Late-Stage Evolution of Low- and Intermediate-Mass Stars
Open Access
- Author:
- Davis, Brian
- Graduate Program:
- Astronomy and Astrophysics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 24, 2022
- Committee Members:
- Robin Ciardullo, Chair & Dissertation Advisor
Jose Fuentes, Outside Unit & Field Member
Randy McEntaffer, Major Field Member
Caryl Gronwall, Major Field Member
Rebekah Ilene Dawson, Professor in Charge/Director of Graduate Studies - Keywords:
- Planetary nebulae
Post-AGB
PAGB stars
Stars
Stellar evolution - Abstract:
- Great strides have been made in recent years to better understand the late stages of stellar evolution for low- and intermediate-mass stars, and, specifically, the asymptotic giant branch (AGB) and post-AGB (PAGB) phases. Even still, there is a great amount of work to be done, as large discrepancies between theory and observations remain. In this dissertation, I perform observational studies of pre- and post-AGB stellar evolution for stars in this mass range, so as to provide better constraints for future theoretical models. For stars with masses <2 Msun, the horizontal-branch (HB) is the phase of evolution immediately preceding the AGB. Stars lying above the HB in the color-magnitude diagrams (CMDs) of globular clusters (GCs) offer valuable observational tests of theoretical evolutionary tracks. I therefore compiled what is to date the most complete survey of above-horizontal-branch (AHB) stars---objects lying above the HB and blueward of the AGB in the CMD---in 97 Galactic and seven Magellanic Cloud GCs. I selected AHB candidates based on photometry in the uBVI system, which is optimized for detection of low-gravity stars with large Balmer jumps, in the color range -0.05 < (B-V)_0 < 1.0. I then used Gaia astrometry and Gaussian-mixture modeling to confirm cluster membership and remove field interlopers. The resulting catalog contains 438 AHB stars, classified and interpreted in the context of post-HB evolution. Generally, I find that theory accounts for the main features of these AHB populations. For example, I find that bluer HBs yield brighter AHB stars, while redder HBs yield fainter AHB stars. However, certain clusters with exceptionally high metallicities tend to also contain blue HB stars and their AHB descendants. I also find an interesting evolutionary sequence in which blue HB and extreme HB stars become bright AHB stars, and then evolve into the instability strip to become W Vir Cepheids. This indicates that the Cepheids are stars evolving toward the AGB, not away from it. After leaving the AGB, these stars enter the PAGB phase of their evolution. Planetary nebulae (PNe) are outstanding observational tools for testing theoretical PAGB models, and I have thus performed multiple studies of PNe in this dissertation. By observing 598 PNe and 137 supernova remnants in M31 and M33, I show that misidentified supernova remnants are not responsible for the known systematic offset of planetary nebula luminosity function (PNLF) distances in distant galaxies. The source of the offset therefore remains a mystery. Given that the success of the use of the PNLF as a standard candle has no real theoretical basis, the sustained mystery of its various issues is unsurprising. Recent theoretical PAGB tracks have come close to correctly predicting the observed PN luminosities in old stellar populations (and have no problem at all in younger populations), but those models have only approached the dust-attenuated brightnesses of observed PNe. With my observations of the brightest 23 PNe in the bulge of M31, I have shown that the de-reddened PNLF of M31's inner bulge (r < 3 arcmin) features PNe that are up to ~1 mag brighter than initially thought, indicating central stars with masses >0.66 Msun and luminosities >11,000 Lsun---much brighter than the target luminosity of these theoretical models. I have also tested theoretical prescriptions for nucleosynthesis and dredge-up on the AGB with my observations of the PN in M31 open cluster B477-D075. The obtained spectra clearly show the PN to be overabundant in nitrogen, by a factor of ~5-6 relative to the initial composition. My CMD analysis implies the mass and metallicity of the progenitor star is M ~ 3.4 Msun and Z ~ 0.007. This places a hard upper limit on the minimum mass required for the onset of hot-bottom burning. This upper limit is at odds with theoretical models that do not incorporate a prescription for overshooting at the boundary of convective cores, but is in better agreement with models that do.