Violent mergers of nearly equal-mass white dwarf as progenitors of subluminous Type Ia supernovae

Abstract

Context. The origin of subluminous Type Ia supernovae (SNe Ia) has long eluded any explanation, because all Chandrasekhar-mass models have severe problems reproducing them. Recently, it has been proposed that violent mergers of two white dwarfs of 0.9 M could lead to subluminous SNe Ia events that resemble 1991bg-like SNe Ia. Aims. Here we investigate whether this scenario still works for mergers of two white dwarfs with a mass ratio below one. We aim to determine the range of mass ratios for which a detonation still forms during the merger, as only those events will lead to an SN Ia. This range is an important ingredient for population synthesis and one decisive point for judging the viability of the scenario. In addition, we perform a resolution study of one of the models. Finally we discuss the connection between violent white dwarf mergers with a primary mass of 0.9 M and 1991bg-like SNe Ia. Methods. The latest version of the smoothed particle hydrodynamics code Gadget3 was used to evolve binary systems with different mass ratios until they merge. We analyzed the result and looked for hot spots in which detonations can form. Results. We show that mergers of two white dwarfs with a primary white dwarf mass of ≈0.9 M and a mass ratio more than about 0.8 robustly reach the conditions we require for igniting a detonation and thus produce thermonuclear explosions during the merger itself. We also find that, while our simulations do not yet completely resolve the hot spots, increasing the resolution leads to conditions that are even more likely to ignite detonations. Additionally, we compare the abundance structure of the ejecta of the thermonuclear explosion of two merged white dwarfs with data inferred from observations of a 1991bg-like SN Ia (SN 2005bl). The abundance distributions of intermediate mass and iron group elements in velocity space agree qualitatively, and our model reproduces the lack of material at high velocities inferred from the observations. Conclusions. The violent merger scenario constitutes a robust possibility for two merging white dwarfs to produce a thermonuclear explosion. Mergers with a primary white dwarf mass of ≈0.9 M are very promising candidates for explaining subluminous SNe Ia. This would imply that subluminous SNe Ia form a distinct class of objects, which are not produced in the standard single white dwarf scenario for SNe Ia, but instead arise from a different progenitor channel and explosion mechanism.

VLT Spectropolarimetry of the Type Ia SN 2005ke

“…In the context of the current work, the relatively high degree of continuum polarization observed in the subluminous SNe 1999by and 2005ke could imply a high degree of rotation in the progenitor and hence may be consistent with a double-degenerate merger, for which the explosion would be intrinsically asymmetric (e.g. Pakmor et al 2011). …”

“…As of today, although several theoretical scenarios have been put forward (see Pakmor et al 2011, for a recent review), we do not have a clear idea about the origin of this sub-class of Type Ia SNe. On the observational side, there are indications that these objects form a rather homogeneous and distinct class; however, there is growing evidence that transition objects, sharing properties of both subluminous and core-normal SNe, do exist (Maguire et al 2011).…”

“…Merger of two WDs (Iben & Tutukov 1984;Webbink 1984;Benz et al 1990;Pakmor et al 2010Pakmor et al , 2011 has recently drawn attention because of perceived advantages for solving the Type Ia progenitor problem (Ruiter et al 2009;Schaefer & Pagnotta al. 2012).…”

“…2012). There is evidence that mergers may form a subclass among SNe Ia (Höflich & Khokhlov 1996;Quimby et al 2007;Pakmor et al 2010Pakmor et al , 2011.…”

Three-dimensional simulations of gravitationally confined detonations compared to observations of SN 1991T

“…Since the spatial scales relevant to the initiation of a detonation cannot be resolved in full-star multi-dimensional explosion simulations (see the discussion in Seitenzahl et al 2009a), it is common practice (e.g. Jordan et al 2008Jordan et al , 2012aGuillochon et al 2010;Pakmor et al 2011Pakmor et al , 2012b to pick a certain critical density ρ crit and temperature T crit that a cell composed of nuclear fuel must exceed for a detonation to be initiated. However, these critical values are no more than informed guesses based on separate high-resolution onedimensional detonation initiation calculations (e.g.…”