Astronomy & Astrophysics

Free access article

A&A 440, 305-320 (2005)
DOI: 10.1051/0004-6361:20042229

Effects of gravitational darkening on the determination of fundamental parameters in fast-rotating B-type stars

Y. Frémat¹, J. Zorec², A.-M. Hubert³ and M. Floquet³

¹ Royal Observatory of Belgium, 3 avenue circulaire, 1180 Bruxelles, Belgium
e-mail: yves.fremat@oma.be
² Institut d'Astrophysique de Paris, CNRS, 98bis Boulevard Arago, 75014 Paris, France
³ Observatoire de Paris, Section d'Astrophysique de Meudon, GEPI, FRE K 2459, 5 place Jules Janssen, 92195 Meudon Cedex, France

(Received 22 October 2004 / Accepted 19 March 2005)

Abstract
In this paper we develop a calculation code to account for the effects carried by fast rotation on the observed spectra of early-type stars. Stars are assumed to be in rigid rotation, and the grid of plane-parallel model atmospheres used to represent the gravitational darkening are calculated by means of a non-LTE approach. Attention is paid to the relation between the apparent and parent non-rotating counterpart stellar fundamental parameters and apparent, and true $V\!\sin i$ parameters as a function of the rotation rate $\Omega/\Omega_{\rm c}$ , stellar mass, and inclination angle. It is shown that omitting of gravitational darkening in the analysis of chemical abundances of CNO elements can produce systematic overestimation or underestimation, depending on the lines used, rotational rate, and inclination angle. The proximity of Be stars to the critical rotation is revised while correcting not only the $V\!\sin i$ of 130 Be stars, but also their effective temperature and gravity to account for stellar rotationally induced geometrical distortion and for the concomitant gravitational darkening effect. We concluded that the $V\!\sin i$ increase is accompanied by an even higher value for the stellar equatorial critical velocity, so that the most probable average rate of the angular velocity of Be stars attains $\Omega/\Omega_{\rm c} \simeq 0.88$ .

Key words: stars: abundances -- stars: atmospheres -- stars: early-type -- stars: emission-line, Be -- stars: fundamental parameters

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