Browsing by Subject "Stellar evolution"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Chemical evolution in low-mass star forming cores(2010-08) Chen, Jo-Hsin; Evans, Neal J.; Edwin, Bergin A.; Volker, Bromm; Paul, Harvey M.; Daniel, Jaffe T.; John, Lacy H.In this thesis, I focus on the physical and chemical evolution at the earliest stages of low-mass star formation. I report results from the Spitzer Space Telescope and molecular line observations of 9 species toward the dark cloud L43, a survey of 10 Class 0 and 6 Class I protostars with 8 molecular lines, and a survey of 9 Very Low Luminosity Objects (VeLLOs) with 11 molecular lines. From the observational results, CO depletion is extensively observed with C¹⁸O(2-1) maps. A general evolutionary trend is also seen toward the Class 0 and I samples: higher deuterium fractionation at higher CO depletion. For the VeLLO candidates and starless cores with N₂D⁺(3-2) detection, we found the deuterium ratio of N₂D⁺/N₂H⁺ is higher comparing with the Class 0 and I samples. We use DCO⁺(3-2) maps to trace the velocity structures. Also, HCO⁺(3-2) blue profiles are seen toward the VeLLO candidate L328, indicating possible infall. To test theoretical models and to interpret the observations, we adopt a modeling sequence with self-consistent calculations of dust radiative transfer, gas energetics, chemistry, and line radiative transfer. In the L43 region described in Chapter 2, a starless core and a Class I protostar are evolving in the same environment. We modeled both sources with the same initial conditions to test the chemical characteristics with and without protostellar heating. The physical model consists of a series of Bonner-Ebert spheres describing the pre-protostellar (PPC) stages following by standard inside-out collapse (Shu 1977). The model best matches the observed lines suggests a longer total timescale at the PPC stage, with faster evolution at the later steps with higher densities. In Chapter 3, we modeled the entire group of Class 0 and I protostars. The trend of decreasing deuterium ratio can be seen after the temperature is high enough for CO to evaporate. After the evaporation, the history of heavy depletion (e.g, from longer PPC timescales or different grain surface properties) no longer affects the line intensities of gas-phase CO. The HCO⁺ blue profiles, which are used as infall indicators, are predicted to be observed when infall is beyond the CO evaporation front. The low luminosity of VeLLOs cannot be explained by standard models with steady accretion, and we tested an evolutionary model incorporating episodic accretion to investigate the thermal history and chemical behaviors. We tested a few chemical parameters to compare with the observations and the results from Chapter 2 and 3. The modeling results from episodic accretion models show that CO and N₂ evaporate from grain mantle surfaces at the accretion bursts and can freeze back onto grain surfaces during the long periods of quiescent phases. Deuterated species, such as N₂D⁺ and H₂D⁺, are most sensitive to the temperature. Possible good tracers for the thermal history include the line intensities of gas-phase N₂H+ relative to CO, as well as CO₂ and CO ice features.Item A detailed study of giants and horizontal branch stars in M68 : atmospheric parameters and chemical abundances(2015-05) Schaeuble, Marc-Andre; Sneden, Christopher; Robinson, EdwardIn this thesis, I present a detailed high-resolution spectroscopic study of post main sequence stars in the Globular Cluster M68. My sample, which covers a range of 4000 K in Teff, and 3.5 dex in log(g), is comprised of members from the red giant, red horizontal, and blue horizontal branch, making this the first high-resolution globular cluster study covering such a large evolutionary and parameter space. Initially, atmospheric parameters were determined using photometric as well as spectroscopic methods, both of which resulted in unphysical and unexpected Teff, log(g), vt, and [Fe/H] combinations. I therefore developed a hybrid approach that addresses most of these problems, and yields atmospheric parameters that agree well with other measurements in the literature. Furthermore, my derived stellar metallicities are consistent across all evolutionary stages, with <[Fe/H]> = 2.42 ( sigma = 0.14) from 25 stars. Chemical abundances obtained using my methodology also agree with previous studies and bear all the hallmarks of globular clusters, such as a Na-O anti-correlation, constant Ca abundances, and mild r-process enrichment.Item Superluminous supernovae : theory and observations(2013-05) Chatzopoulos, Emmanouil; Wheeler, J. CraigThe discovery of superluminous supernovae in the past decade challenged our understanding of explosive stellar death. Subsequent extensive observations of superluminous supernova light curves and spectra has provided some insight for the nature of these events. We present observations of one of the most luminous self-interacting supernovae ever observed, the hydrogen-rich SN 2008am discovered by the Robotic Optical Transient Search Experiment Supernova Verification Project with the ROTSE-IIIb telescope located in the McDonald Observatory. We provide theoretical modeling of superluminous supernova light curves and fit the models to a number of observed events and similar transients in order to understand the mechanism that is responsible for the vast amounts of energy emitted by these explosions. The models we investigate include deposition of energy due to the radioactive decays of massive amounts of nickel-56, interaction of supernova ejecta with a dense circumstellar medium and magnetar spin-down. To probe the nature of superluminous supernovae progenitor stars we study the evolution of massive stars, including important effects such as rotation and magnetic fields, and perform multi-dimensional hydrodynamics simulations of the resulting explosions. The effects of rotational mixing are also studied in solar-type secondary stars in cataclysmic variable binary star systems in order to provide an explanation for some carbon-depleted examples of this class. We find that most superluminous supernovae can be explained by violent interaction of the SN ejecta with >1 Msun dense circumstellar shells ejected by the progenitor stars in the decades preceding the SN explosion.