Relativistic Tidal Disruption and Nuclear Ignition of White Dwarf Stars by Intermediate-mass Black Holes

Abstract

Abstract We present results from general relativistic calculations of the tidal disruption of white dwarf stars from near encounters with intermediate-mass black holes. We follow the evolution of 0.2 M ⊙ and 0.6 M ⊙ stars on parabolic trajectories that approach 103–104 M ⊙ black holes as close as a few Schwarzschild radii at periapsis, paying particular attention to the effect that tidal disruption has on thermonuclear reactions and the synthesis of intermediate-mass to heavy elements. These encounters create diverse thermonuclear environments that are characteristic of Type I supernovae and capable of producing both intermediate-mass and heavy elements in arbitrary ratios, depending on the strength (or proximity) of the interaction. Nuclear ignition is triggered in all of our calculations, even at weak tidal strengths β ∼ 2.6 and large periapsis radius R P ∼ 28 Schwarzschild radii. A strong inverse correlation exists between the mass ratio of calcium-group to iron-group elements and tidal strength, with β ≲ 5 producing predominantly calcium-rich debris. At these moderate to weak interactions, nucleosynthesis is not especially efficient, limiting the total mass and outflows of calcium-group elements to <15% of available nuclear fuel. Iron-group elements, however, continue to be produced in greater quantity and ratio with increasing tidal strength, peaking at ∼60% mass conversion efficiency in our closest encounter cases. These events generate short bursts of gravitational waves with characteristic frequencies 0.1–0.7 Hz and strain amplitudes from 0.5 × 10−22 to 3.5 × 10−22 at a source distance of 10 Mpc.