularemissiondetectedwithAPEXandJCMTintheO-richAGBWXPsc.
4 L.Decinetal.:Non-equilibriumchemistryinAGBstellarwinds
W Aql W Aql W Aql W Aql
5 6 0.4 0.4
12CO3-2 12CO4-3 13CO3-2 SiO7-6
4 APEX APEX 0.3 APEX APEX
0.3
4
3 0.2
K] K] K] K] 0.2
[B 2 [B 2 [B 0.1 [B
M M M M 0.1
T T T T
1 0.0
0
0.0
0 -0.1
-1 -2 -0.2 -0.1
-100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100
v [km/s] v [km/s] v [km/s] v [km/s]
W Aql W Aql W Aql W Aql
1.5 3 0.8 2
SiO11-10 SiO19-18 HCN4-3 HCN9-8
APEX APEX 0.6 APEX APEX
1.0 2
1
0.4
K] 0.5 K] 1 K] K]
[B [B [B 0.2 [B 0
M 0.0 M 0 M M
T T T T
0.0
-1
-0.5 -1
-0.2
-1.0 -2 -0.4 -2
-100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100
v [km/s] v [km/s] v [km/s] v [km/s]
W Aql W Aql W Aql W Aql
0.2 0.15 0.3 0.20
H13CN4-3 SiS16-15 SiS19-18 12CS6-5
APEX APEX 0.2 APEX 0.15 APEX
0.10
0.1
0.1 0.10
K] K] 0.05 K] K]
[B 0.0 [B [B 0.0 [B 0.05
M M 0.00 M M
T T T T
-0.1 0.00
-0.1
-0.05
-0.2 -0.05
-0.2 -0.10 -0.3 -0.10
-100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100
v [km/s] v [km/s] v [km/s] v [km/s]
W Aql W Aql W Aql W Aql
0.4 4 0.3 0.5
12CS7-6 CO2-1 13CO2-1 SiO5-4
APEX JCMT JCMT 0.4 JCMT
3
0.2 0.2
0.3
K] K] 2 K] K]
[B 0.0 [B [B 0.1 [B 0.2
M M 1 M M
T T T T
0.1
-0.2 0.0
0
0.0
-0.4 -1 -0.1 -0.1
-100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100
v [km/s] v [km/s] v [km/s] v [km/s]
W Aql W Aql W Aql
0.6 1.2 0.04
SiO6-5 HCN3-2 SiS12-11
JCMT 1.0 JCMT JCMT
0.4 0.02
0.8
K] K] 0.6 K]
[MB 0.2 [MB 0.4 [MB 0.00
T T T
0.2
0.0 -0.02
0.0
-0.2 -0.2 -0.04
-100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100
v [km/s] v [km/s] v [km/s]
L.Decinetal.:Non-equilibriumchemistryinAGBstellarwinds 5
II Lup II Lup II Lup II Lup
5 5 6 1.5
12CO3-2 12CO4-3 12CO7-6 13CO3-2
APEX APEX APEX APEX
4 4
4 1.0
3 3
K] K] K] K]
T [MB 2 T [MB 2 T [MB 2 T [MB 0.5
1 1
0 0.0
0 0
-1 -1 -2 -0.5
-150 -100 -50 0 50 100 -150 -100 -50 0 50 100 -150 -100 -50 0 50 100 -150 -100 -50 0 50 100
v [km/s] v [km/s] v [km/s] v [km/s]
II Lup II Lup II Lup II Lup
0.8 3 5 0.4
SiO7-6 HCN4-3 H13CN4-3 SiS19-18
APEX APEX APEX APEX
0.6 4 0.3
2
3 0.2
K] 0.4 K] K] K]
T [MB 0.2 T [MB 1 T [MB 2 T [MB 0.1
1 0.0
0
0.0 0 -0.1
-0.2 -1 -1 -0.2
-150 -100 -50 0 50 100 -150 -100 -50 0 50 100 -150 -100 -50 0 50 100 -150 -100 -50 0 50 100
v [km/s] v [km/s] v [km/s] v [km/s]
II Lup II Lup
1.0 1.5
12CS6-5 12CS7-6
0.8 APEX APEX
0.6 1.0
K] 0.4 K]
T [MB 0.2 T [MB 0.5
0.0 0.0
-0.2
-0.4 -0.5
-150 -100 -50 0 50 100 -150 -100 -50 0 50 100
v [km/s] v [km/s]
Figure3.MolecularemissiondetectedwithAPEXintheC-richAGBIILup.
V Cyg V Cyg V Cyg V Cyg
3.0 0.3 0.4 0.6
CO2-1 13CO2-1 SiO5-4 SiO6-5
JCMT JCMT JCMT JCMT
2.5
0.2 0.3
0.4
2.0
0.1 0.2
K] 1.5 K] K] K]
T [MB T [MB T [MB T [MB 0.2
1.0 0.0 0.1
0.5 0.0
-0.1 0.0
0.0
-0.2 -0.1 -0.2
-100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100
v [km/s] v [km/s] v [km/s] v [km/s]
V Cyg V Cyg V Cyg
2.0 0.6 0.08
HCN3-2 CS5-4 SiS12-11
JCMT JCMT JCMT
0.06
1.5 0.4
0.04
1.0 0.2
K] K] K]
T [MB T [MB T [MB 0.02
0.5 0.0
0.00
0.0 -0.2
-0.02
-0.5 -0.4 -0.04
-100 -50 0 50 100 -100 -50 0 50 100 -100 -50 0 50 100
v [km/s] v [km/s] v [km/s]
Figure4.MolecularemissiondetectedwithJCMTintheC-richAGBVCyg.
6 L.Decinetal.:Non-equilibriumchemistryinAGBstellarwinds
Table 1. Overviewof the velocity-integratedintensities( T dv in K km/s) forthe observedline transitions. The frequencyis
mb
listedinGHzandthelowerenergylevelincm−1.TheintegratedintensityofdetectedlineswithalowS/N-ratioisgivenbetween
R
brackets.Incaseofanon-detection,anupperlimitontheintegratedintensityiscomputedas3σ×expectedlinewidth,withσ the
noiseonthedata.A‘−’indicateslinesthatwerenotobserved.APEX-dataarereportedupright,JCMT-dataaregiveninitalics.
transition 12CO(2-1) 12CO(3-2) 12CO(4-3) 12CO(7-6) 13CO(2-1) 13CO(3-2) 12CS(5-4) 12CS(6-5) 12CS(7-6) 12CS(10-9) 12CS(17-16)
frequency 230.538 345.795 461.040 806.651 220.398 330.587 244.935 293.912 342.882 489.759 832.061
Elow 3.84 11.54 23.07 80.74 3.67 11.03 16.34 24.51 34.32 75.53 222.15
WXPsc 58.0 64.3 40.2 22.4 9.9 11.3 <2.4 [0.8] 1.3 <8.1 <42.6
WAql 80.6 118.4 95.9 <178 4.6 8.1 − 2.4 4.6 <54.1 <203
IILup − 145.0 130.0 99.7 − 32.5 − 20.4 35.2 − −
VCyg 45.6 − − − 4.2 − 7.4 − − − −
transition 13CS(7-6) 13CS(10-9)H12CN(3-2)H12CN(4-3)H12CN(9-8)H13CN(4-3) SiO(5-4) SiO(6-5) SiO(7-6) SiO(11-10)
frequency 323.684 462.334 265.886 354.505 797.433 345.340 217.105 260.518 303.926 477.504
Elow 32.39 69.41 8.87 17.74 106.42 17.28 14.48 21.73 30.42 79.65
WXPsc <67.4 <12.7 9.9 8.9 <117 [2.6] 9.2 11.1 10.4 <15.0
WAql − <13.0 21.7 12.2 [34.1] 3.1 7.1 8.9 5.2 13.8
IILup − − − 60.7 − 64.2 − − 19.6 −
VCyg − − 25.7 − − − 3.8 5.1 − −
transitionSiO(19-18)SiS(12-11) SiS(14-13) SiS(16-15) SiS(19-18) SO(11-10) SO(65-54)SO(78-67)SO(89-88)SO(1011-1010)
frequency 824.235 217.817 254.102 290.380 344.778 286.34 219.949 340.714 254.573 336.597
Elow 247.57 39.96 55.10 72.66 103.53 1.00 16.98 45.10 60.80 88.08
WXPsc <92.2 3.2 <1.2 5.4 4.8 <0.9 1.6 <1.4 <2.0 [0.9]
WAql [22.3] − − 1.2 [3.4] − <1.2 <3.4 − −
IILup − − − − 7.8 − − <4.2 − −
VCyg − 0.9 <14.8 − − − <1.3 − − −
main-beambrightnesstemperaturesfora fictitious10′′ FWHM whereN ,N ,g ,g aretheupperandlowerstatecolumnden-
u l u l
source,i.e,Tb =1/f ×Tmb. sities and degeneracies, Tkin the kinetic temperature, ncrit the
Little interferometric data exist for the molecules that we criticaldensity,andnH2 thehydrogendensity.Foramulti-level
haveobservedinanycircumstellarenvelope.Wenote,however, system,thecriticaldensityisgivenby(Tielens2005)
that4′′–6′′resolutionobservationsoftheHCNJ =1−0linein
theMiravariablesTXCamandIKTau(Marvel2005)barelyre- ncrit = l<uAul , (5)
solvetheemissiondistibutionsintheseobjects.Since,first,those γ
Pl6=u ul
objectsareclosertotheSunthanourtargetstarsand,second,our
higher exciationlines most likely arise from more compactre- whereAPrepresentstheEinsteincoefficient,andγ thecolli-
ul ul
gionsthanthe1−0line,thecolumndensitiesderivedarestrict sionalratecoefficients.Criticaldensitieswerecalculatedatdif-
lowerlimits. fering temperatures using the data available in the LAMDA-
Assumingthatthelinesareopticallythinandthatthe exci- database (Scho¨ieretal. 2005) (see Table 2). Applying Eq. (4)
tation temperature,T , between upperand lower level is such togetherwith the columndensities fromTable 2, the minimum
ex
that Tex≫Tbg (with Tbg the temperature of any background density requirementsfor nH2 to achieve the observed number-
source,e.g.2.7K),theintegrationofthestandardradiativetrans- densityratiosaretabulatedinTable3.Notethatthelistedvalues
ferequationshowsthat(Goldsmith&Langer1999) areatTkin = 300K,andthatforlowertemperaturesthecritical
densityincreases.
1.67×1014 Thegastemperature,densityandvelocitystructurewascal-
N = T dv , (2)
u ν µ2 b culatedinaself-consistentwayusingtheGASTRoNOoM-code
(cid:18)Z (cid:19) (Decinetal. 2006) to determine where in the envelope the gas
withN thecolumndensityoftheuppertransitionstateincm−2, densityfallsbelowthedensityrequirementsgiveninTable3(see
u
µ the transition dipole moment in Debye, ν the transition fre- Fig.5).TheassumedstellarparametersarelistedinTable4,and
quencyinGHz, T themain-beamtemperatureinKelvinand the derived (maximum) radius of the emitting region is listed
mb
v the velocity in km/s. For rotational transitions, the transition in Table 3. If collisionalexcitation is assumed, it appearsfrom
momentfordiatomicandpolyatomicmoleculeisgivenby Table3thatthe‘parent’moleculesSiO,HCNandCSareexcited
intheinner(<∼20R∗)andintermediate(<∼70R∗)regionsofthe
J +1 circumstellar envelopeand trace regionsafter the dust conden-
2 2
µ (J +1,J)= µ , (3)
2J +3 e sationzonewheretheyhavebeeninjectedfromdeeperlayers.
It is also of interest to consider the case where collisional
where µe is the permanent dipole moment of the molecule. excitationisignored,andthe moleculesareexcitedbyinfrared
Using the velocity–integrated intensities of Table 1, we calcu- radiationfromthestar.Onecanderivethat(Tielens2005)
latetheresultingupperstatecolumndensities(seeTable2).
Assuming purely collisional excitation, and ignoring radia- Nl = gl exp(hν/kT∗)−1 +1, (6)
tiontrapping,onecanderivefromsolvingthestatisticalequilib- N g W
u u
riumequationthat(Tielens2005)
where W is the geometrical dilution, being (R /2R)2 when
∗
NNul = ggul (cid:18)nncHri2t +1(cid:19) exp(hν/kTkin), (4) Rem≫itRti∗n.gAzsonseeefnofrroSmiOTbaebilneg3<∼,r9adRia∗t,ivfoereHxcCitNati<∼on1c3oRns∗t,raainndstfhoer
L.Decinetal.:Non-equilibriumchemistryinAGBstellarwinds 7
Table 2. Velocity-integrated intensities (in Kkm/s), upper state column densities in cm−2 calculated using Eq. (2) and critical
densites in cm−3 for the detected multiple-transitions ‘parent’ molecules used in the excitation analysis. In a few cases, extra
integrated-intensityvalues(listedinitalics)asfoundinliteratureareaddedtoperformtheexcitationanalysis.Literaturereferences
aregiveninthefootnote.
SiO(5-4) SiO(6-5) SiO(7-6) SiO(8-7) HCN(3-2) HCN(4-3) CS(5-4) CS(6-5) CS(7-6)
WXPsc R Tmbdv 9.2 11.1 10.4 9.9 8.9 2.4 [0.8]† 1.3
N 7.96×1012 7.91×1012 5.15×1012 7.96×1012 4.22×1012 2.53×1012 1.08×1012 1.41×1012
u
WAql R Tmbdv 7.1 8.9 5.2 21.7 12.2 8.5a 2.3 4.6
N 6.19×1012 6.36×1012 2.57×1012 1.74×1013 5.81×1012 8.95×1012 3.03×1012 5.00×1012
u
IILup R Tmbdv 19.62 20.4b 139.76c 60.69 32.34c 20.37 35.24
N 6.96×1012 8.74×1012 1.24×1014 2.88×1013 6.85×1013 2.61×1013 3.83×1013
u
VCyg R Tmbdv 3.8 5.1 8.4d 25.7 31.9d 7.4
N 3.33×1012 3.51×1012 5.28×1012 2.06×1013 1.51×1013 1.41×1013
u
at40K ncrit 3.06×106 5.26×106 8.41×106 1.27×107 5.83×106 1.32×107 1.85×106 3.22×106 5.09×106
at100K ncrit 2.00×106 3.44×106 5.57×106 8.18×106 4.22×106 9.63×106 1.35×106 2.37×106 3.76×106
at300K ncrit 1.22×106 2.10×106 3.44×106 5.12×106 2.35×106 5.65×106 8.35×105 1.48×106 2.37×106
aBujarrabaletal.(1994);bScho¨ieretal.(2006);cWoodsetal.(2003);dBiegingetal.(2000)
†linedetectedhoweverwithsmallS/Nsothattheintegratedintensityisquiteuncertain
Table 3. For each target, the first row lists the minimum analogousexpressionasforEq.(6)canbederivedforaradiation
number density for nH2 in cm−3 as derived using Eq. (4) at fieldcharacterizedbyadusttemperatureTdatthecondensation
Tkin = 300K and the second row gives the corresponding radiusRinner,withthedilutionfactorthenbeing(Rinner/2R)2.
maximum radius for the emission regions obtained using the Using the dust condensation radii listed in Table 4 and a dust
GASTRoNOoM-code.Thethirdrowgivesthemaximumradius temperature of 800K the derived emitting regions are a factor
fortheemittingregioncalculatedusingEq.(6). 3.2 larger for WX Psc, a factor 4.3 for W Aql, a factor 4 for
IILup,andafactor2forVCyg,resultinginsimilarradiiasin
SiO HCN CS
n 5.0×106 4.4×106 9.0×105 caseofcollisionalexcitation.
WXPsc RH[2Eq.(4)] 17R 19R 33R AlthoughthenumbersinTable3canonlybeusedasrough
∗ ∗ ∗
R[Eq.(6)] 4.5R 6R 7.5R guidelines, they suggest in both cases a sequence in the exci-
∗ ∗ ∗
n 2.1×106 2.1×106 6.34×105 tation pattern,SiO beingthe species emitting the closest to the
H2
WAql R[Eq.(4)] 40R 40R 70R star,followedbyHCNandCS.
∗ ∗ ∗
R[Eq.(6)] 9R 10R 11R
∗ ∗ ∗
n 2.7×107 1.3×106 7.6×105
IILup RH[2Eq.(4)] 13R 53R 70R 4. Conclusions
∗ ∗ ∗
R[Eq.(6)] 2.5R 13R 10R
∗ ∗ ∗ From the above analysis, one can draw the following conclu-
n 2.8×107 8.7×106
H2 sions:
VCyg R[Eq.(4)] 5R 9R
∗ ∗
R[Eq.(6)] 4R 1. Theobservationsreportedinthisletterconfirmthestatusof
∗
‘parent’moleculesforCO,SiO,HCN,andCSinAGBstars,
whoseobservedmolecularlinesformclosetothestarinsup-
Table 4. Stellar parameters used as input for the
portofthetheoreticalpredictionsofCherchneff(2006).The
GASTRoNOoM-code. The terminal velocity, v , is de-
∞ excitationanalysissuggeststhatSiOisemittedclosesttothe
rived from the CO lines; the stellar radius from the stellar
star,followedbyHCNandCS.
luminosity and temperature. The envelope density falls off as
2. High-excitationlinesofSiSaredetectedinallstars,imply-
∼r−2.Literaturereferencesaregiveninthefootnote.
ingthatSiStooformsclosetothestar,whateverthestageof
WXPsc WAql IILup VCyg stellarevolution.However,athoroughlineanalysisisneces-
T [K] 2000a 2800 2400f 1900f sarytoproveordisprovethatSiSismoreabundantinCstars
∗
R [1013cm] 5.5 2.4 3.8 5.1 thaninO-richMiras,aspredictedbyCherchneff(2006).
∗
L [103L ] 10a 6.8b 8.8f 6.3f 3. SOappearstobetypicalofO-richAGBsonly.Withonlytwo
∗ ⊙
[CO]/[H2][10−4] 3c 6c 8c 8c excitationlinesdetected,noinformationcanbedrawnonthe
distance[pc] 833a 230d 500f 310f locusofSOformation.However,theSO(1011-1010)linehas
Rinner[R∗] 5a 8e 4f 2f an estimated velocity of only 10km/sin WX Psc when the
v∞[km/s] 18 17.5 21 10.5 windterminalvelocityforthisobjectis19.2km/s.Thisfact
M˙ [10−6M⊙yr−1] 6a,† 2.5b 9f 1.2g suggeststhat the line excitation occursclose to star. In any
aDecinetal.(2007);bRamstedtetal.(2006);cKnapp&Morris case,theinnerchemistryofO-richAGBenvelopesappears
(1985);dBiegingetal.(2000);eDanchietal.(1994);f Scho¨ieretal. tobeasrich,ifnotricher,thanthatofC-richstars.
(2006);gScho¨ier&Olofsson(2001)
†referstotheinnerregion(A)inDecinetal.(2007) Adetailedlineanalysisofthepresentdatacoupledtofurther
observationalcampaignswithJCMT andAPEXareplannedto
corroboratetheseresults.
CS<∼11R∗.Ingeneral,theseregionsaresmallerthanderivedin Acknowledgements. LDandSDacknowledgefinancialsupportfromtheFund
caseofcollisionalexcitation,indicatingthattheradiationfieldof
forScientificResearch-Flanders(Belgium),ICacknowledgessupportfromthe
thestardoesnothaveenoughenergytosustaintheexcitation.An SwissNational Funds forScience through aMarie-Heim-Vo¨gtlin Fellowship,
8 L.Decinetal.:Non-equilibriumchemistryinAGBstellarwinds
Figure5. The structure of the CSE as derived using the GASTRoNOoM-code is shown for the four studied targets WX Psc, W
Aql,IILup,andVCyg.Sincethefocusisontheinnerandintermediateregions,onlythefirst200R aredisplayed.Upperpanel:
⋆
Estimatedtemperatureprofile,middlepanel:estimatedgasvelocitystructure,andbottompanel:estimatedhydrogendensitynH2.
The dashed line indicates the dust condensationradius, Rinner. The variable mass-loss rate of WX Psc, mostly noticeable in the
hydrogendensity,isdiscussedinDecinetal.(2007).
andSHacknowledgesfinancialsupportfromtheInteruniversityAttractionPole Scho¨ier,F.L.&Olofsson,H.2001,A&A,368,969
oftheBelgianFederalSciencePolicyP5/36.WethankRemoTilanus(JCMT) Scho¨ier,F.L.,vanderTak,F.F.S.,vanDishoeck,E.F.,&Black,J.H.2005,
forhissupportduringtheobservationsandreductionofthedata. A&A,432,369
Tielens,A.G.G.M.2005,ThePhysicsandChemistryoftheInterstellarMedium
(CambridgeUniversityPress)
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