THE ASTROPHYSICAL JOURNAL, 518:428È441, 1999 June 10 ( 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. A 2.3 DAY PERIODIC VARIABILITY IN THE APPARENTLY SINGLE WOLF-RAYET STAR WR 134: COLLAPSED COMPANION OR ROTATIONAL MODULATION? THIERRY MOREL,1,2 SERGEY V. MARCHENKO,1 PHILIPPE R. J. EENENS,3 ANTHONY F. J. MOFFAT,1,4 GLORIA KOENIGSBERGER,5 IGOR I. ANTOKHIN,6 THOMAS EVERSBERG,1,7 GAGHIK H. TOVMASSIAN,8 GRANT M. HILL,9 OCTAVIO CARDONA,10 AND NICOLE ST-LOUIS1 Received 1998 April 17; accepted 1999 January 19 ABSTRACT The apparently single WN 6 type star WR 134 (HD 191765) is distinguished among the Wolf-Rayet star population by its strong, presumably cyclical (P B 2.3 day) spectral variations. A true periodicityÈ which is still very much debatedÈwould render WR 134 a prime candidate for harboring either a col- lapsed companion or a rotating, large-scale, inhomogeneous outÑow. We have carried out an intensive campaign of spectroscopic and photometric monitoring of WR 134 from 1989 to 1997 in an attempt to reveal the true nature of this object. This unprecedentedly large data set allows us to conÐrm un- ambiguously the existence of a coherent 2.25 ^ 0.05 day periodicity in the line-proÐle changes of He II j4686, although the global pattern of variability is di†erent from one epoch to another. This period is only marginally detected in the photometric data set. Assuming the 2.25 day periodic variability to be induced by orbital motion of a collapsed companion, we develop a simple model that aims to investigate (1) the e†ect of this strongly ionizing, accreting companion on the Wolf-Rayet wind structure, and (2) the expected emergent X-ray luminosity. We argue that the predicted and observed X-ray Ñuxes can only be matched if the accretion on the collapsed star is signiÐcantly inhibited. Additionally, we performed simu- lations of line-proÐle variations caused by the orbital revolution of a localized, strongly ionized wind cavity surrounding the X-ray source. A reasonable Ðt is achieved between the observed and modeled phase-dependent line proÐles of He II j4686. However, the derived size of the photoionized zone substan- tially exceeds our expectations, given the observed low-level X-ray Ñux. Alternatively, we explore rota- tional modulation of a persistent, largely anisotropic outÑow as the origin of the observed cyclical variability. Although qualitative, this hypothesis leads to greater consistency with the observations. Subject headings: stars: individual (WR 134) È stars: mass loss È stars: Wolf-Rayet 1. INTRODUCTION O ] c ] WR ] c ] c. Because of the recoil of the Ðrst supernova explosion leading to c ] O, the system generally The recognition in the 1980s that some apparently single acquires a high systemic velocity. If massive enough, the Wolf-Rayet (WR) stars exhibit seemingly periodic line secondary in turn evolves into a WR star, at which point the proÐle and/or photometric variations argued for the exis- system may have reached an unusually high Galactic lati- tence of systems made up of a WR star and a collapsed tude for a Population I star. These two peculiarities companion (hereafter WR ] c, where c stands either for a (runaway velocity and position), along with the existence of neutron star or a black hole), as predicted by the general a surrounding ring nebula (presumably formed by matter theory of massive close-binary evolution (e.g., van den ejected during the secondary mass exchange), were among Heuvel & de Loore 1973): O O ] WR O ] c ] ] ] the criteria initially used to select WR ] c candidates (van den Heuvel 1976; Mo†at 1982; Cherepashchuk & Aslanov 1 De partementde Physique, Universite deMontre al, C.P. 6128, Succ. 1984).11 Centre-Ville,Montre al, Que bec H3C 3J7, Canada, and Observatoire du A major breakthrough in the qualitative scenario Mont Me gantic; morel=astro.umontreal.ca, sergey=astro.umontreal.ca, described above comes from a recent redetermination of the mo†at=astro.umontreal.ca, eversber=astro.umontreal.ca, stlouis= astro.umontreal.ca. distribution of the radio pulsar runaway velocities. In sharp 2 Present address: Astrophysics Group, Imperial College of Science, contrast with earlier estimations (100È200 km s~1; Gunn & Technology, and Medicine, Blackett Laboratory, Prince Consort Road, Ostriker 1970; Lyne, Anderson, & Salter 1982), these new London, SW7 2BZ, UK; morel=ic.ac.uk. values imply a mean pulsar kick velocity at birth of about 3 Departamento deAstronom• a, Universidad de Guanajuato, Apartado Postal 144, 36000 Guanajuato Gto, Mexico; 450 km s~1 (Lyne & Lorimer 1994; Lorimer, Bailes, & eenens=carina.astro.ugto.mx. Harrison 1997). Although such a high kick velocity impart- 4 Killam Research Fellow of the Canada Council for the Arts. ed at birth is still much debated (e.g., Hansen & Phinney 5 Instituto deAstronom• a, UNAM, Apartado Postal 70-264, Me xico 1997; Hartman 1997), this result tends to showÈas sug- D.F. 04510, Me xico; gloria=astroscu.unam.mx. gested by the apparent scarcity of ““ runaway ÏÏ OB stars with 6 Sternberg Astronomical Institute, Universiteskij Prospect 13, 119899, Moscow, Russia; igor=sai.msu.su. compact companions (Kumar, Kallman, & Thomas 1983; 7 Present address: Feinfocus Medizintechnik GmbH, Im Bahlbrink 11-13, 30827, Garbsen, Germany; t–eversberg=feinfocus.com. 8 Instituto deAstronom• a, Apartado Postal 877, C. P. 22860, Ensenada, 11 Since then, only Cyg X-3 has been shown to probably belong to this B. C., Me xico; gag=bufadora.astrosen.unam.mx. class (van Kerkwijk et al. 1996; Schmutz, Geballe, & Schild 1996; but see 9 McDonald Observatory, HET, P. O. Box 1337, Fort Davis, TX; Mitra 1998). Besides this system, the single-line WN 7 star WR 148 (HD grant=astro.as.utexas.edu 197406) constitutes one of the most promising candidates (Marchenko et 10 Instituto Nacional deAstrof• sica, Optica y Electro nica, Apartado al. 1996a). Two candidates for WR ] c in 30 Doradus have also been Postal 51, Puebla, Pue. 72000, Me xico; ocardona=inaoep.mx. proposed by Wang (1995). 428 PERIODIC VARIABILITY IN WR 134 429 Gies & Bolton 1986; Philp et al. 1996; Sayer, Nice, & Kaspi was the Ðrst to investigate photometric variability of WR 1996)Èthat the number of systems that would survive the 134 on di†erent timescales. Irregular night-to-night light Ðrst supernova explosion is probably considerably lower Ñuctuations (with rms amplitude pB0.014 mag) were than initially thought (see Brandt & Podsiadlowski 1995 vs. observed, without evidence for rapid (Dhourly) or long- Hellings & de Loore 1986). Indeed, the incorporation of term (Dmonthly) changes. The Ðrst claim of periodic varia- up-to-date physics of supernova explosions in the most bility with P \ 7.44 ^ 0.10 days was made by Antokhin, recent population synthesis models of massive binaries Aslanov, & Cherepashchuk (1982) on the basis of a 2 month leads to a very small number of observable WR ] c systems interval of broadband photometric monitoring. The in the Galaxy(N [ 5; De Donder, Vanbeveren, & van authors also reported small-amplitude radial velocity varia- Bever 1997). This number contrasts signiÐcantly with the tions present in simultaneously acquired spectroscopic number of observationally selected candidates (N B 15; data(K B 20È40 km s~1), consistent with the above Vanbeveren 1991), especially considering the necessarily period.WR This period was subsequently improved to incomplete nature of this sample. At face value, this discrep- P \ 7.483 ^ 0.004 days by Antokhin & Cherepashchuk ancy might imply that most of these WR ] c candidates are (1984). Zhilyaev & Khalack (1996) established that the spurious or, more interestingly, that other physical mecha- short-term stochastic variability presented by WR 134 is nisms are at work to induce the large-scale, periodic varia- not related to this period, as it would be in the case of an bility inherent to some objects. orbiting collapsed companion. Mo†at & Shara (1986) were At least in O stars, the progenitors of WR stars, it appears unable to identify this period in their photometric data, that large-scale periodic variability may be induced by although they found evidence for a 1.8 day periodicity, Ðrst rotating aspherical and structured winds (e.g., Fullerton et tentatively reported by Lamontagne (1983) on the basis of al. 1997; Kaper et al. 1997). The incidence of asymmetric radial velocity measurements. Following the recognition outÑows among the WR star population is probably lower, that the level of continuum Ñux from WR 134 is also irregu- although recent spectropolarimetric (Harries, Hillier, & larly variable on a timescale of weeks to months (Antokhin Howarth 1998), UV (St-Louis et al. 1995), optical & Volkov 1987), periodic changes of the equivalent widths (Marchenko et al. 1998a), and radio observations (White & (EWs) of He II j4859 with P \ 1.74 ^ 0.38 days were Becker 1996; Williams et al. 1997) have now independently reported by Marchenko (1988). Then Robert (1992) report- revealed this peculiarity in a substantial number of WR ed the existence of a 2.34 day periodicity in radial velocity stars. In order to account for the periodic variability variations, which could be a 1 day alias of the 1.8 day period inferred for the suspected WR ] c candidates, the rotation- claimed in previous studies. A 2 day quasi-periodicity in al modulation of a persistent, largely inhomogeneous photometric data was also reported by Antokhin et al. outÑow could thus constitute in some cases an attractive (1992). Although not obvious in the spectroscopic data set alternative to the binary scenario, and it would be consis- obtained by Vreux et al. (1992) because of an unfortunate tent with the lack of strong, accretion-type X-ray emission correspondence to a badly sampled frequency domain (see (Wessolowski 1996).