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تسجيل مستخدم جديدProperties of Carbon-Oxygen White Dwarf Merger Remnants
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Recent studies have shown that for suitable initial conditions both super- and sub-Chandrasekhar mass carbon-oxygen white dwarf mergers produce explosions similar to observed SNe Ia. The question remains, however, how much fine tuning is necessary to produce these conditions. We performed a large set of SPH merger simulations, sweeping the possible parameter space. We find trends for merger remnant properties, and discuss how our results affect the viability of our recently proposed sub-Chandrasekhar merger channel for SNe Ia.
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White dwarf (WD) binary mergers are possible progenitors to a number of unusual stars and transient phenomena, including type Ia supernovae. To date, simulations of mergers have not included magnetic fields, even though they are believed to play a si
gnificant role in the evolution of the merger remnant. We simulated a 0.625 - 0.65 $M_{odot}$ carbon-oxygen WD binary merger in the magnetohydrodynamic moving mesh code Arepo. Each WD was given an initial dipole field with a surface value of $sim10^3$ G. As in simulations of merging double neutron star binaries, we find exponential field growth within Kelvin-Helmholtz instability-generated vortices during the coalescence of the two stars. The final field has complex geometry, and a strength $>10^{10}$ G at the center of the merger remnant. Its energy is $sim2times10^{47}$ ergs, $sim0.2$% of the remnants total energy. The strong field likely influences further evolution of the merger remnant by providing a mechanism for angular momentum transfer and additional heating, potentially helping to ignite carbon fusion.
The merger of two carbon-oxygen white dwarfs can lead either to a spectacular transient, stable nuclear burning or a massive, rapidly rotating white dwarf. Simulations of mergers have shown that the outcome strongly depends on whether the white dwarf
s are similar or dissimilar in mass. In the similar-mass case, both white dwarfs merge fully and the remnant is hot throughout, while in the dissimilar case, the more massive, denser white dwarf remains cold and essentially intact, with the disrupted lower mass one wrapped around it in a hot envelope and disk. In order to determine what constitutes similar in mass and more generally how the properties of the merger remnant depend on the input masses, we simulated unsynchronized carbon-oxygen white dwarf mergers for a large range of masses using smoothed-particle hydrodynamics. We find that the structure of the merger remnant varies smoothly as a function of the ratio of the central densities of the two white dwarfs. A density ratio of 0.6 approximately separates similar and dissimilar mass mergers. Confirming previous work, we find that the temperatures of most merger remnants are not high enough to immediately ignite carbon fusion. During subsequent viscous evolution, however, the interior will likely be compressed and heated as the disk accretes and the remnant spins down. We find from simple estimates that this evolution can lead to ignition for many remnants. For similar-mass mergers, this would likely occur under sufficiently degenerate conditions that a thermonuclear runaway would ensue.
We present a large parameter study where we investigate the structure of white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD masses and compositions are explored and we also probe the effect of different initial conditions.
We investigated the degree of mixing between the WDs, the conditions for detonations as well as the amount of gas ejected. We find that systems with lower mass ratios have more total angular momentum and as a result more mass is flung out in a tidal tail. Nuclear burning can affect the amount of mass ejected. Many WD binaries that contain a helium-rich WD achieve the conditions to trigger a detonation. In contrast, for carbon-oxygen transferring systems only the most massive mergers with a total mass above ~2.1 solar masses detonate. Even systems with lower mass may detonate long after the merger if the remnant remains above the Chandrasekhar mass and carbon is ignited at the centre. Finally, our findings are discussed in the context of several possible observed astrophysical events and stellar systems, such as hot subdwarfs, R Coronae Borealis stars, single massive white dwarfs, supernovae of type Ia and other transient events. A large database containing 225 white dwarf merger remnants is made available via a dedicated web page.
We investigate properties of carbon-oxygen white dwarfs with respect to the composite uncertainties in the reaction rates using the stellar evolution toolkit, Modules for Experiments in Stellar Astrophysics (MESA) and the probability density function
s in the reaction rate library STARLIB. These are the first Monte Carlo stellar evolution studies that use complete stellar models. Focusing on 3 M$_{odot}$ models evolved from the pre main-sequence to the first thermal pulse, we survey the remnant core mass, composition, and structure properties as a function of 26 STARLIB reaction rates covering hydrogen and helium burning using a Principal Component Analysis and Spearman Rank-Order Correlation. Relative to the arithmetic mean value, we find the width of the 95% confidence interval to be $Delta M_{{rm 1TP}}$ $approx$ 0.019 M$_{odot}$ for the core mass at the first thermal pulse, $Delta$$t_{rm{1TP}}$ $approx$ 12.50 Myr for the age, $Delta log(T_{{rm c}}/{rm K}) approx$ 0.013 for the central temperature, $Delta log(rho_{{rm c}}/{rm g cm}^{-3}) approx$ 0.060 for the central density, $Delta Y_{rm{e,c}} approx$ 2.6$times$10$^{-5}$ for the central electron fraction, $Delta X_{rm c}(^{22}rm{Ne}) approx$ 5.8$times$10$^{-4}$, $Delta X_{rm c}(^{12}rm{C}) approx$ 0.392, and $Delta X_{rm c}(^{16}rm{O}) approx$ 0.392. Uncertainties in the experimental $^{12}$C($alpha,gamma)^{16}rm{O}$, triple-$alpha$, and $^{14}$N($p,gamma)^{15}rm{O}$ reaction rates dominate these variations. We also consider a grid of 1 to 6 M$_{odot}$ models evolved from the pre main-sequence to the final white dwarf to probe the sensitivity of the initial-final mass relation to experimental uncertainties in the hydrogen and helium reaction rates.
Double white dwarf (double-WD) binaries may merge within a Hubble time and produce high-mass WDs. Compared to other high-mass WDs, the double-WD merger products have higher velocity dispersion because they are older. With the power of Gaia data, we s
how strong evidence for double-WD merger products among high-mass WDs by analyzing the transverse-velocity distribution of more than a thousand high-mass WDs (0.8--1.3 $M_odot$). We estimate that the fraction of double-WD merger products in our sample is about 20 %. We also obtain a precise double-WD merger rate and its mass dependence. Our merger rate estimates are close to binary population synthesis results and support the idea that double-WD mergers may contribute to a significant fraction of type Ia supernovae.
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