692 research outputs found
The thermonuclear explosion of chandrasekhar mass white dwarfs
No full textThe flame born in the deep interior of a white dwarf that becomes a Type Ia supernova is subject to several instabilities. We briefly review these instabilities and the corresponding flame acceleration. We discuss the conditions necessary for each of the currently proposed explosion mechanisms and the attendant uncertainties. A grid of critical masses for detonation in the range - g cm is calculated and its sensitivity to composition explored. Prompt detonations are physically improbable and appear unlikely on observational grounds. Simple deflagrations require some means of boosting the flame speed beyond what currently exists in the literature. 'Active turbulent combustion' and multi-point ignition are presented as two plausible ways of doing this. A deflagration that moves at the 'Sharp-Wheeler' speed, , is calculated in one dimension and shows that a healthy explosion is possible in a simple deflagration if the front moves with the speed of the fastest floating bubbles. The relevance of the transition to the 'distributed burning regime' is discussed for delayed detonations. No model emerges without difficulties, but detonation in the distributed regime is plausible, will produce intermediate mass elements, and warrants further study
Off-center deflagrations in Chandrasekhar mass type Ia supernova models
No full textA series of two dimensional numerical simulations of explosive nuclear burning is presented for white dwarfs near the Chandraskhar mass. We assume that the burning begins as a slow deflagration front at or near the center of the star, and continues until the density in the burning regions has declined to about 10 g cm, where the flame is essentially extinguished. We employ a novel numerical representation of the turbulent flame brush based upon ideas previously developed for modelling laboratory combustion and explore in some detail the sensitivity of the outcome to the manner in which burning is initiated. In particular, we simulate 1) a centrally ignited deflagration, 2) off-center ignition at a single ``point", and 3) simultaneous off-center ignition at five ``points". We find that the amount of $^{56} that is produced and other observable properties depend sensitively upon how the fuel is ignited
A study of U Aquarii and the nucleosynthesis of neutrons and S-process elements in evolved stars of low mass
No full textThe origin of the extreme hydrogen deficiency observed in the R
Coronae Borealis (RCrB) stars remains poorly understood. A likely
mechanism for producing this deficiency is one whereby the original
hydrogen envelope of the star is engulfed and destroyed in its
interior. One of the few RCrB stars so far analysed, U Aquarii, in fact
shows emphatic evidence of such an event having indeed occurred. This
evidence takes the form of the large s-process enhancements observed in
U Aquarii. Such enhancements are caused by the production of
significant neutron fluxes which are directly produced by envelope
engulfment
giving rise to the ¹²C(p, γ) ¹³N(β⁺γ) ¹³C(α,n) ¹⁶O sequence of
nuclear reactions. In order to obtain vital information regarding the
nature of the envelope mixing event from observed s-process
enhancements, detailed nucleosynthesis calculations investigating
neutron production and s-process synthesis are carried out using a
series of nuclear reaction networks and covering a wide range of
parameter space. These calculations are mainly based on low-mass AGB
and post-AGB stellar models since it is widely believed that the RCrB's
are in some way related to this group of stars. It is shown how the
ingestion rate of envelope material, the initial abundances and the type
of mixing model used have a large influence on the neutron and s-process
production.
New spectral observations of U Aquarii are presented, and it is
shown how these new observations allow a new interpretation of the
mixing event which occurred in this star and how further improved
abundance data of the star will lead to an unambiguous determination of
the nature of the mixing event. The RCrB stars are also thought to be
related to the extreme helium stars. Discovery of variability in two of
these latter stars is presented
Multi-spot ignition in type Ia supernova models
No full textWe present a systematic survey of the capabilities of type Ia
supernova explosion models starting from a number of flame seeds
distributed around the center of the white dwarf star. To this end
we greatly improved the resolution of the numerical simulations in
the initial stages. This novel numerical approach facilitates a
detailed study of multi-spot ignition scenarios with up to hundreds
of ignition sparks. Two-dimensional simulations are shown to be
inappropriate to study the effects of initial flame
configurations. Based on a set of three-dimensional models, we
conclude that multi-spot ignition
scenarios may improve type Ia supernova models towards
better agreement with observations. The achievable effect
reaches a maximum at a limited number of flame ignition kernels as
shown by the numerical models and corroborated by a simple
dimensional analysis
Type Ib And Ic Supernovae: Models And Spectra
No full textthis paper we consider the properties of massive star models for Type Ib and Ic supernovae. There are two ways that a massive star can lose its hydrogen envelope - binary mass exchange and stellar wind. These are not mutually exclusive. A star might lose its envelope to a binary companion and still suffer appreciable mass loss as a detached Wolf-Rayet star. On the other hand, if the companion is very close, possibly due to common envelope interaction, binary mass exchange may continue even after the envelope is lost. In some cases the star's mass may even change appreciably even after carbon has ignited. It turns out that common Type Ib supernovae require progenitors that, at the time of their explosion, have relatively small masses - about 3 - 4 M fi . The value has drifted down over the years since Ensman & Woosley (1988) first placed a limit of 6 M fi on Type Ib progenitors owing to difficulty getting a fast enough light curve with realistic physics (see also Shigeyama et al. 1990; Swartz et al. 1993). The lower limit is set by the requirement that enoug
BANK PRODUCTS IS PREDOMINANTLY LOCAL. IT IS DEFINED BY A GROUP OF PRODUCTS RATHER THAN BY INDIVIDUAL ONES. AND IT IS SERVED PRIMARILY BY COMMERCIAL BANKS. AT THEIR SIMPLEST,
No full textWoosley is an examiner in the policy and supervisory studies section of the Atlanta Fed; King is a former vice president and associate director of the Atlanta Fed’s research department; and Padhi is a senior economic analyst in the department’s financial section. They would like to thank Larry Wall, Tom Cunningham, and Tony Cyrnak for helpful comments
Recommended from our members
Neutrino induced light element synthesis
No full textAs the core of a massive star collapses to form a neutron star, the flux of neutrinos in the overlying shells of heavy elements becomes so great that, despite the small cross section, substantial nuclear transmutation is induced. Neutrinos, especially the higher energy {mu}- and {tau}-neutrinos, excite heavy elements and even helium to particle unbound levels. The evaporation of a single neutron or proton, and the back reaction of these nucleons on other species present, significantly alters the outcome of traditional nucleosynthesis calculations leading to a new process: {nu}-nucleosynthesis. The process was first studied by Domogatsky et al. and Woosley. Recent work by Epstein, Colgate, and Haxton and Woosley and Haxton suggested that a large number of elements could owe their existence in nature to {nu}-induced reactions in supernovae. A parametrized study of this process including shock wave propagation was carried out by Woosley et al. for selected zones of a 20 M{sub {circle dot}} star. Here we give preliminary results for a 25 M{sub {circle dot}} star, including all {nu}-reactions in all stellar zones
Distributed burning in Type Ia supernovae: A statistical approach
No full textWe present a statistical model which shows the influence of turbulence on a thermonuclear flame propagating in C + O white dwarf matter. Based on a Monte Carlo description of turbulence, it provides a method for investigating the physics in the so-called distributed burning regime. Using this method, we perform numerical simulations of turbulent flames and show that in this particular regime the flamelet model for the turbulent flame velocity loses its validity. In fact, at high turbulent intensities, burning in the distributed regime can lead to a deceleration of the turbulent flame and thus induce a competing process to turbulent effects that cause a higher flame speed. It is also shown that in dense C + O matter turbulent heat transport is described adequately by the Peclet number rather than by the Reynolds number, which means that flame propagation is decoupled from small-scale turbulence. Finally, at the onset of our results we argue that the available turbulent energy in an exploding C + O white dwarf is probably too low to make a deflagration-to-detonation transition possible
High-resolution simulations of convection preceding ignition in Type 1a supernovae using adaptive mesh refinement
No full text- …
