Observational Evidence of Binary Wolf - Rayet Star Evolution

~\ large, high-velocity interstell~rshell has been discovered inlhe direction of the \Volr-Raye'! star HD 5089,6. Two dlslmct lllgh~\!eloelty IIIlerstdlar compOllenls at approximately -80 and -12:5 km s I were detected In the IOW-IOniZati,o!l resonance lines of fUE spectn.\ for HD 50896 and 1\ neighhoring field stars, prech!dmg the POSSibilIty that thIS 11Igh-veloclty gas is related to S308, the pfOlll:lnetll r:inlJ: nebula surroundmg HD 5(~896, The data collected 1'(11' HD 50896, at a distance of 1,5 kpc, lllld for ~ 9 fi~ld sta,rs located up to 4· lro~l HD 50896 with distallce~ ranging fnlill 0,.6 to 2.9 kilt, indicnl.e IIlal the 11Igh-vdoclty I~lterstellar gas hes at a. distance of 1.0 :c. 0:2 kpc, has lit. diameter in. excess of 90 pc, and is probably phySIcally assocIated with the c1~ster ~r 121, We conc!udt, that thi,s high-veilocity expanding Interstellar shell represents a p~evlOusly 11l1llden tlfied, Wt:lI-evolved supernOVl'1 remnant. Abundance esti· mates for tIllS gas mdlCate. relatively low depletion consistent with shock processing of interstellar grallls,.~urthenJlore,famt ddfuse opt,lcal 10 HI 1emission detected III!his direoC!ion appears well COrre· lated With the locatlor~ and extent of the observed high-velocity componenls, and thus IIlay represent weak optlc~1 shock emISSll?n. Implications for a possible association of 1he Sllpernovll rCIIHHlnl with H D 50896 and Its suspected bUlary complllllon llre discussed, L INTRODUCTION high-velocity gas was nol associaledwilh I hi:: ring nebula, S308, but inslead rcprt~sented a shocked rnedium some dis­ Ultraviolet interstellar absorption features in the spectra tance rromHD 50896. He speculated thai i his shocked gns of early-type stars have proven 10 be invaluable probes of mlghl be the resuh of either II supemOV!il I'crnnan! in the line high-velocily intersteHar medium phenomena, yielding ele­ 01' siight to HD 50896 or high-vdocllY gas feInted to some ment abundance, energy, and spatial information. High-ve­ previous stellar mass-loss episode, !'Iowever, Smilh, Willis, locity interstellar gas (i.e., v> 25 km s-.1 ) has been detecited and Wilson ( 1(80), using fUB speci mol' FlD 50896, aUri· via ultraviolet resonance absorption lines in the line of sight buted the high-velocily component at _. 80 kill S I seen in to several dift:erent types of objects, including supernova low-ionization lines to the ring nebula S308 on lhe basis of remnants (Vela: Jenkins, Silk, and Wallerstein 1976; len­ similar expansion velocities for other WolI~Rayet I'iing nebu­ hns, Wallerstein, and Silk 1984; S147: Phillips, Gondhale­ lae. kar, and Blades 1981; Philhps and Gondhalekm' 1983), OB Detection of a possible supernova remnant iill tbe line of associations (Cowie, Songaila, and York 1979; Cowie et al. sight to, and possibly associated with, the Wolf-Rayet star I9S1; Phillips and Gondhalekar 1981; Phillips, Welsh, and HD 508% would be of considerable interest. HD 50896 is a Pettini 1984; Laurent, Paul, and Pettini \982;. Walborn, v~riable star ( = EZ Canis Majoris) with a reported period Heckathorn, and Hesser 1984; McLachlan and Nandy 01 3.76 ~lays, ~uggestll1g the existence of a compact binary 1985), and Wolf-Rayet stars (Shullil 1977;. Huber et al. compal1lon (Flrmalll el al. 1980). Evolutionary models have 1979) . been proposed in which a binary system cOlltaining a Wolf­ One of the first stars found to exhibit high-velocity inter­ Rayet -\compact companion represent 11 "second" Wolf­ stellar line components is the Wo]f-Rayet star HD 50896. Rayet stage (van den Heuvel 19'76; de Loore \982). This Classified as WN5 (Smith 1973), this star is surrounded by phasewould have been preceded by a supernova, which pro­ the optical ring nebula S308 ( = RCW 11) with a cliameter duced the compact companiion, i.e., neutron star or black of 40'. The nebula has an observed expansion velocity of 45­ h?Ie. Fin?ing a supernova remnant in Ihe vicinity of at WR 60 km S~-.1 (Chu et ai, 1982; Pismis and Quintao 1982), and bmary mIght be Viewed as supponing evidence for this see· is beIieved to be a wind-blown interstellar bubbllie produced nano, as none are presentlly known. At an. estimated distance by HD 50896 (Chu et al. 1982). However, ultraviolet ab­ of I.S kpe (SIi1ith 1?73), ~D 50896 is 260 pc off the galactic sorption features exhibiting significantly higher velocities plane producmg a hne of SIght that is less confused than that are present in the spectrum of HD 50896 and were first de­ of other WolrRayet strurs nearer the galactic plane. This, tected by Shun (1977) from an analysis of Copernicus ultra­ loge~her with its suspected binary nature and tIle presence of violet spectral data. Ultraviolet lines of low ionization were pOSSible high-velocity interstellar UV absorption lines, made observed at 78 and -96 km sI with estimated electron ~D 50896 a prime candidate t:or investigation using u!tTal­ densities of n e = 0.3 em 3. ShuB found iron and silicon VIOlet spectl'Oscopy, abundances near cosmic levels with Si HI shortward absorp­ III this. paper, o~r initial discovery of an apparent large tIOn and N n/N t and Si IUISi II ratios suggestive of eoHi­ shell of 11lgh-vclocl~y mterstellar gas in the line-of-sight di­ sional ionization. He concluded from these data that this rectIOn of and posslb.ly related to HD 50896 (Heckathorn and Fesen 1984a,b) is more fuBy described. Ultraviolet .rUE "j GlIest Investigator witb the [mernational VI/muiolel Explorer Salelli·te. data and analysis are described in Sec. H for 19 stars located 642 Astron. J. 92 (3), September 1986 0004-6256/86/030642-11 $00,90 @) 1986 Am. Aslroll, Soc. 642 643 14 J. NICHOLS-BOHlIN AND R. A. FESEN: HD 50896 within 4° oLHD 50896, with velocity displacement and equi­ valent-width measurements tabulated for aU low-ionization high-velocity interstellar lines observed. Where possible, column densities have been derived using a curve-or-growth analysis, In Sec. HI, a distance estimate for Ihis high-velocity interstellar medium is given along wilh a spatial map and a discussion of its inferred structure. We Ihen dis,cuss in Sec. IV its physical properties" its possible association with HD 50896, and scenar,ios rm its origin.