PI Name & Contact Information:
The student will join an existing collaboration between Rachael Beaton (STScI, Asst Astronomer) and Sten Hasselquist (STScI, Sr Staff Scientist) in Baltimore, with collaborators at several other institutions including graduate students, undergraduates, and professors.
Project Duration:
This is a ~1 year project for a graduate student; no prior experience with this topic or these datasets is required. There are several avenues after this initial project that could become a thesis project.
Title:
Clarifying Internal Age Spreads in Globular Clusters with Spectroscopy & Photometry
Project Abstract:
Globular Clusters are now known to have multiple generations of stars. The evidence largely comes from the observation of internal chemical spreads that emerge from either chemically-sensitive photometry, largely focusing on the subgiant and main sequence, or in detailed chemical abundances for giant stars. The physical interpretation is that nucleosynthetic products from the first generation of stars enriches the subsequent generations, but to-date the observational constraints cannot identify a unique process. This project uses a wealth of existing spectroscopic and imaging data on two metal-rich globular clusters in the Milky Way, 47 Tuc and M4, along-side state-of-the-art isochrones that use stellar atmospheres with variable chemical mixtures. The central question is to determine if detailed chemical abundance measurements can better constrain the ages in globular clusters. We anticipate being able to estimate an age spread and being able to comment directly on what tools need to be developed to improve the work in this project.
From left to right:
(1) C/N birth abundances versus surface gravity for 47 Tuc used to identify a high, intermediate, and low C/N population.
(2) Comparison of the light-element abundance ratios Al/Fe to Mg/Fe and the correspondence with the three C/N populations.
(3) Distribution of Fe/H for the three populations, showing overall correspondence.
(4) The physical properties of the stars, largely, overlap in logg-Teff space. Filters sensitive to CNO variations will show difference for the three populations.
Work the student will do:
The student will combine SDSS-IV/APOGEE stellar parameters and abundances, archival wide-field UBVRI photometry catalogs, and archival HST catalogs for the globular clusters M4 and 47 Tuc into a single merged dataset (all datasets are in hand and ready for scientific analysis). Using this dataset, the student will use APOGEE data and stellar mixing models to constrain CNO abundances of the “first” and “second” generation stars sufficiently to identify isochrones from the Victoria-Regina models that use atmospheres with multiple chemical abundance mixtures. Using these specialized isochrones, the student will constrain the range of ages that best match the photometric data. Anticipated results are more accurate constraints on the age spreads of these globular clusters as well as providing an important framework for extending this analysis to all GCs, ultimately allowing for constraints on formations processes at a range of metallicity/ages.
The student will develop skills in (1) the use of large datasets from surveys of different scales, (2) generating code for visualizations, (3) scientific writing and communication. The student will become familiar with using datasets on: (1) stellar chemical abundances, (2) ground-based photometry, (3) space-based photometry, and (4) pre-computed stellar tracks (isochrones). These skills and topical knowledge areas can be applied more broadly in astrophysical research.
From left to right:
(1) Stellar tracks matched to CNO chemistry limited in age to span the distribution of stars in U+I+B versus B color-magnitude space. The index U+I+B are sensitive to C and O features in the stellar spectrum.
(2) Correspondence between the U+B+I residual to the N/H variation in the cluster.
(3) Broad-band color-magnitude diagram of U versus U-I on the subgiant branch where the multiple populations spread on the subgiant branch.
(4) Correspondence of isochrones on from ground-based photometry to high-precision HST photometry where age spreads can be measured precisely.
