On the formation of massive stars by accretion

At present, there are two scenarios for the formation of massive stars: 1) The accretion scenario and 2) The coalescence scenario, which implies the m erging of intermediate mass stars. We examine here some properties of the f irst one. Radio and IR observations by Churchwell (1999) and Henning et al. (2000) of mass outflows around massive Pre-Main Sequence (PMS) stars show an increase by several orders of magnitudes of the outflow rates with stell ar luminosities, and thus with stellar masses. As typically, a fraction of 1/3 to 1/6 of the infalling material is estimated to be accreted, this sugg ests that the accretion rate is also quickly increasing with the stellar ma ss. We calculate three different sets of birthlines, i.e. tracks followed by a continuously accreting star. First, three mod els with a constant accretion rate (M-accr = 10(-6), 10(-5), 10(-4) Mo yr(-1)). Then several birthlines following the accretion models of Bernasconi & Maeder (1996), which have M- accr increasing only slightly with mass. Finally we calculate several birth lines for which (M)over dot(accr) = (M)over dot(ref) (M/M-circle dot)(phi), with values of phi equal to 0.5, 1.0 and 1.5 and also for different values of (M)over dot(ref). The best fit to the observations of PMS stars in the HR diagram is achieved for phi between 1.0 or 1.5 and for (M)over dot(ref) similar or equal to 10(-5) M-circle dot yr(-1). Considerations on the lifet imes favour values of phi equal to 1.5. These accretion rates do well corre spond to those derived from radio and IR observations of mass outflows. Mor eover they also lie in the ''permitted region" of the dynamical models give n by Wolfire & Cassinelli (1987). We emphasize the importance of the accretion scenario for shaping the IMF a nd in particular for determining the upper mass limit of stars. In the accr etion scenario, this upper mass limit will be given by the mass for which t he accretion rate is such that the accretion induced shock luminosity is of the order of the Eddington luminosity.