Detonations in Sub-Chandrasekhar Mass C+O White Dwarfs

arXiv:1003.2917v1 [astro-ph.HE] 15 Mar 2010 DRAFT VERSION OCTOBER 8, 2018 Preprint typeset using LATEX style emulateapj v. 11/10/09 DETONATIONS IN SUB-CHANDRASEKHAR MASS C+O WHITE DWARFS S. A. SIM, F. K. RÖPKE, W. HILLEBRANDT, M. KROMER, R. PAKMOR, M. FINK, A. J. RUITER, I. R. SEITENZAHL Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany Draft version October 8, 2018 ABSTRACT Explosions of sub-Chandrasekhar-mass white dwarfs are one alternative to the standard Chandrasekhar-mass model of Type Ia supernovae. They are interesting since binary systems with sub-Chandrasekhar-mass primary white dwarfs should be common and this scenario would suggest a simple physical parameter which determines the explosion brightness, namely the mass of the exploding white dwarf. Here we perform one-dimensional hydrodynamical simulations, associated post-processing nucleosynthesis and multi-wavelength radiation trans- port calculations for pure detonations of carbon-oxygen white dwarfs. The light curves and spectra we obtain from these simulations are in good agreement with observed properties of Type Ia supernovae. In particular, for white dwarf masses from 0.97–1.15 M⊙we obtain 56Ni masses between 0.3 and 0.8 M⊙, sufficient to capture almost the complete range of Type Ia supernova brightnesses. Our optical light curve rise times, peak colours and decline timescales display trends which are generally consistent with observed characteristics al- though the range of B-band decline timescales displayed by our current set of models is somewhat too narrow. In agreement with observations, the maximum light spectra of the models show clear features associated with intermediate mass elements and reproduce the sense of the observed correlation between explosion luminosity and the ratio of the Si II lines at λ6355 and λ5972. We therefore suggest that sub-Chandrasekhar mass ex- plosions are a viable model for Type Ia supernovae for any binary evolution scenario leading to explosions in which the optical display is dominated by the material produced in a detonation of the primary white dwarf. Subject headings: radiative transfer — supernovae: general — white dwarfs 1. INTRODUCTION In recent years, considerable work has been devoted to the study of the Chandrasekhar-mass (MCh) model of Type Ia supernovae (SNe Ia). As shown by Arnett et al. (1971), prompt detonations of MCh carbon/oxygen (C+O) white dwarfs (WDs) in hydrostatic equilibrium mainly produce iron group elements (IGEs). Thus, they cannot account for the significant amounts of intermediate-mass elements (IMEs; e.g. silicon and sulphur) responsible for the features which dominate the maximum light spectra. To obtain these, pre- expansion of the WD is necessary such that burning partially takes place under low-density conditions where IMEs can be synthesized. One way of achieving this is provided by mod- els in which the flame ignites as a deflagration which releases sufficient energy to expand the star before a deflagration-to- detonation transition occurs (Khokhlov 1991). An alternative to this pre-expansion is the detonation of a sub-Chandrasekhar mass (sub-MCh) WD starting from a hydrostatic configuration. Here, a variety of density profiles can be realized, determined by the WD mass. Close to MCh, the detonation produces pri- marily IGEs and few IMEs, while for less massive WDs more IMEs and less IGEs will be synthesized. Detonation of a sub-MCh WD cannot occur spontaneously but must be triggered by external compression. The most widely discussed mechanism for sub-MCh explosions has been the double detonation model. Here, a C+O WD accretes from a companion star and develops a helium-rich outer shell. This may occur for binaries with helium-rich donors or hydrogen- rich donors where the accreted hydrogen is burned to he- lium. If the helium-shell becomes sufficiently massive, it can become unstable and detonate. Subsequent compres- sion of the core by inward propagating shocks may pro- duce a secondary carbon detonation which explodes the WD (e.g. Woosley & Weaver 1986; Fink et al. 2007). Detonations in helium-rich surface layers have also been discussed for the case of rapid dynamical mass transfer in binary systems containing a C+O WD with a helium-rich WD companion (Guillochon et al. 2010). In that case instabilities in the accre- tion seed dense knots which, by impacting on the underlying WD surface, might trigger a detonation in the accreted helium leading to a potential secondary core detonation. It has also been speculated that sub-MCh explosions may arise during vi- olent accretion in mergers of C+O WD binaries. Here, the C+O accretion may lead to an edge-lit detonation or carbon flashes that trigger a core detonation (see e.g. Shigeyama et al. 1992 but for a different result see Lorén-Aguilar et al. 2009). Most previous work on testing sub-MCh models has fo- cused on cases in which the core detonation is triggered by detonation in an overl

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