Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with mass ∼0.9M⊙

Abstract

, a mass of 0.89 M ʘ , a composition of equal parts by mass of carbon and oxygen and an initial temperature of 5× 10 5 K. The initial orbit is circular with a period of 28s. This corresponds to the state when tidal forces deform the white dwarfs sufficiently to make the system unstable (prior evolution is driven by gravitational wave emission and is not simulated). This progenitor system is set up as the initial conditions for a simulation using a smoothed particle hydrodynamics code 12 (SPH). In the initial conditions, marginal asymmetries were deliberately introduced since perfect symmetry is not expected in Nature. In this first simulation, we follow the 3 inspiral and subsequent merger of the binary system (Figure 1). After two orbits one of the two white dwarfs is disrupted. This unequal evolution of the white dwarfs originates from the symmetry-breaking in the initial conditions. The disrupted white dwarf violently merges with the remaining white dwarf and material is heated by compression.In the hottest regions carbon burning begins and releases additional energy which further heats the material. A hotspot, which is resolved by several SPH particles, forms with a temperature of 2.9× 10 9 K in high-density material (3.8× 10 6 g cm -3). Highresolution small-scale simulations 13 show that under such conditions a detonation ignites.In the second step of our simulation sequence, we impose the triggering of a detonation at the hottest point and follow it with a grid-based hydrodynamics code 14,15 as it crosses the merged object. The energy release from the nuclear burning in the detonation disrupts the system (see Figure 1) Finally, we use the structure of the ejecta and the detailed chemical abundances to calculate synthetic light curves and spectra using a Monte Carlo radiative transfer code 19 as required to quantitatively test this model against observations. Owing to the small 56 Ni mass synthesized during the nuclear burning, the synthetic light curves (Figure 2) are faint and decline rapidly compared to those of normal SNe Ia, despite the large total ejecta mass of our simulation (1.8 M ʘ ). Given that there has been no fine-tuning of the explosion model, the light curves agree remarkably well with those of the 1991bg-like SNe Ia -in both absolute magnitude and colour evolution. Moreover, our model naturally predicts the lack of secondary maxima in the near-infrared (J, H and K) light curves which is a peculiarity of 1991bg-like objects compared to normal SNe Ia.In detail, however, there are some discrepancies between our model light curves and the observational data. Comparing the difference in brightness at B band maximum and 15 days thereafter we find values between 1.4 and 1.7 -depending on the line-ofsight. This is less than typically observed for 1991bg-like objects (1.9), but at worst similar to the fastest declining normal SNe Ia and substantially faster than for objects which have previously been claimed as possible super-Chandrasekhar explosions (e.g. The total mass of the system is essential...

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

“…Composition of the ejecta Figure 5 shows the ejecta composition of the thermonuclear explosion of the merger of two 0.89 M as described in Pakmor et al (2010), which is close to the merger with a mass ratio of 0.99 described above. The final composition contains 0.03 M of carbon, 0.54 M of oxygen, 1.05 M of intermediate-mass elements, and 0.1 M of iron group elements.…”

“…A more quantitative comparison based on synthetic lightcurves and spectra can be found in Pakmor et al (2010).…”

“…This is a result of the density structure of the merged object through which the detonation propagates. Consequently, lightcurves and spectra of this explosion are expected to show considerable viewing angle dependence (for viewing angle dependent lightcurves see Pakmor et al 2010). …”

“…Recently, Pakmor et al (2010) have proposed that these SNe Ia are the result of a violent merger of two equally massive white dwarfs. Once a binary system of two white dwarfs has formed, gravitational wave emission leads to continuous shrinking of its orbit.…”

Probing thermonuclear supernova explosions with neutrinos

“…For a single-degenerate channel, the nuclear fuel is expected to burn first subsonically (Nomoto et al 1976) with a likely transition to detonation at a later time (Khokhlov 1991;Woosley & Weaver 1994). It is much less clear what the ultimate fate of the merger is (Hachisu et al 1986;Saio & Nomoto 1985;Yoon et al 2007;Pakmor et al 2010), and perhaps additional routes to an explosion are admissible (Podsiadlowski et al 2008;Podsiadlowski 2010, and references therein). These questions along with the role that SN Ia play in studies of the early universe (Sandage & Tammann 1993;Riess et al 1998;Phillips 2005;Wood-Vasey et al 2007;Ellis et al 2008;Riess et al 2009;Kessler et al 2009) motivate our search for additional sources of information about thermonuclear supernovae, and in particular about the explosion process.…”