Fundamental parameters, helium abundance and distance of X Persei

Astronomy and Astrophysics – Astronomy

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Stars: Abundances, Circumstellar Matter, Stars: Emission-Line, Be, Stars: Fundamental Parameters, Stars: Individual: X Per

Scientific paper

We present spectral observations of the Be/X-ray binary system X Persei/4U 0352+30 taken during the recent (1989-91) low-luminosity discless phase. On the basis of new spectroscopic and photometric data the following self-consistent values of effective temperature T_eff, surface gravity g and interstellar absorption A_v for the visible component of the system are determined: T_eff=29500+/-1500 K, log g=4.0+/-0.2 and A_v=1.05+/-0.02. The corresponding spectral classification of the star is B0 V. The helium overabundance He/H=0.19 is found, as well as the enhanced nitrogen abundance log epsilon(N)~8.4. Two possible explanations of the He excess are proposed: (a) internal enrichment of the outer layers of a massive star by CNO-cycle products because of a mixing on the main-sequence evolutionary phase or (b) external accretion during the evolution of a massive close binary. The star has a rotational velocity vsini=215+/-10km s^-1 and is located at a distance d=700+/-300pc. The X-ray flux corresponding to this revised distance is of the order of 1-2x10^34erg s^-1 in the normal state, and around 10^35erg s^-1 in the only bright state observed (1974-75). Non-LTE analysis of more recently obtained high- resolution profiles of the He ii lambda4686 line shows that there is no evidence for additional heating during the high-luminosity phase.

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