Fundamental parameters, helium abundance and distance of X Persei

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Abstract
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 Teff, surface gravity g and interstellar absorption Av for the visible component of the system are determined: Teff = 29500±1500 K, log g = 4.0 ± 0.2 and Av = 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 ε(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 v sin i = 215 ±10 km s−1 and is located at a distance d = 700 α 300 pc. The X-ray flux corresponding to this revised distance is of the order of 1–2 × 1034 erg s−1 in the normal state, and around 1035 erg s−1 in the only bright state observed (1974–75). Non-LTE analysis of more recently obtained high-resolution profiles of the He ii 14686 line shows that there is no evidence for additional heating during the high-luminosity phase.

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