132 research outputs found
Toroidal vortices as a solution to the dust migration problem
PublishedJournal Article© 2016 The Authors.In an earlier letter, we reported that dust settling in protoplanetary discs may lead to a dynamical dust-gas instability that produces global toroidal vortices. In this Letter, we investigate the evolution of a dusty protoplanetary disc with two different dust species (1 mm and 50 cm dust grains), under the presence of the instability. We show how toroidal vortices, triggered by the interaction of mm grains with the gas, stop the radial migration of metre-sized dust, potentially offering a natural and efficient solution to the dust migration problem.The figures were created using SPLASH (Price 2007), an SPH visualization tool publicly available at http://users.monash.edu.au/∼dprice/splash.
This Letter was supported by the STFC consolidated grant ST/J001627/1, and by the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013 grant agreement no. 339248). This Letter used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure. This Letter also used the University of Exeter Supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS and the University of Exeter
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
Smoothed Particle Hydrodynamics simulations of the core-degenerate scenario for Type Ia supernovae
The core-degenerate (CD) scenario for type Ia supernovae (SN Ia) involves the
merger of the hot core of an asymptotic giant branch (AGB) star and a white
dwarf, and might contribute a non-negligible fraction of all thermonuclear
supernovae. Despite its potential interest, very few studies, and based on only
crude simplifications, have been devoted to investigate this possible scenario,
compared with the large efforts invested to study some other scenarios. Here we
perform the first three-dimensional simulations of the merger phase, and find
that this process can lead to the formation of a massive white dwarf, as
required by this scenario. We consider two situations, according to the mass of
the circumbinary disk formed around the system during the final stages of the
common envelope phase. If the disk is massive enough, the stars merge on a
highly eccentric orbit. Otherwise, the merger occurs after the circumbinary
disk has been ejected and gravitational wave radiation has brought the stars
close to the Roche lobe radius on a nearly circular orbit. Not surprisingly,
the overall characteristics of the merger remnants are similar to those found
for the double-degenerate (DD) scenario, independently of the very different
core temperature and of the orbits of the merging stars. They consist of a
central massive white dwarf, surrounded by a hot, rapidly rotating corona and a
thick debris region.Comment: 17 pages, 10 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
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
White dwarf constraints on a varying
A secular variation of 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 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 , and on the value
of when the white dwarf was born. We show as well that the pulsational
properties of variable white dwarfs can be used to constrain .
Finally, we also show that the ensemble properties of of white dwarfs in
clusters can also be used to set upper bounds to . Precisely, the
tightest bound --- yr --- 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 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 yr, and
yr, 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
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