Professor James Binney FRS

Abstract:

The Hipparcos mission has prompted a thorough reanalysis of the kinematics of the Solar neighborhood. Reliable proper motions are now available for a photometrically complete sample of similar to 10(6) stars, but the largest complete sample of stars with good parallaxes contains only similar to 10(4) stars. The latter sample yields precise values for the first and second moments of near main-sequence stars as a function of color. The effects of the secular increase in velocity dispersion are very evident. From these moments one can redetermine the velocity of the LSR and the age of the Solar neighborhood rather precisely. One can also determine how the density of stars in velocity space varies in the neighborhood of the LSR. This density distribution proves to be significantly more complex than the Schwarzschild distribution and may be affected by the Galactic bar. The larger sample for which only proper motions can be obtained allows one to probe gradients in the large-scale streaming motions that the Oort constants aim to describe. Here again the data imply a significantly more complex situation than has been considered previously. It is probable that these complexities are in part caused by spiral arms and/or the central Galactic bar.

Abstract:

The amount of mass contained in low-mass objects is investigated anew. Instead of using a mass-luminosity relation to convert a luminosity function to a mass function, I predict the mass-luminosity relation from assumed mass functions and the luminosity functions of Jahreiss & Wielen (1997) and Gould et al (1997). Comparison of the resulting mass-luminosity relations with data from binary stars constrains the permissible mass functions. If the mass function is assumed to be a power law, the best fitting slope lies either side of the critical slope, -2, below which the mass in low-mass objects is divergent, depending on the luminosity function adopted. If these power-law mass functions are truncated at 0.001Msun, the contribution to the local density of stars lies between 0.016 and 0.039 Msun pc^-3, in conformity with the density measured dynamically from Hipparcos stars. If the mass function is generalized from a power law to a low-order polynomial in log(M), the mass in stars with M<0.1Msun is either negligible or strongly divergent, depending on the order of the polynomial adopted.