The Galactic Chemical Evolution of Chlorine and Phosphorus

• Main Author: Maas, Zachary G
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2020
• Subjects: Astronomy
• ISBN: 9798662317208

Galactic chemical evolution impacts multiple areas of research in astrophysics, from interpreting isotopic ratios of presolar grains to characterizing stellar populations in distant galaxies. Of particular interest are the light elements, from carbon to argon. While most α-elements have been well measured and have well-known nucleosynthesis, the astrophysical production of the odd-Z elements is poorly understood. This dissertation focuses on the two odd-Z elements phosphorus and chlorine in order to understand their production in the universe. To achieve this, multiple observational studies of stars within 1 kpc of the sun have been conducted using high resolution IR spectrometers Phoenix and CSHELL on the KPNO Mayall 4m telescope, the Gemini South Observatory, and the NASA Infrared Telescope Facility. I have measured 35Cl abundances in 68 M stars and 35Cl/37Cl in seven M giants using HCl ro-vibrational lines at 3.7 μm. Phosphorus abundances have been measured in 43 FGK dwarfs and giants using P I features at 1.06 μm. Abundances were determined by calculating the best fitting synthetic spectrum to observations. I conclude that Cl matches theoretical models over the metallicity range of −0.6 < [Fe/H] < 0.2. Additionally, [Cl/Ca] ratios are consistent over the metallicity range suggesting that Cl is primarily produced in core collapse supernovae with a significant contribution from Type Ia SN. The average isotope ratio measured in seven stars is 2.66 ± 0.58 with a range of 1.76 < 35Cl/37Cl < 3.42 found in the sample. The spread in Cl isotope ratios in the seven star sample is consistent with values measured in the interstellar medium. The average Cl isotope ratio most closely matches yields from massive rotating stars. For phosphorus, I confirm that a chemical evolution model with empirical yields enhanced by a factor of 2.75 best matches observations over a metallicity range of −1 < [Fe/H] < 0.2. The average abundance for the dwarf stars in the Hyades cluster was <[P/Fe]> = −0.01 ± 0.06 and <[P/Fe]> = 0.03 ± 0.03 dex for the three giants. The consistency suggests that abundances derived using the 1.06 μm P I lines are not subjected to temperature- or luminosity-dependent systematic effects at high metallicities. I find that [P/O], [P/Mg], [P/Si], and [P/Ti] ratios are consistent with the solar ratio over a range of −1.0 < [Fe/H] < 0.2 with the [P/Si] ratio increasing by ~0.1−0.2 dex at the lowest [Fe/H] ratios. Finally, the evolution of [P/Fe] with age is similar to other α-elements, providing evidence that P is produced at the same sites. I suggest phosphorus is made entirely in core collapse supernovae with no significant production in Type Ia SN. The possible corrections to chemical evolution models are constrained from the measured abundances and I suggest any adjustment must match the original metallicity dependence, with a uniform enhancement to the yields, to accurately model the Galactic chemical evolution of phosphorus.