Asymmetrical thermonuclear supernovae triggered by the tidal disruption of white dwarfs

TL;DR

The paper demonstrates that tidal disruption events of white dwarfs by intermediate-mass black holes can transition from standard TDEs to thermonuclear supernovae-like explosions depending on the encounter depth, with $^{56}$Ni production ranging from negligible to substantial. Using AREPO with a 55-isotope network, the authors show that deeper encounters induce runaway nuclear burning, producing highly asymmetric, Ni-rich ejecta that form a central cavity due to fallback. They couple the hydrodynamics to radiative transfer with CMFGEN (1D) and LONG_POL (2D) to predict light curves and spectra, revealing strong viewing-angle dependencies and nebular-line shifts that differ from standard SNe Ia. The work suggests WD-TDEs could explain a class of highly asymmetric, SN-like transients and lays groundwork for more sophisticated 3D radiative-transfer studies and exploration across WD and IMBH parameter space.

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] λλ 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.

Figures (15)

• Figure 1: Snapshots of TDEs of $0.6 \,\mathrm{M}_\odot$ WDs, due to $500 \,\mathrm{M}_\odot$ IMBHs (white points) for eight different values of impact parameter $b$ at $\sim 500 \,\mathrm{s}$. The top two (bottom two) panels show slices of densities and temperatures for $b = 0.20, 0.19, 0.18, 0.17$ ($b = 0.16, 0.15, 0.12, 0.10$), respectively. As $b$ decreases, the ejecta are more spread out and increasingly unlike a standard TDE, indicating runaway nuclear burning, e.g., the case of $b = 0.10$. The masses of the unbound ejecta are shown in Table \ref{['tab:mass_energy']}.
• Figure 2: Isotope mass fractions (those with abundances $> 10^{-3}$) of the WD ejecta after periapsis passage for different values of $b$. As $b$ decreases, the fractions of $^{12}\rm{C}$ and $^{16}\rm{O}$ after TDE decrease due to more nuclear burning, and the fraction of $^{56}\rm{N}$i produced increases quickly. Intermediate-mass isotopes (Ne to Ca) first increase in fraction with decreasing $b$, and then decrease due to more favorable conditions for massive isotope (e.g., $^{56}\rm{N}$i) production.
• Figure 3: Snapshots of slices of $^{16}\rm{O}$, $^{28}$Si and $^{56}\rm{N}$i densities, respectively, similar to Figure \ref{['fig:collage_quants']}. As $b$ decreases, a higher abundance of $^{56}\rm{N}$i is produced, at the expense of $^{16}\rm{O}$ (and $^{12}\rm{C}$), due to runaway nuclear burning. Abundances of $^{28}$Si (and other isotopes from Ne to Ca) initially increase with decreasing $b$, peaking around $b \sim 0.16$ -- $0.17$, and decrease after (see also Figure \ref{['fig:nuc_tab']}). When nuclear burning is significant, the inner, intermediate, and outer regions of the ejecta (relative to the centers of the plumes of debris) are dominated by $^{56}\rm{N}$i, $^{28}$Si (also other intermediate isotopes), and $^{16}\rm{O}$ (also $^{12}\rm{C}$), respectively. The transition from a standard TDE ($b = 0.20$) to TDE+SN is clear.
• Figure 4: Polar plots of plane-projected masses within wedges of opening angle $10^{\circ}$, $m_{\mathrm{proj,10^{\circ}}}$, unbound from the IMBHs at $\sim 500 \,\mathrm{s}$. The solid and dashed lines represent the $b = 0.15$ TDE when the NRN is enabled and disabled, respectively. The three subplots represent three perpendicular planes, $xy$, $yz$, and $zx$, passing through the IMBHs (plot centers). The cylindrical angles are defined relative to the $x$, $y$, and $z$ axes, respectively. Note that the values denote the sum of the projected column masses for each angle. A standard TDE ensues when the NRN is disabled, while nuclear fusion occurs when the NRN is enabled. Due to energy injected from nuclear reactions, the ejected material, in this case, spreads out more.
• Figure 5: Polar plots of $m_{\mathrm{proj,10^{\circ}}}$ similar to Figure \ref{['fig:mass_asym_0.15']} with the NRN enabled for different TDE $b$ values. It is clear that the ejected material is asymmetrically distributed in all cases. As $b$ decreases, the ejecta spread out more due to greater energy injection from nuclear fusion. This change is most drastic between $b = 0.20$ (standard TDE with negligible nuclear burning) and $b = 0.19$ (some nuclear burning).