Detonations in white dwarf dynamical interactions

In old, dense stellar systems collisions of white dwarfs are a rather frequent phenomenon. Here we present the results of a comprehensive set of Smoothed Particle Hydrodynamics simulations of close encounters of white dwarfs aimed to explore the outcome of the interaction and the nature of the final remnants for different initial conditions. Depending on the initial conditions and the white dwarf masses, three different outcomes are possible. Specifically, the outcome of the interaction can be either a direct or a lateral collision or the interaction can result in the formation of an eccentric binary system. In those cases in which a collision occurs, the infalling material is compressed and heated such that the physical conditions for a detonation may be reached during the most violent phases of the merger. While we find that detonations occur in a significant number of our simulations, in some of them the temperature increase in the shocked region rapidly lifts degeneracy, leading to the quenching of the burning. We thus characterize under which circumstances a detonation is likely to occur as a result of the impact of the disrupted star on the surface of the more massive white dwarf. Finally, we also study which interactions result in bound systems, and in which ones the more massive white dwarf is also disrupted as a consequence of the dynamical interaction. The sizable number of simulations performed in this work allows to find how the outcome of the interaction depends on the distance at closest approach, and on the masses of the colliding white dwarfs, and which is the chemical pattern of the nuclearly processed material. Finally, we also discuss the influence of the masses and core chemical compositions of the interacting white dwarfs and the different kinds of impact in the properties of the remnants.Comment: 18 pages, 6 figures. Accepted for publication in MNRA

Spiral Disk Instability Can Drive Thermonuclear Explosions in Binary White Dwarf Mergers

Thermonuclear, or Type Ia supernovae (SNe Ia), originate from the explosion of carbon--oxygen white dwarfs, and serve as standardizable cosmological candles. However, despite their importance, the nature of the progenitor systems that give rise to SNe Ia has not been hitherto elucidated. Observational evidence favors the double-degenerate channel in which merging white dwarf binaries lead to SNe Ia. Furthermore, significant discrepancies exist between observations and theory, and to date, there has been no self-consistent merger model that yields a SNe Ia. Here we show that a spiral mode instability in the accretion disk formed during a binary white dwarf merger leads to a detonation on a dynamical timescale. This mechanism sheds light on how white dwarf mergers may frequently yield SNe Ia.Comment: Final version (as in ApJL) with minor edit

White dwarf constraints on a varying GG

A secular variation of $G$ modifies the structure and evolutionary time scales of white dwarfs. Using an state-of-the-art stellar evolutionary code, an up-to-date pulsational code, and a detailed population synthesis code we demonstrate that the effects of a running $G$ are obvious both in the properties of individual white dwarfs, and in those of the white dwarf populations in clusters. Specifically, we show that the white dwarf evolutionary sequences depend on both the value of $\dot G/G$, and on the value of $G$ when the white dwarf was born. We show as well that the pulsational properties of variable white dwarfs can be used to constrain $\dot G/G$. Finally, we also show that the ensemble properties of of white dwarfs in clusters can also be used to set upper bounds to $\dot G/G$. Precisely, the tightest bound --- $\dot G/G \sim -1.8 10^{-12}$ yr$^{-1}$ --- is obtained studying the population of the old, metal-rich, well populated, open cluster NGC 6791. Less stringent upper limits can be obtained comparing the theoretical results obtained taking into account the effects of a running $G$ with the measured rates of change of the periods of two well studied pulsating white dwarfs, G117--B15A and R548. Using these white dwarfs we obtain $\dot G/G\sim -1.8\times 10^{-10}$ yr$^{-1}$, and $\dot G/G\sim -1.3\times 10^{-10}$ yr$^{-1}$, respectively, which although less restrictive than the previous bound, can be improved measuring the rate of change of the period of massive white dwarfs.Comment: 6 pages, 3 figures. To be published in the proceedings of the conference "Varying fundamental constants and dynamical dark energy" (8 - 13 July 2013, Sexten Center for Astrophysics

Self-gravitating barotropic equilibrium configurations of rotating bodies with SPH

We present a novel relaxation method to build three-dimensional rotating structures of barotropic bodies using the SPH technique. The method is able to relax gaseous structures in rigid as well as differential rotation. The relaxation procedure strongly relies on the excellent conservation of angular momentum that characterizes the SPH technique. The method has been successfully applied to a variety of zero-temperature white dwarfs and polytropic self-gravitating structures. Our SPH results have been validated by comparing the main features (energies, central densities and the polar to equatorial radius ratio) to those obtained with independent, albeit grid-based methods, as for example, the self-consistent field method, showing that both methods agree within few percents.Comment: 12 pages, 6 figures, 4 Tables, accepted for publication in Astronomy and Astrophysic

An upper limit to the secular variation of the gravitational constant from white dwarf stars

A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound Ġ/G ~ −1.8 × 10−12 yr−1. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function.Instituto de Astrofísica de La Plat

An upper limit to the secular variation of the gravitational constant from white dwarf stars

A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound G'/G ~ -1.8 10^{-12} 1/yr. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function.Comment: 15 pages, 4 figures, accepted for publication in JCA

Double degenerate mergers as progenitors of high-field magnetic white dwarfs

High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mechanism that produces the magnetic field are still unknown. Here, we show that the hot, convective, differentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores can produce magnetic fields of the required strength that do not decay for long timescales. Using a state-of-the-art Monte Carlo simulator, we also show that the expected number of high-field magnetic white dwarfs produced in this way is consistent with that found in the solar neighborhood.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

One-armed spiral instability in double-degenerate post-merger accretion disks

Increasing observational and theoretical evidence points to binary white dwarf (WD) mergers as the origin of some, if not most, normal Type Ia supernovae (SNe Ia). In this paper, we discuss the post-merger evolution of binary WD mergers and their relevance to the double-degenerate channel of SNe Ia. We present 3D simulations of carbon–oxygen (C/O) WD binary systems undergoing unstable mass transfer, where we vary both the total mass and the mass ratio. We demonstrate that these systems generally give rise to a one-armed gravitational spiral instability. The spiral density modes transport mass and angular momentum in the disk even in the absence of a magnetic field and are most pronounced in systems with secondary-to-primary mass ratios larger than 0.6. We further analyze carbon burning in these systems to assess the possibility of detonation. Unlike the case of a $1.1+1.0\,{M}_{\odot }$ C/O WD binary, we find that WD binary systems with lower mass and smaller mass ratios do not detonate as SNe Ia up to ~8–22 outer dynamical times. Two additional models do, however, undergo net heating, and their secular increase in temperature could possibly result in a detonation on timescales longer than those considered here.Peer ReviewedPostprint (author's final draft