Summary

I am a postdoc in the Department of Astronomy and Astrophysics at the University of California, Santa Cruz, working in Natalie Batalha's group. I'm trying to understand planet formation and evolution by studying the demographics of exoplanets in binary star systems, as well as young stars. I am particularly interested in understanding the impact of multiplicity on star and planet formation and evolution. Papers I have either first- or co-authored are listed on ADS here.

My CV is here. My email is ksulliv4 (at) ucsc.edu.

Research

I work on a variety of projects related to young stellar populations and binary stars that host exoplanets. My star formation interests include using simulations to improve the methods used to derive population-wide statistics in star-forming regions, and working to better understand the relationships between accretion, disk properties, and planetary system formation. My planet demographics work is primarily focused on understanding the impact of stellar multiplicity on planet formation and evolution using exoplanet demographics.

An unresolved, spectrum of a binary can be accurately deconvolved into two components with well-constrained values using a Monte Carlo algorithm (Sullivan, Kraus, and Mann 2022).

Measuring Binary Star Properties

Multiplicity affects a planetary system's environment. For example, close multiplicity (separations less than 50 AU) reduces the planet occurrence rate in Kepler planets by a factor of 3, but the physical process causing this effect is not known. One complicating factor in understanding the mechanisms governing planet survival in binary stars is difficulty accurately characterizing the components of close binary stars. Although small numbers planet-hosting binary systems have been analyzed in detail, a more efficient methodology is necessary for developing a statistically robust sample of planet-hosting binaries with accurate properties.

The revised radius distribution for a population of small circumstellar planets in binaries (blue) with comparable draws from the California Kepler Survey sample of small planets around single stars. The two distributions do not match, and we do not observe a radius gap in our sample. We think this is because the radius gap could be separation-dependent and thus blurred out in a sample with a wide range of physical separations (Sullivan et al. 2023)

I developed a technique to retrieve the properties of spectroscopically-unresolved binary stars and am applying it to a variety of samples of planet-hosting binary stars from the Kepler mission. I began by analyzing archival data (Sullivan, Kraus, and Mann 2022), and have ongoing observational programs at the Hobby-Eberly Telescope at McDonald Observatory to collect data on a larger sample of Kepler binary star planet hosts. As a first scientific step, I published a paper that found that most supposed super-Earths in binaries are actually sub-Neptunes (Sullivan and Kraus 2022b).

Next, we explored the population of small planets in or near the radius gap and in binary stars, where we did not detect the radius gap in a population of 120 small planets. We suggested that this is the result of the radius gap potentially having a separation-dependent location because of the impact of the secondary star on planet formation and evolution (Sullivan et al. 2023). Recently, we followed up this effort with a larger sample and found that the sub-Neptune population of planets is suppressed in close binaries, suggesting that binaries are indeed inhibiting large plaent formation (Sullivan et al. 2024).

One way binary stars impact population statistics is by appearing younger than they truly are. (Sullivan and Kraus 2021)

Understanding Observational Biases

All studies have observational biases introduced by imperfect survey methods or true observational limitations. One way to better understand the impact of observational bias is by using synthetic surveys, where both the input ("true") and output ("observed") population parameters, such as age, binary fraction, and mass distribution, are known. I built a detailed, observationally-anchored population synthesis model to simulate and observe spectroscopic surveys of star-forming regions with different properties to try to understand whether some observed anomalous star-forming region properties could be caused by observational biases.

Starspots produce a uniform age gradient, rather than introducing an apparent age gradient as previously suggested in the literature (Sullivan & Kraus 2024).

Next, we added starspots into the simulation to investigate whether the presence of spots could introduce an apparent mass-dependent age gradient. Instead, we found that the starspots introduce a uniform age change, and that precise distances from the Gaia spacecraft can mitigate the impact of binaries on producing an apparent mass-dependent age gradient.

Spectral energy distributions of binary and single star dust disk excesses from an analysis of archival data (Sullivan and Kraus 2022a). Single and binary stars have similar SEDs, indicating that the inner disk structure is unaffected by multiplicity.

Observing and Quantifying Accretion

Young stars host circumstellar disks comprised of gas that is accreting onto their surfaces and dust that is forming planets or being burned up at the inner edge of the disk. These violent processes produce myriad distinctive observational signatures, but the connections between star and disk evolution and the role of accretion in disk dispersal remain unclear even after decades of work. I am interested in using both large population studies and in-depth studies of individual stellar systems to understand the role that accretion plays in disk dispersal and stellar evolution. My most recent work on the topic has found that the inner disks of binary and single stars are extremely similar, although the sample was not adequate to separate out close binary stars, where the impact of stellar multiplicity on the inner disk properties should be most pronounced.

About

I received a B.S. in physics and astronomy from the University of Massachusetts, Amherst. While at UMass I studied blue compact dwarf galaxies and the magnetic fields of spiral galaxies, before beginning to work on young binary stars during an REU at Lowell Observatory in Flagstaff AZ with Lisa Prato. After graduating, I worked at Lowell for another six months before moving to Austin, Texas for grad school in the fall of 2018. In 2023, I received my PhD in astronomy from the University of Texas at Austin, where I worked with Adam Kraus to study the properties of young star-forming regions and binary stars that host exoplanets. I am currently a postdoc at UC Santa Cruz, working in Natalie Batalha's group and continuing to study exoplanets in binary star systems to improve our understanding of planet formation.

Beyond research, I have been involved in outreach and mentoring within astronomy since undergrad. I was the diversity officer and then the president of the UMass Astronomy Club, and the co-director of the Orchard Hill Observatory, which is a student-run observatory on the UMass campus that opens for the public weekly. I also mentored junior undergraduates. At Texas, I was a near-peer mentor for several undergraduates. In the summer, I ran an Introduction to Astronomy Jargon lecture series for TAURUS and REU students, assisted with an introductory Python workshop run by another graduate student, and was the lead organizer for the summer student professional development workshops. I also co-led the Graduate-Undergraduate Mentoring in Astronomy program. Outside of work, I like hanging out with my two cats, reading, finding new things to cook, and doing pretty much any form of exercise except for running.