In 2011 a promising candidate for an RR Lyrae star in an eclipsing binary system was found. Till that time not even one case of RR Lyrae star in a binary system has been known. The pulsator's mass is 0.26 Mꙩ  which is not enough to burn helium in the core, as RR Lyrae stars do. The presence of a more massive companion is a clue that the mass transfer had to occur in the past. Therefore, Binary Evolution Pulsating (BEP) star, while having RR Lyr-like light curve, has completely unlike internal structure. The bulk of the star's mass was lost during the red giant phase due to mass transfer and the partially degenerated helium core with thin hydrogen burning shell was revealed. The BEP object has been captured inside the instability strip (IS) in the RR Lyrae area and thus it is confused with classical RR Lyrae pulsators. Therefore, the BEP star is the evidence of a new evolutionary channel to produce RR Lyr-like oscillations. In simulations made with StarTrack code we trace the evolution of a sample of binaries and examine properties of the system required for pulsation phase to occur. We suggest that the stars created via this new evolutionary channel can in part explain the existence of UV up-turn, low-mass C-O WD and He WD

Karczmarek, P.

Advances in Astronomy and Space Physics

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

Binary evolution pulsating star  new evolutionary channelto produce RR Lyr-like pulsationsP.KarczmarekAdvances in Astronomy and Space Physics, 2, 135-138 (2012)© P.Karczmarek, 2012Centre for Astronomy, Nicolaus Copernicus University, 11 Gagarina st., 86-100 Toru«, PolandIn 2011 a promising candidate for an RR Lyrae star in an eclipsing binary system was found. Till that time noteven one case of RR Lyrae star in a binary system has been known. The pulsator's mass is 0.26M which is notenough to burn helium in the core, as RR Lyrae stars do. The presence of a more massive companion is a clue thatthe mass transfer had to occur in the past. Therefore, Binary Evolution Pulsating (BEP) star, while having RRLyr-like light curve, has completely unlike internal structure. The bulk of the star's mass was lost during the redgiant phase due to mass transfer and the partially degenerated helium core with thin hydrogen burning shell wasrevealed. The BEP object has been captured inside the instability strip (IS) in the RR Lyrae area and thus it isconfused with classical RR Lyrae pulsators. Therefore, the BEP star is the evidence of a new evolutionary channelto produce RR Lyr-like oscillations. In simulations made with StarTrack code we trace the evolution of a sampleof binaries and examine properties of the system required for pulsation phase to occur. We suggest that the starscreated via this new evolutionary channel can in part explain the existence of UV up-turn, low-mass C-O WD andHe WD.Key words: stars: variables: RR Lyrae  binaries: eclipsing  stars: evolutionintroductionEclipsing binaries are perfect stellar laboratoriesto determine the most important stellar parame-ters: age, mass, radius, metallicity. The light curveand radial velocities analysis of binary componentsnow provide the stellar parameters with the accu-racy of 1% [1]. Moreover, thanks to pulsating starsin eclipsing binaries, the problem of dynamical massof the classical Cepheids was solved  the result wasin a better agreement with the predictions of thestellar pulsation theory than the stellar evolutionone [6]. Determination of the parameters of othertypes of eclipsing pulsators, as  Scuti stars or clas-sical Cepheids, improves the understanding of stel-lar physics and imposes the constraints on the keystellar parameters. In case of standard candles, it al-lows for a better calibration of the cosmic distances.The growing number of discoveries of the eclipsingpulsating stars is being reported nowadays, yet noRRLyrae star has been found.the discoveryIn 2011, Soszy«ski et al. [8] observed an eclips-ing binary with the RR Lyrae component, namedOGLE-BLG-RRLYR-02792, with an orbital periodPorb = 15:24 d and the pulsation period Ppul =0:627d. The method used to untangle the pulsa-tion and eclipse curves is based on tting a func-tion to the data via the Fourier series. The programdesigned for this task nds the best t of the lightcurve phased with pulsational period (with alreadyremoved eclipse points) to the data, subtracts thenewly-found function from the light curve which con-tains all points (eclipse points present). Subtrac-tion eliminates the pulsation variability and retainsonly eclipses.Fig. 1: Pulsational I-band light curve of the primarycomponent of the binary system OGLE-BLG-RRLYR-02792, folded on a pulsation period of 0.627548 days.The shape of the light curve is mimicking that of a clas-sical RRLyr star. The outlier points are responsible forthe eclipsing variability. The gure excerpted from [8].paulina.karczmarek@astri.uni.torun.pl135Advances in Astronomy and Space Physics P.KarczmarekFig. 2: Orbital I-band light curve (617 epochs col-lected over 10 years) of the binary system OGLE-BLG-RRLYR-02792, after removal of the intrinsic brightnessvariation of the pulsating component (data points), to-gether with the solution (solid line), as obtained withthe 2007 version of the standard Wilson-Devinney code[9, 10]. Top panel: the residuals of the observed magni-tudes from the computed orbital light curve. The gureexcerpted from [7].Table 1: Orbital and physical parameters of the OGLE-BLG-RRLYR-02792 system, together with their uncer-tainties as obtained from the modelling of the spectro-scopic and photometric data. The table excerpted from[7].Parametera Primary (pulsating) SecondaryM [M] 0.261  0.015 1.67  0.06R [R] 4.24  0.24 4.27  0.31Te [K] 7320  160 5000  150a [R] 32.20  0.32Porb [d] 15.24350  0.00021Ppul [d] 0.627548  0.000008dP=dt  2:3 10 8e 0:0072 0:0029a The parameters are as follows: stellar mass, stel-lar radius, eective temperature, orbit size, orbitalperiod, pulsational period, rate of period change, ec-centricity.Figs. 1 and 2 give the general view on the star'svariability. Collected data was carefully analysed byPietrzy«ski et al. [7] to determine the orbital andphysical parameters of the system (Table 1) and thetype of pulsations using the Fourier parameters. Allindicators: the Fourier parameters, the pulsationalperiod and the location on Hertzsprung-Russel (HR)diagram, implied with no doubts that the pulsatingcomponent is RRLyrae star but the pulsator's masswas not enough (0.26M) for the star to undergo thehelium ash and thus to burn helium in the core, asit is usually takes place in canonical RRLyrae stars.The presence of the companion could explain thetroublesome mass of the pulsating component. Inthe past the system has undergone the mass trans-fer episode and as a result the donor has strippedits almost entire hydrogen envelope o revealing thehot helium core. The physical properties of the pul-sator happen to place it in the same instability strip(IS) of the HR diagram occupied by RRLyrae stars.It is noteworthy that the pulsations were generatednot as a result of the single star evolution but dueto the evolution of the binary system, therefore thenew type of binary star gained the name Binary Evo-lution Pulsator (BEP). BEPs are considered to passthe IS two orders of magnitude faster than canonicalRRLyrae stars, and always towards higher temper-atures.simulationsThe simulations made with the StarTrack code(manual to the code is available in [2]) enables totrack the evolution of the binary from Zero Age MainSequence (ZAMS) till the white dwarf (WD) phase.If at any time of the evolution, any of two com-ponents crosses the RRLyrae instability strip whilehaving the mass M < 0:5M, this object at thisparticular stage of its evolution is called BEP andinitial parameters of the binary system (masses ofboth components, orbital period) are collected. TheRRLyrae instability area is determined in the termsof eective temperatures (Te) and luminosities (L)as follows [3]:16L < L < 100L5000K < Te < 7400K:(1)Two dierent kinds of BEP objects crossing the ISwere found:(i) M . 0:3M. The primary component startsits evolution as a main sequence (MS) star and endsas He WD after 5-7Gyrs. While being red giant(RG) it transfers the mass to the companion (MSstar). After the mass transfer ended, primary com-ponent enters the Horizontal Branch without goingthrough the helium ash. The luminosity remainsthe same as the radius decreases and the tempera-ture increases. The primary component pulsates asBEP while crossing the IS in the time interval ofabout a million years. The secondary componentgoes through the RG phase, expands and overlls itsRoche Lobe. The unstable mass transfer to the pri-mary leads to the common envelope episode. At thenal stage the binary consists of two helium whitedwarfs. Fig. 3 shows the evolutionary track of theprimary.(ii) 0:3M . M . 0:5M. The primary compo-nent starts its evolution as a MS star and ends as C-OWD after 1.5-2.5Gyrs. While being RG it transfers136Advances in Astronomy and Space Physics P.Karczmarekthe mass to the companion (MS star) until reachesthe tip of the RG branch and ignites helium in thecore. Then the mass transfer ends and the primarydescend the RG branch and resides in the Horizon-tal Branch. The luminosity remains the same as theradius decreases and the temperature increases. Theprimary pulsates as BEP while crossing the IS in thetime interval of about a million years. The secondarycomponent goes through the Red Giant phase, ex-pands and overlls its Roche Lobe. The unstablemass transfer to the primary leads to the commonenvelope episode. At the nal stage the binary con-sists of two C-O WD or the C-O and hybrid WD.Fig. 4 shows the evolutionary track of the primary.The rst (i) low-mass case is available for initialparameters: the mass of the primary and the orbitalperiod, in ranges0:4 M .M0 . 2:0 M1:4 d . P0 . 9:0 d; (2)while the (ii) high-mass case occurs when2:0 M .M0 . 3:0 M4:0 d . P0 . 18:0 d: (3)The constraints are uent and require further exam-inations to be estimated more accurate.-5.0-4.0-3.0-2.0-1.00.01.02.03.03.54.04.55.0log(L/L❍•)log(Teff) [K]M0A = 1.4 M0B = 0.8P0 = 2.9Fig. 3: Evolutionary track of the primary componentwhich undergoes the BEP phase having the mass M .0:3M. Thick line on the evolutionary path indicatesthe mass transfer phase. The trapezium shaped area isthe IS of RRLyrae stars. The initial masses and periodof the binary are given.-5.0-4.0-3.0-2.0-1.00.01.02.03.03.54.04.55.0log(L/L❍•)log(Teff) [K]HeM0A = 2.0 M0B = 1.6P0 = 12.9Fig. 4: Evolutionary track of the primary compo-nent which undergoes the BEP phase having the mass0:3M & M & 0:5M. Thick line on the evolution-ary path indicates the mass transfer phase. Dashed lineshows the helium core burning stage which starts on thetip of the RG branch just after the mass transfer episodeends. The trapezium shaped area is the IS of RRLyraestars. The initial masses and period of the binary aregiven.results and conclusionsLong time-base observations of stars inside theIS allow to determine the rate of pulsation pe-riod change dP=dt. Because the star is movingat a constant luminosity across the IS towardshigher (lower) temperatures, its radius should be-come smaller (larger) and therefore its pulsation pe-riod should steadily decrease (increase). For clas-sical RRLyrae stars jdP=dtRRj  10 11   10 9 (inalternative notation 0:01   1:0 d/Myr) [5] and canbe positive or negative as they cross IS two times,rst going towards lower temperatures, next  to-wards higher temperatures. BEPs, in turn, cross theIS only once, always towards higher temperatures,that means dP=dtBEP < 0 and as they are movingrapidly,  2:2  10 8 . dP=dtBEP .  5:5  10 9(equals to dP=dtBEP  2   8 d/Myr). This dis-tinct dierence can be used to distinguish BEPs fromRRLyrae stars, especially in case when the eclipsesare not seen in a system.BEP is just a part of life of the object which there-after turns into He WD or C-O WD (depending onthe initial mass). Such evolution can in part explainthe abundance of He WD and low-mass C-O WD.The single star simulations show that these objects137Advances in Astronomy and Space Physics P.Karczmarekcan be created from the red giant provided articiallyhigh mass loss [4]. Simulations with StarTrack codeperformed for the binary case show that the highmass loss rate is fully explained by the mass transferthat happens exactly in the RG phase. As the result,the hot (helium or carbon) core stripped from hy-drogen envelope tends to radiate in short-wavelengthspectrum band. To sum up, the binary scenario ac-counts for existence of He WD, C-O WD and UVup-turn.The contamination of RR Lyrae stars by BEPswas estimated to be 0.2% [7]. This means that for1000 objects classied as RRLyrae stars, two canactually be BEPs, only showing the RR Lyr-likelight curve. Nevertheless, detailed calculations arerequired to conrm this number. The higher percentof contamination might increase the observed spreadin luminosity of the RRLyrae stars and aect dis-tance measurements based on them. As the BEPsare much younger than the RRLyrae stars, the ageof old galaxies and globular clusters hosting themmight be redened as younger. It is also possiblethat the contamination by BEPs may concern notonly the RR Lyrae stars but also the other pulsatorsin the IS, like classical Cepheids or  Scuti stars.The discovery of OGLE-BLG-RRLYR-02792points to the new evolutionary channel to createRRLyr-like oscillations. This encourages to improveboth evolutionary and pulsation theories. The pri-ority task is to proceed tracking pulsating binariesand to double check the stellar catalogues which canalready contain the fake RRLyrae stars and otherbinaries. Finally, it is a chance to enrich the evo-lutionary and pulsation codes with new aspects ofbinary evolution and to calibrate them for better pre-dictions of stars' behaviour.acknowledgementIt is a pleasure to thank G.Pietrzy«ski for sup-porting this project. I am grateful to K.Belczy«skifor sharing the StarTrack code and for many usefuland valuable instructions. The research presentedhere was supported by the TEAM subsidies of theFoundation for Polish Science (FNP).references[1] Andersen J., Clausen J.V., NordstromB., Tomkin J. &MayorM. 1991, A&A, 246, 99[2] BelczynskiK., KalogeraV. & BulikT. 2002, ApJ, 572,407[3] BonoG., CaputoF., Cassisi S., IncerpiR. & MarconiM.1997, ApJ, 483, 811[4] CastellaniM., CastellaniV. & Prada Moroni P.G. 2006,A&A, 457, 569[5] KunderA., WalkerA. S., PeterB. et al. 2011, AJ, 141,15[6] Pietrzy«skiG., Thompson I. B., GierenW. et al. 2010,Nature, 468, 542[7] Pietrzy«skiG., Thompson I. B., GierenW. et al. 2012,Nature, 484, 75 2[8] Soszy«ski I., DziembowskiW.A., UdalskiA. et al. 2011,Acta Astronomica, 61, 1[9] Van HammeW. & WilsonR.E. 2007, ApJ, 661, 1129[10] WilsonR.E. & DevinneyE. J. 1971, ApJ, 166, 605138

Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations

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Binary evolution pulsating star - new evolutionary channel to produce RR Lyr-like pulsations

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)