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

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

Double-Degenerate Carbon-Oxygen and Oxygen-Neon White Dwarf Mergers: A New Mechanism for Faint and Rapid Type Ia Supernovae

This is the author accepted manuscript. The final version is available from the American Astronomical Society via the DOI in this record.Type Ia supernovae (SNe Ia) originate from the thermonuclear explosion of carbon-oxygen white dwarfs (CO WDs), giving rise to luminous optical transients. A relatively common variety of subluminous SNe Ia events, referred to as SNe Iax, are believed to arise from the failed detonation of a CO WD. In this paper, we explore failed detonation SNe Ia in the context of the double-degenerate channel of merging white dwarfs. In particular, we have carried out the first fully three-dimensional simulation of the merger of a ONe WD with a CO WD. While the hot, tidally-disrupted carbon-rich disk material originating from the CO WD secondary is readily susceptible to detonation in such a merger, the ONe WD primary core is not. This merger yields a failed detonation, resulting in the ejection of a small amount of mass, and leaving behind a kicked, super-Chandrasekhar ONe WD remnant enriched by the fallback of the products of nuclear burning. The resulting outburst is a rapidly-fading optical transient with a small amount of radioactive 56Ni powering the light curve. Consequently, the ONe-CO WD merger naturally produces a very faint and rapidly-fading transient, fainter even than the faintest Type Iax events observed to date, such as SN 2008ha and SN 2010ae. More massive ONe primaries than considered here may produce brighter and longer-duration transients.The work of P.L-A. and E.G.-B. was partially funded by the MINECO AYA2014-59084-P grant and by the AGAUR. RTF thanks the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics for visiting support during which a portion of this work was undertaken. RTF acknowledges support from NASA 80NSSC18K1013. This work used the Extreme Science and Engineering Discovery Environment (XSEDE) Stampede 2 supercomputer at the University of Texas at Austin’s Texas Advanced Computing Center through allocation TG-AST100038, supported by National Science Foundation grant number ACI-1548562 (Towns et al. 2014)

Evidence for a merger of binary white dwarfs: the case of GD 362

GD 362 is a massive white dwarf with a spectrum suggesting a H-rich atmosphere which also shows very high abundances of Ca, Mg, Fe and other metals. However, for pure H-atmospheres the diffusion timescales are so short that very extreme assumptions have to be made to account for the observed abundances of metals. The most favored hypothesis is that the metals are accreted from either a dusty disk or from an asteroid belt. Here we propose that the envelope of GD 362 is dominated by He, which at these effective temperatures is almost completely invisible in the spectrum. This assumption strongly alleviates the problem, since the diffusion timescales are much larger for He-dominated atmospheres. We also propose that the He-dominated atmosphere of GD 362 is likely to be the result of the merger of a binary white dwarf.Comment: 4 pages, 3 figures. Accepted for publication in Astrophysical Journal Letter

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

Lithium production in the merging of white dwarf stars

The origin of R Coronae Borealis stars has been elusive for over 200 years. Currently, two theories for their formation have been presented. These are the final flash scenario, in which a dying asymptotic giant branch star throws off its atmosphere to reveal the hydrogen poor, heavily processed material underneath, and the double degenerate scenario, in which two white dwarfs merge to produce a new star with renewed vigour. Some theories predict that the temperatures reached during the latter scenario would destroy any lithium originally present in the white dwarfs. The observed lithium content of some R Coronae Borealis stars, therefore, is often interpreted as an indication that the final flash scenario best describes their formation. In this paper, we show that lithium production can, indeed, occur in the merging of a helium white dwarf with a carbon-oxygen white dwarf if their chemical composition, particularly that of 3He, is fully considered. The production mechanism is described in detail, and the sensitivity of lithium production to the merger environment is investigated. Nucleosynthesis post-processing calculations of smoothed-particle hydrodynamics (SPH) tracer particles are performed to show that any lithium produced in these environments will be concentrated towards the cloud of material surrounding the R CrB star. Measurements of the lithium content of these clouds would, therefore, provide a valuable insight into the formation mechanism of R CrB stars.Facultad de Ciencias Astronómicas y Geofísica

Smoothed Particle Hydrodynamics simulations of white dwarf collisions and close encounters

The collision of two white dwarfs is a quite frequent event in dense stellar systems, like globular clusters and galactic nuclei. In this paper we present the results of a set of simulations of the close encounters and collisions of two white dwarfs. We use an up- to-date smoothed particle hydrodynamics code that incorporates very detailed input physics and an improved treatment of the artificial viscosity. Our simulations have been done using a large number of particles (~ 4 \times 10^5) and covering a wide range of velocities and initial distances of the colliding white dwarfs. We discuss in detail when the initial eccentric binary white dwarf survives the closest approach, when a lateral collision in which several mass transfer episodes occur is the outcome of the newly formed binary system, and which range of input parameters leads to a direct collision, in which only one mass transfer episode occurs. We also discuss the characteristics of the final configuration and we assess the possible observational signatures of the merger, such as the associated gravitational waveforms and the fallback luminosities. We find that the overall evolution of the system and the main characteristics of the final object agree with those found in previous studies. We also find that the fallback luminosities are close to 10^48 erg/s. Finally, we find as well that in the case of lateral and direct collisions the gravitational waveforms are characterized by large-amplitude peaks which are followed by a ring-down phase, while in the case in which the binary white dwarf survives the closest approach, the gravitational pattern shows a distinctive behavior, typical of eccentric systems.Comment: 16 pages, 12 figures. Accepted for publication in MNRA

Nucleosynthesis during the merger of white dwarfs and the origin of R coronae borealis stars

Many hydrogen-deficient stars are characterized by surface abundance patterns that are hard to reconcile with conventional stellar evolution. Instead, it has been suggested that they may represent the result of a merger episode between a helium and a carbon-oxygen white dwarf. In this Letter, we present a nucleosynthesis study of the merger of a 0.4 M O helium white dwarf with a 0.8 M O carbon-oxygen white dwarf, by coupling the thermodynamic history of Smoothed Particle Hydrodynamics particles with a post-processing code. The resulting chemical abundance pattern, particularly for oxygen and fluorine, is in qualitative agreement with the observed abundances in R Coronae Borealis stars.Facultad de Ciencias Astronómicas y Geofísica