Stellar mergers are common

Abstract

The observed Galactic rate of stellar mergers or the initiation of common\nenvelope phases brighter than M_V=-3 (M_I=-4) is of order 0.5 (0.3)/year with\n90% confidence statistical uncertainties of 0.24-1.1 (0.14-0.65) and factor of\n2 systematic uncertainties. The (peak) luminosity function is roughly dN/dL\nL^(-1.4+/-0.3), so the rates for events more luminous than V1309 Sco (M_V=-7\nmag) or V838Mon (M_V=-10 mag) are lower at r~0.1/year and 0.03/year,\nrespectively. The peak luminosity is a steep function of progenitor mass, L\nM^(2-3). This very roughly parallels the scaling of luminosity with mass on the\nmain sequence, but the transients are ~2000-4000 times more luminous at peak.\nCombining these, the mass function of the progenitors, dN/dM M^(-2.0+/-0.8), is\nconsistent with the initial mass function, albeit with broad uncertainties.\nThese observational results are also broadly consistent with the estimates of\nbinary population synthesis models. While extragalactic variability surveys can\nbetter define the rates and properties of the high luminosity events,\nsystematic, moderate depth (I>16 mag) surveys of the Galactic plane are needed\nto characterize the low luminosity events. The existing Galactic samples are\nonly ~20% complete and Galactic surveys are (at best) reaching a typical\nmagnitude limit of <13 mag.\n