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  • X-Ray and UV Observations of the Young Sun HIP 67522: Evidence of Lyα Absorption within the Planetary System
    We present ultraviolet (UV) spectroscopy of the 17 Myr, G0V star HIP 67522. The UV spectrum is characterized by strong chromospheric and transition region emission lines. There was moderate spectral variability during the observations consisting of 15% stochastic fluctuation and two small flares releasing EUV ≃ 2–4 × 1032 erg in each event. We compare the broadband spectral energy distribution (SED; 4.7 Å–13.0 μm) of the star first presented in P. C. Thao et al. to the solar SED and show that the X-ray and UV (XUV) flux density at 1 au is 102–105 stronger (from 1000 Å down to 5 Å) in the young star compared to the present-day Sun. Attempts to reconstruct the intrinsic Lyα emission of the star failed to return physically realistic results. The discrepancy appears to arise from a population of neutral hydrogen within the system itself, extending to >±500 km s−1. The absorption could be due to outflow from exoplanet atmospheric loss or from a stellar component; such a picture would require high spectral resolution observations and/or UV transit spectroscopy to confirm. Finally, we examine the evolution of the XUV emission from solar-type stars from ages of 17 Myr–9.4 Gyr and derive a scaling relation between far-UV Lyα and extreme-UV (EUV) emission as a function of stellar age. X-ray (1–100 Å) and EUV (100–911 Å) contributions to high-energy emission are 329 and 672 erg cm−2 s−1 at 1 au, respectively, suggesting that both may contribute to exoplanet heating at this epoch. The XUV emission levels at 17 Myr combined with the low density of the planet HIP 67522 b are consistent with models that predict that solar-type stars born with high rotation and activity levels will drive substantial heating and escape on close-in, gaseous planets.

  • Discovery of 178 Open Clusters with Gaia DR3
    We present 178 new open cluster candidates identified through the application of the HDBSCAN clustering algorithm to astrometric data from the Gaia DR3 catalog. The analysis was performed over 5770 1 deg2 fields centered on OB stars, which serve as tracers of Galactic spiral arms. Using the membership probabilities provided by HDBSCAN, we derived the mean proper motions, parallaxes, and fundamental parameters for each cluster. For a subset of 84 clusters, mean radial velocities were also determined using Gaia data. The cluster parameters (distances, ages, extinctions, and metallicities) were estimated through our nonsubjective multidimensional global optimization code, which fits theoretical isochrones to Gaia photometric data. The quality of the results and their uncertainty support the reality of the clusters, which span distances from 739 to 12,000 pc. Age estimates indicate that 59 clusters are younger than 50 Myr, 20 have intermediate ages, and 99 are classified as old clusters. These results highlight that there is still work to be done on open cluster discovery and reinforce the importance of regular updates to open cluster catalogs.

  • The Gaia–Kepler–TESS-host Stellar Properties Catalog: Uniform Physical Parameters for 10022 Host Stars and 10189 Planets
    We present the first homogeneous catalog of Kepler, K2, and TESS host stars and the corresponding catalog of exoplanet properties, which contain 10022 stars and 10189 planets, respectively. We used isochrone fitting and Gaia DR3 photometry, parallaxes, and spectroscopic metallicities to compute precise, homogeneous Teff, , masses, radii, mean stellar densities, luminosities, ages, distances, and V-band extinctions for 3387, 618, 6017 Kepler, K2, and TESS host stars, respectively. We compared our stellar properties to studies using fundamental and precise constraints, such as interferometry and asteroseismology, and find residual scatters of 2.8%, 5.6%, 5.0%, and 31%, with offsets of 0.2%, 1.0%, 1.2%, and 0.7% between our Teff, radii, masses, and ages and those in the literature, respectively. In addition, we compute planet radii, semimajor axes, and incident fluxes for as many as 4285, 678, and 5226 Kepler, K2, and TESS planets, respectively, and find that the exoplanet radius gap is less prominent in the K2, TESS, and combined samples than in the Kepler sample alone. We suspect this difference is due to heterogeneous planet-to-star radius ratios, shorter time baselines of K2 and TESS, smaller sample sizes, and the different gap locations hinted at by the K2 (∼1.5 R⊕) and TESS (∼1.8 R⊕) planet radius distributions. Finally, we identify a clear radius inflation trend in thousands of hot Jupiters and find 151 hot sub-Neptunian desert planets, in addition to a population of 233 young (≲1 Gyr) host stars as potential opportunities for testing theories of planet formation and evolution.

  • A High-resolution Spectroscopic Survey of Directly Imaged Companion Hosts. II. Diversity in C/O Ratios among Host Stars
    The era of JWST has enabled measurements of abundances of elements such as C, O, and even Na, S, K, and Fe in planetary atmospheres to very high precisions (∼0.1 dex). Accurate inference of planet formation using these elemental abundances requires the corresponding abundance measurements for the host star. We present the second set of results from our high-resolution spectroscopic survey of directly imaged companion host stars, measuring abundances of 16 elements (including C, O, Na, Mg, Si, S, K, and Fe) for five directly imaged companion host stars. Using both the spectral fitting and the equivalent width methods, we find solar C/O ratios for HR 2562 (0.58 ± 0.09), AB Pic (0.50 ± 0.14), and YSES 1 (0.45 ± 0.05), and subsolar C/O ratios for PZ Tel (0.28 ± 0.05) and β Pic (0.22 ± 0.06). The 4σ subsolar C/O detections for PZ Tel and β Pic highlight the importance of accurate stellar C/O estimates for constraining planet formation. Subsequently, we combine our abundances with those from our previous work to measure population-level average elemental abundances. We find supersolar carbon and oxygen for this stellar population, indicating that the protoplanetary disks around these stars were potentially rich in volatiles. We compare stellar C/O to those of their companions, revealing superstellar C/O for several objects that suggest planet-like formation mechanisms. We also compare the C/O of our directly imaged companion host star population with other planet host stars using the Kolmogorov–Smirnov Test, which indicates insufficient evidence to differentiate between the various stellar populations.

  • Simulated Impact on LSST Data of Starlink V1.5 and V2 Satellites
    The new Starlink V2 satellites incorporate improvements to the chassis brightness through dielectric mirrors, off-pointing solar arrays, and blackened exposed components. For the general case in which the reflectivities are initially unknown, we simulate Legacy Survey of Space and Time (LSST) operations and repeated photometry of every satellite in simulated model constellations. We derive a brightness model of the Starlink V2 satellite and study the simulated apparent brightness as a function of the satellite position relative to the observer and the Sun. Simulated V2 Starlink satellites appear brightest at two distinct positions in the sky: toward the Sun at low elevations where light is specularly reflected, and nearly overhead where the satellite is closest to the observer. A simulation of Starlink V2 satellites at 550 km height distributed across a series of Walker constellations with varying inclinations was analyzed to study the impact on LSST observations. For every thousand V2 Starlink satellites observed, we find only 0.93 will appear brighter than 7th magnitude. The off-pointed solar array and reduced diffuse reflection of the chassis mitigate the brightness. Finally, we simulate lowering this Walker constellation to 350 km. Only 0.56 V2 Starlink satellites per thousand brighter than 7 AB magnitude will be observed in the first hour at this height. This is a 40% reduction in number of bright satellites entering the focal plane compared to the constellation at 550 km height. We find that a combination of factors yield an apparent surface brightness of these satellites for LSST operations only 5% brighter than at 550 km orbit.