The i-process and CEMP-r/s stars

© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. We investigate whether the anomalous elemental abundance patterns in some of the C-enhanced metal-poor-r/s (CEMP-r/s) stars are consistent with predictions of nucleosynthesis yields from the i-process, a neutron-capture regime at neutron densities intermediate between those typical for the slow (s) and rapid (r) processes. Conditions necessary for the i-process are expected to be met at multiple stellar sites, such as the He-core and He-shell flashes in low-metallicity low-mass stars, super-AGB and post-AGB stars, as well as low-metallicity massive stars. We have found that single-exposure one-zone simulations of the i-process reproduce the abundance patterns in some of the CEMP-r/s stars much better than the model that assumes a superposition of yields from s and r-process sources. Our previous study of nuclear data uncertainties relevant to the i-process revealed that they could have a significant impact on the i-process yields obtained in our idealized one-zone calculations, leading, for example, to ∼ 0:7dex uncertainty in our predicted [Ba/La] ratio. Recent 3D hydrodynamic simulations of convection driven by a He-shell flash in post-AGB Sakurai's object have discovered a new mode of non-radial instabilities: the Global Oscillation of Shell H-ingestion. This has demonstrated that spherically symmetric stellar evolution simulations cannot be used to accurately model physical conditions for the i-process

The i-process and CEMP-r/s stars

© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. We investigate whether the anomalous elemental abundance patterns in some of the C-enhanced metal-poor-r/s (CEMP-r/s) stars are consistent with predictions of nucleosynthesis yields from the i-process, a neutron-capture regime at neutron densities intermediate between those typical for the slow (s) and rapid (r) processes. Conditions necessary for the i-process are expected to be met at multiple stellar sites, such as the He-core and He-shell flashes in low-metallicity low-mass stars, super-AGB and post-AGB stars, as well as low-metallicity massive stars. We have found that single-exposure one-zone simulations of the i-process reproduce the abundance patterns in some of the CEMP-r/s stars much better than the model that assumes a superposition of yields from s and r-process sources. Our previous study of nuclear data uncertainties relevant to the i-process revealed that they could have a significant impact on the i-process yields obtained in our idealized one-zone calculations, leading, for example, to ∼ 0:7dex uncertainty in our predicted [Ba/La] ratio. Recent 3D hydrodynamic simulations of convection driven by a He-shell flash in post-AGB Sakurai's object have discovered a new mode of non-radial instabilities: the Global Oscillation of Shell H-ingestion. This has demonstrated that spherically symmetric stellar evolution simulations cannot be used to accurately model physical conditions for the i-process

Multi-faceted Investigation of the Supernova Ia Progenitor Problem

Type Ia supernovae are generally agreed to arise from thermonuclear explosions of carbon-oxygen white dwarfs, which are led to explode via interaction with a companion star in a binary system. The actual path to explosion, however, remains uncertain, with numerous plausible parent systems and explosion mechanisms suggested. Observationally, Type Ia supernovae have multiple subclasses, distinguished by their light curves and spectra. This raises the question of whether these differences reflect multiple mechanisms occurring in nature or, instead, that explosions can be described by variations of a few physical properties. In this thesis, I use a spectral modeling package to investigate whether the spectra of events of distinct subclasses can be understood as part of a spectral sequence, where one parameter is varied at a time. I find that a single ejecta structure is sufficient to provide reasonable fits for spectra that are prototypical of subluminous and normal events. These spectra can be obtained provided that the luminosity (and thus temperature) of the ejecta are adjusted appropriately. Using a similar method, I also study the distribution of unburned material in SN 2011fe, finding a relatively large range of solutions, which does not rule out the predictions of most physical models. Lastly, I investigate the time-dependent rate of supernovae resulting from a burst of star formation. This analysis can, in principle, discriminate among proposed evolutionary paths, because it is sensitive to the nature of the companion star. By modeling the rates as a power-law, I find a normalization constant that indicates that field and cluster galaxies may be explained by the same distribution, contrary to previous results. In addition, I also provide a state-of-the-art measurement of the slope of this distribution, finding that is intermediate between the various predictions and does not yet constrain the evolutionary path leading to Type Ia supernovae.Ph.D