Prelude to a Double Degenerate Merger: The Onset of Mass Transfer and Its Impact on Gravitational Waves and Surface Detonations

Abstract

We present the results of a systematic numerical study of the onset of mass transfer in double degenerate binary systems and its impact on the subsequent evolution. All investigated systems belong to the regime of direct impact, unstable mass transfer. In all of the investigated cases, even those considered unstable by conventional stability analysis, we find a long-lived mass transfer phase continuing for as many as several dozen orbital periods. This settles a recent debate sparked by a discrepancy between earlier SPH calculations that showed disruptions after a few orbital periods and newer grid-based studies in which mass transfer continued for tens of orbits. The number of orbits a binary survives sensitively depends on the exact initial conditions. We find that the approximate initial conditions that have been used in most previous SPH calculations have a serious impact on all stages of the evolution from the onset of mass transfer up to the final structure of the remnant. We compare "approximate" initial conditions where spherical stars are placed at an initial separation obtained from an estimate of the Roche lobe size with "accurate" initial conditions that were constructed by carefully driving the binary system to equilibrium by a relaxation scheme. Simulations that use the approximate initial conditions underestimate the initial separation when mass transfer sets in, which yields a binary that only survives for only few orbits and thus a rapidly fading gravitational wave signal. Conversely, the accurate initial conditions produce a binary system in which the mass transfer phase is extended by almost two orders of magnitude in time, resulting in a gravitational wave signal with amplitude and frequency that remain essentially constant up until merger. The mass transfer also shows a unique oscillatory signal that is most pronounced for binary systems that are marginally unstable. As we show that these binaries can survive at small separation for hundreds of orbital periods, their associated gravitational wave signal should be included when calculating the gravitational wave foreground (although expected to below LISA's sensitivity at these high frequencies). We also show that the inclusion of the entropy increase associated with shock-heating of the accreted material reduces the number of orbits a binary survives given the same initial conditions, although the effect is not as pronounced when using the appropriate initial conditions. The use of accurate initial conditions and a correct treatment of shock heating allows for a reliable time evolution of the temperature, density, and angular momentum, which are important when considering thermonuclear events that may occur during the mass transfer phase and/or after merger. Our treatment allows us to accurately identify when surface detonations may occur in the lead-up to the merger, as well as the properties of final merger products. arXiv:1101.5132v1 [astro-ph.HE] 26 Jan 2011 spin, accretor / J tot t / orbit t / orbit

A SYSTEMATIC STUDY OF CARBON–OXYGEN WHITE DWARF MERGERS: MASS COMBINATIONS FOR TYPE Ia SUPERNOVAE

“…We also compare our results of M 0.9 0.6 +  and k M 100 1 - with those from the previous studies of Yoon et al (2007), Dan et al (2011), andZhu et al (2013). Since these studies have used similar resolutions (a few 10 5 SPH particles in all), they are suitable for comparison.…”

“…If the initial separation is smaller when the RLOF starts, the mass transfer tends to occur more violently and the secondary is completely disrupted within a few orbital periods (Dan et al 2011). As a result, accreted matter is more strongly shockheated and dynamic carbon burning easily occurs.…”

“…We suppose that the difference in the highest temperature comes from the difference in the initial condition. Dan et al (2011) reported that the morphology of a merger remnant could be affected by the initial condition. They found that a remnant whose initial separation is larger has a longer trailing arm than one whose initial separation is smaller (see Figure 10 of Dan et al 2011).…”

“…Dan et al (2011) reported that the morphology of a merger remnant could be affected by the initial condition. They found that a remnant whose initial separation is larger has a longer trailing arm than one whose initial separation is smaller (see Figure 10 of Dan et al 2011). By contrast, we find that the highest temperature in the remnant phase is barely affected by the initial condition (see Figure 7).…”

Solar abundance of manganese: a case for near Chandrasekhar-mass Type Ia supernova progenitors

“…The latter case would result in a near-M Ch SN Ia event, with the same potential explosion mechanisms listed above. (2) Recent multi-dimensional hydrodynamical simulations have shown that an accretion disc need not form, and the resulting violent merger of the two WDs may lead to a detonation in the primary (Pakmor et al 2010(Pakmor et al , 2011(Pakmor et al , 2012Dan et al 2011;Raskin et al 2012). In this violent merger model, the explosion is essentially driven by a pure detonation of a nearly hydrostatic sub-M Ch WD.…”