On the recent star formation history of the Milky Way disk

We have derived the star formation history of the Milky Way disk over the last 2 Gyr with a time resolution of 0.05 Gyr from the age distribution diagram of a large sample of open clusters comprising more than 580 objects. By interpreting the age distribution diagram using numerical results from an extensive library of N-body calculations carried out during the last ten years, we reconstruct the recent star formation history of the Milky Way disk. Under the assumption that the disk has never been polluted by any extragalactic stellar populations, our analysis suggests that superimposed on a relatively small level of constant star formation activity mainly in small-N star clusters, the star formation rate has experienced at least 5 episodes of enhanced star formation lasting about 0.2 Gyr with production of larger clusters. This cyclic behavior shows a period of 0.4±0.1 Gyr and could be the result of density waves and/or interactions with satellite galaxies. On the other hand, the star formation rate history from a volume-limited sample of open clusters in the solar neighbourhood appears to be consistent with the overall star formation history obtained from the entire sample. Pure continuous star formation both in the solar neighbourhood and the entire Galactic disk is strongly ruled out. Our results also indicate that, in the Milky Way disk, about 90% of open clusters are born with N ≤ 150 and the slope in the power-law frequency distribution of their masses is about-2.7 when quiescent star formation takes place. If the above results are re-interpreted taking into consideration accretion events onto the Milky Way, it is found that a fraction of the unusually high number of open clusters with ages older than 0.6 Gyr may have been formed in disrupted satellites. Problems arising from the selection effects and the age errors in the open cluster sample used are discussed in detail.