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1. Mutual Inclination of Ultra-short-period Planets with Time-varying Stellar J ₂ Moments

   https://doi.org/10.3847/1538-4357/ac6024  

   Chen, Chen; Li, Gongjie; Petrovich, Cristobal (May 2022, The Astrophysical Journal)

   Abstract Systems with ultra-short-period (USP) planets tend to possess larger mutual inclinations compared to those with planets located farther from their host stars. This could be explained due to precession caused by stellar oblateness at early times when the host star was rapidly spinning. However, stellar oblateness reduces over time due to the decrease in the stellar rotation rate, and this may further shape the planetary mutual inclinations. In this work, we investigate in detail how the final mutual inclination varies under the effect of a decreasing J 2 . We find that different initial parameters (e.g., the magnitude of J 2 and planetary inclinations) will contribute to different final mutual inclinations, providing a constraint on the formation mechanisms of USP planets. In general, if the inner planets start in the same plane as the stellar equator (or coplanar while misaligned with the stellar spin axis), the mutual inclination decreases (or increases then decreases) over time due to the decay of the J 2 moment. This is because the inner orbit typically possesses less orbital angular momentum than the outer ones. However, if the outer planet is initially aligned with the stellar spin while the inner one is misaligned, the mutual inclination nearly stays the same. Overall, our results suggest that either USP planets formed early and acquired significant inclinations (e.g., ≳30° with its companion or ≳10° with its host star spin axis for Kepler-653 c) or they formed late (≳Gyr) when their host stars rotated slower. 

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2. A 4 Gyr M-dwarf Gyrochrone from CFHT/MegaPrime Monitoring of the Open Cluster M67

   https://doi.org/10.3847/1538-4357/ac90be  

   Dungee, Ryan; van Saders, Jennifer; Gaidos, Eric; Chun, Mark; García, Rafael A.; Magnier, Eugene A.; Mathur, Savita; Santos, Ângela R. (October 2022, The Astrophysical Journal)

   Abstract We present stellar rotation periods for late K- and early M-dwarf members of the 4 Gyr old open cluster M67 as calibrators for gyrochronology and tests of stellar spin-down models. Using Gaia EDR3 astrometry for cluster membership and Pan-STARRS (PS1) photometry for binary identification, we build this set of rotation periods from a campaign of monitoring M67 with the Canada–France–Hawaii Telescope’s MegaPrime wide-field imager. We identify 1807 members of M67, of which 294 are candidate single members with significant rotation period detections. Moreover, we fit a polynomial to the period versus color-derived effective temperature sequence observed in our data. We find that the rotation of very cool dwarfs can be explained by simple solid-body spin-down between 2.7 and 4 Gyr. We compare this rotational sequence to the predictions of gyrochronological models and find that the best match is Skumanich-like spin-down,P_rot∝t^0.62, applied to the sequence of Ruprecht 147. This suggests that, for spectral types K7–M0 with near-solar metallicity, once a star resumes spinning down, a simple Skumanich-like relation is sufficient to describe their rotation evolution, at least through the age of M67. Additionally, for stars in the range M1–M3, our data show that spin-down must have resumed prior to the age of M67, in conflict with the predictions of the latest spin-down models. 

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3. Astronomical Cardiology: A Search For Heartbeat Stars Using $\text{Gaia}$ and $\text{TESS}$

   https://doi.org/10.33232/001c.142226  

   Callahan, Jowen; Rowan, D M; Kochanek, C S; Stanek, K Z (January 2025, The Open Journal of Astrophysics)

   Heartbeat stars are a subclass of binary stars with short periods, high eccentricities, and phase-folded light curves that resemble an electrocardiogram. We start from the $\text{𝐺𝑎𝑖𝑎}$catalogs of spectroscopic binaries and use $\text{𝑇𝐸𝑆𝑆}$photometry to identify 112 new heartbeat star systems. We fit their phase-folded light curves with an analytic model to measure their orbital periods, eccentricities, inclinations, and arguments of periastron. We then compare these orbital parameters to the $\text{𝐺𝑎𝑖𝑎}$spectroscopic orbital solution. Our periods and eccentricities are consistent with the $\text{𝐺𝑎𝑖𝑎}$solutions for 85 $\%$of the single-line spectroscopic binaries but only 20 $\%$of the double-line spectroscopic binaries. For the two double-line spectroscopic binary heartbeat stars with consistent orbits, we combine the $\text{𝑇𝐸𝑆𝑆}$phase-folded light curve and the $\text{𝐺𝑎𝑖𝑎}$velocity semi-amplitudes to measure the stellar masses and radii with $\text{𝙿𝙷𝙾𝙴𝙱𝙴}$. In a statistical analysis of the HB population, we find that non-giant heartbeat stars have evolved off the main sequence and that their fractional abundance rises rapidly with effective temperature. 

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4. A Monte Carlo Method for Evaluating Empirical Gyrochronology Models and Its Application to Wide Binary Benchmarks

   https://doi.org/10.3847/1538-4357/ac6035  

   Otani, Tomomi; von Hippel, Ted; Buzasi, Derek; Oswalt, T. D.; Stone-Martinez, Alexander; Majewski, Patrice (May 2022, The Astrophysical Journal)

   Abstract Accurate stellar ages are essential for our understanding of the star formation history of the Milky Way and Galactic chemical evolution, as well as to constrain exoplanet formation models. Gyrochronology, a relationship between stellar rotation and age, appears to offer a reliable age indicator for main-sequence (MS) stars over the mass range of approximately 0.6–1.3 M ⊙ . Those stars lose their angular momentum due to magnetic braking and as a result their rotation speeds decrease with age. Although current gyrochronology relations have been fairly well tested for young MS stars with masses greater than 1 M ⊙ , primarily in young open clusters, insufficient tests exist for older and lower mass MS stars. Binary stars offer the potential to expand and fill in the range of ages and metallicity over which gyrochronology can be empirically tested. In this paper, we demonstrate a Monte Carlo approach to evaluate gyrochronology models using binary stars. As examples, we used five previously published wide binary pairs. We also demonstrate a Monte Carlo approach to assess the precision and accuracy of ages derived from each gyrochronology model. For the traditional Skumanich models, the age uncertainties are σ age /age = 15%–20% for stars with B − V = 0.65 and σ age /age = 5%–10% for stars with B − V = 1.5 and rotation period P ≤ 20 days. 

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5. Measuring Long Stellar Rotation Periods (>10 days) from TESS FFI Light Curves is Possible: An Investigation Using TESS and ZTF

   https://doi.org/10.3847/1538-3881/add0ab  

   Hattori, Soichiro; Angus, Ruth; Foreman-Mackey, Daniel; Lu, Yuxi Lucy; Colman, Isabel (June 2025, The Astronomical Journal)

   Abstract The rotation period of a star is an important quantity that provides insight into its structure and state. For stars with surface features like starspots, their periods can be inferred from brightness variations as these features move across the stellar surface. TESS, with its all-sky coverage, is providing the largest sample of stars for obtaining rotation periods. However, most of the periods have been limited to shorter than the 13.7 days TESS orbital period due to strong background signals (e.g., scattered light) on those timescales. In this study, we investigated the viability of measuring longer periods (>10 days) from TESS light curves for stars in the Northern Continuous Viewing Zone (NCVZ). We first created a reference set of 272 period measurements longer than 10 days for K and M dwarfs in the NCVZ using data from the Zwicky Transient Facility (ZTF) that we consider as the “ground truth” given ZTF’s long temporal baseline of 6+ years. We then used theunpopularpipeline to detrend TESS light curves and implemented a modified Lomb–Scargle (LS) periodogram that accounts for flux offsets between observing sectors. For 179 out of the 272 sources (66%), the TESS-derived periods match the ZTF-derived periods to within 10%. The match rate increases to 81% (137 out of 170) when restricting to sources with a TESS LS power that exceeds a threshold. Our results confirm the capability of measuring periods longer than 10 days from TESS data, highlighting the data set’s potential for studying slow rotators. 

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