The galactic disk mass budget. I. Stellar mass function and density

In this paper, we use the general theory worked out within the past few yea rs for the structure and the evolution of low-mass stars to derive the stel lar mass function in the Galactic disk down to the vicinity of the hydrogen -burning limit from the observed nearby luminosity functions. The accuracy of the mass-magnitude relationships derived from the aforementioned theory is examined by comparison with recent, accurate observational relationships in the M-dwarf domain. The mass function is shown to flatten out below sim ilar to1 M-circle dot but to keep rising down to the bottom of the main seq uence. Combining the present determination below 1 M-circle dot and J. M. S calo's mass function for larger masses, we show that the mass function is w ell described over the entire stellar mass range, from similar to 100 M-cir cle dot to similar to0.1 M-circle dot by three functional forms, namely, a two-segment power law, a lognormal form, or an exponential form, all normal ized to the Hipparcos sample at 0.8 M-circle dot. Integration of this mass function yields a reasonably accurate census of the entire stellar populati on in the Galactic disk and its volume and surface mass density.