arXiv subjects

Kendall Sullivan

Publications and source records attributed to Kendall Sullivan.

At least 19 recordsLinked to original sources

arXiv2026

Improving exoplanet mass characterisation with Bayesian model selection using the Learned Harmonic Mean Estimator

Radial velocity (RV) analyses require modelling choices (such as eccentricity treatment, noise model, velocity trends, and number of planets) that can significantly affect derived planetary masses. Current practice often relies on information criteria to compare and select models, but these have known limitations: they lack the built-in Occam's razor of Bayesian model comparison, and they do not incorporate prior information. Computing the Bayesian evidence needed for Bayes factor model comparison has traditionally required dedicated algorithms such as nested sampling. The learned harmonic mean estimator (LHME) offers an alternative, estimating the Bayesian evidence directly from MCMC posterior samples, with less computational cost and with no modification to the fitting procedure. We present the first application of the LHME to RV model selection, fitting 18 model variants -- comparing circular and eccentric orbits, white noise and Gaussian Process noise models, and long-term velocity trends -- to six single-planet systems, and 72 variants to a seventh system for an $N$ versus $N+1$ planet model comparison. We find that no single model is universally preferred, reinforcing the need for model comparison to select the most appropriate model for a system, thereby ensuring robust mass characterisation. The LHME, implemented in the open-source harmonic package, makes rigorous Bayesian model comparison accessible to existing MCMC-based RV workflows, and we encourage its wider use for other model comparisons in astrophysics.

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arXiv2026

Determining the Host Stars of Planets in Binary Star Systems with Asterodensity Profiling: Investigating the Canonical Radius Gap

Over the past 30 years, thousands of exoplanets have been discovered, revealing detailed demographics of planets outside the Solar System. One of the most dramatic features of the planet radius distribution is the radius gap, a lack of planets between $\sim$1.8-2 $R_\oplus$. The radius gap is thought to mark the distinction between rocky and gas planets. Recent research has found that the radius gap may not be present in binary star systems. In past studies of planets in binary star systems, the common assumption has been that all of the planets are hosted by the primary star. In many cases, the radius of the planet would be significantly larger if it were orbiting the companion star, which could potentially affect the true radius distribution. It is possible to identify the host stars of planets through stellar density estimates obtained from transit fitting. Using this method, we made probabilistic estimates for the host stars of a sample of 15 transiting exoplanets across 10 binary star systems hosting either 1 or 2 planets, at least one of which would reside in the canonical radius gap if it was circumprimary. We found that 5 of the planets are highly likely to be circumprimary, while the remainder have ambiguous host stars. The lack of unambiguously circumsecondary planets is caused by physical and observational biases that favor circumprimary planets. Nonetheless, the summed posterior probabilities suggest that the canonical radius gap appears less vacant for planets in binaries.

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arXiv2026

Planetary Architectures of Kepler Compact Multis with Binary Star Companions

Planets in binary-star systems exhibit demographic differences compared to planets in single-star systems. In particular, planets with binary-star hosts have a lower overall occurrence rate compared to their single-star counterparts, as well as a suppressed relative occurrence rate for sub-Neptunes ($R_p=2{-}4R_{\oplus}$) compared to super-Earths ($R_p=1.0{-}1.5R_{\oplus}$). These differences are most pronounced in close separation binaries ($\rho < 100$ au) which has been interpreted as a result of binary stars disrupting the protoplanetary disks of their stellar companions. The architectures of planetary systems -- i.e. the arrangements of planet sizes and orbits -- provide additional information about system formation and evolution. Architectures of single-star planetary systems are well studied, but architectures of binary-star planetary systems have not been investigated in detail. In this work, we analyzed a large sample of Kepler planetary systems (162 planets in 118 binary-star systems; 880 planets in 544 single-star systems) to compare their architectures as a function of stellar multiplicity. We found that planets with binary-star hosts follow a similar ``peas-in-a-pod'' tendency toward uniformity in planet radii and log-uniformity in period spacing as planets with single-star hosts. However, we also detected modest ($2.5-3\sigma$) differences in period spacing and planet multiplicity, with binary-star systems having higher typical gap complexities (indicating more uneven spacing) and a higher prevalence of single planets. We interpret these results as evidence that binary stars primarily influence the planetary architectures of their stellar companions by shaping the protoplanetary disk at formation, with subsequent dynamical processing more gently altering the system architectures over secular timescales.

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arXiv2026

Barium Stars Across the Milky Way: Probing Their Origins via the GALAH Survey

Barium stars are unusually enriched in barium ([Ba/Fe] >= 1.0 dex) and not predicted by current Galactic chemical evolution models. Previous observations of barium stars have found evidence that they form through mass transfer from a companion asymptotic giant branch (AGB) star or through radiative levitation. The chemical abundance and kinematic information of barium stars may help constrain AGB stellar nucleosynthesis, binary star evolution, and internal evolutionary processes that affect surface abundances. Using ~450,000 stars from the GALactic Archaeology with Hermes (GALAH) survey, we identify nearly 3000 new barium-rich stars and separate them into hot (Teff > 6000 K) and cool (Teff < 6000 K) populations. Cross-matching with Gaia DR3, we find that 47.7% of our barium stars within 1 kpc have elevated re-normalized unit weight error (RUWE >= 1.4), compared to 16.3% of a comparable sample of the GALAH field, suggesting multiplicity plays an important role in the formation of both populations of barium stars. A subset of hot barium stars exhibit low RUWE (RUWE < 1.2) and [alpha/Fe] < -0.2, supporting radiative levitation as an origin as well. We determine Galactic memberships using both kinematics and chemistry and find that barium stars exist in the thin disk, thick disk, and halo though they are slightly more prevalent at lower metallicities. Overall, we show evidence for barium stars produced by mass transfer and for those produced by radiative levitation, with both formation mechanisms occurring ubiquitously across the Galaxy.

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arXiv2025

Are We There Yet? Challenges in Quantifying the Frequency of Earth Analogs in the Habitable Zone

Searching for life elsewhere in the universe is one of the most highly prioritized pursuits in astronomy today. However, the ability to observe evidence of Earth-like life through biosignatures is limited by the number of planets in the solar neighborhood with conditions similar to Earth. The occurrence rate of Earth-like planets in the habitable zones of Sun-like stars, $\eta_{\oplus}$, is therefore crucial for addressing the apparent lack of consensus on its value in the literature. Here we present a review of the current understanding of $\eta_{\oplus}$. We first provide definitions for parameters that contribute to $\eta_{\oplus}$. Then, we discuss the previous and current estimated parameter values and the context of the limitations on the analyses that produced these estimates. We compile an extensive list of the factors that go into any calculation of $\eta_{\oplus}$, and how detection techniques and surveys differ in their sensitivity and ability to accurately constrain $\eta_{\oplus}$. Understanding and refining the value of $\eta_{\oplus}$ is crucial for upcoming missions and telescopes, such as the planned Habitable Worlds Observatory and the Large Interferometer for Exoplanets, which aim to search for biosignatures on exoplanets in the solar neighborhood.

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arXiv2025

The First Occurrence Rate Estimates for Exoplanets in Small-Separation Binary Star Systems: Planet Occurrence is Suppressed in Binary Stars

Exoplanet occurrence rates facilitate comparisons between observations of planets and theoretical models of planet formation. Despite their deductive power, exoplanet occurrence rates for half the stars in the sky are missing because occurrence rate studies systematically exclude binary star systems. We assembled a large sample of high-likelihood binaries from the Kepler mission to calculate occurrence rates for circumstellar (S-type) planets in small-separation binary star systems ($\lesssim 100$ au) for the first time. For a sample of high-likelihood small-separation binaries, we found binaries to host 58% fewer planets per system than single stars to 11.4$\sigma$ significance within 1-4 $R_{\oplus}$ and 1-50 d, and 50% fewer planets compared to single stars when integrating over the full parameter space of 1-10 $R_{\oplus}$ and 1-100 d to 3.8$\sigma$ significance.. We found no evidence for a radius valley or radius cliff, instead detecting a smooth decline in planet occurrence with increasing planetary radius. The difference between the single-star planet radius distribution and the binary-star planet radius distribution is 4.3$\sigma$ significant from a Kolomogorov-Smirnov test. These results suggest significantly different planet formation and survival outcomes in binaries compared to single stars, and support other studies that have measured a deficit of observed planets in binary star systems.

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arXiv2024

Quantifying the Contamination From Nearby Stellar Companions in Gaia DR3 Photometry

Identifying and removing binary stars from stellar samples is a crucial but complicated task. Regardless of how carefully a sample is selected, some binaries will remain and complicate interpretation of results, especially via flux contamination of survey photometry. One such sample is the data from the Gaia spacecraft, which is collecting photometry and astrometry of more than $10^{9}$ stars. To quantify the impact of binaries on Gaia photometry, we assembled a sample of known binary stars observed with adaptive optics and with accurately measured parameters, which we used to predict Gaia photometry for each stellar component. We compared the predicted photometry to the actual Gaia photometry for each system, and found that the contamination of Gaia photometry because of multiplicity decreases non-linearly from near-complete contamination ($\rho \leq 0''.15$) to no contamination (binary projected separation, or $\rho > 0''.3$). We provide an analytic relation to analytically correct photometric bias in a sample of Gaia stars using the binary separation. This correction is necessary because the Gaia PSF photometry extraction does not fully remove the secondary star flux for binaries with separations with $\rho \lesssim 0''.3$. We also evaluated the utility of various Gaia quality-of-fit metrics for identifying binary stars and found that RUWE remains the best indicator for unresolved binaries, but multi-peak image fraction probes a separation regime not currently accessible to RUWE.

astro-ph.IM↗

arXiv2024

Orbital Architectures of Planet-Hosting Binaries III. Testing Mutual Inclinations of Stellar and Planetary Orbits in Triple-Star Systems

Transiting planets in multiple-star systems, especially high-order multiples, make up a small fraction of the known planet population but provide unique opportunities to study the environments in which planets would have formed. Planet-hosting binaries have been shown to have an abundance of systems in which the stellar orbit aligns with the orbit of the transiting planet, which could give insights into the planet formation process in such systems. We investigate here if this trend of alignment extends to planet-hosting triple-star systems. We present long-term astrometric monitoring of a novel sample of triple-star systems that host Kepler transiting planets. We measured orbit arcs in 21 systems, including 12 newly identified triples, from a homogeneous analysis of our Keck adaptive optics data and, for some systems, Gaia astrometry. We examine the orbital alignment within the nine most compact systems ($\lesssim500$ au), testing if either (or both) of the stellar orbits align with the edge-on orbits of their transiting planets. Our statistical sample of triple systems shows a tendency toward alignment, especially when assessing the alignment probability using stellar orbital inclinations computed from full orbital fits, but is formally consistent with isotropic orbits. Two-population tests where half of the stellar orbits are described by a planet-hosting-binary-like moderately aligned distribution give the best match when the other half (non-planet-hosting) has a Kozai-like misaligned distribution. Overall, our results suggest that our sample of triple-star planet-hosting systems are not fully coplanar systems and have at most one plane of alignment.

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arXiv2024

Revising Properties of Planet-Host Binary Systems. IV. The Radius Distribution of Small Planets in Binary Star Systems is Dependent on Stellar Separation

Small planets ($R_{p} \leq 4 R_{\oplus}$) are divided into rocky super-Earths and gaseous sub-Neptunes separated by a radius gap, but the mechanisms that produce these distinct planet populations remain unclear. Binary stars are the only main-sequence systems with an observable record of the protoplanetary disk lifetime and mass reservoir, and the demographics of planets in binaries may provide insights into planet formation and evolution. To investigate the radius distribution of planets in binary star systems, we observed 207 binary systems hosting 283 confirmed and candidate transiting planets detected by the Kepler mission, then recharacterized the planets while accounting for the observational biases introduced by the secondary star. We found that the population of planets in close binaries ($\rho \leq 100$ au) is significantly different from the planet population in wider binaries ($\rho > 300$ au) or single stars. In contrast to planets around single stars, planets in close binaries appear to have a unimodal radius distribution with a peak near the expected super-Earth peak of $R_{p} \sim 1.3 R_{\oplus}$ and a suppressed population of sub-Neptunes. We conclude that we are observing the direct impact of a reduced disk lifetime, smaller mass reservoir, and possible altered distribution of solids reducing the sub-Neptune formation efficiency. Our results demonstrate the power of binary stars as a laboratory for exploring planet formation and as a controlled experiment of the impact of varied initial conditions on mature planet populations.

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arXiv2024

Starspots and Undetected Binary Stars Have Distinct Signatures in Young Stellar Associations

Young stars form in associations, meaning that young stellar associations provide an ideal environment to measure the age of a nominally coeval population. Isochrone fitting, which is the typical method for measuring the age of a coeval population, can be impacted by observational biases that obscure the physical properties of a population. One feature in isochrone fits of star-forming regions is an apparent mass-dependent age gradient, where lower-mass stars appear systematically younger than higher-mass stars. Starspots and stellar multiplicity are proposed mechanisms for producing the mass-dependent age gradient, but the relative importance of starspots versus multiplicity remains unclear. We performed a synthetic red-optical low-resolution spectroscopic survey of a simulated analog to a 10 Myr stellar association including mass-dependent multiplicity statistics and age-dependent starspot coverage fractions. We found that undetected starspots alone do not produce an apparent mass-dependent age gradient, but instead uniformly reduce the average measured age of the population. We also found that binaries continue to produce an apparent mass-dependent age gradient, and introduce more scatter in the age measurement than spots, but are easily removed from the population as long as there are good distance measurements to each target. We conclude that it is crucial to incorporate treatments of both starspots and undetected stellar multiplicity into isochrone fits of young stellar associations to attain reliable ages.

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arXiv2023

Revising Properties of Planet-Host Binary Systems. III. There is No Observed Radius Gap For Kepler Planets in Binary Star Systems

Binary stars are ubiquitous; the majority of solar-type stars exist in binaries. Exoplanet occurrence rate is suppressed in binaries, but some multiples do still host planets. Binaries cause observational biases in planet parameters, with undetected multiplicity causing transiting planets to appear smaller than they truly are. We have analyzed the properties of a sample of 119 planet-host binary stars from the Kepler mission to study the underlying population of planets in binaries that fall in and around the radius valley, which is a demographic feature in period-radius space that marks the transition from predominantly rocky to predominantly gaseous planets. We found no statistically significant evidence for a radius gap for our sample of 122 planets in binaries when assuming the primary stars are the planet hosts, with a low probability ($p < 0.05$) of the binary planet sample radius distribution being consistent with the single-star small planet population via an Anderson-Darling test. These results reveal demographic differences in the planet size distribution between planets in binary and single stars for the first time, showing that stellar multiplicity may fundamentally alter the planet formation process. A larger sample and further assessment of circumprimary versus circumsecondary transits is needed to either validate this non-detection or explore other scenarios, such as a radius gap with a location that is dependent on binary separation.

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arXiv2022

Revising Properties of Planet-host Binary Systems II: Apparent Near-Earth Analog Planets in Binaries Are Often Sub-Neptunes

Identifying rocky planets in or near the habitable zones of their stars (near-Earth analogs) is one of the key motivations of many past and present planet-search missions. The census of near-Earth analogs is important because it informs calculations of the occurrence rate of Earth-like planets, which in turn feed into calculations of the yield of future missions to directly image other Earths. Only a small number of potential near-Earth analogs have been identified, meaning that each planet should be vetted carefully and then incorporated into the occurrence rate calculation. A number of putative near-Earth analogs have been identified within binary star systems. However, stellar multiplicity can bias measured planetary properties, meaning that apparent near-Earth analogs in close binaries may have different radii or instellations than initially measured. We simultaneously fit unresolved optical spectroscopy, optical speckle and near-infrared AO contrasts, and unresolved photometry, and retrieved revised stellar temperatures and radii for a sample of 11 binary Kepler targets that host at least one near-Earth analog planet, for a total of 17 planet candidates. We found that 10 of the 17 planets in our sample had radii that fell in or above the radius gap, suggesting that they are not rocky planets. Only 2 planets retained super-Earth radii and stayed in the habitable zone, making them good candidates for inclusion in rocky planet occurrence rate calculations.

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arXiv2022

Revising Properties of Planet-host Binary Systems I.: Methods and Pilot Study

To fully leverage the statistical strength of the large number of planets found by projects such as the Kepler survey, the properties of planets and their host stars must be measured as accurately as possible. One key population for planet demographic studies is circumstellar planets in close binaries ($\rho < 50$au), where the complex dynamical environment of the binary inhibits most planet formation, but some planets nonetheless survive. Accurately characterizing the stars and planets in these complex systems is a key factor in better understanding the formation and survival of planets in binaries. Toward that goal, we have developed a new Markov Chain Monte Carlo fitting algorithm to retrieve the properties of binary systems using unresolved spectra, unresolved photometry, and resolved contrasts. We have analyzed 8 Kepler Objects of Interest in M star binary systems using literature data, and have found that the temperatures of the primary stars (and presumed planet hosts) are revised upward by an average of 200 K. The planetary radii should be revised upward by an average of 20% if the primary star is the host, and 80% if the secondary star is the planet host. The average contrast between stellar components in the Kepler band is 0.75 mag, which is small enough that neither star in any of the binaries can be conclusively ruled out as a potential planet host. Our results emphasize the importance of accounting for multiplicity when measuring stellar parameters, especially in the context of exoplanet characterization.

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arXiv2022

Optical and Near-Infrared Excesses are Correlated in T Tauri Stars

Accretion is one of the defining characteristics of classical T Tauri stars, fueled by the presence of a circumstellar disk comprised of dust and gas. Accretion produces a UV and optical excess, while re-radiated emission at the inner edge of the dust component of the disk produces a near-infrared (NIR) excess. The interplay between stars and their disks helps regulate protoplanetary disk evolution and dispersal, which is key to a full understanding of planet formation. To investigate the relations between NIR excess and optical excess in both single and binary stars, we used an archival sample of spectroscopically characterized members of the Taurus star-forming region ($\tau \sim$ 1-2 Myr) with measured luminosities, spectral types, and optical veiling. We combined the archival sample with 2MASS and WISE NIR photometry and high-resolution imaging surveys. We found that NIR and optical excesses are correlated in multiple NIR photometric bands, suggesting that they are closely related, likely because more massive disks have higher inner dust disk walls and are also associated with higher accretion rates. We also found that multiplicity has no impact on accretion or inner disk properties in a sample with a wide range of separations, but the sample was too small to specifically investigate close binaries, where the effects of multiplicity on disk properties should be most significant.

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arXiv2021

Undetected Binary Stars Cause an Observed Mass Dependent Age Gradient in Upper Scorpius

Young stellar associations represent a key site for the study of star formation, but to accurately compare observations to models of stellar evolution, the age of an association must be determined. The Upper Scorpius region is the youngest section of the Scorpius-Centaurus OB association, which is the largest collection of nearby, young, low-mass stars. The true age of Upper Scorpius is not clear, and an observed mass-dependent age gradient in Upper Scorpius, as well as in other star-forming regions, complicates age measurements. The age gradient may indicate a genuine astrophysical feature or may be an artifact of unrecognized systematic effects in stellar age measurements. We have conducted a synthetic red-optical low-resolution spectroscopic survey of a simulated analog to the Upper Scorpius star-forming region to investigate the effects of unresolved binary stars (which have mass-dependent demographics) on age measurements of a stellar population. We find that the observed mass-dependent age gradient in Upper Scorpius can be explained by a population of undetected binary stars. For a simulated population with an age of 10 (RMS = 2) Myr, we measure an age of 10.5 (RMS = 3.5) Myr for F stars, and 7.5 (RMS = 5.8) Myr for M stars. This discrepancy is caused by the mass-dependent mass ratio distribution and the variable steepness of the mass-luminosity relation. Our results support the previously suggested 10 Myr age for Upper Scorpius, with a small intrinsic age spread.

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arXiv2021

Projected Rotational Velocities and Fundamental Properties of Low-Mass Pre-Main Sequence Stars in the Taurus-Auriga Star Forming Region

The projected stellar rotational velocity ($v \sin i$) is critical for our understanding of processes related to the evolution of angular momentum in pre-main sequence stars. We present $v \sin i$ measurements of high-resolution infrared and optical spectroscopy for 70 pre-main sequence stars in the Taurus-Auriga star-forming region, in addition to effective temperatures measured from line-depth ratios, and stellar rotation periods determined from optical photometry. From the literature, we identified the stars in our sample that show evidence of residing in circumstellar disks or multiple systems. The comparison of infrared $v \sin i$ measurements calculated using two techniques shows a residual scatter of $\sim$ 1.8 km s$^{-1}$, defining a typical error floor for the $v \sin i$ of pre-main sequence stars from infrared spectra. A comparison of the $v \sin i$ distributions of stars with and without companions shows that binaries/multiples typically have a higher measured $v \sin i$, which may be caused by contamination by companion lines, shorter disk lifetimes in binary systems, or tidal interactions in hierarchical triples. A comparison of optical and infrared $v \sin i$ values shows no significant difference regardless of whether the star has a disk or not, indicating that CO contamination from the disk does not impact $v \sin i$ measurements above the typical $\sim$ 1.8 km s$^{-1}$ error floor of our measurements. Finally, we observe a lack of a correlation between the $v \sin i$, presence of a disk, and H-R diagram position, which indicates a complex interplay between stellar rotation and evolution of pre-main sequence stars.

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arXiv2019

S and VV Corona Australis: Spectroscopic Variability in Two Young Binary Star Systems

We used high-resolution near-infrared spectroscopy from the NIRSPEC instrument on the Keck II telescope, taken over multiple epochs spanning five years, to examine two young binary T Tauri star systems, S Corona Australis and VV Corona Australis. The stars in these 1-2" separation systems have optically thick circumstellar disks and high extinctions at optical and near-infrared wavelengths. Using a combination of new and archival data, we have determined the spectral types of all the stars in these two systems for the first time, examined the variable NIR veiling, measured the emission line equivalent widths, and created spectral energy distributions. They have similar spectral types (K7-M1) and are at approximately the same evolutionary stage, allowing comparison of the four stars in the two systems. We conclude that S CrA and VV CrA are young binary systems of stars bridging the Class I and Class II evolutionary stages, characterized by high accretion luminosities and variable emission lines.

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arXiv2019

CO Detected in CI Tau b: Hot Start Implied by Planet Mass and M$_K$

We acquired high resolution IR spectra of CI Tau, the host star of one of the few young planet candidates amenable to direct spectroscopic detection. We confirm the planet's existence with a direct detection of CO in the planet's atmosphere. We also calculate a mass of 11.6 M$_J$ based on the amplitude of its radial velocity variations. We estimate its flux contrast with its host star to get an absolute magnitude estimate for the planet of 8.17 in the K band. This magnitude implies the planet formed via a "hot start" formation mechanism. This makes CI Tau b the youngest confirmed exoplanet as well as the first exoplanet around a T Tauri star with a directly determined, model-independent, dynamical mass.

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