Thermonuclear Flashes on H/He Accreting Co White Dwarfs and Structure of Exotic Nuclei
Mitchell, Joseph P. (author)
Hoeﬂich, Peter (professor directing thesis)
Rogachev, Grigory (professor directing thesis)
Humayun, Munir (university representative)
Gerardy, Christopher (committee member)
Volya, Alexander (committee member)
Wiedenhover, Ingo (committee member)
Department of Physics (degree granting department)
Florida State University (degree granting institution)
We studied H-shell flashes on CO WDs accreting Hydrogen rich matter in regimes where they are believed to be on the border of stable accretion and of having dynamical mass loss. These systems are believed to be progenitors of SNe Ia, however, there is still some question of what range of accretion rates and WD masses allow for growth to the Chandrasekhar mass, if any do at all. Flashes that result in mass loss are also of interest as they enrich the Inter Stellar Medium. Initial models are calculated using a stationary evolution code that starts with a 0.5 M WD. Accretion of material of solar metalicity is employed, and nuclear burning is allowed, until the model reaches the desired masses of 0.8 M and 1.0 M for our study. The resultant initial models have structures that are in agreement with the previous works of [93, 26, 96] for accreting hot WD models. The flashes were calculated with an explicit hydrodynamics code that utilizes the PPM method, a second order Godunov scheme, allowing for high time resolution during the flash. This code is used in the co-moving frame to avoid chemical advection over time. Due to a recurrence of flashes of years to thousands of years, however, periods of steady nuclear burning were evolved in a quasi-static method, allowing for longer time steps than are possible in an explicit hydro code. Thus, evolution of the models followed an alternating quasi-static/explicit hydro evolution scheme until flash conditions were met, at which point the explicit hydro was employed. Use of an explicit hydro code has allowed for the observation of a new physical effect from wave dissipation. With our high time resolution, energy transport via waves, and detailed EOS, we found that at the onset of the flash, a reduction in the degeneracy pressure due to electron captures, results in a reduction of the total pressure. With a gravitational acceleration on the order of 108 in the shell, a reduction of the total pressure by 1% results in an in fall acceleration of 10 km . With such a strong in fall, compressional heating results in a hotter flash, with results showing temperatures over a billion degrees in all models. These high temperatures had consequences on the nucleosynthesis, as they allowed for rp-breakout during the flash. The effect of a "double" flash was found in one model. This resulted when the flash stalled in the H-shell, resulting in high temperature burning in only a portion of the shell. Once the H was exhausted in the flash region, cooling occurred and there was contraction of the H exhausted region. This contraction caused an in fall of the un-exhausted region which via compressional heating resulted in the flash to occur in the un-exhausted region. Such an effect may happen in any progenitor system in which the flash stalls and compression afterwards is suitable for a re-start of the flash. This effect may be observable with the current generation of instruments. With the high temperatures found in the flashes, rp-breakout nucleosynthesis was found to occur. Occurrence of rp-nucleosynthesis in these objects may make important sources of the chemical enrichment of isotopes below the iron group that are not know to be synthesized in hydrostatic stellar burning. The existence of rp-breakout in the flashes, shows the importance of nuclear physics in these objects. More precise nuclear reaction rate data are needed for proper energy generation and chemical evolution. With the occurrence of rp-nucleosynthesis in our models, it is especially advantageous to study radioactive proton rich nuclei. These studies are not without many difficulties in the laboratory, as many of the studies require the use of low intensity radioactive beams making clean, high statistic studies difficult. To address this issue, the hybrid target technique was used. This target technique was found to be a great tool for studying resonant proton scattering with exotic beams. It has been used to measure elastic and inelastic excitation functions in the study of 8 B via 7 Be+p scattering, as well as 12 N+p elastic scattering. With such success, the hybrid target technique can be a very useful tool for studying reactions that are important in the rp-process. We have studied the structure of the astrophysically important, radioactive isotope 8 B. Three new resonances have been suggested, a 0+ , 2+ , and 1+ which were predominantly in the inelastic channel and never before seen in previous studies. However, due to their high excitation energies and narrow width, none of the resonances are expected to effect the astrophysically important 7 Be(p,γ) reaction rate. Results were compared to continuum shell model as well as ab initio calculations and found to be in good agreement with both sets of predictions, with the notable exception of the 2+ state. The structure of 13 O, an isotope important in the pp-chain breakout was studied by 12 N+p elastic scattering. This work extended the 13 O excitation function to higher energies than the previous work of , however, the cross section was found to be rather flat. Due to the very low intensity of the radioactive 12 N beam, the experiment had very low statistics, 1+ making the observation of any states other than the known 2 very difficult.
September 4, 2012.
A Thesis submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Peter Hoeﬂich, Professor Directing Thesis; Grigory Rogachev, Professor Directing Thesis; Munir Humayun, University Representative; Christopher Gerardy, Committee Member; Alexander Volya, Committee Member; Ingo Wiedenhover, Committee Member.
Florida State University
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