Nucleosynthesis in tidally disrupted white dwarfs
Final Report Abstract
Tidal disruptions of stars by massive black holes have long been recognizsed as a way to infer the properties of otherwise undetectable, “dormant” black holes. Within the funded project, we studied the possibility to infer the existence of so-called intermediate mass black holes via tidal disruptions of white dwarf stars with a particular focus on aspects related to the nuclear composition. We found that white dwarfs of all masses may be brought to a thermonuclear explosion by the action of tidal forces from the black hole, provided that they enter deeply enough into the tidal radius of the black hole. This mechanism is viable for white dwarfs of all masses and compositions and it does not depend on the Chandrasekhar mass. It is restricted, though, to black holes with masses below ∼ 105 M , otherwise the white dwarf is swallowed as a whole. In a first step, we simulated tidal disruptions in hydrodynamics-plus-nuclear-network calculation to obtain the dynamics and nuclear composition of bound and unbound debris. In a second step, we performed a Monte Carlo radiative transfer calculation to infer light curves and spectra of such thermonuclear transients. The transient’s emission emerges in the optical, with light curves and spectra reminiscent of a Type I supernova. The properties are strongly viewing-angle dependent, and key spectral signatures are ≈ 10000 km/s 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 fuelled by accretion and arise from the nascent accretion disk and relativistic jet. If BHs 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 detection rate or the non-detection of these transients by current and upcoming surveys can thus be used to place meaningful constraints on the extrapolation of the BH mass function to moderate masses.
Publications
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Optical Thermonuclear Transients from Tidal Compression of White Dwarfs as Tracers of the Low End of the Massive Black Hole Mass Function. The Astrophysical Journal 819, 17 (2016)
M. MacLeod; J. Guillochon; E. Ramirez-Ruiz; D. Kasen; S. Rosswog
