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