Bibcode
Herrero, A.; Vilardell, F.; Norton, A. J.; Castro, N.; Negueruela, I.; Lorenzo, J.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 562, id.A18, 10 pp.
Fecha de publicación:
2
2014
Revista
Número de citas
24
Número de citas referidas
19
Descripción
Context. The current generation of X-ray satellites has discovered many
new X-ray sources that are difficult to classify within the
well-described subclasses. The hard X-ray source IGR J11215-5952 is a
peculiar transient, displaying very short X-ray outbursts every 165
days. Aims: To characterise the source, we obtained
high-resolution spectra of the optical counterpart, HD 306414, at
different epochs, spanning a total of three months, before and around
the 2007 February outburst with the combined aims of deriving its
astrophysical parameters and searching for orbital modulation.
Methods: We fit model atmospheres generated with the fastwind code to
the spectrum, and used the interstellar lines in the spectrum to
estimate its distance. We also cross-correlated each individual spectrum
to the best-fit model to derive radial velocities. Results: From
its spectral features, we classify HD 306414 as B0.5 Ia. From the model
fit, we find Teff ≈ 24 700 K and log g ≈ 2.7, in good
agreement with the morphological classification. Using the interstellar
lines in its spectrum, we estimate a distance to HD 306414 d ≳ 7
kpc. Assuming this distance, we derive R∗ ≈ 40
R⊙ and Mspect ≈ 30 M⊙
(consistent, within errors, with Mevol ≈ 38
M⊙, and in good agreement with calibrations for the
spectral type). Analysis of the radial velocity curve reveals that
radial velocity changes are not dominated by the orbital motion, and
provide an upper limit on the semi-amplitude for the optical component
Kopt ≲ 11 ± 6 km s-1. Large variations
in the depth and shape of photospheric lines suggest the presence of
strong pulsations, which may be the main cause of the radial velocity
changes. Very significant variations, uncorrelated with those of the
photospheric lines are seen in the shape and position of the Hα
emission feature around the time of the X-ray outburst, but large
excursions are also observed at other times. Conclusions: HD
306414 is a normal B0.5 Ia supergiant. Its radial velocity curve is
dominated by an effect that is different from binary motion, and is most
likely stellar pulsations. The data available suggest that the X-ray
outbursts are caused by the close passage of the neutron star in a very
eccentric orbit, perhaps leading to localised mass outflow.
Based on observations collected at the European Southern Observatory, La
Silla, Chile (ESO 078.D-0172).
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