We present X-ray observations of the close binary nucleus of the planetary nebula (PN) PN G135.9+55.9 obtained with the XMM satellite. The nebula is the most oxygen-poor PN known to date and is located in the Galactic halo. It is known to harbor a close binary nucleus of which only one component can be observed in optical-UV range. New X-ray observations show that the invisible component is a very hot compact star. This finding allows us to reconstruct the immediate past of the object and predict its future. The parameters of the binary components we determine strongly suggest that the precursor was a symbiotic supersoft X-ray source that finished its life by Roche lobe overflow. PN G135.9+55.9 is an excellent candidate for a future type Ia supernova

Napiwotzki, R.

Pena, M.

Richer, M.G.

Stasinska, G.

Tomsick, J.

Tovmassian, G.

Yungelson, L.

English

arXiv

University of Hertfordshire Research Archive

An objet that dees stereotypes: X-rayobservations of SBS 1150+599A, the binarynuleus of PNG135.9+55.9G.Tovmassian1, J.Tomsik2, R.Napiwotzki3, L.Yungelson4,G.Stasi«ska5, M.Peña1and M.Riher11Institute of Astronomy, Universidad Naional Autonoma de Méxio.gag,riherastrosen.unam.mx, miriamastrosu.unam.mx2University California Berkeley, USA jtomsikssl.berkeley.edu3University of Hertfordshire, UK r.napiwotzkiherts.a.uk4Institute of Astronomy, RAS, Russia lryinasan.ru5Observatoire de Meudon, Frane grazyna.stasinskaobspm.frSummary. We present X-ray observations of the lose binary nuleus of the plan-etary nebula (PN) PNG135.9+55.9 obtained with the XMM satellite. The nebulais the most oxygen-poor PN known to date and is loated in the Galati halo.It is known to harbor a lose binary nuleus of whih only one omponent anbe observed in optial-UV range. New X-ray observations show that the invisibleomponent is a very hot ompat star. This nding allows us to reonstrut theimmediate past of the objet and predit its future. The parameters of the binaryomponents we determine strongly suggest that the preursor was a symbioti su-persoft X-ray soure that nished its life by Rohe lobe overow. PNG135.9+55.9is an exellent andidate for a future type Ia supernova.Key words: Planetary Nebula; Close, Interative Binary, Symbioti Stars; SuperSoft X-Ray Soure1 IntrodutionSBS 1150+599A was identied as a planetary nebula (PN) in [15℄ and subsequentlydesignated as PNG135.9+55.9. The objet is unusual and is renown for its extremelylow oxygen ontent [15, 2, 9, 13℄. It is also loated far and above the Galati plane,whih plaes it among a handful of known halo PNe. Diret images obtained by [10, 2℄and a study of dynamis of the nebula [11℄ onrmed its PN identiation, but didnot make its explanation easier. Another, if not unusual but outstanding feature ofthe PN is that it harbors a lose binary system, only one omponent of whih isobservable in the optial and UV [16℄. The far-UV observations by FUSE establisheda temperature range for that visible omponent of 110 000 - 120 000K. The lowerlimit of this range was established by a minimum temperature needed to produethe [NeV℄ emission line, while the upper limit was dedued from a ontinuum t516 Tovmassian et al.of atmosphere models to the observed data [16℄. The extremely high temperatureoupled with high log g led us to assume that the opt/UV omponent was the entralstar of planetary nebula (CSPN), i.e. the post-AGB star that lost its envelope andwas the soure of its ionization. Péquignot & Tsamis [9℄ laimed that the ionizingstar must be hotter in order to inrease the oxygen abundane to more ommonlevels ([O/H℄≈ −0.9 instead of [O/H℄≤ −2.2). It was an extreme solution: to justifya temperature of 120 000K [16℄ a higher extintion orretion than estimated for thediretion of PNG135.9+55.9 is required. To resolve this ontroversy, we [8℄ obtainedphotometri light urves of the binary ore of PNG135.9+55.9. The orbital periodof the system is 3.919 hr, and to explain the double-humped shape of the lighturve, one should invoke a Rohe-lobe lling opt/UV omponent. Additionally, thedepths of the minima in the light urve are uneven, whih in turn assumes that thevisible omponent should be irradiated by an even more energeti (hot) soure. Thedynamis required that this invisible omponent must be another ompat objet ofat least 0.85M⊙ [8℄.Here we report the analysis and results of X-ray observations of SBS1150+599A,the lose binary ore of PNG135.9+55.9 made with orbital telesope XMM-Newtonand make use of publily available HST UV-data.2 Observations and spetral energy distribution (SED)On 2006 Nov 1, a 25 kse exposure of SBS 1150+599A was obtained with XMM-Newton. We redued the observations using the latest version of SAS7.0. The objetwas deteted with XMM with a ount rate of 0.035 /s in the PN detetor, 0.0029/s in the MOS1 detetor, and 0.0048 /s in the MOS 2 detetor. SBS 1150+599Aappears to be emitting only very soft photons, the absolute majority of whih wereregistered in a narrow range between 0.1 to 0.3 keV. This soft end of spetral rangesuers alibration problems [14℄, but the extrated spetra in all three detetors arehighly onsistent with eah other and are well-desribed with a blak-body (BB).For a better understanding of the nature of the objet and omprehensive tof the SED we also use the UV data obtained by HST in 2003 (Obs. ID #9466,PI Garnavih). Here, we use the ontinuum t to the stellar spetra of the objetobtained by STIS in near and far UV ranges. The STIS spetrum helps to ll thegap between optial and extremely far UV data obtained previously with FUSE.More importantly it eliminates the large unertainty regarding extrapolation of theinterstellar extintion law in the FUSE range and in general provides a more reliableux alibration. Thus, we have observational data ontinuously overing the wholespetral range from about 10 000 to 900Å.The SED over a wide range of frequenies from infrared (IR) to X-rays is pre-sented in Fig 1. The optial and UV data an be tted with a single blak body(BB), while, for the X-rays, a seond BB with muh higher temperature is required.The disovery of a seond, hotter omponent in the system means that we do notneed to artiially elevate the extintion towards SBS1150+599A in order to justifythe presene of an ionization soure of at least 110 000K. Therefore in a revisionof previous results reported in [16℄ we apply a regular extintion in this diretionE(B-V)=0.029 [12℄ and t the observed points with two BB using a χ2 method. Oneof the possible solutions is presented in Fig. 1. Other solutions with omparableAn objet that dees stereotypes 517Fig. 1. Spetral energy distribution of SBS1150+599A. Open irles with error barsare X-ray data. Filled and open diamonds are optial & UV data. The observed dataare tted with two blak bodies presented with dotted & dashed lines and their sumby a solid line. The open triangles and open squares are SED of opt/UV and X-omponent respetively alulated by Nightfall for a number of solutions. They allare adjusted to the ux of opt/UV omponent. The disrepany between Nightfalland double BB ts an be explained by lak of onsideration of irradiation, geometryand eets of limb-darkening et., in BB tting. The solutions with muh hotter X-omponent produe a larger disrepany and are ruled out.χ2 are viable by inrease of the temperature/derease of size of the X-ray ompo-nent, due to some exess of emission at the high-energy end of the spetrum. Weexplain below why we hose the solution with the lower temperature. Aording tothis t the opt/UV omponent has a temperature of ≈ 58 000K and ratio of theradius to the distane (r/D)opt/UV = 3.8 × 10−13, while the X-ray omponent hasTX = 170 000K and (r/D)X = 5.6× 10−14.3 Parameters and the nature of the systemAs the next step, we may try to determine binary system parameters, making use ofthe radial veloity (RV) urve and lighturves in dierent olors. The rough knowl-edge of omponent's temperatures greatly improves our hanes of nding otherparameters of the system orretly. For this purpose we use a binary modeling odeNightfall provided by R. Wihmann.6The program allows tting a large number of6http://www.hs.uni-hamburg.de/DE/Ins/Per/Wihmann/Nightfall.html.Nightfall is based on a physial model that takes into aount the nonspherial518 Tovmassian et al.Fig. 2. Evolution traks of low mass post-AGB stars aording to [1℄. The plae ofopt/UV omponent of SBS1150+599A is marked by an arrow.unknown parameters of the system: total mass, mass ratio of omponents, size of theomponents measured in frations of orresponding Rohe lobe, temperatures andseparation of the system omponents. As an input for tting we provided RV urveas measured in [16℄ and light urves in three olors (roughly orresponding to U, B,R desribed in [8℄). Setting all parameters free fores the program immediately toseek solutions in a high mass stellar binary (> 20M⊙). However we know for surethat the system is omprised of ompat objets (WD-like) so the total mass shouldhover around 1.4M⊙. Fixing the total mass and temperature(s) of the omponentsnarrows the number of possible solutions, but a variety of dierent ongurationsresult in a similarly good t to the data. Therefore, additional onstraints are ne-essary to pin down the orret omposition of SBS 1150+599A.One of the most important parameters to know are the masses of the om-ponents. We derive the mass of the opt/UV omponent from the fat that it isthe envelope-shedding omponent on its trak from AGB to WD. Aording to [1℄,a ore of post-AGB star an reah temperatures ≥ 55 000K in a reasonable time(< a few×104 yr) only if it mass exeeds 0.56M⊙ (Fig 2). On the other hand we anevaluate the upper limit to the mass of opt/UV omponent from the onsiderationof ommon envelope (CE) evolution. In an 3.9 hr Pop. II binary, a 0.56M⊙ visibleomponent may desend from AGB star that had ZAMS mass 0.9-1.1M⊙. Forma-tion of a double degenerate with given orbital period through ommon enveloperequires a ertain ombination of the binding energy λ and CE expulsion eienyαCE, e.g. [19℄. We searhed solutions for a range of progenitor masses using ode[4℄ like in [17℄. The grid of solutions is presented in Fig 3. The mass of the opt/UVomponent progenitor should not exeed 1.1M⊙, provided that SBS1150+599A isPop. II objet, and it is not a strange invader aptured by merging of a satellitegalaxy. Fig 3 shows that this results in Mopt/UV < 0.58M⊙. These mass limits or-shape of stars in lose binary systems, as well as mutual irradiane of both stars,and a number of additional physial eets, like albedo or limb darkening. Night-fall an handle a large range of binary star ongurations, inluding overontat(ommon envelope) systems, eentri orbits, surfae spots and synhronous ro-tation, and the possible existene of a third star in the system.An objet that dees stereotypes 519Fig. 3. Possible ombinations of initial and nal masses of opt/UV omponent ofSBS 1150+599A (left panel) and orresponding αλ ombinations of binding energyλ & expulsion eieny αCE that result in orbital period of 3.9 hr (right panel).respond to αCEλ = 0.01 − 0.02, whih is not unreasonable for stars with relativelylarge ores and tenuous envelopes.We also an establish the lower limit for the mass of the X-ray omponent. Ap-parently the X-ray omponent is a WD remnant of a star, whih was more massivethan the progenitor of opt/UV omponent and has evolved through its AGB phasemuh earlier. The system most probably passed through the rst CE phase and fora ertain time appeared as a symbioti system, see, e. g., [7℄. In the symbioti phase,the X-omponent ould arete mass at a high rate (∼ 10−7M⊙/yr) and manifestitself as a supersoft X-ray soure (SSS), see, e. g. [5℄. The areting WD in suhsystems gets heated to extremely high temperatures by steady nulear burning ofareted matter at their surfae. Only aretors with Mwd > 0.7M⊙ reah temper-atures high enough to be deteted as SSS [18℄. The estimate of amount of matterareted during symbioti phase varies ([6, 7, 3℄), but at least 0.1-0.15M⊙ must beaumulated as a result. Therefore, we an assume that the total mass of the systemis very lose to or exeeding the Chandrasekhar limit.Introduing these additional onstraints to the Nightfall, we nd the best solu-tions desribing the light and radial veloity urves at the same time. The parametersof the system dedued by our analysis are presented in the Table 1.Table 1. The parameters of binary omponents of SBS 1150+599A.opt/UV-omponent X-omponentM (M⊙) 0.565 0.85 (Mtot = 1.41M⊙)R (R⊙) 0.40  0.48 0.16  0.05T (K) 65 - 57 000 165  245 000Corresponding ts to the RV and light urves are presented in Fig 4. The best tsare ahieved with inlination angle i = 50◦±4. The wide range of values in the rightolumn of Tab. 1 indiates that similarly good ts an be obtained with a variety520 Tovmassian et al.Fig. 4. Simultaneous ts to RV and light urves in three olors by Nightfall. Theparameters of the system dedued from χ2 ts are presented in Tab 1of temperature/radius ombination for the X-omponent. What matters for the tsis the luminosity of X-omponent to ensure ertain irradiation of the Rohe-lobelling opt/UV-omponent. Thus, the X-ray omponent may be ooler and larger orhotter and smaller. But, regardless of the unertainties of the XMM alibration andthe double blakbody tting to the SED, the orret solution is a rather ool andlarge X-ray omponent, beause the ux ratio of the omponents should obey theratio alulated by Nightfall for the Raleigh-Jeans tail and presented in Fig 1 byopen triangles for the opt/UV and by the open squares for X-ray omponent. Anextremely hot solution learly deviates from the presented set of solutions. However,even a hot solution requires a size obviously larger than an ordinary WD. It onrmsthe suggestion that the X-omponent underwent a period of intense aretion andis surrounded by a shell-like atmosphere, heated by the surfae burning.4 ConlusionsNew X-ray observations oupled with the previous UV and optial spetral energydistribution reveal a seond, previously undeteted omponent, of the lose doubledegenerate binary ore of PNG135.9+55.9. It appears to be a WD that has grownin size from areted material in a previous symbioti/SSS stage and heated by asteady nulear burning on its surfae. The disovery of the X-ray omponent addsanother unpreedented feature to the already unique harateristis of the objet,but at the same time helps to explain its nature. Aording to some senarios,double degenerate ompat binaries, progenitors of SNIa are formed as a result oftwo ommon envelope phases, whih helps to remove signiant amounts of angularmomentum and bring the remnants of the binary system omponents lose enough toAn objet that dees stereotypes 521Fig. 5. Shemati evolutionary trajetory of SBS 1150+599A. The urrent state ofthe objet is marked by 2000AD.merge in a Hubble time. We suggest that in rare ases these systems pass through asymbioti stage and show up for a short time as supersoft X-ray soures. A shematievolutionary senario for suh systems is presented in Fig 5. Currently, we deteta PN formed as a CE released by opt/UV-omponent. But its ionization is due tothe extreme UV photons from both omponents of the binary system, whih meansthat the oxygen abundane should be revised. SBS 1150+599A remains, however,one of the rarest objets of its type, observed as a PN, heated by its extremely hotbinary ore, with an extreme hemial omposition, loated in the very outskirts ofthe Galaxy. Its observation over a wide range of wavelengths allows insight into oneof the peuliar phases of evolution that lasts only a short time and thus is diultto ath.Aknowledgement. GT & JT aknowledge NASA grant NNX07AQ12G , GT is alsofunded by CONACyT. LY is supported by Origin and Evolution of Stars and Galax-ies Program and RFBR grant 07-02-00454. We appreiate expertise and marosrendered by J.Wilms for XMM data redution.Referenes1. Bloeker, T. 1995, A&A, 299, 7552. Jaoby, G. H., Feldmeier, J. J., Claver, C. F., Garnavih, P. M., Noriega-Crespo,A., Bond, H. E., & Quinn, J. 2002, AJ, 124, 33403. Hahisu, I. 2002, The Physis of Catalysmi Variables and Related Objets,261, 605522 Tovmassian et al.4. Hurley, J. R., Pols, O. R., & Tout, C. A. 2000, MNRAS, 315, 5435. Kahabka, P. 2006, Advanes in Spae Researh, 38, 28366. Korrek, K. E., Kellogg, E., & Sokoloski, J. L. 2007, Amerian Institute ofPhysis Conferene Series, 924, 9037. Lü, G., Yungelson, L., & Han, Z. 2006, MNRAS, 372, 13898. Napiwotzki, R., Tovmassian, G., Riher, M. G., Stasi«ska, G., Peña, M., Dreh-sel, H., Dreizler, S., & Rauh, T. 2005, Planetary Nebulae as AstronomialTools, 804, 1739. Péquignot, D., & Tsamis, Y. G. 2005, A&A, 430, 18710. Riher, M. 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An object that defies stereotypes : X-ray observations of SBS 1150+599A - the binary nucleus of PN G135.9+55.9

An object that defies stereotypes : X-ray observations of SBS 1150+599A - the binary nucleus of PN G135.9+55.9

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