Asymmetrical thermonuclear supernovae triggered by the tidal disruption of white dwarfs

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 M ⊙ C/O WD by a 500 M ⊙ 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 ≳ 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 km s −1 . We also performed 1D and 2D radiative-transfer calculations for these WD-TDEs using the codes CMFGEN and LONG_POL , 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 such as [Ca II ] λλ 7291, 7323 may appear both displaced and skewed by many 1000 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.