Issue |
A&A
Volume 457, Number 2, October II 2006
|
|
---|---|---|
Page(s) | 595 - 610 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20065393 | |
Published online | 12 September 2006 |
VLT/UVES spectroscopy of Wray 977, the hypergiant companion to the X-ray pulsar GX301-2
1
Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: [lexk;ameer]@science.uva.nl
2
Instituto de Estructura de la Materia, CSIC, Serrano 121, 29006 Madrid, Spain e-mail: najarro@damir.iem.csic.es
Received:
7
April
2006
Accepted:
28
June
2006
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 , an effective temperature of 18 100 K and a
luminosity of
. 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
yr-1 and 305 km s-1,
respectively. The interstellar
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
kpc. The luminosity derived from the model atmosphere is
consistent with this distance (3 kpc). The luminosity of the
wind-fed X-ray pulsar (
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
km s-1 yielding a mass ratio
. The absence of an X-ray
eclipse results in a lower limit to the mass of Wray 977 of
39
. An upper limit of 68 or 53
is derived for
the mass of Wray 977 adopting a maximum neutron star mass of 3.2 or
2.5
, respectively. The corresponding lower limit to the
system inclination is
, 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
, consistent with the range in mass derived
from the binarity constraints. The mass of the neutron star is
. Time series of spectral lines formed in the
dense stellar wind (e.g.
5876 Å and
)
indicate the presence of a gas stream trailing the neutron star in its
orbit. The long-term behaviour of the
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.
Key words: stars: binaries: close / stars: evolution / stars: individual: Wray 977 / stars: pulsars: individual: GX301-2 / stars: supergiants / X-rays: stars
© ESO, 2006
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