Chemical enrichment and star formation in the Milky Way disk II. Star formation history

A chromospheric age distribution of 552 late-type dwarfs is transformed int o a star formation history by the application of scale height corrections, stellar evolutionary corrections and volume corrections, We show that the d isk of our Galaxy has experienced enhanced episodes of star formation at 0- 1 Gyr, 2-5 Gyr and 7-9 Gyr ago, although the reality of the latter burst is still uncertain. The star sample birthsites are distributed over a very la rge range of distances because of orbital diffusion, and so give an estimat e of the global star formation rate. These results are compared with the me tal-enrichment rate, given by the age-metallicity relation, with the expect ed epochs of close encounters between our Galaxy and the Magellanic Clouds, and with previous determinations of the star formation history. Simulation s are used to examine the age-dependent smearing of the star formation hist ory due to age uncertainties, and the broadening of the recovered features, as well as to measure the probability level that the history derived to be produced by statistical fluctuations of a constant star formation history. We show, with a significance level greater than 98%, that the Milky Way ha ve not had a constant star formation history.