Context. Based mostly on stellar models which do not include rotation, CO white dwarfs which accrete helium at rates of about ∼ 10 −8 M ⊙ /yr have been put forward as candidate progenitors for a number of transient astrophysical phenomena, including supernovae of Type Ia, and the peculiar and fainter Type Iax supernovae. Aims. Here we study the impact of accretion-induced spin-up including the subsequent magnetic field generation, angular momentum transport, and viscous heating on the white dwarf evolution up to the point of helium ignition. Methods. We resolve the structure of the helium accreting white dwarf models with a one dimensional Langrangian hydrodynamic code, modified to include rotational and magnetic effects, in 315 model sequences adopting different mass transfer rates (10 −8 . . . 10 −7 M ⊙ /yr), and initial white dwarf masses (0.54 . . . 1.10 M ⊙ ) and luminosities (0.01 . . . 1 L ⊙ ). Results. We find magnetic angular momentum transport, which leads to quasi solid-body rotation, profoundly impacts the evolution of the white dwarf models, and the helium ignition conditions. Our rotating lower mass (0.54 and 0.82 M ⊙ ) models accrete up to 50% more mass up to ignition compared to the non-rotating case, while it is the opposite for our more massive models. Furthermore, we find that rotation leads to up to 10-times smaller helium ignition densities, except for the lowest adopted initial white dwarf mass. Ignition densities of order 10 6 g/cm 3 are only found for the lowest accretion rates and for large amounts of accreted helium ( 0.4 M ⊙ ). However, correspondingly massive donor stars would transfer mass at much higher rates. We therefore expect explosive He-shell burning to mostly occur as deflagrations and atṀ > 2 · 10 −8 M ⊙ /yr, regardless of white dwarf mass. Conclusions. Our results imply that helium accretion onto CO white dwarfs at the considered rates is unlikely to lead to the explosion of the CO core or to classical Type Ia supernovae, but may instead produce events which belong to the recently identified classes of faint and fast hydrogen-free supernovae.
In recent years, observations have shown that multiple-star systems such as hierarchical triple and quadruple-star systems are common, especially among massive stars. They are potential sources of interesting astrophysical phenomena such as compact object mergers, leading to supernovae, and gravitational wave events. However, many uncertainties remain in their often complex evolution. Here, we present the population synthesis code Multiple Stellar Evolution (mse), designed to rapidly model the stellar, binary, and dynamical evolution of multiple-star systems. mse includes a number of new features not present in previous population synthesis codes: 1) an arbitrary number of stars, as long as the initial system is hierarchical, 2) dynamic switching between secular and direct N-body integration for efficient computation of the gravitational dynamics, 3) treatment of mass transfer in eccentric orbits, which occurs commonly in multiple-star systems, 4) a simple treatment of tidal, common-envelope, and mass transfer evolution in which the accretor is a binary instead of a single star, 5) taking into account planets within the stellar system, and 6) including gravitational perturbations from passing field stars. mse, written primarily in the c++ language, will be made publicly available and has few prerequisites; a convenient Python interface is provided. We give a detailed description of MSE and illustrate how to use the code in practice. We demonstrate its operation in a number of examples.
Context. Type Ia supernovae (SNe Ia) have been an important tool for astronomy for quite some time; however, the nature of their progenitors remains somewhat mysterious. Recent theoretical studies indicated the possibility of producing thermonuclear detonations of carbon-oxygen white dwarfs (CO WDs) at masses less than the Chandrasekhar mass through accretion of helium-rich matter, which would, depending on mass accretion rate, mass, and initial temperature of the WD, spectrally resemble either a normal SN Ia or a peculiar one. Aims. This study aims to further resolve the state of binary systems comprised of a sub-Chandrasekhar-mass CO WD and a helium star at the point where an accretion-induced detonation occurs and constrains the part of the initial parameter space where this kind of phenomenon is possible. Methods. Preexisting data obtained through simulations of single, constantly accreting CO WDs is used as an indicator for the behavior of new binary models in which the WD is treated as a point mass and which include the non-degenerate partner as a fully resolved stellar model. We parameterize the ignition of the accumulated helium layer, changes in the WD temperature, and changes in the CO core mass depending on the mass transfer rate. Results. The initial conditions allowing for detonation do not form a single contiguous area in the parameter space, whose shape is profoundly influenced by the behavior of the donor star. Mass loss due to Nova outbursts acts in favor of detonation. According to our criteria, about 10% of the detonations in this study can be expected to show spectra consistent with ordinary SNe Ia; the rest exhibit peculiar features.
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