Keplerian discs around post-AGB stars: a common phenomenon?

1.4 MB·English

by  S. De Ruyter

1
We aim at showing that the broad-band SED characteris-
tics of our sample of post-AGB stars are best interpreted, as-
suming the circumstellar dust is stored in Keplerian rotating
passivediscs.Wepresentahomogeneousandsystematicstudy
oftheSpectralEnergyDistributions(SEDs)ofasampleof51
post-AGB objects. The selection criteria to define the whole
sample were tuned to cover the broad-bandcharacteristics of
knownbinarypost-AGBstars. The whole sampleincludes20
dusty RVTauri stars from the General Catalogue of Variable
Stars(GCVS).WesupplementedourownGenevaopticalpho-
tometry with literature data to cover a broad range of fluxes
from the UV to the far-IR. All the SEDs display very simi-
larcharacteristics:alargeIRexcesswithadustexcessstarting
near the sublimation temperature,irrespectiveof the effective
6
0 temperatureof the centralstar. Moreover,when available, the
0 longwavelengthfluxesshowa black-bodyslope indicativeof
2 thepresenceofacomponentoflargemmsizedgrains.Wear-

n gue that in all systems, gravitationally bound dusty discs are
a present.Thediscsmustbepuffed-uptocoveralargeopening
J
angleforthecentralstarandwearguethatthediscshavesome

5 similaritywiththepassivediscsdetectedaroundyoungstellar
2
objects.Weinterpretthepresenceofadisctobeasignaturefor

binarity of the central object, but this will need confirmation
1
v bylong-termmonitoringoftheradialvelocities.Wearguethat
8 dusty RVTauri stars are those binarieswhich happento be in
7 thePopulationIIinstabilitystrip.
5
1
Keywords.stars:AGBandpost-AGB–stars:binaries:gen-
0
6 eral–stars:circumstellarmatter
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Astronomy&Astrophysicsmanuscriptno.4062sdr February3,2008
(DOI:willbeinsertedbyhandlater)
Keplerian discs around post-AGB stars:
⋆
a common phenomenon?
S.DeRuyter1,H.VanWinckel2,T.Maas2,3,T.LloydEvans4,L.B.F.MWaters2,5,andH.Dejonghe1
1 SterrenkundigObservatorium,UniversiteitGent,Krijgslaan281S9,9000Gent,Belgium
2 InstituutvoorSterrenkunde,K.U.Leuven,Celestijnenlaan200B,3001Leuven,Belgium
3 DepartmentofAstronomy,UniversityofTexas,Austin,TX78712
4 SchoolofPhysicsandAstronomy,UniversityofSt.Andrews,NorthHaugh,St.Andrews,Fife,ScotlandKY169SS
5 SterrenkundigInstituut‘AntonPannekoek’,UniversiteitAmsterdam,Kruislaan403,1098Amsterdam,TheNetherlands
Received¡date¿/Accepted¡date¿
Abstract.
1. Introduction belinkedtothepole-to-equatordensitycontrast,asdetermined
on the basis of high spatial resolution mid-infrared images
Post-AGB stars are low and intermediate initial mass (≤ 8-
(Meixneretal.1999).RecentlyalsoGledhill(2005)foundev-
9M⊙) stars which have suffered a large and dusty mass-loss
idence for axi-symmetry in the dust density in his polarimet-
phaseattheendoftheAsymptoticGiantBranch(AGB)during
ricimagingsurveyofcandidatepost-AGBstars.Thedetached
whichalmostthewholestellarenvelopewasexpelled.Theyare
shellscorrespondtostarswithanopticallythinexpandingcir-
evolvingatconstantluminosityonafastevolutionarytrackin
cumstellarenvelopewhereasthebipolarandunresolvedtargets
whichthecentralstarcrossestheHR-diagramfromacoolAGB
have optically thick dust structures, probably in the form of
photospheretotheionizingtemperatureofthecentralstarofa
discs. It is suggested once again that this bifurcation in mor-
PlanetaryNebula(PN).Giventhesmallevolutionarytimescale
phologyisrootedinthepresenceorabsenceofabinarycom-
of about104years, post-AGBstars are rare and not manyare
panion,whichdetermineswhetherornotadiscforms.
known(e.gSzczerbaetal.2001;VanWinckel2003).
DiscussionsonthemorphologyofPNeusuallystartwitha Stunningkinematicinformationresultedfromtheextensive
displayoftheaestheticpicturesoftheHubbleSpaceTelescope COsurveyofBujarrabaletal.(2001):itappearstobeafunda-
(HST) showing the complex geometry and structure of the mental property of the omnipresentfast molecular outflow in
nebulae,immediatelyfollowedbythepuzzlingandcontradic- PPNe, that it carries a huge amount of linear momentum, up
tory finding that, on the AGB, the mass-loss is found to be to 1000 times the momentum available for a radiation driven
sphericallysymmetric.Duringthetransitiontime,thestarand wind. Clearly, other momentum sources have to be explored.
circumstellar envelope must undergo fundamental and rapid Some molecular jets are resolved by high spatial resolution
changesinstructure,mass-lossmodeandgeometrywhichare imaging (Sahai 2004). The formation process of the strongly
still badly understood. The debate on which physical mecha- collimatedjetsis,however,stillbadlyunderstood.Anintrigu-
nisms are driving the morphology changes gained even more ingsuggestionisthattheprocessessimilartothejetformation
impetus from the finding that also resolved cooler post-AGB inlow-massyoungstellar objectsoperateandthatthejetsare
starsorProto-PlanetaryNebulae(PPNe)displayasurprisingly borninaccretiondiscs.Thismechanismrequiresasignificant
widevarietyinshapesandstructures,veryearlyintheirpost- amountofmassorbitingthepost-AGBstar.Suchadisccould
AGBevolution(Balick&Frank2002,andreferencestherein). be present, but likely only in binary stars. Testing of such an
hypothesis is severely hampered by the lack of observational
In a survey of 27 PPNe, 21 were found to be resolved
informationonbinarityinPNeandPPNebutalsoonourpoor
(Uetaetal. 2000). Moreover, the degree of asymmetry could
theoreticalunderstandingofAGBevolutioninbinarysystems.
Send offprint requests to: S. De Ruyter, e-mail: NotethatmostobjectswhichweredetectedinCOarestrongly
[email protected] embedded and the sample is probably biased towards more
⋆ BasedonobservationswiththeP7photometeratthe0.7mSwiss massivePPNe.
Telescope,LaSilla(Chile)andattheFlemishMercatorTelescope,La
Palma(Spain)andonobservationswiththeSubmillimetreCommon- ProbingbinarityinPNeandembeddedPPNedirectlywith
User Bolometer Array (SCUBA) at the James Clerk Maxwell radialvelocitymonitoringisnoteasy.Foropticallybrightpost-
Telescope(JCMT),MaunaKea,Hawaii. AGBstars,thisisdifferentandsomefamousexamplesexistin
S.DeRuyteretal.:Kepleriandiscsaroundpost-AGBstars:acommonphenomenon? 3
whichitisclearthatthebinarynatureofthecentralobjectmust Our sample and the selection criteria are presented in
haveplayedakeyroleintheevolutionofthesystem. Sect. 2. In Sect. 3 we present our photometric data comple-
mented with the literature values and the results of the large
ThemostfamousexampleiscertainlyHD44179,thecen-
all sky surveys. In Sect. 4 we present the detailed Spectral
tral star of the Red Rectangle nebula. It displays a huge and
Energy Distribution (SED) construction of all individual ob-
broad IRexcess. The IR luminosity is 33 times stronger than
jects. Distances to the objects are estimated in Sect. 5. We
the optical luminosity (Leinert&Haas 1989; Waelkensetal.
end this extensive study with a detailed discussion in Sect. 6.
1996).VanWinckeletal.(1995)haveshownthatHD44179is
ConclusionsaresummarizedinSect.7.
aspectroscopicbinarywithanorbitalperiodof298days.The
eccentricityisaremarkablyhighe=0.37.Sinceitsdiscovery
by Cohenetal. (1975), HD44179 has often been used as an 2. Programmestars
archetypicalexampleofaC-richpost-AGBobject,butitisnow
Our total sample of 51 stars was defined on several observa-
generally accepted that many of the remarkable phenomena
tional criteria inspired by the characteristics of known post-
andthepeculiarmorphologyofthenebula(foranoverviewsee
AGB stars in binary systems. The whole sample is given
Cohenetal.2004)arecloselyrelatedtothepresenceofastable
in Table 1. Besides these proven binaries, we discuss also
circumbinarydiscaroundthebinarycentralstar.Thelongevity
RVTauristarswithIRASdetectionsofgoodqualityandasam-
ofthediscwasdramaticallyconfirmedbythedetectionofcool
ple of newly identified objects, originally found by one of us
O-richcrystallinesilicate dustgrainsin thedisc (Watersetal.
(T.LloydEvans)insearchofnewcandidateRVTauristars.
1998).Themixedchemistryisbestexplainedassumingthefor-
mationoftheO-richdiscpredatedthemorerecentC-richtran-
sitionofthecentralobject.RecentSpitzerdatashow,however,
that also far from the central star, the nebula appears to have
a dust component which is O-rich (Markwick-Kemperetal.
2005). The (chemical) history of the binary, disc and neb-
ula is therefore far from understood. The disc is resolved in
ground-based high spatial-resolution imaging at optical and
near-IRwavelengths(Men’shchikovetal.2002,andreferences
therein)aswellasinHSTopticalimages(Cohenetal.2004).
The disc was also resolved in interferometric CO(2-1) maps,
andtheKepleriankinematicsofthediscweredirectlydetected
(Bujarrabaletal.2003,2005).HD44179showsaconsiderable
amount of dust processing in the disc with indications of the
presence of very large grains (Juraetal. 1997) and possibly
evenmacro-structures(Jura&Turner1998).
Anotherremarkableevolvedobjectwithalong-liveddiscis
HR4049.Itisabinarywithanorbitalperiodof430dayswith
aremarkablyhigheccentricityofe=0.30.Alsointhisobject,
thecircumstellarmaterialshowsa mixedchemistrywith both
carbon rich and oxygenrich features. The SED of the dust is
also very peculiar as it can be fitted with a single black-body
ofabout1150K,from1µmdownto850µm.TheseSEDchar-
acteristicsareveryconstrainingandthebestmodelforthecir-
cumstellarmaterialis,thatthedustistrappedinaveryopaque
dusttorusatKeplerianrotation(Dominiketal.2003).Itisclear
that also in this object the dusty disc plays a lead role in the
(future) evolution. Other examplesexist and there is substan-
tial observationalevidence that the systems are all likely sur-
roundedby a circumstellar orbitingdisc (VanWinckel 2003).
NotethatsofaronlyfortheRedRectangletheKepleriandisc
is spatio-kinematically resolved by CO interferometric maps
(Bujarrabaletal.2005).
Togaininsightintheevolutionofbinarysystemsandtheir
circumstellar material, we reportin this contributionon a ho-
mogeneous and systematic study of a sample of objects with
similarIRcharacteristicsastheknownbinarypost-AGBstars.
Themainaimistostudythebroad-bandSEDsinasystematic
way, in order to gain insight in the possible evolutionarylink
betweenthedifferentobjects.
Table 1. The IRASnumber,the HDnumber or the name from the GCVS, the spectral type, the equatorialcoordinatesα and δ(J2000), the effective temperature T , the 4
eff
surfacegravitylogg,themetallicity[Fe/H],thereferenceforthemodelparameters,thetypeofobject(post-AGB,RVTauriorNewSample)andareferencewheretheorbital
motionoftheobjectcanbefound,aregiven.NotethatIRAS11472−0800isastronglydepletedobjectaddedtooursamplestars.
1 2 3 4 5 6 7 8 9 10 11 12
No IRASnumber HDnumberor Spectral α(J2000) δ(J2000) Teff logg [Fe/H] ReferenceModelParameters Type ReferenceBinarity
GCVSname type (hms) (◦′′′) (K) (cgs)
1 IRAS04166+5719 TWCam G3I 042048.1 [phone] 0.0 −0.5 Giridharetal.(2000) RVTauri
2 IRAS04440+2605 RVTau G2I 044706.8 [phone] 0.0 −0.4 Giridharetal.(2000) RVTauri
3 IRAS05208−2035 M0e+F 052259.423 −203253.03 4000 0.5 0.0 fromspectraltype NewSample
4 IRAS06034+1354 DYOri G0I 060612.3 [phone] 1.5 −2.0 Gonzalezetal.(1997b) RVTauri
5 IRAS06072+0953 CTOri F9I 060957.4 +095235 5500 1.0 −2.0 Gonzalezetal.(1997a) RVTauri S.
6 IRAS06108+2743 SUGem G5I 061400.8 [phone] 1.125 −0.7 Wahlgren(1992) RVTauri D
7 IRAS06160−1701 UYCMa G0V 061816.367 −170234.72 5500 1.0 0.0 fromspectraltype RVTauri eR
8 IRAS06176−1036 HD44179 F1I 061958.2160 −103814.691 7500 0.8 −3.3 Waelkensetal.(1992) post-AGB VanWinckeletal.(1995) u
9 IRAS06338+5333 HD46703 F3I 063752.4253 +533101.957 6250 1.0 −1.5 fromspectrum post-AGB Hrivnak,privatecommunication yte
10 IRAS06472−3713 STPup F7I 064856.4131 −371633.332 5750 0.5 −1.5 Gonzalez&Wallerstein(1996) RVTauri Gonzalez&Wallerstein(1996) r
11 IRAS07008+1050 HD52961 F6I 070339.6314 +104613.067 6000 0.5 −4.8 Waelkensetal.(1991b) post-AGB Waelkensetal.(1992) et
1132 IIRRAASS0077218440−−02934201 USAMOo1n73329 FG50II 007713600487..2571 −−203924760317.61 75000000 10..50 −−00..88 VGairnidWhainrcekteall.(1(2909070)) pRoVstT-AauGriB VPoalnlaWrdin&ckCeoltettrealll.((11999955)) al.:
K
14 IRAS08011−3627 ARPup F0I 080301.1 −363547 6000 1.5 −1.0 Gonzalezetal.(1997b) RVTauri e
1156 IIRRAASS0089504640−−42483017 FF35 008950681140..1182 −−424841391100.73 76255000 11..55 −−00..55 MMaaaasseettaall..((22000035)) NNeewwSSaammppllee Maasetal.(2003) pler
17 IRAS09144−4933 G0 091609.1 −494606 5750 0.5 −0.5 Maasetal.(2005) NewSample ian
18 IRAS09256−6324 IWCar F7I 092653.4 −633748 6700 2.0 −1.0 Giridharetal.(1994) RVTauri d
19 IRAS09400−4733 M0 094152.9 −474703 NewSample is
c
20 IRAS09538−7622 G0 095358.5 −763653 5500 1.0 −0.5 Maasetal.(2005) NewSample s
21 IRAS10158−2844 HR4049 A6I 101807.5903 −285931.201 7500 1.0 −4.5 Dominiketal.(2003) post-AGB Waelkensetal.(1991b) ar
22 IRAS10174−5704 K:rr 101918.1 −571936 NewSample ou
23 IRAS10456−5712 HD93662 K5 104738.3965 −572802.679 4250 0.5 0.0 fromspectraltype NewSample nd
24 IRAS11000−6153 HD95767 F0I 110204.314 −620942.84 7600 2.0 0.1 VanWinckel(1997) post-AGB VanWinckeletal.(1995) p
25 IRAS11118−5726 GKCar M0I 111401.3 −574309 RVTauri os
26 IRAS11472−0800 F5I 114948.5 −081721 5750 1.0 −2.5 fromspectrum t-A
27 IRAS12067−4508 RUCen F6I 120923.7 −452535 6000 1.5 −2.0 Maasetal.(2002) RVTauri G
28 IRAS12185−4856 SXCen G3V 122112.6 −491241 6000 1.0 −1.0 Maasetal.(2002) RVTauri Maasetal.(2002) B
2390 IIRRAASS1123222528−−48615023 HD108015 FG35IbDe 113232140573..1501 −−841710893007.51 7000 1.5 −0.1 VanWinckel(1997) Npoeswt-ASaGmBple star
31 ENTrA F2Ib 145700.6847 −685022.879 6000 1.0 −0.5 VanWinckel(1997) RVTauri VanWinckeletal.(1995) s:
32 IRAS15469−5311 F3 155044.0 −532044 7500 1.5 0.0 Maasetal.(2005) NewSample ac
33 IRAS15556−5444 F8 155932.1 −545318 NewSample o
m
34 IRAS16230−3410 F8 162620.29 −341712.3 6250 1.0 −0.5 Maasetal.(2005) NewSample m
35 IRAS17038−4815 G2p(R)e 170736.3 −481908 4750 0.5 −1.5 Maasetal.(2005) NewSample o
36 IRAS17233−4330 G0p(R) 172657.5 −433313 6250 1.5 −1.0 Maasetal.(2005) NewSample n
37 IRAS17243−4348 LRSco G2 172756.1 −435048 6250 0.5 0.0 Maasetal.(2005) NewSample ph
38 IRAS17534+2603 89Her F3I 175525.1889 +260259.966 6500 1.0 0.0 Watersetal.(1993) post-AGB Watersetal.(1993) en
39 IRAS17530−3348 AISco G4I 175618.5 −334847 5000 0.0 0.0 fromspectrum RVTauri om
40 IRAS18123+0511 G5 181449.4 +051255 5000 0.5 0.0 fromspectraltype NewSample e
41 IRAS18158−3445 F6 181913.6 −344432 6500 1.5 0.0 fromspectraltype NewSample no
42 IRAS18281+2149 ACHer F4I 183016.2 [phone] 0.5 −1.5 VanWinckeletal.(1998) RVTauri VanWinckeletal.(1998) n?
43 IRAS18564−0814 ADAql G8I 185908.1 −081014 6300 1.25 −2.1 Giridharetal.(1998) RVTauri
44 IRAS19125+0343 F2 191500.8 +034841 7750 1.0 −0.5 Maasetal.(2005) NewSample
45 IRAS19163+2745 EPLyr A4I 191817.5 [phone] 2.0 −1.5 Gonzalezetal.(1997b) RVTauri
46 IRAS19157−0247 F3 191822.5 −024209 7750 1.0 0.0 Maasetal.(2005) NewSample
47 IRAS20056+1834 QYSge G0De 200754.8 [phone] 0.7 −0.4 Raoetal.(2002) NewSample
48 IRAS20117+1634 RSge G0I 201403.8 [phone] 0.0 −0.5 Gonzalezetal.(1997b) RVTauri
49 IRAS20343+2625 VVul G8I 203631.8 [phone] 1.0 0.0 fromspectraltype RVTauri
50 IRAS22327−1731 HD213985 A2I 223527.5259 −171526.889 8250 1.5 −1.0 fromspectrum post-AGB VanWinckeletal.(1995)
51 BD+39◦4926 A9I 224611.2273 +400626.294 7500 1.2 −2.9 VanWinckeletal.(1995) post-AGB Kodairaetal.(1970)
S.DeRuyteretal.:Kepleriandiscsaroundpost-AGBstars:acommonphenomenon? 5
2.1. Confirmedbinarypost-AGBstars WiththeirhighluminosityandoftenlargeIRexcessesdue
to thermal radiation from circumstellar dust, there is general
Thegroupofbinarypost-AGBstarsinoursamplewasassem-
agreement that RVTauri stars are low-mass objects in tran-
bled in a rather coincidentalway on the basis of independent
sition from the Asymptotic Giant Branch (AGB) to white
detailedstar-to-staranalyses.We includedtheprovenbinaries
dwarfs (Jura 1986). It was noted already in the seventies that
in our programme star sample and in Col. 12 of Table 1 we manyRVTauristarsshowaconsiderablenear-IRexcesscaused
giveareferencewheretheorbitalmotionoftheobjectswasdis-
by a hot dust component (Gehrz&Ney 1972; Gehrz 1972;
cussed.TheorbitalelementsthemselvesarelistedinTableB.1.
LloydEvans1985),whichwasattributedtothepossiblepres-
The photospheres of several binary post-AGB stars enceofadustydisc.Morris(1987)suggestedthatcircumstel-
(e.g. HR4049 (Lambertetal. 1988; Waelkensetal. lar dust could indeed exist in a disc structure in binary sys-
1991a), HD44179 (Waelkensetal. 1992), HD52961 tems, either in a circumbinary disc or in a disc around one
(VanWinckeletal. 1992) and BD+39◦4926 (Kodairaetal. ofthecomponents.TheweakcircumstellarCOemission(e.g.
1970)) are strongly affected by a poorly understood selective Bujarrabaletal. 1988) together with the black-body spectral
depletion process. The basic scenario of this process is that index at long wavelengths observed in some RVTauri stars
circumstellar gas is separated from the dust and subsequently (DeRuyteretal.2005)corroboratetheconclusionthatthecir-
re-accretedontothestar(Mathis&Lamers1992;Watersetal. cumstellardustisnotfreelyexpandingbutconfined.
1992).Sincethatgasisdevoidofrefractories,thephotosphere
In recent years it has become clear that also many
willbealteredchemically,andthismayresultinveryFe-poor
RVTauri photospheres show chemical anomalies pointing to
stars which are rich in non-refractories like Zn and S. This
an efficient depletion of refractory elements (Giridharetal.
process is very efficient in the four named binaries where
1994; Giridhar&Ferro 1995; Giridharetal. 1998, 2000;
there is observational evidence for the presence of a dusty
Gonzalezetal. 1997b,a; VanWinckeletal. 1998; Maasetal.
stablereservoir(VanWinckeletal.1995).Efficientseparation
2002).Themanyaffectedstarsshowthatthedepletionprocess
of circumstellar gas and dust is not evident and Watersetal.
isaverycommonphenomenoninevolvedstars.Giridharetal.
(1992) argued that the most favourable circumstances may
(2005)showthatdepletioninRVTauristarsislessstrongwhen
occur if the circumstellar dust is indeed trapped in a stable
disc.Notethatforonestronglydepletedobject,BD+39◦4926, the centralstar is cooler,which is interpretedas pointingto a
stronger dilution due to a deeper convective envelope in the
therewasnoIRexcessdetectedbyIRAS.
coolerstars. Thedepletionpatternsarenotseeninobjectsfor
In the few cases where ISO spectra are available,the dust
which the intrinsic metallicity is smaller than aboutone tenth
processingisstrong,whichresultsinalargecrystallinityfrac-
solar([Fe/H]≤ −1.0).
tionofthegrains(e.g.Molsteretal.2002).Alsothedustgrain
sizedistributionisdifferentindiscsthaninoutflowsandwhere It is clear that the observational restriction that depletion
long wavelength fluxes are available, they indicate the pres- isonlyactiveinbinarystars,whichwasformulatedwhenonly
ence of large, µm sized and even cm sized dust grains (e.g. fourextremelydepletedobjectswereknown,hasbecomemuch
Shentonetal.1995). less evident with the many new detections. Direct evidence
Obviouslysuchadiscmustplayanimportantroleinthese for binarity,from radial velocity measurements,is difficultto
systems and, although the indirect observationalevidence for obtain since RVTauristars have large pulsationalamplitudes.
the presence of a stable disc is well established in the binary Moreover,theyoftenshowthepresenceofshocksintheline-
stars,theactualstructureletalonetheformation,stabilityand forming region of the photosphere making the very determi-
evolutionarenotwellunderstood.NotethatonlyforHD44179 nation of the radial velocity difficult (e.g. Gilletetal. 1990).
isthisdiscspatiallyresolved.Forallotherobjects,thepresence Nonethelessorbitalelementshavebeendeterminedforquitea
ofthediscwaspostulated. fewclassicalRVTauristars:UMon(Pollard&Cottrell1995),
ACHer(VanWinckeletal.1998),ENTrA(VanWinckeletal.
1999), SXCen (Maasetal. 2002) and orbital motion is also
2.2. Classical RVTauristars from the General foundforIWCar(Pollardetal.1997),EPLyr(Gonzalezetal.
CatalogueofVariableStarswith strong 1997b)andRUCen(Maasetal.2002).
IRexcess
Inthiscontributionweanalysethebroad-bandSEDsofthe
RVTauri stars form a class of classical pulsating variables. RVTauristarsoftheGCVSwithareliableIRASmeasurement
They are luminous (I-II) supergiant pulsating variables, the at60µm(e.g.Jura1986)andcomparethemwiththeSEDsof
lightcurveofwhichshowsalternatingdeepandshallowmin- similarobjects,whicharenotinthePopIICepheidinstability
ima with a formal period (measured between one deep mini- strip.
mumandthenext)of30to150daysandabrightnessrangeof
uptofourmagnitudes.ThespectraltypeistypicallyFtoGat
minimum and G to K at maximum. There are two main pho- 2.3. New SampleofcandidateRVTauristars
tometric varieties of RVTauri stars: the RVa types maintain a
roughlyconstant mean brightness;RVb types show on top of LloydEvans (1985) and Raveendran (1989) showed that the
theirpulsationalperiodalongerterm(600to1500days)peri- RVTauri stars are located in a well-defined and relatively
odicity. thinly-populatedpartoftheIRAScolour-colourdiagram.The
6 S.DeRuyteretal.:Kepleriandiscsaroundpost-AGBstars:acommonphenomenon?
definingrectangleis: Genevaopticalphotometry
WeacquiredGenevaopticalphotometryatrandomepochswith
[12]−[25]=1.56+2.5log[F(25)/F(12)]=1.0−1.55 the 0.7m Swiss Telescope at La Silla and with the Flemish
(cid:26)[25]−[60]=1.88+2.5log[F(60)/F(25)]=0.20−1.0 MercatorTelescopeatLaPalma,usingtherefurbishedGeneva
(1) photometer P7 (Raskinetal. 2004). Our total data set was
scanned for the maximum and minimum magnitudes (see
LloydEvans(1997)searchedfornewexamplesofRVTauri
Table C.1). Observation dates, the number of measurements
stars, as stars with dusty discs, in the IRAS Catalogue, and
andthetotaltimebasesofthesemaximaandminimaaregiven
foundthestarsofthe‘newsample’discussedinthispaper.The
aswell.AdditionalGenevaopticalphotometrywasfoundinthe
presenceinthissampleofstarswiththeinfraredpropertiesof
Geneva database: the General Catalogue of Photometric Data
theGCVSsampleofRVTauristars,butwhichfelloutsidethe
(GCPD,http://obswww.unige.ch/gcpd/gcpd.html).
instability strip as judgedfromtheir spectral typesand which
Notethesedataareonlygivenforcompleteness,butwerenot
lackedthelarge-amplitudeRVTaurivariability,suggestedthat
used in our analysis; except for IRAS10456 for which the
RVTauri stars are those stars with dusty discs which are cur-
GCPDdataareouronlyGenevafluxes.
rentlylocatedwithintheinstabilitystrip(LloydEvans1999).
Maasetal.(2005)presentedachemicalabundanceanalysis
onthebasisofhighsignal-to-noiseandhighresolutionspectra UBVRI photometry
for12starsofthenewlydefinedsample.Theyfoundthat9stars
UBVRI photometry for the newly-identified objects was
areaffectedbythedepletionprocess.Inadetailedstudyofone
obtained from the South African Astronomical Observatory
object, IRAS08544 (Maasetal. 2003); orbital elements were
(SAAO) service observing programme(Table C.2). This uses
found which show that this star must have undergone severe
the 0.5m Telescope and the 0.75m Automatic Photometric
binaryinteractionwhenitwasan(asymptotic)giant.
Telescope(APT)atSutherland.IftheUBVRI dataofthestar
Inthiscontributionweanalysethebroad-bandSEDsofall show large variations mainly due to a large variability of the
20newlycharacterizedstarsofthissample. star, we plotboth minima and maxima.If the numberof data
pointsavailableistooloworiftherearenobigvariations,we
useameanUBVRI.
2.4. TotalSample
WealsosearchedforJohnsonandCousinsbroad-bandpho-
tometryin the literatureandthese dataare givenin TableC.2
The 51 objects of our sample are presented in Table 1.
aswell.
In Col. 11 we differentiate between the several subtypes.
Statisticalstudiesontheoccurrenceandcharacteristicsofpost-
AGBstarswithrespecttoAGBstarsand/orPlanetaryNebulae Near-IR data
(PNe)donotexistyet.Inthemostextensivecataloguedescrip-
JHKL photometry of the newly-identified objects was ob-
tion found in the literature, Szczerbaetal. (2001) consider a
tainedbyT.LloydEvanswiththe0.75mreflectoroftheSouth
Galactic sample of about 220 post-AGB stars known at that
AfricanAstronomicalObservatory.
time. Although the overlap between the specific sample dis-
These data points were complemented with near-IR data
cussedhere,andthetotalsampleofSzczerbaetal.(2001)isfar
from the 2MASS and DENIS projects and with other
fromcomplete,thesampleof51objectsdiscussedhere,forms
asignificantpopulationofpost-AGBstarsknowntodate.Note JHKLM datapointsfromtheliterature(TableC.3).
thatoursamplewasdefinedonthebasisofaspecificsetofcri-
teriaexplainedaboveandoursampleisnotmeanttocoverall Far-IR data
knownevolvedstarswithsimilarSEDcharacteristics.
For the characterization of the longer wavelength part of
the SEDs we use far-IR data from the IRAS Point Source
Catalogue (Beichmanetal. 1988, Table C.4); for some
3. Broad-bandphotometricdata stars data from the MSX Infrared Point Source Catalogue
(Eganetal. 2003) are also available (Table C.5). The IRAS
In order to reconstructthe complete SEDs of our programme satellitehadfourpassbands,at12,25,60and100µm,respec-
stars, we combined different sets of broad-band photomet- tively. The SPIRITIII instrument on board the MSX satellite
ric data: our own Geneva optical photometry, supplemented had6passbands,at4.29,4.35,8.3,12.1,14.7and21.3µm.
with UV, optical, near-IR, far-IR and submillimetre photom-
etryfoundintheliterature.
Submillimetredata
The main difficulty in constructing the SEDs of pulsating
stars with large amplitudes, like the RVTauri objects, is the For six stars (TWCam, RVTau, SUGem, UYCMa, UMon
acquisitionofequallyphaseddataoverawidespectraldomain. and ACHer) submillimetre data (see Table C.6) are available
Sincethesedataarenotavailable,welimitedourstudyofthe from DeRuyteretal. (2005), for HR4049 and IRAS20056
broad-bandenergetics to the phases of minimal and maximal weusedatafromtheotherliterature.ForHR4049,HD52961
coveredbrightness. and89Her,wehavesomenewlydeterminedSCUBA850mi-
S.DeRuyteretal.:Kepleriandiscsaroundpost-AGBstars:acommonphenomenon? 7
croncontinuummeasurements.Theobservationswerecarried AnappropriatephotosphericKuruczmodel-basedonT ,
eff
out with the 15m James Clerk Maxwell Telescope (JCMT) logg and [Fe/H]- is used for the unattenuated stellar photo-
atMaunaKea,Hawaii,duringMarch-April1999(programme spheres(Kurucz1979).
M99AN08). SCUBA (Hollandetal. 1999) was operated in
photometrymodetosimultaneouslyobtaindataat850µmand
4.2. Colourexcess E(B −V) determination
450µm. For the data reduction, the standard software SURF
wasusedwithMarsasafluxcalibrator.Table2detailsthenew Lightcomingfromastarisattenuatedandreddenedbymate-
850µmcontinuummeasurements,whilefor450µm,nosignif- rialintheline-of-sight.Notethatthetotalline-of-sightextinc-
icant signal was detected. Note that the new 850µm SCUBA tionislikelytocontainbothacircumstellarandaninterstellar
datapointforHR4049is,withintheerrors,thesameastheone component.
inDominiketal.(2003). WeestimatethetotalcolourexcessE(B−V)byusingthe
average interstellar extinction law given by Savage&Mathis
(1979) to deredden the observed maximal UV-optical fluxes.
Table2.Newobservationaldataat850µmfromSCUBA. E(B−V)isfoundbyminimizingthedifferencebetweenthe
dereddenedobservedfluxesin the UV-optical,and the appro-
No Name F850(mJy) priate Kurucz model (Kurucz 1979). We scale to the J filter
11 HD52961 2.8±1.9
whichisthereddestfilterwherenodustexcesscanbeexpected.
21 HR4049 8.7±2.8
Weassumethecircumstellarreddeninglawtobesimilartothe
38 89Her 40.9±2.4
ISMlaw.DeterminingtheE(B−V)inthiswayimpliesthat
wedon’tcorrectforthecontributionofthegreyextinction.
Results are in Table A.1. The erroron E(B −V) is typi-
For ACHer we have a flux point at 1.1mm from
cally0.1.Butchangingthestellarmodelsby±250Kineffec-
Shentonetal.(1995).
tive temperature causes a change in E(B −V) of about 0.2.
Thus,togetherwiththeerrorof0.2inducedbytheuncertainty
Other ofthe temperatureoftheunderlyingphotosphere,we havean
uncertaintyof0.3onthetotalextinctionduringmaximallight.
At the short wavelength side, we use the IUE data
Adistributionofthetotalcolourexcessesfoundforoursample
(InternationalUltravioletExplorer,0.115µm-0.320µm)from starsisshownin Fig.1. Notethatthetotalreddeningissmall
the newly extractedspectral data release (INES Archive Data
formostofthestars.Wemaythusassumethatasituationwith
Server).Ifthereweremultiplespectraavailable,weonlycon-
onlygreyextinctionwouldberatherexceptional.
sideredthemaximalflux.
4. SpectralEnergyDistributions
We analyse the SEDs of our sample stars in a homogeneous
andsystematicway.
4.1. Thestars photosphere:modelparameters
ThedeterminationofthemodelatmosphericalparametersT
eff
and logg, and the overall metallicity [Fe/H] is based on the
analysisofhighresolutionspectrausedinthechemicalanaly-
sesofthestars.Thosespectraarepreferentiallytakenatmax-
imum light, because of the minimal molecular veiling during
thehotterphaseinthelightcurve(Giridharetal.2000).
For most stars we use model parameters deduced from
our own spectra (e.g VanWinckel 1997; Maasetal. 2003,
Fig.1. The distribution of the total E(B −V) found for our
2005), for others we use values found in the literature. For
sample.Remarkthatmostcolourexcessesarelessthan1.0.
someofoursamplestars(IRAS05208,UYCMa,IRAS10456,
IRAS18123, IRAS18158, VVul), however, we don’t have a
spectrum nor do we find any estimates for the photospheric
parameters in the literature. Here we deduce the parame-
4.3. A first-order approximation:an opticallythin
ters on the basis of the spectral type of the star. And for
dust fit
someothers(IRAS09400,IRAS10174,GKCar,IRAS13258,
IRAS15556)welackopticalphotometry.Themodelparame- Abroad-bandSEDhaslimiteddiagnosticvalueforconstrain-
tersaregiveninTable1,Cols.7to9.Thereferenceswherewe ing the chemistry and spatial distribution of the circumstellar
foundtheparametersareshowninCol.10. dust. We apply an optically thin dust modelas a simple first-
8 S.DeRuyteretal.:Kepleriandiscsaroundpost-AGBstars:acommonphenomenon?
order approximation to fit the SEDs with the prime goal to and IRAS20056) are omitted in the figure. Like HD44179
quantifythedifferencesinSEDcharacteristicsbetweenthedif- (Cohenetal. 2004) and IRAS20056 (Menzies&Whitelock
ferentobjects.We use theopticallythindustmodeldescribed 1988; Raoetal. 2002) we suspect that we only see the pho-
bySopkaetal.(1985).Inthismodelthedustisassumedtobe tosphere of those sources throughscattered light. This means
distributedsphericallysymmetricaroundthestarfrominnerra- thatweseethesesystemsnearlyedge-on.
diusr to outer radiusr . Since we applyan opticallythin
in out
model, the geometry of the dust is not constrained as we ob-
servethewholevolumeandalldustparticlescontributetothe
observedflux.
For moredetails of the modelwe refer to DeRuyteretal.
(2005).ThefluxF isdeterminedbyfiveparameters:thenor-
ν
malizationtemperatureT ,theinnerradiusofthedustshellr ,
0 in
theouterradiusofthedustshellr ,thespectralindexpand
out
thedensityparameterm.Applyingaleastsquareminimization
thefreeparametersr ,r ,pandmaredetermined.Results
in out
aregiveninTableA.1.
4.4. SEDs
InFig 2 the SEDsof a selectionofpost-AGBstarsare given.
The other SEDs are givenin Fig. A.1. The dereddenedfluxes
areplottedtogetherwiththescaledphotosphericmodelrepre-
senting the unattenuated stellar photosphere. We plotted both Fig.3.ThedistributionoftheenergyconversionratioLIR/L∗
minimal(opentriangles) and maximal(openrectangles)pho- foundforoursample.
tometry.
HD44179andARPuparetworatherexceptionalstars,for For those stars where we have a submillimetre data point
whichweadoptedanotherstrategytodeterminetheSEDs.The (see Table C.6) the SED followsa Rayleigh-Jeansslope from
dust excess for HD44179 and ARPup starts even at shorter the 60-100µm flux point redwards. Assuming that the dust
wavelengths making it impossible to scale the model photo- emissivity at far-IR wavelengths follows a power law (Q ∼
ν
spheretotheJ filter.WethereforescaletotheGenevaGband νp),thespectralindexpasdeterminedfromtheslopebetween
datapointinourleastsquaresprocedure. the 60-100µmemission andthe 850µmflux pointisclose to
TheIRexcessduetothepresenceofcircumstellardust,is zero.Thesmallspectralindicesintherangefrom0.0to0.5for
inallcasesverysignificantwithBD+39◦4926anoticeableex- most objectsare consistentwith the presence of a component
ception for which no IRexcess was found. A generalcharac- oflargegrains(radius&0.1mm)inthecircumstellarenviron-
teristic is that the dust energy distribution peaks at very high mentofthestars.Forthosestarswherewelackasubmillimetre
temperaturesandthatthereisnoevidenceforlargeamountsof data point, the spectral index of the long wavelength slope is
cool(T ≤100K)dust:thepeakofthedustSEDsliesaround notwellconstrained.
d
10µm and in some cases even bluer. In most cases, the dust Note that the 100µm IRAS fluxes for some stars are af-
excessstartsnearthedustsublimationtemperature. fected by Galactic cirrus, so they present upper limits. In the
To determine the amount of energy reprocessed by the figuresanarrowisdrawn.Thismakesitstillmoredifficulttobe
dust grains in the circumstellar environment,we compute the sureabouttheslopefromthe60-100µmfluxpointredwards.
energy ratio LIR/L∗. We first calculate the stellar flux L∗ From all SED characteristicsmentioned,we infer thatthe
by numerically integrating the scaled Kurucz model between dustexcessin the SEDs isclearly differentfromwhatwe ex-
145nm and 850µm. This gives a good estimate of the unat- pecttoobserveinapost-AGBstarwheretheexcessrepresents
tenuated stellar flux. The energy radiated at even shorter and the expandingand cooling relic of the strong AGB mass-loss
longer wavelengths is only a negligible fraction (. 10−4) of episode(s).
the total stellar flux so we evaluate our wavelength integral
boundaries as adequate. Integrating over the IRexcess model
5. Distanceestimates
described in Section 4.3 yields L . We note that the ratio
IR
LIR/L∗(TableA.1)ishighfor78%oftheobjects(largerthan The typical luminosity of a lower mass post-AGB star is ex-
20%).Assumingthepresenceof20%greyornon-selectiveex- pectedtobebetween1000and10000L⊙.Accurateluminosi-
tinction and a mean E(B −V) of 0.5, reduces LIR/L∗ with tiesofindividualpost-AGBobjectsarestilllargelyunavailable
25%, which remains, nevertheless, still large. The absorption sincetheyaregenerallytoofartoobtainreliableparallaxes.
andthermalre-radiationofthestellarradiationbythecircum- In the few cases where reliable Hipparcos parallaxes are
stellardustisonaverageveryefficient.InFig.3thedistribution availablewehaveadirectprobeofthedistanceandhencelu-
oftheenergyconversionratiosisshown.Notethatratioslarger minosity.ResultsaregiveninTableA.2.Notethelargeerrors
than 1.0 (for HD44179, ARPup, IRAS17233, IRAS18158 ontheparallaxestranslateinalargeerrorboxforthedistance
S.DeRuyteretal.:Kepleriandiscsaroundpost-AGBstars:acommonphenomenon? 9
Fig.2.TheSEDsofaselectionofoursampleofpost-AGBstars:HD52961andHD213985areexamplesofconfirmedbinary
post-AGBstars,RVTauandIWCararegenuineRVTauristarsandIRAS17233andIRAS18123areRVTaurilikeobjectsfrom
thenewlistofT.LloydEvans.ThedereddenedfluxesaregiventogetherwiththescaledphotosphericKuruczmodelrepresenting
the unattenuatedstellar photosphere(solid line). An optically thin dust modelwas used to fit the IRexcess (dottedline). Data
foundintheliteraturetogetherwithour7bandGenevaphotometry(onlythemaxima)areplottedastriangles.Theminimaldata
points(squares)werenotusedforthedeterminationofE(B−V).Theyareshowntogiveanindicationoftheamplitudeofthe
pulsations.Whereavailable,crossesrepresentour850µmSCUBAdatapoint.Errorbarsonthe850µmSCUBAdatapointare
plottedaswell,forsomeobjectsthoughsmallerthanthesymbols.Thearrowatthe100µmfluxpointsignifiesanupperlimit.
and the luminositydeterminations.The parallaxof HD44179 seeonlyscatteredlight(Cohenetal.2004).Thesameisprob-
cannotbeusedtoinferaluminositybecauseweknowthatwe ablytrueforARPup(seeSect.4.4).
10 S.DeRuyteretal.:Kepleriandiscsaroundpost-AGBstars:acommonphenomenon?
Alcocketal. (1998) reported the discovery of RVTauri against the effectivetemperaturefor all stars and in all cases,
stars in the Large Magellanic Cloud (LMC). In light- and thereisdustatorverynearsublimationtemperature.Withtyp-
colour-curve behaviour, the classical RVTauri stars appeared icalluminosityestimates,thissublimationtemperatureedgeis
to be a direct extension of the type II Cepheids to longer pe- atadistancesmallerthanabout10AUfromthecentralsource.
riods.WeusetheP-LrelationofAlcocketal.(1998)-derived Moreover,theSEDsrevealthatthepresenceofdust,veryclose
for the LMC RVTauri stars- to derive the luminosity for the totheobject,isirrespectiveoftheeffectivetemperatureofthe
pulsatingobjectsinoursample.ForvariableswithP/2>12.6 centralstar.Notethatnoneoftheobjectsshowsevidencefora
daysweuse present-daydustymass-loss.Wethereforeinferthatpartofthe
dust must be gravitationally bound:any typical AGB outflow
M =2.54(±0.48)−(3.91(±0.36))log(P/2), velocitywouldbringthedusttocoolerregionswithinyears.
V
σ =0.35. (2) Withinour sampleof post-AGBobjectsconsidered- con-
firmedbinarypost-AGBstars,classicalRVTauristarsandnew
Notethattheintrinsicscatterislarge,implyingverysignif- RVTauri-likeobjects-thereisawiderangeinthestrengthof
icantuncertaintiesintheestimatedabsolutemagnitudesofthe the total IRexcess, but the shape of the IRexcess thus indi-
objects.InEq.2weusethefundamentalperiodP/2,definedas cates that in all systems, the circumstellar shell is not freely
the timebetweena deepand a shallowminimum.Theformal expandingbutstoredinthesystem.Wearguethereforethatthe
periodP isshowninTableA.2.Weapplythisextensionofthe same inner geometryas in the resolvedsystem HD44179ap-
period-luminosityrelationknownfortypeIICepheids,forthe plies to the whole sample: the objects seem to be surrounded
RVTauri stars from the GCVS to derive an estimate for their byaKepleriandisc.
luminosities.Results,togetherwiththebolometriccorrections From the dust modelling fit it is also clear that the outer
BCV (Besselletal.1998)used,arelistedinTableA.2.Theer- radiiarenotverylargeeither.Objectssimilartotheenigmatic
rorpropagationstartingfromtheerrorsintheP-Lrelationare extreme HR4049 are UYCMa, UMon and IRAS17233 for
giveninthetableaswell.Theobjectsareindeedluminous,as whichthedusttemperaturegradientobservedisverysmall.
canbeexpectedfrompost-AGBobjects,butthelargescatterin
theP-Lrelationpreventsmoreaccurateluminosityestimates.
Fortheotherobjects,wherewedon’thaveapulsationpe-
riod determination,we take L=5000 ± 2000L⊙. Note that
itisnotknownwhethertheP-LrelationoftheLMCstarsisdi-
rectlyapplicabletoGalacticstars.Ingeneral,theluminosities
foundare smaller than,or equalto, about5000L⊙ indicating
thatthepopulationisofratherlowinitialmass.
By comparing the integrated fluxes of the scaled Kurucz
model with the luminosity deduced from the P-L relation or
the default assumed value of L=5000L⊙, we calculate the
distanceD tothestar. Thedetermineddistancesareshownin
Table A.2. The propagated error of the poorly calibrated P-L
relation, yields very uncertain luminosities and therefore also
uncertaindistances.
Remark that the luminosities determined by the P-L re-
lation will not be appropriate to stars seen with the dusty
Fig.4.Theinnerradiiareverysmall.Thedustmustbelocated
disc nearly edge-on. Besides for the well-known example
veryclosetothestar:thedustexcessstartsfornearlyallobjects
HD44179, this is likely the case for ARPup, IRAS17233,
nearthedustsublimationtemperature.AllSEDsshowa clear
IRAS18158andIRAS20056forwhichthedistanceslistedin
near-IRexcess.
TableA.2areclearlyupperlimits.
Other indirect indications for the presence of gravitionally
6. Discussion
bounddustare:
We presented the SEDs of all stars in our sample based on
literature data supplemented with our own Geneva photome- - Despite the significant IRexcess, the total E(B −V) for
tryand-whereavailable-continuummeasurementsat850µm moststarsisverysmall.Efficientredistributionofthestel-
(Fig. 2 and Fig. A.1). Despite the differentcriteria for the se- larfluxisclearlyincontradictionwith thelackofline-of-
lectionofthethreesubsamples,itisclearthatthebroad-band sight reddening.This implies a significant grey extinction
SEDs display a high degree of uniformity.Between extremes and/oranon-sphericallysymmetricdustdistribution.Since
likeHR4049(mostcompactSEDwithasingledusttempera- theobjectsallhavearathersmalltotalextinction,thelatter
ture)andHD44179(largestLIR/L∗),theshapeoftheSEDsis ismoreplausible.AgoodindependentestimateoftheISM
verysimilarforallstars. contributiontothetotalextinctionintheline-of-sightofall
Oneofthemostremarkablefeaturesisthestartofthedust individualobjectsisstilllacking.Itmust,however,beare-
excess.InFig.4weplottheinnerradiiofthedustcomponents wardingexperimenttomeasurethosesinceitwouldhelpin