SDSSJ212531.92-010745.9 - the first definite PG1159 close binary system

0.41 MB·English

by  T. Nagel

Astronomy&Astrophysicsmanuscriptno.Hl146 (cid:13)c ESO2008
February5,2008
−
SDSSJ212531.92 010745.9 - the first definite PG1159 close
binary system
6
0 T.Nagel1,S.Schuh2,D.-J.Kusterer1,T.Stahn2,S.D.Hu¨gelmeyer2,S.Dreizler2,B.T.Ga¨nsicke3,andM.R.
0
Schreiber4
2

n
a 1 Institutfu¨rAstronomieundAstrophysik,Eberhard-Karls-Universita¨tTu¨bingen,Sand1,72076Tu¨bingen,Germany
J 2 Institutfu¨rAstrophysik,Georg-August-Universita¨tGo¨ttingen,Friedrich-Hund-Platz1,37077Go¨ttingen,Germany

3 3 DepartmentofPhysics,UniversityofWarwick,Coventry,CV47AL,GreatBritain
2 4 DepartamentodeFisicayMeteorologia,FacultaddeCiencias,UniversidaddeValparaiso,Valparaiso,Chile

1 Receivedxx.xx.xx/Acceptedxx.xx.xx
v
2
ABSTRACT
1
5
1 Aims.The archival spectrum of SDSSJ212531.92−010745.9 shows not only the typical signature of a PG1159 star, but also indicates the
0 presenceofacompanion.Ouraimwastheproofofthebinarynatureofthisobjectandthedeterminationofitsorbitalperiod.
6 Methods.Weperformed time-seriesphotometry ofSDSSJ212531.92−010745.9. Weobserved theobject during10nights, spreadover one
0 month,withtheTu¨bingen 80cmandtheGo¨ttingen50cmtelescopes.Wefittedtheobservedlightcurvewithasineandsimulatedthelight
/ curve of this system with the nightfall program. Furthermore, we compared the spectrum of SDSSJ212531.92−010745.9 with NLTE
h
models,theresultsofwhichalsoconstrainthelightcurvesolution.
p
- Results.Anorbitalperiodof6.95616(33)hwithanamplitudeof0.354(3)magisderivedfromourobservations.Apulsationperiodcouldnot
o bedetected.ForthePG1159starwefound,aspreliminaryresultsfromcomparisonwithourNLTEmodels,T ∼90000K,logg∼7.60,and
eff
r theabundanceratioC/He∼0.05bynumberfraction.Forthecompanionweobtainedwithameanradiusof0.4±0.1R ,amassof0.4±0.1M ,
t ⊙ ⊙
s andatemperatureof8200Kontheirradiatedside,goodagreementbetweentheobservedlightcurveandthenightfallsimulation,butwe
a
donotregardthosevaluesasfinal.
:
v
i
X
Keywords.stars:AGBandpost-AGB–whitedwarfs–binaries:close
r
a
1. Introduction Currently, 37 PG1159 stars are known. Figure1 shows
their position in a logT -logg-diagram. Two of them have
PG1159 stars are hot hydrogen-deficient (pre-)white dwarfs eff
been foundto be binarystars. These are NGC246 (e.g.Bond
witheffectivetemperaturesbetween75000and200000K,and
& Ciardullo 1999), which is a resolved visual binary, and
logg=5.5–8.0 (Werner 2001). They are in the transition be-
PG2131+066(Wesemaeletal.1985).Concerningthelatter,it
tween the asymptotic giant branch (AGB) and cooling white
isstillunclearwhetheritisaclosebinary(Paunzenetal.1998)
dwarfs.SpectraofPG1159starsaredominatedbyabsorption
or a resolved visual binary with an M2V star as companion
linesofHe,CandO.
(Reedetal.2000).
Currenttheorysuggests(e.g.Werner2001)thattheyarethe
outcomeofalatehelium-shellflash,aphenomenonthatdrives
the currently observed fast evolutionary rates of three well-
2. ThespectrumofSDSSJ212531.92−010745.9
known objects (FG Sge, Sakurai’s object, V605 Aql). Flash-
inducedenvelopemixingproducesaH-deficientstellarsurface.
The spectrum of SDSSJ212531.92−010745.9 (u=17.15,
The photosphericcompositionthen essentially reflects that of
g=17.54,r=17.75,i=17.79,z=17.83),takenonSept.6th2002,
theregionbetweentheH-andHe-burningshellsintheprecur-
is from the Sloan Digital Sky Survey (SDSS) archive Data
sorAGBstar. TheHe-shellflash forcesthestarbackontothe
Release (DR) 4. The spectrum shows significant featuresthat
AGB.Thesubsequent,secondpost-AGBevolutionexplainsthe
are typical for PG1159 stars, for example the strong C ab-
existenceofWolf-Rayetcentralstarsofplanetarynebulaeand
sorption lines at 4650 − 4700Å and He at 4686Å (Fig.2).
theirsuccessors,thePG1159stars.
Furthermore, the spectrum shows features which indicate the
Sendoffprintrequeststo:T.Nagel presence of a companion. The Balmer series of hydrogen is
Correspondenceto:[email protected] seeninemission,Hα-Hδcanclearlybeidentified.Thisisprob-
2 T.Nagelatal.:ThefirstdefinitePG1159closebinarysystem
Fig.2. Spectrum of SDSSJ212531.92−010745.9 (gray line, exposure time 3703s), a PG1159 NLTE model spectrum with
T =90000K, logg=7.60, and C/He=0.05, N/He=0.01 (thin black line), a blackbody spectrum with T=8200K (dashed
eff
line),representingthecontributionfromtheirradiatedcompanion,andthetotalmodelspectrum(thickblackline).TheBalmer
seriesinemission(top),belongingtothecompanion,andsomeheliumandcarbonlines(bottom),belongingtothePG1159star,
aremarked.
ablyduetoacoolcompanionwhichisheatedupbyirradiation Table1.Observationlog.Allobservationsareperformedwith
fromthehydrogen-deficientPG1159star. clearfilter.
Figure2 shows the observed spectrum (t = 3703s)
exp Date t [s] t [s] Duration[s] Telescope
of SDSSJ212531.92−010745.9. Overlayed are a PG1159 exp cycle
NLTE model spectrum with T =90000K, logg=7.60, 2005/09/21 90 98 18900 80cm
eff
C/He=0.05, and N/He=0.01, a blackbody model spectrum 2005/09/22 90 98 18899 80cm
with T=8200K for the irradiated companion, and the sum 2005/09/23 90 98 21758 80cm
2005/09/23 180 194 14873 50cm
ofthetwomodelspectra.Theparametersofbothstellarcom-
2005/10/06 240 248 10202 50cm
ponents are estimates obtained from a qualitative comparison
2005/10/07 240 246 14897 50cm
of our NLTE models to the single SDSS spectrum. Detailed
2005/10/08 240 248 9298 50cm
parameters for both stars need to be derived from a full two-
2005/10/10 90 98 19852 80cm
component analysis of orbital phase resolved spectroscopy. 2005/10/11 240 248 17872 50cm
Theeffectivetemperatureinparticularmaybelowerorhigher 2005/10/18 90 98 16532 80cm
by20000K.Thesurfacetemperatureofthecompanion’sirra- 2005/10/26 90 98 20095 80cm
diatedsidewasalsoconstrainedwithnightfallsimulations,
seebelow.
toτ =1,whichcausestheobservedemissionlinespectrum
The overall shape of the observed spectrum is well fitted Ross
(Barmanetal.2004).
with the combinationof a PG1159 star and a cool, irradiated
companion,butespeciallytheCspectrallinesofthePG1159
model atmosphere are not strong enough. There is another 3. PhotometryofSDSSJ212531.92−010745.9
PG1159starshowingthisphenomenon(Hu¨gelmeyeretal.,in
PhotometricobservationsofSDSSJ212531.92−010745.9were
prep.),andalsononeofthedeepabsorptionlineswhichsome
performedduring10nights(Tab.1)usingtheTu¨bingen80cm
DOwhitedwarfsshowcanbefitted(e.g.Werneretal.1995).
f/8 telescope with an SBIGST-7E CCD camera and the
The spectral signatures of an A star, as one would expect Go¨ttingen 50cm f/10 telescope with an SBIGSTL-6303E
for the companionwith 8200K surface temperatureat the ir- CCD camera.Toachievegoodtimeresolutionwechoseclear
radiated side, cannotbe seen in the observation.This may be filter exposures with a binning of 2x2 pixels to reduce read-
becausetheirradiationfromthePG1159leadstoatemperature outtime.Theexposuretimewast =90sfortheobservations
exp
inversionintheupperlayersofthecompanion’satmosphereup with the 80cm telescope. In the case of the 50cm telescope,
T.Nagelatal.:ThefirstdefinitePG1159closebinarysystem 3
−0.5
0.0
0.5
−0.5 1 2 3 4
0.0
0.5
mag] −0.5 16 17 18 19
m [ 0.0
∆ 0.5
−0.5 21 22 23 24
0.0
0.5
−0.5 26 27 28 29
0.0
0.5
32 33 34 35
JD−2453635 [days]
Fig.3.Lightcurveofallnights,overplottedthebestsinefitwithaperiodof6.95616h.
Teff [kK] 2003). The relative flux of the object was calculated with re-
200 100 50
5 spect to the same two comparison stars (SDSS J212530.60-
010921.0 and SDSS J212528.83-010828.5) for all nights,
whichweretestedforstability.Theresultinglightcurveisdis-
playedinFig.3.
6
Toanalysethecombinedlightcurveofallnights,weused
] CAFE(CommonAstronomicalFitEnvironment,Go¨hler,priv.
-2s comm.),acollectionofroutineswritteninIDL.Thebrightness
m
c variation is probably caused by a reflection effect. The com-
g [ 0.565
g 7 panionis,duetothesmallseparation,heatedupononesideby
o
l irradiationfromthe PG1159star, and the orbitalmotionthen
0.546
leads to a variable light curve. We fitted the combined light
0.605 curveofallnightswithasine,achievingbestresultsforape-
0.6
8 0.7 riod of 6.95616(33)h (Fig.3). The observed variability has a
0.76
meanamplitudeof0.354(3)mag.
0.89
To check if the observed light curve can be explained by
5.6 5.4 5.2 5.0 4.8 4.6
a PG1159 star and an irradiated companion and for an im-
log T [K]
eff
pression of what the system geometry might look like we
Fig.1. Positions of the known PG1159 stars in the log Teff- simulated the light curve of the binary system for an orbital
logg-diagram. The two known binary systems are shown as period of 6.95616h with the program nightfall. Figure4
black dots, the new one is shown as square. Post-AGB evo- shows the simulated and observed light curves of all nights,
lutionary tracks are taken from Scho¨nberner (1983, dashed foldedontotheorbitalperiod.ForthePG1159starweassumed
lines, 0.546M⊙ and 0.565M⊙), Blo¨cker (1995, dashed line, Teff=90000K, a mass of 0.6M⊙ and a radius of 0.1R⊙. For
0.605M⊙), and Wood & Faulkner (1986, solid lines) (labels: thecompanionwevariedthemassfrom0.1M⊙ to0.7M⊙.We
massinM⊙).Thedashed-dottedlinesrepresentthetheoretical foundthattheobservedlightcurvecanbereproducedbestwith
red(Quirionetal.2004)andblueedge(Gautschypriv.comm.) an M dwarfwith an effectivetemperatureof3500±150K,a
oftheGWVirinstabilitystrip. meanradiusof0.4±0.1R andamassofabout0.4±0.1M .
⊙ ⊙
For the inclination of this system we obtained 70± 5◦. Due
to the irradiation by the PG1159star the surface of the com-
the exposure time was texp=180s and texp=240s. The observ- panionwouldbeheateduptoasurfacetemperatureof8200K,
ingconditionsweregoodduringthenights,consideringthatthe which, in combination with the PG1159 star, reproduces the
telescopesarelocatedinthecitiesofTu¨bingenandGo¨ttingen. overall shape of the observed spectrum quite well, as can be
Allimageswerebiasanddarkcurrentcorrected,thenaper- seeninFig.2.Thebroaddipattheminimumofthelightcurve
turephotometrywasperformedusingourIDLsoftwareTRIPP iswellreproducedbythissystemconfiguration,too.InTable2
(Time Resolved Imaging Photometry Package, Schuh etal. welistallstellarandsystemparametersassumedandderived.
4 T.Nagelatal.:ThefirstdefinitePG1159closebinarysystem
Table 2. Stellar and system parameters of
SDSSJ212531.92−010745.9, assumed (normal font) or
derivedfromcomparisonwithNLTEmodelspectra(boldface),
photometricanalysis(*)andnightfallsimulation(italic).
Parameter PG1159 Companion System
T [K] ∼90000 3500±150
eff
T [K] 8200
eff,irr
logg[cm/s2] ∼7.6
m [M ] 0.6 0.4±0.1 1.0±0.1
⊙
r [R ] 0.1 0.4±0.1
⊙
P [h] 6.95616(33)*
orb
∆m [mag] 0.354(3)*
a[R ] 1.85
⊙
i [◦] 70±5
Fig.4.Simulatedlightcurveofabinarysystem,consistingofa
PG1159star(Teff=90000K)andanMdwarf(Teff=3500K, 3. Froma firstcomparisonwith NLTEmodelspectrawe de-
heatedupto8200K),calculatedwithnightfall(blackline) rived, as preliminary results, an effective temperature of
and the observedlight curve of all nights, folded onto the or- 90000K,logg ∼7.60andtheabundanceratioC/He∼0.05
bital period. The shape of the light curve is well resampled, forthePG1159component.Adetailed,quantitativeNLTE
especiallythebroaddips. spectral analysis of the PG1159 star and the irradiated
companionhas to be done next. We will reporton the re-
sultsinasubsequentpaper.
We found that ellipsoidal variation due to geometrical defor- 4. We simulated the light curve of the binary system with
mation of the stars cannot generate the observed light curve. an orbital period of 6.95616h using nightfall. A good
Intheaboveconfiguration,calculatedbynightfallaccordingto agreementwith the observedlight curve was obtained for
thegeometryinDjurasevic1992,theequatorialradiusoftheM ameanradiusof0.4±0.1R⊙,amassof 0.4±0.1M⊙ and
dwarfisonly4.5%largerthanitspolarradius,andthePG1159 atemperatureoftheirradiatedsurfaceofabout8200Kfor
starisnotaffectedbydeformationabovethenumericallimitof thecompanion.
nightfall.
To determine the system parameters more pre-
Because the object is positioned in the GWVir instabil-
cisely, high-resolution phase-resolved spectroscopy of
ity strip (Fig.1), we also looked for pulsation periods below
SDSSJ212531.92−010745.9 is necessary. It should then be
two hours in the light curve of SDSSJ212531.92−010745.9.
possible to derive both the companion’s variable light contri-
Therefore, we calculated a Lomb-Scargle periodogram
butiontotheoverallspectrumaswellasdynamicalmassesfor
(Scargle 1982) for the night with the best S/N (2005/10/26).
both components from radial velocity measurements of their
But observational noise precludes detection of any periodic-
distinctlinesystems.
ity with amplitudesbelowabout50mmag. Theamplitudesof
pulsatingPG1159starsnormallyareoftheorderofafewper-
Acknowledgements. We thank T.-O. Husser, R. Lutz and E. Nagel
centofamagnitude,andSDSSJ212531.92−010745.9isfainter
for supporting the observations. We acknowledge the use of CAFE
thanHE1429-1209,forwhichwerecentlydiscoveredpulsation
5.1, an astronomical fit enviroment, written by Eckart Go¨hler. We
with the Tu¨bingen 80cm telescope (Nagel & Werner 2004).
acknowledge the use of the nightfall program for the light
Our80cmtelescopemightthereforejustbetoosmalltodetect curve synthesis of eclipsing binaries written by Rainer Wichmann
pulsationbelow50mmaginthiscase. (http://www.lsw.uni-heidelberg.de/∼rwichman/Nightfall.html). BTG
wassupportedbyaPPARCAdvancedFellowship.
4. Conclusions
References
1. The spectrum of SDSSJ212531.92−010745.9 from DR4
Barman,T.S.,Hauschildt,P.H.,&Allard,F.2004,ApJ,614,338
of the Sloan Digital Sky Survey shows the signature of a
Blo¨cker,T.1995,A&A,299,755
PG1159starplusemissionfromacoolirradiatedcompan-
Bond,H.E.,&Ciardullo,R.1999,PASP,111,217
ion.
Djurasevic,G.1992,Ap&SS,196,241
2. We performed time-series photometry during 10 nights
Nagel,T.&Werner,K.2004,A&A,426,L45
with the Tu¨bingen 80cm and the Go¨ttingen 50cm tele-
Paunzen,E.,Ko¨nig,M.,&Dreizler,S.1998,A&A,331,162
scopesanddetectedaperiodof6.95616(33)hwithanam- Quirion,P.O.,FontaineG.,&BrassardP.2004,ApJ,610,436
plitude of0.354(3)mag.Thisrepresentsthe orbitalperiod Reed,M.D.,Kawaler,S.D.,&O’Brien,M.S.2000,ApJ,545,429
ofthebinarysystem.Thus,SDSSJ212531.92−010745.9is Scargle,J.D.1982,ApJ,263,835
thefirstclosePG1159binarywithoutanydoubts. Scho¨nberner,D.1983,ApJ,272,708
T.Nagelatal.:ThefirstdefinitePG1159closebinarysystem 5
Schuh, S. L., Dreizler, S., Deetjen, J. L., & Go¨hler, E. 2003, Baltic
Astronomy,12,167
Werner,K.,Dreizler,S.,Heber,U.,Rauch,T.,Wisotzki,L.,&Hagen,
H.-J.1995,A&A,293,75
Werner,K.2001,Ap&SS,275,27
Wesemael,F.,Green,R.F.,&Liebert,J.1985,ApJS,58,379
Wood,P.R.,&Faulkner,D.J.1986,ApJ,307,659