A SCHMIDT-KENNICUTT LAW FOR STAR FORMATION IN THE MILKY WAY DISK

We use a new method to trace backward the star formation history of the Milky Way disk, using a sample of M dwarfs in the solar neighborhood that is representative for the entire solar circle. M stars are used because they show Ha emission until a particular age, which is a well-calibrated function of their absolute magnitudes. This allows us to reconstruct the rate at which disk stars have been born over about half the disk's lifetime. Our star formation rate (SFR) agrees well with those obtained by using other independent methods and seems to rule out a constant SFR. The principal result of this study is to show that a relation of the Schmidt-Kennicut type (which relates the SFR to the interstellar gas content of galaxy disks) has pertained in the Milky Way disk during the last 5 Gyr. The SFR we derive from the M dwarfs and the interstellar gas content of the disk can be inferred as a function of time from a model of the chemical enrichment of the disk, which is well constrained by the observations indicating that the metallicity of the Galactic disk has remained nearly constant over the timescales involved. We demonstrate that the SFR and gas surface densities over the last 5 Gyr can be accurately described by a Schmidt Kennicutt law with an index of Gamma = 1.45(-0.09)(+0.22). This is, within statistical uncertainties, the same value found for other galaxies.