On the very long term evolutionary behavior of hydrogen-accreting Low-Mass CO white dwarfs

Hydrogen-rich matter has been added to a CO white dwarf of initial mass 0.516 \msun at the rates $10^{-8}$ and $2\times 10^{-8}$ \msun \yrm1, and results are compared with those for a white dwarf of the same initial mass which accretes pure helium at the same rates. For the chosen accretion rates, hydrogen burns in a series of recurrent mild flashes and the ashes of hydrogen burning build up a helium layer at the base of which a He flash eventually occurs. In previous studies involving accretion at higher rates and including initially more massive WDs, the diffusion of energy inward from the H shell-flashing region contributes to the increase in the temperature at the base of the helium layer, and the mass of the helium layer when the He flash begins is significantly smaller than in a comparison model accreting pure helium; the He shell flash is not strong enough to develop into a supernova explosion. In contrast, for the conditions adopted here, the temperature at the base of the He layer becomes gradually independent of the deposition of energy by H shell flashes, and the mass of the He layer when the He flash occurs is a function only of the accretion rate, independent of the hydrogen content of the accreted matter. When the He flash takes place, due to the high degeneracy at the base of the He layer, temperatures in the flashing zone will rise without a corresponding increase in pressure, nuclear burning will continue until nuclear statistical equilibrium is achieved; the model will become a supernova, but not of the classical type Ia variety.Comment: 14 pages and 3 Postscript figures, Accepted for publication on ApJ Letter

Hydrogen-Accreting Carbon-Oxygen White Dwarfs of Low Mass: Thermal and Chemical Behavior of Burning Shells

Numerical experiments have been performed to investigate the thermal behavior of a cooled down white dwarf of initial mass M_{\rm WD} = 0.516 M_{\sun} which accretes hydrogen-rich matter with Z = 0.02 at the rate $\dot{M}=10^{-8}$ \msun \yrm1, typical for a recurrent hydrogen shell flash regime. The evolution of the main physical quantities of a model during a pulse cycle is examined in detail. From selected models in the mass range $M_{\rm WD} = 0.52\div 0.68$ \msunend, we derive the borders in the $M_{\rm WD}$ - $\dot{M}$ plane of the steady state accretion regime when hydrogen is burned at a constant rate as rapidly as it is accreted. The physical properties during a hydrogen shell flash in white dwarfs accreting hydrogen-rich matter with metallicities Z = 0.001 and Z = 0.0001 are also studied. For a fixed accretion rate, a decrease in the metallicity of the accreted matter leads to an increase in the thickness of the hydrogen-rich layer at outburst and a decrease in the hydrogen-burning shell efficiency. In the $M_{\rm WD}$-$\dot{M}$ plane, the borders of the steady state accretion band are critically dependent on the metallicity of the accreted matter: on decreasing the metallicity, the band is shifted to lower accretion rates and its width in $\dot{M}$ is reduced.Comment: 31 pages and 10 Postscript figures; Accepted for publication on Ap

Carbon-Oxygen White Dwarfs Accreting CO-Rich Matter I: A Comparison Between Rotating and Non-Rotating Models

We investigate the lifting effect of rotation on the thermal evolution of CO WDs accreting CO-rich matter. We find that rotation induces the cooling of the accreting star so that the delivered gravitational energy causes a greater expansion with respect to the standard non-rotating case. The increase in the surface radius produces a decrease in the surface value of the critical angular velocity and, therefore, the accreting WD becomes gravitationally unbound (Roche instability). This occurrence is due to an increase in the total angular momentum of the accreting WD and depends critically on the amount of specific angular momentum deposited by the accreted matter. If the specific angular momentum of the accreted matter is equal to that of the outer layers of the accreting structure, the Roche instability occurs well before the accreting WD can attain the physical conditions for C-burning. If the values of both initial angular velocity and accretion rate are small, we find that the accreting WD undergoes a secular instability when its total mass approaches 1.4 Msun. At this stage, the ratio between the rotational and the gravitational binding energy of the WD becomes of the order of 0.1, so that the star must deform by adopting an elliptical shape. In this case, since the angular velocity of the WD is as large as 1 rad/s, the anisotropic mass distribution induces the loss of rotational energy and angular momentum via GWR. We find that, independent of the braking efficiency, the WD contracts and achieves the physical conditions suitable for explosive C-burning at the center so that a type Ia supernova event is produced.Comment: 39 pages, 22 eps-figures; accepted for publication in Astrophysical Journa

Synthesis of the elements in stars: forty years of progress

Forty years ago Burbidge, Burbidge, Fowler, and Hoyle combined what we would now call fragmentary evidence from nuclear physics, stellar evolution and the abundances of elements and isotopes in the solar system as well as a few stars into a synthesis of remarkable ingenuity. Their review provided a foundation for forty years of research in all of the aspects of low energy nuclear experiments and theory, stellar modeling over a wide range of mass and composition, and abundance studies of many hundreds of stars, many of which have shown distinct evidence of the processes suggested by B2FH. In this review we summarize progress in each of these fields with emphasis on the most recent developments

Part I. Higher Order Effects in Beta-Decay. Part Ii. The Beta-Gamma Correlation and Time Reversal Invariance

118 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1958.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Neutron-capture nucleosynthesis in the He-flash convective zone in extremely metal-poor stars

We investigate the nucleosynthesis in the helium flash convective zone, triggered by the hydrogen mixing, for extremely metal-poor stars of low and intermediate mass. Mixed hydrogen is converted into neutron through 12C(p,γ) 13N(e +ν) 13C(α ,n) 16O and the doubly neutron-recycling reactions 12C(n,γ) 13C(α,n) 16O(n,γ) 17O(α,n) 20Ne operate. In addition to oxygen and neon, not only light elements from sodium through phosphorus but also the s-process elements, heavier than iron, are synthesized via successive neutron captures with 20Ne as seeds even in the stars originally devoid of metals. We follow the both the doubly neutron-recycling reactions and the s-process nucleosynthesis up to Pb and Bi by varying model parameters such as the amount of mixed 13C. The resultant abundance patterns is shown to reproduce the observed enhancement not only of oxygen, the light elements but also Sr observed from HE 0107-5240 and HE 1327-2326. © 2006 American Institute of Physics.SCOPUS: cp.pInternational Symposium on Origin of Matter and Evolution of Galaxies 2005: New Horizon of Nuclear Astrophysics and Cosmology; Tokyo; Japan; 8 November 2005 through 11 November 2005.info:eu-repo/semantics/publishe