arXiv subjects

D. M. Townsley

Publications and source records attributed to D. M. Townsley.

14 recordsLinked to original sources

arXiv2019

Partly burnt runaway stellar remnants from peculiar thermonuclear supernovae

We report the discovery of three stars that, along with the prototype LP40-365, form a distinct class of chemically peculiar runaway stars that are the survivors of thermonuclear explosions. Spectroscopy of the four confirmed LP 40-365 stars finds ONe-dominated atmospheres enriched with remarkably similar amounts of nuclear ashes of partial O- and Si-burning. Kinematic evidence is consistent with ejection from a binary supernova progenitor; at least two stars have rest-frame velocities indicating they are unbound to the Galaxy. With masses and radii ranging between 0.20-0.28 Msun and 0.16-0.60 Rsun, respectively, we speculate these inflated white dwarfs are the partly burnt remnants of either peculiar Type Iax or electron-capture supernovae. Adopting supernova rates from the literature, we estimate that ~20 LP40-365 stars brighter than 19 mag should be detectable within 2 kpc from the Sun at the end of the Gaia mission. We suggest that as they cool, these stars will evolve in their spectroscopic appearance, and eventually become peculiar O-rich white dwarfs. Finally, we stress that the discovery of new LP40-365 stars will be useful to further constrain their evolution, supplying key boundary conditions to the modelling of explosion mechanisms, supernova rates, and nucleosynthetic yields of peculiar thermonuclear explosions.

astro-ph.SR↗

arXiv2018

Evidence for mass accretion driven by spiral shocks onto the white dwarf in SDSS J123813.73-033933.0

We present high-time-resolution photometry and phase-resolved spectroscopy of the short-period ($P_\mathrm{orb} = 80.52\,\mathrm{min}$) cataclysmic variable SDSS J123813.73-033933.0, observed with the $\mathit{Hubble}$ $\mathit{Space}$ $\mathit{Telescope}$ $\mathit{(HST)}$, the $\mathit{Kepler/K2}$ mission and the Very Large Telescope (VLT). We also report observations of the first detected super-outburst. SDSS J1238-0339 shows two types of variability: quasi-regular brightenings recurring every $\simeq 8.5$ h during which the system increases in brightness by $\simeq 0.5$mag, and a double hump quasi-sinusoidal modulation at the orbital period. The detailed $\mathit{K2}$ light curve reveals that the amplitude of the double-humps increases during the brightenings and that their phase undergoes a $\simeq 90^{\circ}$ phase shift with respect to the quiescent intervals. The $\mathit{HST}$ data unambiguously demonstrate that these phenomena both arise from the heating and cooling of two relatively large regions on the white dwarf. We suggest that the double-hump modulation is related to spiral shocks in the accretion disc resulting in an enhanced accretion rate heating two localised regions on the white dwarf, with the structure of the shocks fixed in the binary frame explaining the period of the double humps. The physical origin of the 8.5 h brightenings is less clear. However, the correlation between the observed variations of the amplitude and phase of the double-humps with the occurrence of the brightenings is supportive of an origin in thermal instabilities in the accretion disc.

astro-ph.SR↗

arXiv2018

Astrophysical Validation

We present examples of validating components of an astrophysical simulation code. Problems of stellar astrophysics are multi-dimensional and involve physics acting on large ranges of length and time scales that are impossible to include in macroscopic models on present computational resources. Simulating these events thus necessitates the development of sub-grid-scale models and the capability to post-process simulations with higher-fidelitymethods.We present an overview of the problem of validating astrophysical models and simulations illustrated with two examples. First, we present a study aimed at validating hydrodynamicswith high energy density laboratory experiments probing shocks and fluid instabilities. Second, we present an effort at validating code modules for use in both macroscopic simulations of astrophysical events and for post processing Lagrangian tracer particles to calculate detailed abundances from thermonuclear reactions occurring during an event.

astro-ph.SR↗

arXiv2018

Anatomy of the hyper-runaway star LP 40-365 with Gaia

LP 40-365 (aka GD 492) is a nearby low-luminosity hyper-runaway star with an extremely unusual atmospheric composition, which has been proposed as the remnant of a white dwarf that survived a subluminous Type Ia supernova (SN Ia) in a single-degenerate scenario. Adopting the Gaia Data Release (DR2) parallax, 1.58 +/- 0.03 mas, we estimate a radius of 0.18 +/- 0.01 Rsun, confirming LP 40-365 as a subluminous star that is ~ 15 times larger than a typical white dwarf and is compatible with the SN Ia remnant scenario. We present an updated kinematic analysis, making use of the Gaia parallax and proper motion, and confirm that Lp 40-365 is leaving the Milky Way at about 1.5 times the escape velocity of the Solar neighbourhood with a rest-frame velocity of 852 +/- 10 km/s. Integrating the past trajectories of LP 40-365, we confirm it crossed the Galactic disc 5.0 +/- 0.3 Myr ago in the direction of Carina, likely coming from beneath the plane. Finally, we estimate that LP 40-365 was ejected from its progenitor binary with a velocity of at least 600 km/s, which is compatible with theoretical predictions for close binaries containing a white dwarf and a helium-star donor.

astro-ph.SR↗

arXiv2018

Further insight on the hypervelocity white dwarf, LP 40-365 (GD 492): a nearby emissary from a single-degenerate Type Ia supernova

The recently discovered hypervelocity white dwarf LP 40-65 (aka GD 492) has been suggested as the outcome of the failed disruption of a white dwarf in a sub-luminous Type Ia supernova (SN Ia). We present new observations confirming GD 492 as a single star with unique spectral features. Our spectroscopic analysis suggests that a helium-dominated atmosphere, with ~ 33 percent neon and 2 percent oxygen by mass, can reproduce most of the observed properties of this highly unusual star. Although our atmospheric model contrasts with the previous analysis in terms of dominant atmospheric species, we confirm that the atmosphere of GD 492 is strongly hydrogen deficient, log(H/He) < -5, and displays traces of eleven other alpha- and iron-group elements (with sulfur, chromium, manganese, and titanium as new detections), indicating nuclear processing of carbon and silicon. We measure a manganese-to-iron ratio seven times larger than Solar. While the observed abundances of GD 492 do not fully match any predicted nuclear yields of a partially-burned supernova remnant, the manganese excess strongly favors a link with a single-degenerate SN Ia event over alternative scenarios.

astro-ph.SR↗

arXiv2016

GW Librae: A unique laboratory for pulsations in an accreting white dwarf

Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of HST ultraviolet spectroscopy taken in 2002, 2010 and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in May~2013, we obtained new HST/COS ultraviolet observations that displayed unexpected behaviour: besides showing variability at ~275s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits high-amplitude variability on a ~4.4h time-scale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to ~30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star

astro-ph.SR↗

arXiv2014

Heavy metals in a light white dwarf: Abundances of the metal-rich, extremely low-mass GALEX J1717+6757

Using the Hubble Space Telescope, we detail the first abundance analysis enabled by far-ultraviolet spectroscopy of a low-mass (~0.19 Msun) white dwarf (WD), GALEX J1717+6757, which is in a 5.9-hr binary with a fainter, more-massive companion. We see absorption from nine metals, including roughly solar abundances of Ca, Fe, Ti, and P. We detect a significantly sub-solar abundance of C, and put upper limits on N and O that are also markedly sub-solar. Updated diffusion calculations indicate that all metals should settle out of the atmosphere of this 14,900 K, log(g) = 5.67 WD in the absence of radiative forces in less than 20 yr, orders of magnitude faster than the cooling age of hundreds of Myr. We demonstrate that ongoing accretion of rocky material that is often the cause of atmospheric metals in isolated, more massive WDs is unlikely to explain the observed abundances in GALEX J1717+6757. Using new radiative levitation calculations, we determine that radiative forces can counteract diffusion and support many but not all of the elements present in the atmosphere of this WD; radiative levitation cannot, on its own, explain all of the observed abundance patterns, and additional mechanisms such as rotational mixing may be required. Finally, we detect both primary and secondary eclipses using ULTRACAM high-speed photometry, which we use to constrain the low-mass WD radius and rotation rate as well as update the ephemeris from the discovery observations of this WD+WD binary.

astro-ph.SR↗

arXiv2013

KIC11911480: the second ZZ Ceti in the $Kepler$ field

We report the discovery of the second pulsating hydrogen-rich (DA) white dwarf in the $Kepler$ field, KIC11911480. It was selected from the $Kepler$-INT Survey (KIS) on the basis of its colours and its variable nature was confirmed using ground-based time-series photometry. An atmosphere model fit to an intermediate-resolution spectrum of KIC11911480 places this DA white dwarf close to the blue edge of the empirical boundaries of the ZZ Ceti instability strip: $T_\mathrm{eff} = 12\,160 \pm 250$ K and $\log{g} = 7.94 \pm 0.10 $. Assuming a mass-radius relation and cooling models for DA white dwarfs, the atmospheric parameters yield: M$_{\rm WD}$ = 0.57 $\pm$ 0.06 M$_\odot$. We also obtained two quarters (Q12 and Q16) of nearly uninterrupted short-cadence $Kepler$ data on this star. We detect a total of six independent pulsation modes with a $\geq$ 3$σ$ confidence in its amplitude power spectrum. These pulsations have periods ranging between 172.9 s and 324.5 s, typical of the hotter ZZ Ceti stars. Our preliminary asteroseismic study suggest that KIC11911480 has a rotation rate of 3.5$\pm$0.5 days.

astro-ph.SR↗

arXiv2012

The Detonation Mechanism of the Pulsationally-Assisted Gravitationally-Confined Detonation Model of Type Ia Supernovae

We describe the detonation mechanism comprising the "Pulsationally Assisted" Gravitationally Confined Detonation (GCD) model of Type Ia supernovae SNe Ia. This model is analogous to the previous GCD model reported in Jordan et al.(2008); however, the chosen initial conditions produce a substantively different detonation mechanism, resulting from a larger energy release during the deflagration phase. The resulting final kinetic energy and nickel-56 yields conform better to observational values than is the case for the "classical" GCD models. In the present class of models, the ignition of a deflagration phase leads to a rising, burning plume of ash. The ash breaks out of the surface of the white dwarf, flows laterally around the star, and converges on the collision region at the antipodal point from where it broke out. The amount of energy released during the deflagration phase is enough to cause the star to rapidly expand, so that when the ash reaches the antipodal point, the surface density is too low to initiate a detonation. Instead, as the ash flows into the collision region (while mixing with surface fuel), the star reaches its maximally expanded state and then contracts. The stellar contraction acts to increase the density of the star, including the density in the collision region. This both raises the temperature and density of the fuel-ash mixture in the collision region and ultimately leads to thermodynamic conditions that are necessary for the Zel'dovich gradient mechanism to produce a detonation. We demonstrate feasibility of this scenario with three 3-dimensional (3D), full star simulations of this model using the FLASH code. We characterized the simulations by the energy released during the deflagration phase, which ranged from 38% to 78% of the white dwarf's binding energy. We show that the necessary conditions for detonation are achieved in all three of the models.

astro-ph.HE↗

arXiv2010

Multi-site Observations of Pulsation in the Accreting White Dwarf SDSS J161033.64-010223.3 (V386 Ser)

Nonradial pulsations in the primary white dwarfs of cataclysmic variables can now potentially allow us to explore the stellar interior of these accretors using stellar seismology. In this context, we conducted a multi-site campaign on the accreting pulsator SDSS J161033.64-010223.3 (V386 Ser) using seven observatories located around the world in May 2007 over a duration of 11 days. We report the best fit periodicities here, which were also previously observed in 2004, suggesting their underlying stability. Although we did not uncover a sufficient number of independent pulsation modes for a unique seismological fit, our campaign revealed that the dominant pulsation mode at 609s is an evenly spaced triplet. The even nature of the triplet is suggestive of rotational splitting, implying an enigmatic rotation period of about 4.8 days. There are two viable alternatives assuming the triplet is real: either the period of 4.8 days is representative of the rotation period of the entire star with implications for the angular momentum evolution of these systems, or it is perhaps an indication of differential rotation with a fast rotating exterior and slow rotation deeper in the star. Investigating the possibility that a changing period could mimic a triplet suggests that this scenario is improbable, but not impossible. Using time-series spectra acquired in May 2009, we determine the orbital period of SDSS J161033.64-010223.3 to be 83.8 +/- 2.9 min. Three of the observed photometric frequencies from our May 2007 campaign appear to be linear combinations of the 609s pulsation mode with the first harmonic of the orbital period at 41.5min. This is the first discovery of a linear combination between nonradial pulsation and orbital motion for a variable white dwarf.

astro-ph.SR↗

arXiv2007

Flame Evolution During Type Ia Supernovae and the Deflagration Phase in the Gravitationally Confined Detonation Scenario

We develop an improved method for tracking the nuclear flame during the deflagration phase of a Type Ia supernova, and apply it to study the variation in outcomes expected from the gravitationally confined detonation (GCD) paradigm. A simplified 3-stage burning model and a non-static ash state are integrated with an artificially thickened advection-diffusion-reaction (ADR) flame front in order to provide an accurate but highly efficient representation of the energy release and electron capture in and after the unresolvable flame. We demonstrate that both our ADR and energy release methods do not generate significant acoustic noise, as has been a problem with previous ADR-based schemes. We proceed to model aspects of the deflagration, particularly the role of buoyancy of the hot ash, and find that our methods are reasonably well-behaved with respect to numerical resolution. We show that if a detonation occurs in material swept up by the material ejected by the first rising bubble but gravitationally confined to the white dwarf (WD) surface (the GCD paradigm), the density structure of the WD at detonation is systematically correlated with the distance of the deflagration ignition point from the center of the star. Coupled to a suitably stochastic ignition process, this correlation may provide a plausible explanation for the variety of nickel masses seen in Type Ia Supernovae.

astro-ph↗

arXiv2006

Capturing the Fire: Flame Energetics and Neutronizaton for Type Ia Supernova Simulations

We develop and calibrate a realistic model flame for hydrodynamical simulations of deflagrations in white dwarf (Type Ia) supernovae. Our flame model builds on the advection-diffusion-reaction model of Khokhlov and includes electron screening and Coulomb corrections to the equation of state in a self-consistent way. We calibrate this model flame--its energetics and timescales for energy release and neutronization--with self-heating reaction network calculations that include both these Coulomb effects and up-to-date weak interactions. The burned material evolves post-flame due to both weak interactions and hydrodynamic changes in density and temperature. We develop a scheme to follow the evolution, including neutronization, of the NSE state subsequent to the passage of the flame front. As a result, our model flame is suitable for deflagration simulations over a wide range of initial central densities and can track the temperature and electron fraction of the burned material through the explosion and into the expansion of the ejecta.

astro-ph↗

arXiv1999

Simulations of metastable decay in two- and three-dimensional models with microscopic dynamics

We present a brief analysis of the crossover phase diagram for the decay of a metastable phase in a simple dynamic lattice-gas model of a two-phase system. We illustrate the nucleation-theoretical analysis with dynamic Monte Carlo simulations of a kinetic Ising lattice gas on square and cubic lattices. We predict several regimes in which the metastable lifetime has different functional forms, and provide estimates for the crossovers between the different regimes. In the multidroplet regime, the Kolmogorov-Johnson-Mehl-Avrami theory for the time dependence of the order-parameter decay and the two-point density correlation function allows extraction of both the order parameter in the metastable phase and the interfacial velocity from the simulation data.

cond-mat.mtrl-sci↗

arXiv1997

Projected dynamics for metastable decay in Ising models

The magnetization switching dynamics in the kinetic Ising model is projected onto a one-dimensional absorbing Markov chain. The resulting projected dynamics reproduces the direct simulation results with great accuracy. A scheme is proposed to utilize simulation data for small systems to obtain the metastable lifetime for large systems and/or for very weak magnetic fields, for which direct simulation is not feasible.

cond-mat↗