VLT/UVES spectroscopy of Wray 977, the hypergiant companion to the X-ray pulsar GX301-2

Model atmosphere fits to high-resolution optical spectra of Wray 977 confirm the B hypergiant classification of the massive companion to the X-ray pulsar GX301-2. The models give a radius of 62 R-., an effective temperature of 18 100 K and a luminosity of 5 x 10(5) L-.. These values are somewhat reduced compared to the stellar parameters of Wray 977 measured previously. The deduced mass-loss rate and terminal velocity of the stellar wind are 10(-5) M-. yr(-1) and 305 km s(-1), respectively. The interstellar Na I D absorption indicates that Wray 977 is located behind the first intersection with the Sagittarius-Carina spiral arm ( 1 - 2.5 kpc) and probably belongs to the stellar population of the Norma spiral arm at a distance of 3-4 kpc. The luminosity derived from the model atmosphere is consistent with this distance ( 3 kpc). The luminosity of the wind-fed X-ray pulsar (L-X similar to 10(37) erg s(-1)) is in good accordance with the Bondi-Hoyle mass accretion rate. The spectra obtained with UVES on the Very Large Telescope (VLT) cover a full orbit of the system, including periastron passage, from which we derive the radial-velocity curve of the B hypergiant. The measured radial-velocity amplitude is 10 +/- 3 km s(-1) yielding a mass ratio q = M-X/M-opt = 0.046 +/- 0.014. The absence of an X-ray eclipse results in a lower limit to the mass of Wray 977 of 39 M-.. An upper limit of 68 or 53 M-. is derived for the mass of Wray 977 adopting a maximum neutron star mass of 3.2 or 2.5 M-., respectively. The corresponding lower limit to the system inclination is i > 44 degrees, supporting the view that the dip in the X-ray lightcurve is due to absorption by the dense stellar wind of Wray 977 (Leahy 2002). The spectroscopic mass of Wray 977 is 43 +/- 10 M-., consistent with the range in mass derived from the binarity constraints. The mass of the neutron star is 1.85 +/- 0.6 M-.. Time series of spectral lines formed in the dense stellar wind (e.g. He I 5876 angstrom and H alpha) indicate the presence of a gas stream trailing the neutron star in its orbit. The long-term behaviour of the H alpha equivalent width exhibits strong variations in wind strength; the sampling of the data is insufficient to conclude whether a relation exists between wind mass-loss rate and pulsar spin period.