The Formation Of Massive Stars And The Effects Of Rotation On Star Formation

Geneva Observatory, University of Geneva, 51 chemin des Maillettes, 1290 Versoix, Switzerland

e-mail: andre.maeder@unige.ch

Abstract

We first review the current debates about massive star formation over the last decade. Then we concentrate on the accretion scenario, emphasizing the evidences in favor of it. We study the basic properties of the accretion scenario in the spherical case. In the case of massive stars, the free-fall time is longer than the Kelvin–Helmholtz timescale, so that the massive stars in formation reach thermal equilibrium before the accretion is completed. This is why the history of the accretion rates for massive stars is so critical. We derive analytically the typical accretion rates, their upper and lower limits, showing the importance of dust properties.

We examine the basic properties of the disk, their luminosity and temperature in the stationary approximation, as well as their various components. The results of some recent numerical models are discussed with a particular attention to the effects that favor accretion on the central body relatively to the case of spherical accretion. These effects strongly influence the final stellar mass resulting from a collapsing clump in a cloud. We also show some properties of the pre-main sequence tracks of massive stars in the Hertzsprung-Russell diagram. During the first part of their evolution up to a mass of about 3M_ʘ the forming stars are overluminous, then they are strongly underluminous (with respect to the zero age main sequence) up to a mass of about 10M_ʘ until they adjust after a slight overluminosity to the main sequence values. We consider some rotational properties related to star formation. The angular momentum has to be reduced by a factor of about 10⁶ during star formation. Some effects contributing to this reduction have been studied particularly in the case of low- and intermediate-mass stars: disk locking and magnetic braking. We also discuss the case of massive stars and emphasize the effects of the gravity darkening of rotating stars that may favor the accretion from the disk of massive stars in formation.