Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012a%26a...539a.114r&link_type=abstract
Astronomy & Astrophysics, Volume 539, id.A114
Astronomy and Astrophysics
Astrophysics
Circumstellar Matter, X-Rays: Binaries, Stars: Early-Type
Scientific paper
Aims: We study the optical and near-infrared colour excesses produced by circumstellar emission in a sample of Be/X-ray binaries. Our main goals are exploring whether previously published relations, valid for isolated Be stars, are applicable to Be/X-ray binaries and computing the distance to these systems after correcting for the effects of the circumstellar contamination. Methods: Simultaneous UBVRI photometry and spectra in the 3500-7000 Å spectral range were obtained for 11 optical counterparts to Be/X-ray binaries in the LMC, 5 in the SMC and 12 in the Milky Way. As a measure of the amount of circumstellar emission we used the Hα equivalent width corrected for photospheric absorption. Results: We find a linear relationship between the strength of the Hα emission line and the component of E(B - V) originating from the circumstellar disk. This relationship is valid for stars with emission lines weaker than EW ≈ -15 Å. Beyond this point, the circumstellar contribution to E(B - V) saturates at a value ≈0.17 mag. A similar relationship is found for the (V - I) near infrared colour excess, albeit with a steeper slope and saturation level. The circumstellar excess in (B - V) is found to be about five times higher for Be/X-ray binaries than for isolated Be stars with the same equivalent width EW(Hα), implying significant differences in the physical properties of their circumstellar envelopes. The distance to Be/X-ray binaries (with non-shell Be star companions) can only be correctly estimated by taking into account the excess emission in the V band produced by free-free and free-bound transitions in the circumstellar envelope. We provide a simple method to determine the distances that includes this effect.
Partially based on observations collected at the European Southern Observatory, La Silla, Chile (66.D-0292; 074.D-0529) and the Nordic Optical Telescope.
Negueruela Ignacio
Riquelme M. S.
Torrejon Jose Miguel
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