RV variable, hot post-AGB stars from the MUCHFUSS project - Classification, atmospheric parameters, formation scenarios

0.4 MB·

by  N. Reindl

Astronomy&Astrophysicsmanuscriptno.muchfuss_postagb (cid:13)cESO2016
January6,2016
RV variable, hot post-AGB stars from the MUCHFUSS project
- Classification, atmospheric parameters, formation scenarios
N.Reindl1,S.Geier2,3,4,T.Kupfer5,S.Bloemen6,
V.Schaffenroth7,3,U.Heber3,B.N.Barlow8,andR.H.Østensen9
1 InstituteforAstronomyandAstrophysics,KeplerCenterforAstroandParticlePhysics,EberhardKarlsUniversity,Sand1,72076,
Tübingen,Germany
e-mail:[email protected]
6 2 DepartmentofPhysics,UniversityofWarwick,CoventryCV47AL,UK
1 3 Dr.KarlRemeis-Observatory&ECAP,AstronomicalInstitute,Friedrich-AlexanderUniversityErlangen-Nuremberg, Sternwart-
0
str.7,D96049Bamberg,Germany
2 4 ESO,Karl-Schwarzschild-Str.2,85748GarchingbeiMünchen,Germany
n 5 DivisionofPhysics,Mathematics,andAstronomy,CaliforniaInstituteofTechnology,Pasadena,CA91125,USA
a 6 DepartmentofAstrophysics/IMAPP,RadboudUniversityNijmegen,P.O.Box9010,6500GLNijmegen,TheNetherlands
J 7 InstituteforAstro-andParticlePhysics,UniversityofInnsbruck,Technikerstr.25/8,6020Innsbruck,Austria
8 DepartmentofPhysics,HighPointUniversity,833MontlieuAvenue,HighPoint,NC27262,USA
5
9 InstituteofAstronomy,KULeuven,Celestijnenlaan200D,B-3001Heverlee,Belgium

]
R Received– Accepted–
S
. ABSTRACT
h
p
InthecourseoftheMUCHFUSSprojectwehaverecentlydiscoveredfourradialvelocity(RV)variable,hot(T ≈80000−110000K)
- eff
o post-asymptoticgiantbranch(AGB)stars. Amongthem,wefoundthefirstknownRVvariableO(He)star,theonlysecondknown
r RVvariable PG1159 closebinary candidate, aswellasthefirsttwonaked (i.e., without planetarynebula(PN))H-richpost-AGB
t starsofspectraltypeO(H)thatshowsignificantRVvariations. Wepresentanon-LTEspectralanalysisofthesestarsalongwithone
s
a furtherO(H)-typestarwhoseRVvariationswerefoundtobenotsignificant.Wealsoreportthediscoveryofanfar-infraredexcessin
[ thecaseofthePG1159star. Noneofthestarsinoursampledisplaysnebularemissionlines,whichcanbeexplainedwellinterms
ofaverylatethermalpulseevolutioninthecaseofthePG1159star. The“missing”PNearoundtheO(H)-typestarsseemstrange,

1 sincewefindthatseveralcentralstarsofPNehavemuchlongerpost-AGBtimes. Besidesthenon-ejectionofaPN,theoccurrence
v ofalatethermalpulse,orthere-accretionofthePNinthepreviouspost-AGBevolutionofferpossibleexplanationsforthosestars
4 notharbouringaPN(anymore).IncaseoftheO(He)starJ0757wespeculatethatitmighthavebeenpreviouslypartofacompactHe
7 transferringbinarysystem.Inthisscenario,themasstransfermusthavestoppedafteracertaintime,leavingbehindalowmassclose
8 companionthatcouldberesponsiblefortheextremeRVshiftof107.0±22.0km/smeasuredwithinonly31min.
0
0 Keywords. binaries:spectroscopic–stars:AGBandpost-AGB–stars:evolution–stars:atmospheres
.
1
0
61. Introduction during their post-asymptotic giant branch (AGB) evolution,
1 possiblytriggeredbyaclosecompanion(Reindletal.2014b).
:Thediscoveryandanalysisofclosebinariesthroughoutallevo- Close binary evolution is also needed to explain the formation
v
of low mass He-core white dwarfs (M < 0.5M ), because
ilutionarystagesplaysanimportantroleinvariousastronomical ⊙
X the evolutionary time-scale of the supposed low-mass single
fields. Due to the radiation of gravitational waves, very close
main-sequenceprogenitorstarssignificantlyexceedstheHubble
r(periodslessthanafewhours)binarysystemsmaymergewithin
aa Hubble time. If the system contains two sufficiently massive time. The orbital energy released during a common envelope
whitedwarfsorasubdwarfstarandasufficientlymassivewhite phase,is supposedto rapidlyexpeltheenvelopeofthepre-low
masswhitedwarfandmayterminatethegrowthoftheHe-core
dwarf, this evolutionary path can lead to a type Ia supernova
before it reaches a sufficient mass for He ignition (Paczynski
(Webbink1984),theso-calleddoubledegeneratescenario.Ifthe
1976;Webbink1984;Iben&Tutukov1986).
total mass of the two mergingstars does not exceed the Chan-
Also the formation of hot subdwarfs (sdO/B), located at the
drasekhar limit, He-dominated stars, i.e., R Coronae Borealis
(post-) extreme horizontal branch, may best be understood in
stars (RCB), extreme helium (EHe) stars, He-rich subdwarf O
terms of close binary evolution. The progenitorsof these stars
(He-sdO)starsorO(He)starsmaybeproduced(Webbink1984;
Iben&Tutukov 1984; Justhametal. 2011; Zhang&Jeffery are expected to have undergone a strong mass-loss already on
the red giant branch (RGB), which was likely triggered by the
2012b,a;Zhangetal.2014;Reindletal.2014b). Theexistence
close companion. If the mass-loss was sufficiently high, these
of planetary nebulae (PNe) around every other O(He) star and
starsleavetheRGBandigniteHeonlywhileevolvingtoorde-
the [WN] type central stars, however, questions the possibility
scendingthewhitedwarfcoolingcurve,andwillconsequently,
that all He-dominated stars have a merger origin. Those stars
reach a hotter zero-age horizontal position (Brownetal. 2001;
might have lost their H-rich envelope via enhanced mass-loss
Articlenumber,page1of12
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Lanzetal.2004). as well as their importance for our understanding of the late,
Finally,closebinariesareproposedto playacrucialroleinthe hot stages of stellar evolution we decided to subject these five
formationofasymmetricalPNe,whichmakeupthegreatmajor- stars to a more comprehensiveanalysis and discussion of their
ity(80–85%,Parkeretal.2006)ofallPNe.Theso-calledbinary evolutionarystatus.
hypothesis (DeMarcoetal. 2009) states that a companion is The paper is organized as follows. First, we describe the
needed to account for the non-spherical shapes of these PNe. observations and the discovery of RV variations (Sect.2).
Several research groups are trying to determine the frequency Then, we determine the parameters of the stars in Sect.3. In
of binary central stars of PNe (CSPNe) and the properties of Sect.4 follows a search for a PN and an infrared excess. The
these systems. Most of the known close binary CSPNe were evolutionarystatusofthestarsisdiscussedinSect.5andfinally
detectedviaperiodicfluxvariabilityduetoeclipses,irradiation, weconcludeinSect.6.
and/or ellipsoidal deformation (Bond 2000; Miszalskietal.
2009). This technique is, however, biased against binaries
with periods longer than about two weeks, against binaries
with low inclination angles, and against companions with
small radii (DeMarcoetal. 2008). Large radial velocity (RV)
surveysofCSPNearetelescope-timeexpensiveandnotalways
straightforward, because most of the strongest photospheric
linesareblendedwithnebularemissionlinesandtheextraction
of the pure stellar spectrum may often not be successful in the
caseofverycompactnebulae.
Fortunately, several hundreds of hot subluminous stars, which
generally do not display nebular lines, were observed in the
courseoftheSloanDigitalSkySurvey(SDSS).BecauseSDSS
spectra are co-addedfrom at least three individualintegrations
(with a typical exposure time of 15 min), they offer the ideal
basis for the search for RV variations. The MUCHFUSS
(Massive Unseen Companions to Hot Faint Underluminous
Stars from SDSS) survey made use of that to search for hot
subdwarf stars with massive compact companions (Geieretal.
2011). For that purpose, hotsubdwarfswere selected from the
SDSSDR7bycolor(i.e.,theywererequiredtobepointsources
withu−g < 0.4andg−r < 0.1)andvisualinspectionoftheir
spectra in order to distinguish them from hot white dwarfs or
extragalactic sources. Those stars, whose SDSS sub-spectra
Fig.1. Highest RV shiftbetween individual spectraplottedagainst
revealed high RV variations, were selected as candidates for
time difference between the corresponding observing epochs for the
follow-upspectroscopyto derive the RV curvesand the binary
O(He) star J0757 (red, triangle), the PG1159 star J1556 (black, in-
mass functions of the systems. In this way, more than 100 vertedtriangle),andthethreeO(H)starsJ1000,J2046,andJ0935(blue,
RV-variablesubdwarfshavebeenfound(Geieretal.2015). filledcircles).Thenumbersinthebracketscorrespondtothenumberof
Hot post-AGB stars directly overlap with hot subdwarfs of epochseachstarwasobserved. NotethattheRVshiftsofJ2046were
O-type(sdO)inthecolor-colorplane. Ifhotpost-AGBstarsare foundtobenotsignificant.
not surroundedby a compactnebula and/or do not suffer from
still ongoing strong mass loss (i.e., weak emission line stars,
Wolf Rayet type central stars, whose spectra show or are even
2. Observationsanddetectionofradialvelocity
dominated by emission lines), their spectra may look indistin-
variations
guishablefromthelessluminoussdOstars. Aboutthree-quarter
of all hot post-AGBstars display a normalH-rich composition Individualobservations and RV measurementsof the five stars
andincasetheirspectraaredominatedbyabsorptionlines,they are listed in Geieretal. (2015). The RVs were measured by
canbeclassifedasO(H)orhgO(H)stars(thelattercaseapplies fittinga setofmathematicalfunctions(Gaussians, Lorentzians,
to high gravity stars which are positioned on the white dwarf andpolynomials)tothespectrallinesusingtheFITSB2routine
cooling track, Mendez 1991). Hot, H-deficient stars, which (Napiwotzkietal. 2004). The fraction of false detections pro-
show predominatly absorption lines and lie in the post-AGB duced by random fluctuations and the significance of the mea-
region, are either classified as O(He) stars (typically showing suredRVvariationswerecalculatedasdescribedinMaxtedetal.
more than 90% He, by mass), or PG1159stars, whose spectra (2001).
showbesidesHealsostronglinesofcarbon.Byeye,thespectra InthespectraofJ0757,J0935,J1000,andJ1556wediscovered
of O(H), O(He), and PG1159 stars may look very similar to significantRV shifts, while the RV variationsin the five SDSS
those of the less luminous sdO, He-sdO, and C-strong lined sub-spectra of J2046 were found to be most likely caused by
He-sdO stars, respectively. Thus, the sample of Geieretal. randomfluctuations(false-detectionprobabilityp islargerthan
(2015) also contains a small fraction of hot post-AGB candi- 0.01%). InFig.1weshowtheirhighestRVshiftbetweenindi-
dates, namely SDSSJ075732.18+184329.3 (here after J0757), vidualspectraplottedagainsttimedifferencebetweenthecorre-
SDSSJ093521.39+482432.4 (J0935), SDSSJ100019.98- spondingobservingepochs. Most impressive, we discovereda
003413.3 (J1000), SDSSJ155610.40+254640.3 (J1556), and maximumRVshiftof107.0±22.0km/swithinonly31mininthe
SDSSJ204623.12-065926.8(J2046).Previousanalysesofthese sixSDSSsub-spectra(takenintwoconsecutivenights)ofJ0757.
stars indicated very high effective temperatures (Werneretal. J1000wasobserved16timesinthecourseoftheSDSSandre-
2014;Geieretal.2015).Becauseoftherarenessofsuchobjects vealsamaximumRVshiftof135.0±28.0km/s.Inadditiontothe
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Reindletal.:RVvariable,hotpost-AGBstars
nineSDSSobservationsofJ1556,wehaveobtainedonemedium
resolutionspectrumwithTWINattheCalarAlto3.5mtelescope J0935 J1000 J2046
andfoundamaximumRVshiftof116.0±21.0km/s.ForJ0935 Teff =92649 Teff =108194 Teff =93936
logg= 5.50 logg= 5.92 logg= 5.51
second epoch spectroscopy was obtained with WHT/ISIS. The
X =-0.54 X =-0.49 X =-0.46
maximumRVshiftis38.0±7.5km/s. He He He
To verify the close binary nature of the stars, we tried to de- HeII λ 3887Ao, HI 8
rive orbitalperiodsand RV semiamplitudesof the objectswith
morethantenepochsasdescribedinGeieretal.(2015). Unfor-
tunately, in both cases the RV datasets were insufficient to put HeII λ 3968Ao, Hε
anymeaningfulconstraintsontheorbitalparameters. 10%
HeII λ 4100Ao, Hδ
3. Stellarparametersanddistances
In this section we first perform a non-LTE spectral analysis to
derivetheeffectivetemperatures,surfacegravitiesandthechem- HeII λ 4200Ao
ical compositionof the five stars (Sect.3.1). These parameters
are then used in Sect.3.2 to derive their masses, luminosities,
anddistances. m. flux HeII λ 4340Ao, Hγ
or
n
3.1.Spectralanalysis HeII λ 4542Ao
Quantitativespectralanalysisisthekeytoderivethesurfacepa-
rametersofthestarsandtounderstandtheirevolutionarystatus. HeII λ 4686Ao
Our spectral analysis was based on the SDSS observations of
thestars. Thesinglespectrahavebeencorrectedfortheirorbital
motion and coadded. For the model calculationswe employed
the Tübingen non-LTE model-atmosphere package (TMAP1, HeII λ 4860Ao, Hβ
Werneretal. 2003; Rauch&Deetjen 2003) which allows to
compute plane-parallel, non-LTE, fully metal-line blanketed
HeII λ 5412
model atmospheres in radiative and hydrostatic equilibrium.
Model atoms were taken from the Tübingen model atom
database TMAD2. To calculate synthetic line profiles, we used
Stark line-broadening tables provided by Barnardetal. (1969) HeII λ 6560Ao, Hα
for HeIλλ4026,4388,4471,4921Å, Barnardetal. (1974)
for HeIλ4471Å, and Griem (1974) for all other Hei lines.
For Heii we used the tables provided by Schöning&Butler
(1989),forHitablesprovidedbyTremblay&Bergeron(2009), -30 0 30 -30 0 30 -30 0 30
and for CIV tables provided by Dimitrijevic´etal. (1991) and ∆λ / Ao
Dimitrijevic´&Sahal-Brechot(1992). Fig.2. BalmerandHeiilinesusedtoderiveT , logg, andtheHe
eff
abundance(giveninlogarithmicmassfractions)ofthethreeO(H)-type
stars. Theobservationsareshowningrey,thebestfitmodelsinblack
(redinonlineversion).Theverticalbarindicates10%ofthecontinuum
3.1.1. H-richstars
flux.
ThespectraofJ0935,J1000,andJ2046aredominatedbyhydro-
genBalmerlines. SpectallinesofHeiiareofmoderatestrength, allBalmerandHeiilinesdetectedintheSDSSspectraofthese
but no Hei lines can be detected. However, the signalto noise
of the observations is too poor (S/N ≈ 20-30) to exclude their stars. HeIIλ3968Å/Hǫ andHeIIλ3887Å/HIλ3889Å were
notfittedincaseofJ2046duetothepoorqualityofthebluepart
presencefromtheoutset. Toderiveeffectivetemperatures,sur-
ofthespectra. OurbestfitsareshownFig. 2andthe resultsof
face gravities, and He abundancesof these stars, we calculated
ouranalysisaresummarizedinTable1.
a model grid spanning from T = 40000 − 140000K (step
eff The spectra were previously analysed by Geieretal. (2015)
size 1250K)andlogg = 4.75−6.5(stepsize 0.25dex)forsix
basedonamodelgridcalculatedbyStroeeretal.(2007)andwe
differentHe abundances(He= 0.1,0.2,0.3,0.4,0.5,and0.6,by
findthattheygenerallyunderestimatedT by5to15%.Inaddi-
mass). Modelsabovethe Eddingtonlimit (i.e, T > 90000K eff
eff tion,ourresultsforJ1000lieoutsidethegridusedbyGeieretal.
for logg=4.75, T > 100000K for logg=5.00, and T >
eff eff (2015). WhiletheeffectsofthesmallerstepsizeinT (1250K
120000K for logg=5.25) were not calculated. For Heii 20 eff
insteadof5000K)ofourgrid,arerelativelysmall(T isusu-
levels were considered in non-LTE, and for Hei 29 levels if eff
ally underestimated by 1% when using a finer grid), the main
T < 100000K, and 5 levels if T ≥ 100000K. For Hi 15
eff eff influence can be attributed in the larger extent towards higher
levelswere consideredin non-LTE.The parameterfit was then
T (140000Kinsteadof100000K),aswellasintheuseofthe
performedbymeansofaχ2 minimizationtechniquewithSPAS eff
Starkline-broadeningtablesprovidedbyTremblay&Bergeron
(Spectrum Plotting and Analysing Suite, Hirsch 2009), which
(2009)insteadofthoseprovidedbyLemke(1997).
is based on the FITSB2 routine (Napiwotzki 1999). We fitted
Itisimportanttonote,thattheerrorsgiveninTable1corre-
1 http://astro.uni-tuebingen.de/TMAP spondtoformalfittingerrorsonly.Theneglectofmetalopacities
2 http://astro.uni-tuebingen.de/TMAD inthemodel-atmospherecalculationscanleadtosignificantsys-
Articlenumber,page3of12
Draft Version: January 6, 2016.
ofNvandOv. Constraintsontheeffectivetemperaturecanal-
ready be put from the absence of Hei lines (T ≥ 70000K)
eff
and the CIVλλ5801,5812Å lines, which appear in emission
4 vZ1128 (Teff ≥ 90000K). In addition, the fact that the Nv lines at
M 4604Åand4620Åappearinabsorptionallowsustoputanup-
LSIV-121 0.534 perlimitofTeff ≤115000K(Rauchetal.1994).
ROB162 ForthespectralanalysisofJ1556wefirstlycalculatedamodel
M grid including only the opacities of H, He, and C. For H and
5 0.831 Henon-LTElevelswereusedasdescribedinSect.3.1.1. TheC
BD+182647 modelatomincludestheionizationstagesiii-vand30,54,and1
2) GD605 non-LTElevelswere consideredfor CIII, CIV, and CV, respec-
m/s M tively. Our model grid spans Teff = 90000− 120000K and
gg (/c6 M 0.8571M0 M d(lobugyegtmo=atshs5e).0owc−ceur6er.r7ceonancnsediodCfernaeubdmu,nTedrhiacenacgleriinsdsitniasbt,hihleiotriweasne.vgTeerh,oenffio0tt.t3icn3ogm−opf0l.te0ht1ee
lo 0.618 0.5 0.552 A39 sthpeecptarruammewtearsstohfenthceamrroieddelohuatvienbaenenitecrhaatnivgeedprsouccecsess,siivnewlyhuicnh-
A74 IC1295 tilagoodagreementwiththeobservationwasachieved.Thebest
Sh2-68
7 PSuhW2-e2116 fitwasfoundforTeff = 100000K,logg = 5.33,andC = 0.15.
A61 NotracesofHcouldbedetected.Unfortunately,wedidnotsuc-
ceedincalculatingnumericallystablemodelsthatalsoincluded
Sh2-188 HaWe5 the opacities of N and O at such low surface gravities, which
precludedtheabundancesdeterminationofthoseelements.
The C abundance lies well above the limit set by Werneretal.
HDW4
8 (2014) to distinguish between the PG1159 stars and the He-
dominatedDOwhitedwarfsandO(He)stars, whichonlyshow
up to C = 0.03. Therefore, we classify J155610.40+254640.3
5.2 5.0 4.8 4.6
asa PG1159star. Itis worthwhileto note, that thisis also the
log (T /K)
eff PG1159starwiththelowestsurfacegravityknownsofar. Inter-
Fig. 3. The locations of our three O(H) stars (filled, blue circles estingly,itsCabundanceisratherlowcomparedtothemajority
with error bars) are compared with H-burning post-AGB evolution- of the luminous PG1159 stars (that is they have logg < 7.0)
ary tracks (blue lines, labeled with stellar masses, MillerBertolami whichtypicallyshowC = 0.5(seeFig.4inReindletal.2014a
2015).AlsoshownarethelocationsofnakedO(H)-typestars(filledcir- forcomparison).
cles, Chayeretal. 2015; Fontaineetal. 2008; Bauer&Husfeld 1995;
Heber&Hunger 1987; Heber&Kudritzki 1986), as well asthe loca-
tions of H-richCSPNe(open circles, Ziegler 2012; Herald&Bianchi
3.2.Masses,luminosities,anddistances
2011;Gianninasetal.2010;Rauchetal.2007;Napiwotzki1999).
Toderivethemassesandluminositiesofthefivestars,wecom-
pared their position in the log T – logg plane with different
tematicerrorsintheinferredatmosphericparametersofveryhot eff
evolutionary tracks. Figure3 shows the location of the three
stars. Latouretal.(2010,2015)showedthatforsdOstarshotter
O(H) stars compared with H-burning post-AGB evolutionary
thanabout70000K,T andloggaretypicallyunderestimated
eff tracks(MillerBertolami2015). Becausethesestarsmostlikely
by5to10%and0.15dex,respectively.
belong to the Galactic halo (see below), we used tracks with
However, even considering these effects, all stars lie clearly in
Z = 0.001. We derived M = 0.54M for all O(H)-type stars,
thepost-AGBregionofthelogT −loggdiagram,shortlybe- ⊙
eff whichagreeswellwithintheerrorlimitswiththemeanmassof
fore they will reach their maximum T , to then cool down as
eff 0.551±0.005M ofH-richhalowhitedwarfs(Kalirai2012).
whitedwarfs(Fig.3).TheirHeabundancesareslightlysuperso- ⊙
IntheupperpanelofFig.5thelocationsofthetwoH-deficient
lar(1.2−1.4×solar,solarabundanceaccordingtoAsplundetal.
stars, J0757andJ1556, arecomparedwitha late thermalpulse
2009). InordertodistinguishthesestarsfromsdOstars,which
(LTP) evolutionary track of MillerBertolami&Althaus
are usuallyassociated with AGB-manquéstars, whichare stars
(2007) and very LTP (VLTP) post-AGB tracks of
thatfailtoevolvethroughtheAGB,weemploythespectralclas-
MillerBertolami&Althaus (2006). We derived M = 0.57M
sification scheme of Mendez(1991) andclassify them as O(H) ⊙
for J1556, and M = 0.53M for J0757. For J0757 we ad-
typestars. ⊙
ditionally derived the mass and luminosity using post-double
He white dwarf merger evolutionarytracks of Zhang&Jeffery
3.1.2. H-deficientstars (2012a,b) and found M = 0.73M⊙ (lower panel of Fig.5).
Within the error limits, J0757 could also be a merger of a
J0757 has been analysed by Werneretal. (2014), who derived CO white dwarf and a He white dwarf, however, to this date
T = 80000K,logg = 5.00andaCabundanceofC = 0.006 there are no post-CO+He white dwarf merger tracks available
eff
(by mass). Thus, the star belongsto the class of C-rich O(He) that reach to the region of O(He) type stars. We stress that a
stars. reliablespectroscopicmassdeterminationforJ0757wouldonly
TheClinesinthespectrumofJ1556arestrongerandmorenu- be possible if the evolutionary history of this object would be
merous compared to the ones found in the spectrum of J0757. known.Therefore,theparameterlistedforJ0757inTable1,are
In Fig. 4 the co-addedSDSS spectrumof J1556is shown. Be- onlyvalidifacertainscenarioisassumed.
sidesphotosphericlinesofHeiiandCiv,wecouldidentifylines Based on the derived masses, we calculated the distances of
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Reindletal.:RVvariable,hotpost-AGBstars
HeII
1.2
CaIIK CIV NV CIV OV CIV
i.s.
1.0
0.8
x
u
m. fl 4000 4200 4400 4600 4800 5000 5200 5400 5600 5800 6000
or
n
HeII
1.2
CIV CIV
1.0
0.8
6200 6400 6600 6800 7000 7200 7400 7600 7800 8000 8200 8400
λ / Ao
Fig.4. CoaddedandnormalizedSDSSspectrum(grey)ofthePG1159starSDSSJ155610.40+254640.3. Thebestfitmodel(black,redinonline
version)includingHeandClinesissuperimposedandthepositionsofidentifiedspectrallinesareindicated.
Table1.StellarParameter.Heabundancesaregiveninlogarithmicmassfractions.
name Spectral T logg X M log(L/L ) d z
eff He ⊙
type [K] [M ] [kpc] [kpc]
⊙
J0935 O(H) 92649±11358 5.50±0.12 −0.46±0.09 0.54+0.05 3.5+0.4 16.7+2.7 12.1+1.8
−0.01 −0.2 −2.2 −1.6
J1000 O(H) 108194± 8356 5.92±0.09 −0.49±0.06 0.54+0.02 3.4+0.2 8.0+0.9 5.2+0.6
−0.01 −0.1 −0.8 −0.5
J2046 O(H) 93936±21166 5.51±0.32 −0.54±0.16 0.54+0.08 3.5+0.5 10.8+4.8 −5.2+1.6
−0.01 −0.2 −3.3 −2.3
J1556 PG1159 100000 +15000 5.30±0.30 −0.07±0.03 0.57+0.13 3.8+0.5 16.3+6.7 12.3+5.1
−10000 −0.05 −0.4 −4.8 −3.6
J0757(a) O(He) 80000±20000 5.00±0.30 −0.01±0.01 0.53+0.21(a) 3.7+0.6(b) 26.8+11.1(b) 10.3+4.3(b)
−0.05 −0.3 −7.8 −3.0
0.73+0.27(c) 3.9+1.0(c) 31.5+13.0(c) 12.1+5.0(c)
−0.08 −0.5 −9.2 −3.5
Notes.
(a)AtmosphericparameterstekenfromWerneretal.(2014).(b)Basedon(V)LTPtracksofMillerBertolami&Althaus(2007,2006).(c)Basedon
doubleHe-whitedwarfmergertracksofZhang&Jeffery(2012a,b).
the starsbyusingthe fluxcalibrationof Heberetal. (1984) for 4. Searchfornebularlinesandinfraredexcess
λ =5454Å,
eff
d[pc]=7.11×104· qHν·M×100.4mv0−logg, AllofourfivestarsarelocatedintheregionofCSPNeinthelog
T – logg diagram, which suggests that they mightbe central
eff
withm =m −2.175c,c =1.47E ,andtheEddingtonflux stars as well. The large distances of the stars, however, imply
Vo V B−V
H atλ ofthebest-fitmodelatmospheresofeachstar. Values rathersmallangulardiametersoftheorderofonearcsecoreven
ν eff
forE wereobtainedfromGalacticdustextinctionmapsfrom less(assuminga typicalexpansionvelocityof20km/sanddis-
B−V
Schlafly&Finkbeiner(2011)andvaluesform weretakenfrom tancesandpost-AGBtimesfromTable3). Thus,itisunlikelyto
V
Geieretal. (2015). The distances range from 8.0 to 31.5kpc detectPNearoundthesestarsviaground-basedimaging.There-
anddeviateslightlyfromthevalueslistedinGeieretal.(2015). fore,itismuchmorepromisingtosearchforprominentnebular
Thisisbecausetheyuseddifferentmodelfluxesandassumeda emission lines in the spectra of these stars, i.e., the [Oiii] λλ
canonical mass of 0.6M for all post-AGB stars for their dis- 4959, 5007 Å emission lines, which are typically the strongest
⊙
tancedetermination. We foundthatallfivestarsarelocatedfar lines for a medium to high-excitationPN. Because of the long
aboveorbelowtheGalacticplane(Table1). Sincethethickdisk exposuretimesandthemuchbetterbackgroundsubtractionthat
onlydominatesintheregion1kpc.z.4kpc(Kordopatisetal. canbeachievedinslitorfibrespectroscopycomparedtonarrow-
2011), weconcludethatallstarsfromoursampleprobablybe- band imaging or slitless objective prism imaging spectroscopy,
longtotheGalactichalo. SDSSspectraallowthedetectionofextremelyfaintPN(downto
Articlenumber,page5of12
Draft Version: January 6, 2016.
29−30magarcsec−1,Yuan&Liu2013).However,wecouldnot theIRexcessisasuperpositionofthephotosphericemissionof
detectanyhintfornebularlinesinthespectraofourfivestars. the CSPN and a red companionas well as the dust continuum.
Thedetectionofaninfrared(IR)excesscanputconstraints Thus,itislikelythatmorethanonesourcecontributestotheIR
on the nature of a possible companion, or the presence of a excessofJ1556aswell.
circumstellar disk. To search for possible IR excess emission Finally, we note that the IR excess could also have an extra-
around our five stars, we cross matched their positions with galacticorigin. We correctedtheWISEcolorsofJ1556forthe
the UKIDSS (UKIRT InfraredDeep Sky Survey) DR9 catalog stellarfluxcontributionandobtainedW1 =18.40,W2= 16.54,
(Lawrenceetal. 2013) and the WISE (Wide-field InfraredSur- W3=12.23,andW4= 8.93. ThisleadstoW1−W2=1.86and
veyExplorer)catalog(Cutri&etal.2014). Weappliedasearch W2−W3=4.31,whichcorrespondstotheregionofluminousIR
radius of 2 arcsec. For the O(H) stars J0935 and J2046 no IR galaxiesorSbcstar-burstgalaxiesinthe(W1−W2),(W2−W3)
colors were found, but for J1000 and the PG1159 star J1556 diagram(Fig.12inWrightetal.2010).TheWISE-excessmight
we found J and H entries. For the O(He) star J0757 we found thereforebecausedbyachancealignmentofsuchabackground
a J value. The PG1159 star J1556 is the only object with galaxywithJ1556. However,theprobabilityforsuchanalign-
entries in the WISE catalog. In addition, GALEX FUV and ment of two very peculiar classes of objects seems to be very
NUV (Bianchietal. 2014) and SDSS magnitudes (Ahnetal. unlikely.
2012) were obtained for all objects. Magnitudes and redden-
ingofthefivestarsaresummarizedinTable2. SDSS,UKIDSS,
5. Discussion
andWISEmagnitudeswereconvertedintofluxesasoutlinedin
Verbeeketal.(2014).FortheGALEXmagnitudesconversions3, The high RV variations- partly on short time scales - detected
weused in four of our stars are a strong hint that they are part of close
binary systems. Unlike hot subdwarfs, which are expected to
fFUV[erg s−1cm−2Å−1]=1.40×10−1510−0.4(FUV−18.82) have lost almost their entire envelope already on the RGB via
stableRochelobeoverflowand/orcommonenvelopeevolution,
f [erg s−1cm−2Å−1]=2.06×10−1610−0.4(NUV−20.08). themass-lossontheRGBofpost-AGBclosebinariesmusthave
NUV
beenweaker.Thisisbecauseonlystarswithansufficientlythick
Then, we corrected the TMAP model flux of our best fit for
envelopeareabletomaintainH-shellburningandtoascendthe
each star for interstellar reddening using the reddening law of
AGB.
Fitzpatrick(1999). Again,weusedthevaluesforE fromthe
B−V Thepost-AGBphase(i.e.,theevolutionfromtheAGBtowards
Galactic dustextinctionmapsof Schlafly&Finkbeiner(2011).
the maximum T ) is very short-lived. The evolutionary time
Thereafter,wenormalizedthereddenedmodelfluxtotheSDSS eff
scales depend strongly on the stellar mass, so that a 0.53M
g magnitude and checked if the other color measurements are ⊙
star needs about 80000yr to reach its maximum T , while a
within3σinagreementwiththemodelfluxes. eff
0.83M starsreachesitsmaximumT withinlessthan1000yrs
A perfectagreementwith the color measurementsand the red- ⊙ eff
(MillerBertolami2015). Itisexpectedthateachpost-AGBstar
dened model flux was found for the three O(H) stars and the
eventuallybecomesaCSPN,ifthestarisabletoionizethepre-
O(He)star,i.e. noevidenceforanexcesswasfoundinthenear
viously ejected outer layers before the expandingenvelope be-
IR. The PG1159 star J1556, however, displays a clear excess
comestootenuoustoappearasaPN.
inallWISE colors(Fig.6). DuetothelackofWISEmeasure-
AsmentionedaboveallofourfivestarsarelocatedinthelogT
mentsasearchforanfar-IRexcesswasnotposiblefortheother eff
–loggdiagramjustamongstthe CSPNe. However,nonebular
fourstars. Inthe(W2−W3),(W3−W4)diagram,theWISEcol-
linescanbedetectedintheirspectra. Wewillthereforediscuss
orsofJ1556agreewiththebulkofPNe(Kronbergeretal.2014).
in the following the evolutionary status of these objects taking
However,asmentionedabove,nonebularemissionlinesarevis-
into account the absence of a PN around these stars as well as
ibleintheSDSSspectrum(totalexposuretime≈2.25h).
theirpossibleclosebinarynature.
AnIRexcessthatstronglyshowsupintheSpitzerIRACbands
(Xu&Jura 2012), was found around some warm (i.e., T ≤
eff
25000K) white dwarfs. This IR excess is interpreted as orig- 5.1.ThePG1159starJ1556
inating from a small hot disk due to the infall and destruc-
Thephenomenonofthe“missing”PNiswellknownfortheH-
tion of single asteroids that come within the star’s Roche limit
(Gänsickeetal. 2006). However, we excludethe presence of a deficientPG1159andO(He)stars. Only15ofthe47currently
known PG1159 stars are confirmed CSPNe, which can be un-
debrisdiskinthecaseofJ1556,becauseatsuchhighT ,solids
eff
derstoodwithregardtotheirevolutionaryhistory.PG1159stars
wouldbesublimatedintogaseousdisks(vonHippeletal.2007).
Inadditionthedouble-peakedCaiitriplet,whichisthehallmark arebelievedtobetheresultofa(V)LTPthatoccurseitherdur-
ingthebluewardexcursionofthepost-AGBstar(LTP),ordur-
of a gaseous, rotating disk (Youngetal. 1981; Horne&Marsh
ingits early white dwarfcoolingphase(VLTP). Therelease of
1986),cannotbedetected.
Cold dust disks, which are located much farther from the star nuclear energy by the flashing helium shell forces the already
(≈ 50AU compared to ≤ 0.01AU for the debris disks) have verycompactstar to expandbackto giantdimensions- the so-
called born-againscenario (Fujimoto 1977; Schönberner1979;
been announcedfor a number of hot white dwarfs and CSPNe
and are likely formed by mass loss from the stars during their Ibenetal.1983;Althausetal.2005). Theabsenceofhydrogen
intheatmosphereofJ1556combinedwiththeappearanceofNv
AGBphase(Claytonetal.2014;Chuetal.2011). Thesedisks,
however, were found to be relatively cool (≈ 100K compared linesintheSDSSspectrumsuggestsaVLTP(Werner&Herwig
≈ 1000K for the debrisdisks) and should show up only in the 2006). Since for He-burners a three times longer evolutionary
W4band. TherelativelyflatIRexcessinJ1556cannotbefitted time scale is predicted (Ibenetal. 1983) and given that J1556
is going through the post-AGB phase for the second time, the
with a single blackbodyand looksverysimilar like in the case
“missing”PNcanbeunderstoodverywell.
ofthe CSPN K1-22. Chuetal. (2011) showedthatforK1-22,
Should the RV variations in J1556 be confirmed to orig-
3 http://asd.gsfc.nasa.gov/archive/galex/FAQ/counts_background.html inate from a close companion, then J1556 would be just
Articlenumber,page6of12
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Reindletal.:RVvariable,hotpost-AGBstars
the second known PG1159 close binary system besides cretion. AMCVn stars are He transferringultracompactbinary
SDSSJ212531.92−010745.9 (Nageletal. 2006; Schuhetal. systems with orbitalperiod below 1h. The mass ratio of these
2009;Shimanskyetal.2015). Thecompanioninthelattersys- systems is expected to be sufficently low and therefore, stable
tem is a low mass main sequence star as it is obvious from masstransferoccures.Thesesystempreventthemergerandwith
the reflection effect in the light curve and from the Hydrogen decreasingaccretionrates,theyevolvetowardslongerorbitalpe-
Balmerserieswhichappearsinemissionduetotheradiationof riods Yungelsonetal. 2002; Marshetal. 2004; Nelemansetal.
the PG1159 star (Nageletal. 2006). However, in the spectra 2010).TheaccretoresofthesesytemsmayexperiencestableHe-
ofJ1556,wecannotfindevidenceforsuchemissionlines. The burning (Piersantietal. 2014, 2015). In case the progenitorof
IR excess of J1556 showes up only in the WISE bands, which J0757wasaHeWD,whichignitedHe-burning,thepostaccre-
suggestthat a possible companionto J1556 mightbe even less tionevolutionmightlooksimilarasinthecaseofthepost-double
massive. We stress thatSDSSJ212531.92−010745.9is at least He white dwarf merger models of Zhang&Jeffery (2012a,b)
twoordersofmagnitudelessluminouscomparedtoJ1556,and and, thus, would imply that J0757 must have evolved through
thusitismuchmoredifficulttodetectalowmassmainsequence the regionof the centralHe burningHe-sdO stars (lower panel
starforthelatter. ofFig.5).
Amongst the H-deficient Wolf Rayet type central stars, which Quiteinterestingly,wefindthatthegreatmajority(upto84%)of
are believed to be the progenitors of the PG1159 stars theRV variableHe-sdOstarsdiscoveredbyGeieretal.(2015),
(Werner&Herwig2006), onlythreeclosebinarysystemshave belong to the class of C-rich He-dominated objects5 just like
beendiscoveredsofar(Hajduketal.2010;Manicketal.2015). J0757. Also note that the irregular RV variations detected by
Thisleadstoaknownclosebinaryfractionofabout5%amongst Geieretal.(2015)insomeoftheHe-sdOstars(e.g.,theC-rich
theseC-dominatedobjects4,whichissignificantlylesscompared He-sdO SDSSJ232757.46+483755.2),might even support this
totheoverallclosebinaryfractionofCSPNe(≈15−20%,Bond scenario,ifweassumethattheyarecausedbyastillpresentac-
2000;Miszalskietal.2009). Thissuggests,thatbinaryinterac- cretionstream(Schwarzetal.2010).
tion may not play a fundamentalrole in forming C-dominated
stars. Definitivestatementscan, however,onlybemade aftera
systematicsearchofclosebinariesamongstthesestars. 5.3.H-richstars
J1000 and J0935 are the first RV variable naked O(H) stars
5.2.TheO(He)starJ0757 ever discovered. So far, only five other naked O(H) stars are
known: BD+182647(Bauer&Husfeld1995), ROB162 in the
J0757 is the first RV variable O(He) star ever discovered. globular cluster NGC6397 (Heber&Kudritzki 1986), vZ1128
DeMarcoetal. (2015) recently discovered the O(He)-type intheglobularclusterM3(Chayeretal.2015),GD605,which
CSPNofPa5(PNG076.3+14.1)tohaveaconsistentphotomet- isalso consideredashalostar (Fontaineetal. 2008), aswellas
ric periodof1.12din twelveindividualKeplerquarters. How- the metal poor O(H) star LSIV-121 (Heber&Hunger 1987).
ever, they failed to detect RV shifts larger than 5km/s. They The fact that no PN is present around these stars is rather un-
argue,thatthe photometricvariabilitymaybe explainedby the expected, since in our canonical understanding each star that
influenceofanorbitingplanetormagneticactivity,butmostlik- evolvesthroughthe AGB is expectedto producea PN. A sim-
leybyaevolvedcompanioninanearly-pole-onorbit.Giventhat pleexplanationfortheir“missing”PNecouldbethattheirpost-
there are currently only ten O(He) stars known, the confirma- AGB times are longer than the time it took for the PNe to dis-
tion of close binariesto J0757and Pa5 would alreadyindicate appearintotheinterstellarmedium.Thispossibilityis,however,
a binary fraction of 20%. This may suggest, that close binary putinto doubt, when we have a look in Fig.3, where we show
evolution is, at least for some of the He-dominatedobjects, an in addition to the eight known naked O(H) stars also the loca-
importantformationchannel. tions of H-rich CSPNe found in the literature. We derived the
AlsoamongstHe-dominatedstars,aPNseemstobetheexcep- post-AGBtimes fromthe post-AGBtracksof MillerBertolami
tionratherthantherule. Inaddition,itappearsthatthepresence (2015) for the seven naked O(H) stars as well as the H-rich
ofaPNisrestrictedtoN-richO(He)starsand[WN]-typeCSPNe CSPNe. As it can be seen fromTable 3, we foundthat - com-
only, which suggests that these objects are formed differently pared to the eight naked O(H) stars - there are several CSPNe
than C-rich or C+N-rich He-dominated objects (Reindletal. withapparentlymuchlongerpost-AGBtimes6.
2014b). Thepossibilitythatthelattertwosubclassesareformed On that basis, it is obscure why those stars still harbor a PNe,
viathemergeroftwowhitedwarfsnaturallyexplainsthemiss- but not the seven naked O(H) stars and it raises the question:
ing PN, because of the very long predicted post-merger times Doeseverypost-AGBstar,andinparticularalsoeveryclosebi-
ofthe stars. Though,theconfirmationofa close companionto narypost-AGBsystem,necessarilyproduceaPNe? Thiswould
J0757 would question the merger scenario as triple stars sys- beparticularlyimportanttounderstandinviewoftheproposed
tems are arranged hierarchically. Moreover, it is questionable role of close binary post-AGB stars in the formation of asym-
if it is possible that unstable mass transfer could stop before metrical PNe. We stress that if J1000 and J0935 indeed turn
the merger of two white dwarfs is complete, and thus leaving outtobeclosebinarysystems,thiswouldleadtoaclosebinary
behind a very low mass close companion(Justhametal. 2010;
Han&Webbink1999).
5 ClassificationbasedonthepresenceofClines,butabsenceofNlines
We consider therefore yet another possibility to explain both,
intheSDSSspectra.TheremainingRVvariableHe-sdOstarseitherdo
the “missing”PN aswellasthe extrememaximumRV shiftof
notshowanymetallines(8%)orbelongtothegroupofC+N-richHe-
107.0± 22.0km/s within only 31min (which is already point-
sdOstars(8−12%).
ingtowardaveryclosebinarysystem),namelythatJ0757could 6 NotethatforthetwoDAwhitedwarfCSPNeHaWe5andHDW4,
have evolved from an AMCVn type system which stopped ac- Napiwotzki(1999)evenfoundpost-AGBtimesofseveralmillionyears,
which,however, stronglycontradictedtheveryshortkinematicalages
4 There are currently about 60 Wolf Rayet type central stars known ofbothPNe(lessthan4kyr).Napiwotzki(1999)suggestedthatPNeof
(H.Todt,priv.comm.). thesestarsmightactuallybeejectednovashells.
Articlenumber,page7of12
Draft Version: January 6, 2016.
Table3.Post-AGBtimesasderivedfromH-burningpostAGBtracksof musthavesufferedaLTP(Schönberner2008). Thisstarwasac-
MillerBertolami(2015)oftheeightnakedO(H)starsandsomeCSPNe
tually observed evolving back to the AGB were the H fraction
with very long post-AGB times. For objects that are not covered by
initsatmospheregotdilutedsignificantly(from0.9to0.01,by
thesetracks,wegivelowerlimits.
number,Jeffery&Schönberner2006).
Finally, it is interesting to note that there are also some F or G
name tpost−AGB typesupergiantswhichhavealreadydepartedfromtheAGB,but
LSIV-121(a) 9kyr lackareflectionnebula,andassucharenotexpectedtoevolve
GD605(b) 20kyr intoaCSPN.Thesemuchcoolerobjects(comparedtotheO(H)
J2046(c) 31kyr starsdiscussedabove)showthepresenceofacompactdiskwith
J0935(c) 31kyr aninnerradiusof≈ 15AU(Derooetal.2006,2007)andinad-
dition they were found to reside in binary systems. They have
J1000(c) 33kyr
orbital periods between a hundred and a couple of thousands
vZ1128(d) 38kyr
days, and probably unevolved (very) low initial mass compan-
ROB162(e) 49kyr
ions (vanWinckeletal. 2009). It is thought, that these stars
BD+182647(f) 56kyr
probablydid have a pre-PN before, but that the re-accretionof
PNG name tpost−AGB materialhasstalledtheblue-wardevolutionofthepost-AGBstar
047.0+42.4 A39(g) 73kyr and, thus, providing the circumstellar material enough time to
072.7−17.1 A74(h) 80kyr dispersebeforeitcloudbecameionized(DeMarco2014).
158.9+17.8 PuWe1(h) 80kyr
025.4−04.7 IC1295(h) >80kyr 6. Conclusions
030.6+06.2 Sh2-68(h) >80kyr
Our non-LTE model atmosphere analysis revealed that all five
158.5+00.7 Sh2-216(i) >80kyr
stars, which were recently discovered by the MUCHFUSS
077.6+14.7 A61(g) 90kyr
project as RV stars, lie in the post-AGB region of the HRD.
128.0−04.1 Sh2-188(j) 120kyr
The RV variations were found not to be significant in case of
047.0+42.4 A39(f) 73kyr theO(H)starJ2046,butinthecasesoftheothertwoO(H)stars
072.7−17.1 A74(g) 80kyr (J1000, J0935), the O(He) star (J0757), and the PG1159 star
158.9+17.8 PuWe1(f) 80kyr (J1556). ThisrevealsJ0757as the firstRV variableO(He) star
025.4−04.7 IC1295(g) >80kyr ever discovered, J1556 as the only second known RV variable
030.6+06.2 Sh2-68(g) >80kyr PG1159star,andJ1000andJ0935asthefirstRVvariableO(H)
158.5+00.7 Sh2-216(h) >80kyr starsthatdonotshowanyhintofaPN.
077.6+14.7 A61(f) 90kyr TheabsenceofaPNaroundthePG1159starJ1556canbeex-
128.0−04.1 Sh2-188(j) 120kyr plained well in terms of a VLTP evolution, which can also ac-
countforthehighCabundance.Giventhecurrentlyknownlow
binary fraction of C-dominated stars, it appears at the moment
Notes. Post-AGB times derived from atmospheric parameters, which
weretakenfrom(a) Heber&Hunger (1987),(b) Fontaineetal.(2008), thatbinaryinteractionsdo notplay a crucialrole in the forma-
(c) this work, (d) Chayeretal. (2015), (e) Heber&Kudritzki (1986), tionofthesestars.
(f) Bauer&Husfeld (1995), (g) Napiwotzki (1999), (h) Ziegler (2012), In case of the O(He) star J0757 we speculate that it once was
(i)Rauchetal.(2007),(j)Gianninasetal.(2010), part of a compact He transferring binary systems in which the
masstransferhadstopafteracertaintime,leavingbehindalow
massclosecompanion. Inthiswayonecouldexplainboth,the
fractionof25%amongstnakedO(H)stars,whichisevenhigher absenceofaPNaswellastheRVshiftof107.0±22.0km/smea-
thanwhatisfoundfortheCSPNe. suredwithinonly31min. Surely,thesespeculationsneedmore
An alternative solution to the non-ejection of a PNe of ap- observationalaswellastheoreticalsupport,however,theymight
parently naked O(H) stars was suggested by Heber&Hunger offerayetnotconsideredevolutionarychannelforthemysteri-
(1987),namelythatalsothesestarssufferedaLTP.Contraryto ousHe-dominatedobjects.
the VLTP scenario, which always results in a H-deficient sur- Variousexplanationscould hold for the “missing” PNe around
facecompositionsinceHismixedandburntalreadyduringthe theO(H)-typestars. Thepossibilitythatthepost-AGBtimesof
thermal pulse, the LTP does not necessarily predict a H-poor thenakedO(H)starsarelongerthanittookforthePNetodisap-
star. This is because in the case of a LTP, the convectiveshell pearintotheinterstellarmedium,seemsoddgiventhefactthat
triggered by excessive He burning is not able to penetrate the thereareseveralCSPNewithmuchlongerpost-AGBtimes. Be-
H-rich envelope from below because the entropy jump across sides the non-ejectionof a PN, the occurrenceof a LTP, or the
the He/H interface is too large. Only when the star evolves re-accretionofthePNinthepreviouspost-AGBevolutionoffer
backto itsHayashilimitontheAGB(T .7000K),envelope possibleexplanationsforthosestarsnotharbouringa PN (any-
eff
convection sets in again (Blöcker&Schönberner 1996, 1997; more).
Schönberner 2008). Evolutionary calculations without over- The search for an IR excesswas successful onlyin the case of
shoot (e.g., Blöcker&Schönberner1997) predictonly mild, if J1556,whichdisplaysa clear IRexcessinall WISE colors. In
any,mixing. Theenvelopeconvectiondoesnotreachthelayers ordertounderstanditsnature,spectroscopicfollow-upobserva-
enrichedwithcarbon,andnothirddredgeupoccurs. Latercal- tions are necessary. We should, however, keep in mind that it
culationsbyHerwig(2001) showedthatifovershootisapplied is very difficult to detect a close companion via the IR excess
toallconvectiveregions,AGBmodelsshowefficientdredgeup method given the very high luminosities of stars in post-AGB
evenforverylowenvelopemassesandthusproduceH-deficient region(ourstarshaveseveralthousandtimesthesolarluminos-
stars. Observational proof for the validity of the latter case is ity,Table1). Thus,thepresenceofalatetypecompaniontothe
offeredbyFGSge,theonlystarknowntothisdaythatcertainly otherstarsinoursamplecannotbeexcludedfromtheoutset.
Articlenumber,page8of12
Draft Version: January 6, 2016.
Reindletal.:RVvariable,hotpost-AGBstars
Acknowledgements. N.R. was supported by DFG (grant WE1312/41-1) and Herald,J.E.&Bianchi,L.2011,MNRAS,417,2440
DLR(grant50OR1507). WethankMarceloMiguelMillerBertolamiforpro- Herwig,F.2001,inAstrophysicsandSpaceScienceLibrary,Vol.265,Astro-
viding us with the post-AGB tracks before they were published. We thank physicsandSpaceScienceLibrary,ed.R.Szczerba&S.K.Górny,249
ThomasRauchandPetervanHoofforhelpfuldiscussionsandcomments.Based Hirsch,H.A.2009,PhDthesis,UniversityErlangen-Nürnberg
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theInstitutodeAstrofísicadeAndalucía(CSIC).Basedonobservationswiththe Iben,Jr.,I.,Kaler,J.B.,Truran,J.W.,&Renzini,A.1983,ApJ,264,605
WilliamHerschelandIsaacNewtonTelescopes operatedbytheIsaacNewton Iben,Jr.,I.&Tutukov,A.V.1984,ApJS,54,335
GroupattheObservatoriodelRoquedelosMuchachosoftheInstitutodeAs- Iben,Jr.,I.&Tutukov,A.V.1986,ApJ,311,742
trofisicadeCanarias ontheislandofLaPalma, Spain. FundingforSDSS-III Jeffery,C.S.&Schönberner,D.2006,A&A,459,885
has been provided by the Alfred P.Sloan Foundation, the Participating Insti- Justham,S.,Podsiadlowski,P.,&Han,Z.2011,MNRAS,410,984
tutions, theNational Science Foundation, andtheU.S.Department ofEnergy Justham,S.,Podsiadlowski,P.,Han,Z.,&Wolf,C.2010,Ap&SS,329,3
OfficeofScience.TheSDSS-IIIwebsiteishttp://www.sdss3.org/.SDSS- Kalirai,J.S.2012,Nature,486,90
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the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie ricalPlanetaryNebulaeVIConference,47
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GermanParticipationGroup,HarvardUniversity,theInstitutodeAstrofisicade ApJ,602,342
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kinsUniversity,LawrenceBerkeleyNationalLaboratory,MaxPlanckInstitute icanInstituteofPhysicsConferenceSeries,Vol.1273,AmericanInstituteof
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https://publikationen.uni-tuebingen.de/xmlui/handle/10900/50006 7
8
4
M
0.8
5
)
2
s
m/
c
gg (/6 0.7 M 0.6 M 0.5 M
o
l
7
8
5.2 5.0 4.8 4.6
log (T /K)
eff
Fig. 5. Locations of the O(He) star J0757 (red triangle
with error bars) and the PG1159 star J1556 (black inverted tri-
angle with error bars) and other PG1159 and O(He) stars (black
inverted triangles and red triangles, respectively, DeMarcoetal.
2015; Reindletal. 2014b; Werner&Rauch 2014; Werneretal. 2014;
Gianninasetal. 2010; Wassermannetal. 2010; Werner&Herwig
2006) in the T − logg diagram. Their likely successors, the
eff
H-deficient white dwarfs (Reindletal. 2014a; Werneretal. 2014;
Mahsereci2011;Hügelmeyeretal.2006;Dreizler&Werner1996)are
indicated by filled, red squares. The upper panel compares the lo-
cation of these stars to (V)LTP evolutionary tracks (grey lines) of
MillerBertolami&Althaus(2007,2006). Forclarity,onlytwotracks
arelabeledwithstellarmasses.Intermediatetrackscorrespondto0.530,
0.542, 0.565, 0.584, 0.609, 0.664, 0.741M . The lower panel com-
⊙
paresonlythelocationsoftheC-richO(He)stars(redtriangles)andC-
stronglinedHe-sdOstars(redrhombs)fromthesampleofGeieretal.
(2015)withpost-doubleHewhitedwarfmergertracks(purplelines)of
Zhang&Jeffery(2012a,b).
Articlenumber,page10of12
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