Remnants of Binary White Dwarf Mergers

📝 Abstract

We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

💡 Analysis

We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

📄 Content

Remnants of Binary White Dwarf Mergers Cody Raskin1, Evan Scannapieco1, Chris Fryer2, Gabriel Rockefeller2, & F.X. Timmes1,3 ABSTRACT We carry out a comprehensive smooth particle hydrodynamics simulation survey of double- degenerate white dwarf binary mergers of varying mass combinations in order to establish cor- respondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations. Subject headings: hydrodynamics – nuclear reactions, nucleosynthesis, abundances – supernovae: general – white dwarfs 1. Introduction Type Ia supernovae are commonly accepted to be the observed transient produced after a thermonuclear detonation inside a white dwarf star. While the preferred mechanism for producing SNeIa involves accretion from an evolved main sequence star onto a white dwarf (Whelan & Iben 1973; Nomoto 1982; Hillebrandt & Niemeyer 2000), the observed SNeIa rate is incompatible with the narrow range of helium accretion rates that initiate a carbon detonation as opposed to accretion induced collapse or classical novae (Nomoto & Kondo 1991; Hardin et al. 2000; Pain et al. 2002; Ruiter et al. 2009). Moreover, many recent observations of abnormally luminous SNeIa have been interpreted as having derived from double-degenerate systems involving two white dwarfs. For example, photometric observations of SN 2007if suggest 1.6±0.1 M⊙of 56Ni was formed, implying a progenitor mass of 2.4±0.2 M⊙(Scalzo et al. 2010), which is well above the Chandrasekhar limit (Chan- drasekhar 1931). Spectroscopic observations of SN 2009dc suggest ? 1.2 M⊙of 56Ni (Tanaka et al. 2010), depending on the assumed dust absorption. Since 0.92 M⊙of 56Ni is the greatest yield a Chandrasekhar mass can produce (Khokhlov et al. 1993), this yield also implies a super-Chandrasekhar progenitor mass. And observations of SN 2003fg by Howell et al. (2006) and of SN 2006gz by Hicken et al. (2007) infer ∼1.3 M⊙of 56Ni each. Generally, for the purposes of cosmological measurements, obvious double-degenerate candidates are ex- cluded from SNeIa surveys. The Phillips relation, or the width-luminosity relation (WLR), which established 1School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ, 85287-1404 2Los Alamos National Laboratories, Los Alamos, NM 87545 3The Joint Institute for Nuclear Astrophysics arXiv:1112.1420v1 [astro-ph.HE] 6 Dec 2011 – 2 – SNeIa as standard candles (Phillips 1993), relates the peak luminosity of a SNIa to the change in magnitude after 15 days. The WLR for standard SNeIa indicates that SNeIa with bright peak magnitudes also decay at a slower rate than dimmer SNeIa. This is often thought to be the result of a relationship between the 56Ni yield and the opacity of the ejecta material, assuming a total mass not exceeding the Chandrasekhar mass. However, a double-degenerate system may have up to two times the Chandrasekhar mass, and so the relationship between the 56Ni yield and the ejecta opacity need not be similar to single-degenerate scenarios, and the WLR may not be applicable to these SNeIa. In fact, it is more likely that for a given 56Ni production and energy deposition history, an increased ejecta mass results in an increased opacity, reducing the peak magnitude and broadening the lightcurve in a fashion that is the inverse of the standard WLR (Pinto & Eastman 2000; Mazzali et al. 2001; Mazzali & Podsiadlowski 2006; Kasen, R¨opke, & Woosley 2009). Complicating matters is the possibility that SNeIa deriving from double-degenerate progenitors can have ordinary 56Ni yields, depending on the progenitor mechanism (see e.g. collisional mechanisms in Raskin et al. 2009, 2010 and Rosswog et al. 2009) and the final, central densities of the degenerate material before ignition. Thus, double-degenerate SNeIa may be masquerading as typical SNeIa. If they do not conform to the WLR, they may introduce systematic errors into cosmological surveys. In order to reduce the scatter in the Hubble diagram, we must first establish whether double-degenerate SNeIa are standardizable, and if not, we must identify the tell-tale signatures of a double-degenerate progenitor mechanism. The most probable double-degenerate progenitor scenario involves two white dwarfs in a tight binary, though other progenitor systems have been considered (Benz et al. 1989a; Raskin et al. 2009; Rosswog et al. 2009). Bin

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