Sheila A. Sagear

Welcome to my website! I am a Ph.D. Candidate in Astronomy at the University of Florida. With my advisor Prof. Sarah Ballard, I study exoplanets around the smallest and coolest stars.
I recieved my Bachelor's degree in Astronomy & Physics from Boston University in 2020. In the past, I have been a Flatiron Pre-Doctoral Fellow in the Center for Computational Astrophysics at the Flatiron Institute, an intern at the Kepler/K2 Guest Observer office at NASA Ames Research Center and the FastML group at CERN.
Click here to download my CV.

Research

zoomies: A Tool to Infer Stellar Age from Vertical Action in Gaia Data.

Stellar age measurements are fundamental to understanding a wide range of astronomical processes, including Galactic dynamics, stellar evolution, and planetary system formation. However, extracting age information from main-sequence stars is complicated, with techniques often relying on age proxies in the absence of direct measurements. The Gaia data releases have enabled detailed studies of the dynamical properties of stars within the Milky Way, offering new opportunities to understand the relationship between stellar age and dynamics. In this study, we leverage high-precision astrometric data from Gaia DR3 to construct a stellar age prediction model based only on stellar dynamical properties, namely the vertical action. We calibrate two distinct, hierarchical stellar age–vertical action relations, first employing asteroseismic ages for red-giant-branch stars, then isochrone ages for main-sequence turn-off stars. We describe a framework called zoomies based on this calibration, by which we can infer ages for any star given its vertical action. This tool is open-source and intended for community use. We compare dynamical age estimates from zoomies with age measurements from open clusters and asteroseismology. We use zoomies to generate and compare dynamical age estimates for stars from the Kepler, K2, and TESS exoplanet transit surveys. While dynamical age relations are associated with large uncertainty, they are generally mass independent and depend on homogeneously measured astrometric data. These age predictions are uniquely useful for large-scale demographic investigations, especially in disentangling the relationship between planet occurrence, metallicity, and age for low-mass stars.

The Orbital Eccentricities of Planetary Systems Orbiting M dwarfs

Leveraging the photoeccentric effect , we combine Kepler transit light curves with stellar density information from spectroscopy and parallaxes from Gaia to constrain the orbital eccentricities for over 150 planets around nearby M dwarfs. Within a Bayesian hierarchical framework, we draw out the underlyling eccentricity distribution for planets around M dwarfs and find two distinct populations: single-transit, higher eccentricity planets and multi-transit, low-eccentricity planets. We use planet occurrence information to constrain the volume-limited occurrence rate of eccentric M dwarf planets. Read the paper in PNAS

Upper Limits on Planet Occurrence around Ultracool Dwarfs

As an undergraduate at Boston University, I worked with my advisors Prof. Philip Muirhead and Dr. Julie Skinner to constrain the upper limit of planet occurrence around late-M and early-L dwarfs (or "ultracool dwarfs") in our local neighborhood. We conducted a planet search around 827 ultracool dwarfs observed by NASA's K2 mission, and found none. Using this null result, we constrained the upper limit of planet occurrence around ultracool dwarfs in the local neighborhood, accounting for our transit detection efficiency and transit probability.

Papers

  1. Sheila Sagear, Adrian M. Price-Whelan, Sarah Ballard, Yuxi (Lucy) Lu, Ruth Angus, and David W. Hogg
    zoomies: A Tool to Infer Stellar Age from Vertical Action in Gaia Data (2024)
    The Astrophysical Journal, Volume 977, Number 1
  2. Sagear, Sheila and Ballard, Sarah
    The orbital eccentricity distribution of planets orbiting M dwarfs (2023)
    Proceedings of the National Academy of Sciences, Volume 120, No. 23
  3. Tamburo, Patrick; Muirhead, Philip S.; McCarthy, Allison M.; Hart, Murdock; Gracia, David; Vos, Johanna M.; Bardalez Gagliuffi, Daniella C.; Faherty, Jacqueline; Theissen, Christopher; Agol, Eric; Skinner, Julie N.; Sagear, Sheila
    The Perkins INfrared Exosatellite Survey (PINES) I. Survey Overview, Reduction Pipeline, and Early Results (2022)
    The Astronomical Journal, Volume 163, Number 6
  4. Sagear, Sheila A.; Skinner, Julie N.; Muirhead, Philip S.,
    Upper Limits on Planet Occurrence around Ultracool Dwarfs with K2 (2020)
    The Astronomical Journal, vol. 160, no. 1, id.19, 7 pp., 2020. >
  5. Di Guglielmo, Giuseppe; Duarte, Javier; Harris, Philip; Hoang, Duc; Jindariani, Sergo; Kreinar, Edward; Liu, Mia; Loncar, Vladimir; Ngadiuba, Jennifer; Pedro, Kevin; Pierini, Maurizio; Rankin, Dylan; Sagear, Sheila; Summers, Sioni; Tran, Nhan; Wu, Zhenbin
    Compressing Deep Neural Networks on FPGAs to Binary and Ternary Precision with HLS4ML (2020)
    Machine Learning: Science and Technology, 2, 015001.

Presentations

Software

I published an open-source Python package called zoomies, which is an open-source tool to constrain stellar ages using only kinematic information (vertical action from Gaia), a galactic potential model, and an external stellar calibration sample. The package includes tools to calibrate a stellar age--vertical action relation and apply it to your desired stellar sample. Since the age relation comes from purely kinematic information (galactic orbits), the relation is nearly completely mass-independent, meaning you can use this tool to constrain ages for any stars from supergiants to M dwarfs!

I also published an open-source Python package called photoeccentric, which includes a suite of tools to process Kepler transit lightcurves, calculate stellar densities, and perform transit fitting to constrain eccentricities using only photometric data + stellar density priors.

I contribute to lightkurve, a user-friendly, open-source Python package for working with Kepler, K2, and TESS data.

Contact

Trulli
My lovely group at the OWL Exoplanet Workshop in Santa Cruz, CA.
L to R: Natalia Guerrero, Chris Lam, and Quadry Chance.