THE GALACTIC CLUSTER NGC-188 - W-URSAE MAJORIS CONTACT BINARIES AS A CLUE TO 2 SEPARATE BURSTS OF STAR FORMATION

We revisit the problem of the unique richness in W UMa binaries of the galactic cluster NGC 188, applying our recent results on the dynamical evolution of close binaries (van 't Veer & Maceroni, 1988, 1989; Maceroni & van 't Veer, 1991). We show how the presence of seven contact binaries and the alleged absence of less close ones finds a straightforward explanation in terms of a scenario we developed from the properties of field solar type close binaries. Our model of NGC 188 requires: i) that the dynamical evolution of late-type MS or solar-type close binaries is governed by the loss and transfer of angular momentum, ii) that a second burst of star formation has taken place about 2.5 Gyr ago. This second burst can also explain some other characteristics of this peculiar cluster, for instance the presence of several blue stragglers, which may be the results of the delayed star formation.