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A Hypergiant with Pulsar Companion

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A Hypergiant with Pulsar Companion AA manuscript no will b e inserted by hand later ASTRONOMY AND Your thesaurus co des are ASTROPHYSICS Wray GX Wray GX a hypergiant with pulsar ? companion 1 23 456 45 78 L Kap er HJGLM Lamers E Ruymaekers EPJ van den Heuvel and EJ Zuiderwijk 1 Europ ean Southern Observatory Karl Schwarzschild Str D Garching b ei M unchen Germany 2 Sterrekundig Instituut Princetonplei n Postbus Utrecht The Netherlands 3 SRON Utrecht Sorb onnelaan CA Utrecht The Netherlands 4 Astronomical Institute Anton Pannekoek University of Amsterdam Kruislaan SJ Amsterdam The Netherlands 5 Center for High Energy Astrophysics CHEAF Kruislaan SJ Amsterdam The Netherlands 6 Instituut voor Sterrenkunde KU Leuven Celestijnenlaan B Heverlee Belgium 7 Kapteyn Astronomical Institute Postbus AV Groningen The Netherlands 8 Royal Greenwich Observatory Madingley Road Cambridge CB EZ England Received November Accepted January Abstract Wray the B sup ergiant companion of the They derived a sp ectral type of B Iae for the su Xray pulsar GX should b e classied as B Ia p ergiant and concluded that the massloss rate is probably 6 1 based on a comparison of its optical sp ectrum to that of higher than M yr adding the remark that 1 Sco a wellknown B hypergiant and one of the bright the sp ectrum of Wray do es show more signs of mass est stars in our galaxy The classication of Wray as loss than a normal B sup ergiant Adopting V a hypergiant results in a new distance determination of HammerschlagHensberge et al and absolute mag the binary system ie d kp c previously nitude M for a B Ia star Balona Crampton V kp c The average Xray luminosity of the pulsar is then they derived a distance to Wray of 37 1 erg s in go o d agreement with the predicted X kp c This distance implied an average Xray luminosity of 36 1 ray luminosity resulting from accretion of a dense low erg s for the s pulsar GX 1 velocity v km s stellar wind A massloss rate The orbital p erio d of this highmass Xray binary sys 1 5 1 of M yr is estimated from the H prole A tem HMXB is days Watson et al based new upp er limit for the inclination of the system is de on the p erio dicity in Xray aring events of GX rived which provides a lower limit to the present mass Pulsetiming analysis carried out by White Swank of Wray M Regarding current binary evolution and from a more extended dataset by Sato et al scenarios the empirical lower mass limit for blackhole resulted in the accurate determination of the or M formation in a binary increases to M bital elements of GX The system is characterized Z AM S by a large eccentricity e and the mass function Key words Stars binaries close Stars evolution f m M is the highest known for a HMXB Stars individual Wray Stars pulsars individual with pulsar companion The latter fact was used by Van GX Stars sup ergiants Xrays stars den Heuvel Hab ets to derive an observational astro-ph/9503003 01 Mar 1995 lower mass limit for blackhole formation M Z AM S M in a massive binary Given the sup ergiants radius R Parkes et al the absence of an eclipse in the Xray lightcurve constrains the inclination of the sys tem to i A plausible set of parameters is i Introduction and hence from the mass function M M Sato et al The most recent sp ectral study of Wray BP Cru the Several authors have tried to explain the observed optical counterpart Vidal of the binary Xray pul mo dulation of the Xray luminosity of GX with or sar GX U was presented by Parkes et al bital phase White Swank prop osed that the mass loss from Wray must b e extreme and that the Send oprint requests to L Kap er wind velocity at the neutron stars orbit is much lower Based on observations obtained at the Europ ean Southern Observatory than usually adopted for a normal B sup ergiant Leahy L Kap er et al Wray GX a hypergiant with pulsar companion 1 Fig The optical sp ectrum of Wray b ottom from A and that of the hypergiant Sco top are very similar 1 A line of Wray are stronger and The sp ectrum of Sco was kindly provided to us by Dr O Stahl The H and He i 1 have a P Cygnitype prole suggesting that Wray has an even higher massloss rate than Sco The O i i lines are stronger 1 and the N i i lines weaker than those in the sp ectrum of Sco indicating that Wray has a slightly earlier sp ectral type 1 than Sco ie B Ia The strong O i i lines the relatively weak He i A line in comparison with the Si i i i lines and the strong Balmer emission are consistent with an extremely high luminosity of Wray and Hab erl suggested that a gas stream in using line strength ratios of Si ivHe i Mg i iHe i and the system might b e resp onsible for the observed drastic O i iHe i The great strength of the O i i lines particularly increase in Xray luminosity just b efore each p eriastron O i i A is indicative of a high luminosity Therefore passage In the following we will present arguments that they prop osed that the sp ectral type of Wray is B Ia Wray is not a normal B sup ergiant but a B hyper giant This results in a new distance determination of the system new estimates for the various stellar parameters Table Log of observations of Wray obtained with the are given in Section including a new lower limit to the ESO m telscop e and CASPEC the orbital phase is calcu mass of Wray In the nal section we discuss the im lated from JD mo d given in Sato et 0 plications for massive binary evolution al JD 4 t Orbital exp Sp ectral classication and luminosity of Feb A sec phase Wray In Figs and we show parts of the optical sp ectrum of Wray obtained in February with the Cassegrain sp ectrograph CASPEC mounted on the m telescop e at the Europ ean Southern Observatory in Chile Tab The sp ectra partly cover the wavelength region from to A The Balmer up to H and the strong he In the atlas of optical sp ectra of galactic B sup ergiants lium lines have P Cygnitype proles indicating a strong Lennon et al only one of the early B sup ergiants stellar wind The H emission combined with sp ectral shows emission in the Balmer series HD with sp ec type B usually classies a star as Be but for a Be star the tral type B Ia The classication refers to a class luminosity class should b e in the range VI I I cf Underhill of sup ersup ergiants Keenan initially consisting Doazan Hammerschlag et al and Parkes of the four reddest of the brightest stars in the Large et al classied the optical sp ectrum of Wray Magellanic Cloud Feast Thackeray Van Gen L Kap er et al Wray GX a hypergiant with pulsar companion 1 Fig The optical sp ectrum of Sco and Wray in the wavelength region from to A Note the weaker N i i lines in the sp ectrum of Wray and the stronger emission in the He i prole at A Although a uorescence line of Fe i i i is 1 present in the sp ectrum of Sco the bad SN prohibits a rm detection of this line in this part of the sp ectrum of Wray deren named these stars hypergiants A wellknown N i i lines weaker than their counterparts in the sp ectrum 1 1 earlyB hypergiant is Sco HD one of the most of Sco which is conrmed by the measured EWs listed luminous stars in our galaxy It is lo cated in the neigh in Table Using the atlases of OBstar sp ectra by Lennon b ourho o d of the op en cluster NGC in the asso ciation et al and Walborn Fitzpatrick we con I Scorpii which has a distance mo dulus m M clude that Wray has a slightly earlier sp ectral type 0 1 1 Stothers The sp ectral type of Sco is B Ia than Sco B in stead of B For this sp ectral type T K App enzeller Wolf and the ab the relative strength of the He i A line compared to ef f solute magnitude M Humphreys The the Si i i i lines at provides a go o d indica V ultraviolet P Cygni proles can b e found in Wolf Ap tion of the luminosity class The fact that the He i line is p enzeller and are comprised in the new IUE atlas weaker than the Si i i i lines clearly demonstrates that the of Snow et al luminosity class is Ia see Walborn Fitzpatrick Indeed the sp ectrum of HD BIa resembles the If we compare the optical sp ectrum of Wray with sp ectrum of Wray even more Wolf private commu 1 that of Sco Figs and we note that the two sp ectra nication are very similar To enable a more quantitative compar ison we have measured the equivalent widths EWs of Wolf Stahl found that uorescence lines of the most imp ortant sp ectral lines in the wavelength range Fe i i i are sensitive to luminosity in earlyB sup ergiants A Table The uncertainty in EW due to eg An example of an Fe i i i emission line at A and at 1 the normalization of the sp ectra is estimated to b e A at the b order of the plot in the sp ectrum of Sco or less A careful insp ection of the sp ectra shows that the is shown in Fig in agreement with the hypergiant na O i i lines in the sp ectrum of Wray are stronger and the ture of this star Due to the bad signaltonoise in this L Kap er et al Wray GX a hypergiant with pulsar companion part of the sp ectrum of Wray we can not conrm the 1 Table Line equivalent widths for Sco and Wray The presence of this line although an emission feature might uncertainty in EW is estimated to b e or less Some uniden b e identied at A tied lines may corresp ond to interstellar features Finally the P Cygni shap e of the H Balmer and the strong He i lines is consistent with the extreme luminos Line A EW A EW A Remarks ity of Wray Rosendhal showed that the net 1 Sco Wray strength of H is correlated with luminosity
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