Stellar GADGET: A smooth particle hydrodynamics code for stellar astrophysics and its application to Type Ia supernovae from white dwarf mergers

Mergers of two carbon-oxygen white dwarfs have long been suspected to be progenitors of Type Ia Supernovae. Here we present our modifications to the cosmological smoothed particle hydrodynamics code Gadget to apply it to stellar physics including but not limited to mergers of white dwarfs. We demonstrate a new method to map a one-dimensional profile of an object in hydrostatic equilibrium to a stable particle distribution. We use the code to study the effect of initial conditions and resolution on the properties of the merger of two white dwarfs. We compare mergers with approximate and exact binary initial conditions and find that exact binary initial conditions lead to a much more stable binary system but there is no difference in the properties of the actual merger. In contrast, we find that resolution is a critical issue for simulations of white dwarf mergers. Carbon burning hotspots which may lead to a detonation in the so-called violent merger scenario emerge only in simulations with sufficient resolution but independent of the type of binary initial conditions. We conclude that simulations of white dwarf mergers which attempt to investigate their potential for Type Ia supernovae should be carried out with at least 10^6 particles.Comment: 11 pages, 6 figures, accepted for publication in MNRA

Current and Future White Dwarf Mass-radius Constraints on Varying Fundamental Couplings and Unification Scenarios

We discuss the feasibility of using astrophysical observations of white dwarfs as probes of fundamental physics. We quantify the effects of varying fundamental couplings on the white dwarf mass-radius relation in a broad class of unification scenarios, both for the simple case of a polytropic stellar structure model and for more general models. Independent measurements of the mass and radius, together with direct spectroscopic measurements of the fine-structure constant in white dwarf atmospheres lead to constraints on combinations of the two phenomenological parameters describing the underlying unification scenario (one of which is related to the strong sector of the theory while the other is related to the electroweak sector). While currently available measurements do not yet provide stringent constraints, we show that forthcoming improvements, expected for example from the Gaia satellite, can break parameter degeneracies and lead to constraints that ideally complement those obtained from local laboratory tests using atomic clocks.Comment: 11 pages, 8 figure

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

AR Sco as a possible seed of highly magnetised white dwarf

We explore the possibility that the recently discovered white dwarf pulsar AR Sco acquired its high spin and magnetic field due to repeated episodes of accretion and spin-down. An accreting white dwarf can lead to a larger mass and consequently a smaller radius thus causing an enhanced rotation period and magnetic field. This spinning magnetic white dwarf temporarily can inhibit accretion, spin down, and, eventually, the accretion can start again due to the shrinking of the binary period by gravitational radiation. A repeat of the above cycle can eventually lead to a high magnetic field white dwarf, recently postulated to be the reason for over-luminous type Ia supernovae. We also point out that these high magnetic field spinning white dwarfs are attractive sites for gravitational radiation.Comment: 7 pages including 4 figures; accepted for publication in MNRA

Lead exposure in adult males in urban Transvaal Province, South Africa during the apartheid era

Human exposure to lead is a substantial public health hazard worldwide and is particularly problematic in the Republic of South Africa given the country’s late cessation of leaded petrol. Lead exposure is associated with a number of serious health issues and diseases including developmental and cognitive deficiency, hypertension and heart disease. Understanding the distribution of lifetime lead burden within a given population is critical for reducing exposure rates. Femoral bone from 101 deceased adult males living in urban Transvaal Province (now Gauteng Province), South Africa between 1960 and 1998 were analyzed for lead concentration by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Of the 72 black and 29 white individuals sampled, chronic lead exposure was apparent in nearly all individuals. White males showed significantly higher median bone lead concentration (ME = 10.04 µg·g−1), than black males (ME = 3.80 µg·g−1) despite higher socioeconomic status. Bone lead concentration covaries significantly, though weakly, with individual age. There was no significant temporal trend in bone lead concentration. These results indicate that long-term low to moderate lead exposure is the historical norm among South African males. Unexpectedly, this research indicates that white males in the sample population were more highly exposed to lead

Lost conversations: finding new ways for black and white Australians to lead together

It\u27s time for a game-changer in how black and white Australians relate.   The difficulties we have in coming together—to talk, to work, to lead change—are core to our challenge to reconcile, as a country. But if we want to shift the status quo, if we want to lead change on entrenched Indigenous disadvantage, we don\u27t need another program, initiative or money to try and \u27fix\u27 the problem. We need to start having a different conversation.  The result of two years experience working together as part of a Social Leadership Australia initiative, Lost Conversations brings together the diverse perspectives and personal stories of five Aboriginal and four non-Indigenous authors, all with first-hand knowledge of what happens when black and white Australians come together to try and work on change.  Lost Conversations asks the questions and starts the conversations that we daren\u27t have in Australia ... until now:  What is \u27black\u27 power? What is \u27white\u27 power?  What qualifies someone to lead in this cross-cultural space?  Why is this so hard to talk about?  Can we start to name these things and try to shift the status quo?  Can we change?  Should we?  &nbsp