NLTE-analysis of the sdO star ROB 162 in the globular cluster NGC 6397

Astronomy and Astrophysics – Astrophysics

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Globular Clusters, Nonequilibrium Thermodynamics, O Stars, Planetary Nebulae, Stellar Models, Abundance, Asymptotic Giant Branch Stars, Atmospheric Models, Stellar Evolution, Stellar Luminosity, Ultraviolet Spectra, White Dwarf Stars

Scientific paper

A high-resolution visual blue spectrum of the subluminous O star ROB 162 in the globular cluster NGC 6397 has been analysed for its atmospheric parameters by means of NLTE model atmospheres. The effective temperature Teff = 51000±2000K, the gravity log g = 4.5±0.2 and a normal helium mass fraction (Y = 0.28 (+0.04, -0.06)) resulted. Spectroscopically, ROB 162 appears not to be a population II counterpart of the field sdO stars. Instead, it resembles a (metal-poor) central star of a planetary nebula (CPN) although no nebula has been found so far. From comparison with evolutionary calculations it is concluded that ROB 162, like the CPNs, is a post-AGB star of 0.55 M_sun; - 0.6 M_sun;. It is conjectured that a hypothetical nebula is already dispersed or too faint to be detected.

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