OPTICAL THERMONUCLEAR TRANSIENTS FROM TIDAL COMPRESSION OF WHITE DWARFS AS TRACERS OF THE LOW END OF THE MASSIVE BLACK HOLE MASS FUNCTION

In this paper, we model the observable signatures of tidal disruptions of white dwarf (WD) stars using massive black holes (MBHs) of moderate mass, approximate to 10(3)-10(5) M-circle dot. When the WD passes deep enough within the MBH's tidal field, these signatures include thermonuclear transients from burning during maximum compression. We combine a hydrodynamic simulation that includes nuclear burning of the disruption of a 0.6 M-circle dot C/O WD with a Monte Carlo radiative transfer calculation to synthesize the properties of a representative transient. The transient's emission emerges in the optical, with light. curves and spectra reminiscent of Type I supernovae. The properties are strongly viewing. angle dependent, and key spectral signatures are approximate to 10,000 km s(-1) doppler shifts, due to the orbital motion of the unbound ejecta. Disruptions of He WDs likely produce large quantities of intermediate-mass elements, offering a possible production mechanism for Ca-rich transients. Accompanying multi-wavelength transients are fueled by accretion and arise from the nascent accretion disk and relativistic jet. If MBHs of moderate mass exist with number densities similar to those of supermassive BHs, both high-energy wide-field monitors and upcoming optical surveys should detect tens to hundreds of WD tidal disruptions per year. The current best strategy for their detection may therefore be deep optical follow-up of high-energy transients of unusually long duration. The detection rate or the nondetection of these transients by current and upcoming surveys can thus be used to place meaningful constraints on the extrapolation of the MBH mass function to moderate masses.