An Initial Mass Function for Individual Stars in Galactic Disks

The Initial Mass Function (IMF) has been derived by a variety of methods. Here we propose an IMF form (a Smoothed Two-Power Law psi_STPL) that approaches a power law at both low stellar masses (psi_STPL ~ m^(gamma)) and at high stellar masses (psi_STPL ~ m^(-Gamma)) with a turnover near the characteristic mass m_ch. The values of gamma and m_ch are derived from two integral constraints: i) the ratio of the number density of stars in the range m=0.1-0.6 Msun to that in the range m=0.6-0.8 Msun as inferred from the mass distribution of field stars in the local neighborhood, and ii) the ratio of the number of stars in the range m=0.08-1 Msun to the number of stars in the range m=0.03-0.08 Msun in young clusters. For the latter constraint, we allow for the effect of unresolved binaries. The IMF satisfying the above constraints is characterized by the parameters Gamma=1.35, gamma=0.5, and m_ch=0.35 Msun (which corresponds to an IMF turnover at 0.27 Msun). We show how well the proposed individual star IMF and other popular IMFs fit several observational constraints, including the present day mass function of main sequence and evolved stars. We also consider the effects of the star formation history, the age-metallicity relation and the upper mass cut off of the IMF. The time-averaged value of the star-formation rate at the solar circle, sfr_mean, during the disk age t_0, is derived by using the observed surface mass density of field M dwarfs. The present value of the star formation rate sfr(0) at the solar circle is derived from the formation rate of high-mass stars, which in turn is inferred from the rate of ionizing photon production in the Galaxy; we find sfr(0)/sfr_mean ~ 0.8.


02/07/2010 - 14:00
Antonio Parravano
Universidad de Los Andes, Merida, Venezuela