Does Explosive Nuclear Burning Occur in Tidal Disruption Events of White Dwarfs by Intermediate-mass Black Holes?

Abstract

We investigate nucleosynthesis in tidal disruption events (TDEs) of white dwarfs (WDs) by intermediate mass black holes (IMBHs). We consider various types of WDs with different masses and compositions by means of 3 dimensional (3D) smoothed particle hydrodynamics (SPH) simulations. We model these WDs with different numbers of SPH particles, N , from a few 10 4 to a few 10 7 , in order to check mass resolution convergence, where SPH simulations with N > 10 7 (or a space resolution of several 10 6 cm) have unprecedentedly high resolution in this kind of simulations. We find that nuclear reactions become less active with increasing N , and that these nuclear reactions are excited by spurious heating due to low resolution. Moreover, we find no shock wave generation. In order to investigate the reason for the absence of a shock wave, we additionally perform 1 dimensional (1D) SPH and mesh-based simulations with a space resolution ranging from 10 4 to 10 7 cm, using characteristic flow structure extracted from the 3D SPH simulations. We find shock waves in these 1D high-resolution simulations. One of these shock waves triggers a detonation wave. However, we have to be careful of the fact that, if the shock wave emerged at a bit outer region, it could not trigger the detonation wave due to low density. Note that the 1D initial conditions lack accuracy to precisely determine where a shock wave emerges. We need to perform 3D simulations with 10 6 cm space resolution in order to conclude that WD TDEs become optical transients powered by radioactive nuclei.

Rapid Transients Originating from Thermonuclear Explosions in Helium White Dwarf Tidal Disruption Events

“…We employ 786,432 SPH particles to represent the WD on a parabolic orbit around the BH. This resolution is not enough to resolve shock structure during the tidal compression, and thus the nucleosynthetic results are resolution-dependent (Tanikawa et al 2017). However, we expect that our results do not change significantly.…”

Nuclear Ignition of White Dwarf Stars by Relativistic Encounters with Rotating Intermediate Mass Black Holes

“…Although highly resolved for three dimensions, our calculations do not cover fully the spatial range needed to determine precisely where the shocks emerge, or whether they are responsible for triggering or simply enhancing ignition. Instead we refer to the work of Tanikawa et al (2017) who performed much more highly resolved 1D simulations down to scales of 10 4 cm, adopting dynamical flow conditions from 3D SPH models. Their calculations demonstrate a network of shocks develop within the scale height of stellar matter at different locations above and below the orbital plane.…”

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

“…This is critical in order to achieve sufficient zone resources in the central plane regions and provide the necessary scale height coverage to resolve internal shock and ignition features, which in turn are critical to approach convergent nuclear reactive solutions. A comprehensive study by Tanikawa et al (2017) suggests a resolution of ≤ 10 6 cm is needed to properly resolve nuclear flows for moderate tidal strength interactions (β ≈ 5). Previous three dimensional calculations, (e.g.…”

“…Additionally, recent studies by Tanikawa et al (2017) have shed some doubt on the reliability (or limitation) of multi-dimensional numerical calculations due to the difficulty of allocating sufficient resources in the central ignition region. They argue coarse resolution does not adequately resolve shock heating and can produce spurious heating and artificially trigger ignition.…”

“…One especially intriguing aspect of TDEs that concerns this work is the possibility that tidal compression perpendicular to the orbital plane could trigger explosive thermonuclear reactions and create heavy-ion nuclei if the temperature is raised above ∼ 3 × 10 8 K for He burning or ∼ 3 × 10 9 K for C/O (Luminet & Pichon 1989;Rosswog et al 2009;Haas et al 2012;Tanikawa et al 2017;Kawana et al 2017). This is a likely outcome if the WD is massive enough and tidal compression strong enough.…”