Asymmetrical thermonuclear supernovae triggered by the tidal disruption of white dwarfs

TL;DR

This study models white dwarf tidal disruption events by intermediate-mass black holes, revealing they can trigger thermonuclear supernovae with unique observational signatures and extreme asymmetry.

Contribution

It presents high-resolution hydrodynamic and radiative transfer simulations showing how WD TDEs can produce supernovae with distinctive features and viewing-angle dependent light curves.

Findings

Partial burning of C/O into heavier isotopes varies with impact parameter.

Simulated light curves match SNe Ia rise times and luminosities but show strong asymmetry.

Nebular spectra exhibit shifted and skewed emission lines due to asymmetry.

Abstract

In a dense star cluster core, a tidal disruption event (TDE) of a white dwarf (WD) can occur if the WD passes within the tidal radius of an intermediate-mass black hole (IMBH). Very close encounters cause extreme tidal compression in the WD, raising temperatures enough to induce runaway fusion and produce a thermonuclear supernova (SN). Using the hydrodynamics code AREPO augmented with a 55-isotope nuclear reaction network, we performed high-resolution simulations of the TDE of a $0.6$ Msun C/O WD by a $500$ Msun IMBH for different values of the scaled impact parameter $b$ (i.e., the ratio of periapsis distance to tidal radius). Closer encounters produce combined TDE+SN events, with a partial burning of $^{12}$C and $^{16}$O into heavier isotopes -- the $^{56}$Ni fractions of the disrupted WD material vary from 1% at $b = 0.19$ to 82% at $b = 0.10$, while wider ones ($b \gtrsim 0.20$) lead to standard TDEs. In all cases, the material away from the denser regions remains unburnt, spanning a wide range of radial velocities. Such WD TDEs also exhibit a central cavity, wherein little material is found below a radial velocity of several $1000 \,\mathrm{km s}^{-1}$. We also performed 1D and 2D radiative-transfer calculations for these WD-TDEs using the codes CMFGEN and LONGPOL, respectively, covering epochs from a few days to one hundred days. We recover the typical rise times and peak luminosities of SNe Ia, but with an extremely strong viewing-angle dependence of both light curves and spectra. At nebular times, isolated strong emission lines like [Ca ii] {\lambda}{\lambda} 7291, 7323 may appear both displaced and skewed by many $1000 \,\mathrm{km s}^{-1}$ -- such extreme offsets are harder to identify at earlier times due to optical depth effects and line overlap. WD TDEs may produce a diverse set of transients with extreme asymmetry and peculiar composition.