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Origin of Elements
The origin of elements is a fundamental question in astrophysics. However, the primary astrophysical site(s) for the creation of elements heavier than iron, via the r-process is still unknown. I use the preserved signatures of r-process elements in metal-poor stars from preceding nucleosynthesis events to constrain properties of candidate astrophysical sites.
Near-Field Cosmology
I use metal-poor stars as a near-field probe to constrain the age of the Universe and the assembly history the Milky Way.
Chemical Enrichment and Evolution of the Universe
I am interested in large sample analyses of chemical signatures in stars to understand the chemical enrichment and evolution of the Milky Way and its substructures.
First-Author Publications
4. Shah, S. P., Holmbeck, E. M., Ezzeddine, R., Ji, A. P., and R-Process Alliance. In prep.
3. Shah, S. P., Ezzeddine, R., Roederer, I., et al. 2024. MNRAS, 529, 3
2. Shah, S. P., Ezzeddine, R., Ji, A. P., et al. 2023, ApJ, 948, 122
1. Shah, S. P., Wright, J. T., Isaacson, H., et al. 2018, ApJL, 863, L26
In-Press
astrobite: Age is Just a Number: Radioactive Dating in Stars by Mark Popinchalk, which was also featured in AAS Nova
Thorium Abundances and Actinide Mass Fraction of 53 Metal-Poor Stars
Thorium Abundances and Actinide Mass Fraction of 53 Metal-Poor Stars
Coming Soon!
Actinides are the heaviest group of elements created in the Universe. Their yields are have been found to be uniquely sensitive to the astrophysical conditions of the r-process events. In fact, the detection of actinide elements in future JWST observations of kilonovae is highly anticipated. In turn, abundances of actinides, such as thorium, in metal-poor stars, have been very limited, with only 30 abundance determination in the last 3 decades, typically with one thorium abundance published in one study, leading to an inhomogeneous sample.
In this project, I have customized LESSPayne to homogeneously determine thorium abundances in over 50 metal-poor stars, a first of its kind large sample. A result of this is also the first chemical evolution model of thorium spanning 2 dex in metallicity. I confirm that actinides-to-lanthanides ratios vary between stars over an order of magnitude. I also collaborated with Erika Holmbeck to determine the actinide mass fraction of the progenitor r-process events of these stars, enabling constraints for future JWST observations of actinides in kilonovae.
Abundances of 23 r-process elements for J0051-1053, including rarely-examined elements like cadmium, platinum, and gold with UV and optical spectroscopy.
The Chemical Composition of a Metal-Poor Star from Lithium to the Actinides
In this work, I have used UV and optical spectroscopy to obtain abundances of 42 elements for a metal-poor star, ranging from lithium to the actinides. The results include abundance determinations of rarely-examined elements such as germanium and cadmium, and only the 6th abundance determination of gold in a star.
This work enables a probe of nuclear physical processes in r-process nucleosynthesis (e.g., fissioning), different astrophysical sites, and effects of NLTE. UV spectroscopy is critical since several of these elements can not be accessed in the optical domain.
Recent related work: Roederer et al. 2022a; Roederer et al. 2022b; Holmbeck et al. 2022.
Shah, Shivani P., Ezzeddine, Rana, Ji, Alex P., Hansen, Terese T., Catelan, Marcio, Roederer, Ian U., Hackshaw, Zoe, Holmbeck, Erika M., Beers, Timothy C., and Surman, Rebecca. 2023. Accepted to ApJ.
Uranium Abundances and Ages of R-process Enhanced Stars with Novel U II Lines
Nucleo-cosmochronometry, a radioactive dating technique, enables stellar age determination independent of stellar evolution models. However, it is challenging to determine the abundance of radioactive elements, uranium and thorium. Specifically, the only U II line canonically used in the literature is blended and frequently enables only an upper limit on the U abundance.
In this work, I have performed the first homogeneous U abundance analysis of four r-process enhanced stars with two novel U II lines at 4050 and 4090 Å, along with the canonical 3859 Å line. Poster presented at JINA-CEE 2022 conference.
Number of galaxies per mass bin that are positively grouped. A galaxy is positively grouped by the observational algorithm if it is associated to the same group by the simulation as well.
Methods to Examine the Robustness of the H0 Measurements Using Standard Sirens
(Masters Thesis. Advised by Prof. Paul Torrey. Collaborators: Profs. Steve Eikenberry and Anthony Gonzalez)
The GW170817 event offered a unique opportunity to determine the Hubble constant using a standard siren distance measurement and an estimated Hubble velocity. The Hubble velocity was obtained by correcting for the peculiar velocity of the host-galaxy based on its observed membership to a group of galaxies.
For my masters thesis, I developed methods to investigate the accuracy of the observational algorithms used to assign group membership and therefore the peculiar velocity to galaxies similar to the host-galaxy of GW170817. For this purpose, I generated ARCID, a suite of dark-matter only simulations with constrained initial conditions. I also developed statistical tools including Bayesian sampling for the post-processing of the simulation data (see).
This project is now being led by David Wright, a graduate student at the University of Central Florida, who is applying these methods to IllustrisTNG for its higher resolution. David presented his results in a poster at AAS 241.
Shah, Shivani P., Wright, Jason T., Isaacson Howard, Howard, Andrew W., and Curtis, Jason L. 2018, ApJL, 863, 2. https://doi.org/10.3847/2041-8213/aad40c
HD 4915: A Maunder Minimum Candidate
(Shah et al. 2018. Advised by Prof. Jason Wright.)
In this work, I discovered a Sun-like star with decreasing amplitude of its magnetic cycle, given by Ca II H & K line strengths, reminiscent of the Solar Maunder-Minimum period from 1645-1715 AD.
The cause of such sudden decreasing amplitude in magnetic cycles remains unknown, although there is evidence of several such periods for the Sun including Dalton Minimum. Studying such low activities in other Sun-like stars provides a unique opportunity to investigate the phenomenon.
See some recent work on this topic: Luhn et al. 2022; Baum et al. 2022.
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