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Abstract The element abundances of stars, particularly the refractory elements (e.g., Fe, Si, and Mg), play an important role in connecting stars to their planets. Most Sun-like stars do not have refractory abundance measurements since obtaining a large sample of high-resolution spectra is difficult with oversubscribed observing resources. In this work we infer abundances for C, N, O, Na, Mn, Cr, Si, Fe, Ni, Mg, V, Ca, Ti, Al, and Y for solar analogs with Gaia Radial Velocity Spectrometer (RVS) spectra (R= 11,200) usingTheCannon, a data-driven method. We train a linear model on a reference set of 34 stars observed by Gaia RVS with precise abundances measured from previous high-resolution spectroscopic efforts (R> 30,000–110,000). We then apply this model to several thousand Gaia RVS solar analogs. This yields abundances with average upper limit precisions of 0.04–0.1 dex for 17,412 stars, 50 of which are identified planet (candidate) hosts. We subsequently test the relative refractory depletion of these stars with increasing element condensation temperature compared to the Sun. The Sun remains refractory depleted compared to other Sun-like stars regardless of our current knowledge of the planets they host. This is inconsistent with theories of various types of planets locking up or sequestering refractories. Furthermore, we find no significant abundance differences between identified close-in giant planet hosts, giant planet hosts, and terrestrial/small planet hosts with the rest of the sample within our precision limits. This work demonstrates the utility of data-driven learning for future exoplanet composition and demographics studies. 

Abstract AU Microscopii (AU Mic) is an active 24 ± 3 Myr pre-main-sequence M dwarf in the stellar neighborhood (d= 9.7 pc) with a rotation period of 4.86 days. The two transiting planets orbiting AU Mic, AU Mic b and c, are warm sub-Neptunes on 8.5 and 18.9 day periods and are targets of interest for atmospheric observations of young planets. Here we study AU Mic’s unocculted starspots using ground-based photometry and spectra in order to complement current and future transmission spectroscopy of its planets. We gathered multicolor Las Cumbres Observatory (LCO) 0.4 m SBIG photometry to study the star's rotational modulations and LCO Network of Robotic Echelle Spectrographs high-resolution spectra to measure the different spectral components within the integrated spectrum of the star, parameterized by three spectral components and their coverage fractions. We find AU Mic’s surface has at least two spectral components: aT_amb= ${4003}_{-14}^{+15}$K ambient photosphere and cool spots that have a temperature ofT_spot= ${3003}_{-71}^{+63}$K, covering a globally averaged area of 39% ± 4% which increases and decreases by 5.1% ± 0.3% from the average throughout a rotation. We also detect a third flux component with a filling factor less than 0.5% and a largely uncertain temperature between 8500 and 10,000 K that we attribute to flare flux not entirely omitted when time averaging the spectra. We include measurements of spot characteristics using a two-temperature model, which we find agree strongly with the three-temperature results. Our expanded use of various techniques to study starspots will help us better understand this system and may have applications for interpreting the transmission spectra for exoplanets transiting stars of a wide range of activity levels. 

Abstract The characterization of young planets (<300 Myr) is pivotal for understanding planet formation and evolution. We present the 3–5μm transmission spectrum of the 17 Myr, Jupiter-size (R∼10R_⊕) planet, HIP 67522b, observed with JWST NIRSpec/G395H. To check for spot contamination, we obtain a simultaneousg-band transit with the Southern Astrophysical Research Telescope. The spectrum exhibits absorption features 30%–50% deeper than the overall depth, far larger than expected from an equivalent mature planet, and suggests that HIP 67522b’s mass is <20M_⊕irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of 13.8 ± 1.0M_⊕. This challenges the previous classification of HIP 67522b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of <0.10 g cm⁻³, HIP 67522 b is one of the lowest-density planets known. We find strong absorption from H₂O and CO₂(≥7σ), a modest detection of CO (3.5σ), and weak detections of H₂S and SO₂(≃2σ). Comparisons with radiative-convective equilibrium models suggest supersolar atmospheric metallicities and solar-to-subsolar C/O ratios, with photochemistry further constraining the inferred atmospheric metallicity to 3 × 10 solar due to the amplitude of the SO₂feature. These results point to the formation of HIP 67522b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01–0.03M_⊕Myr⁻¹), sufficient for envelope destruction within a gigayear. This highlights the dramatic evolution occurring within the first 100 Myr of its existence. 

Astronomers have found more than a dozen planets transiting stars that are 10–40 million years old1, but younger transiting planets have remained elusive. The lack of such discoveries may be because planets have not fully formed at this age or because our view is blocked by the protoplanetary disk. However, we now know that many outer disks are warped or broken2; provided the inner disk is depleted, transiting planets may thus be visible. Here we report observations of the transiting planet IRAS 04125+2902 b orbiting a 3-million-year-old, 0.7-solar-mass, pre-main-sequence star in the Taurus Molecular Cloud. The host star harbours a nearly face-on (30 degrees inclination) transitional disk3 and a wide binary companion. The planet has a period of 8.83 days, a radius of 10.7 Earth radii (0.96 Jupiter radii) and a 95%-confidence upper limit on its mass of 90 Earth masses (0.3 Jupiter masses) from radial-velocity measurements, making it a possible precursor of the super-Earths and sub-Neptunes frequently found around main-sequence stars. The rotational broadening of the star and the orbit of the wide (4 arcseconds, 635 astronomical units) companion are both consistent with edge-on orientations. Thus, all components of the system are consistent with alignment except the outer disk; the origin of this misalignment is unclear. 

Abstract Young (<500 Myr) planets are critical to studying how planets form and evolve. Among these young planetary systems, multiplanet configurations are particularly useful, as they provide a means to control for variables within a system. Here, we report the discovery and characterization of a young planetary system, TOI-1224. We show that the planet host resides within a young population we denote as MELANGE-5. By employing a range of age-dating methods—isochrone fitting, lithium abundance analysis, gyrochronology, and Gaia excess variability—we estimate the age of MELANGE-5 to be 210 ± 27 Myr. MELANGE-5 is situated in close proximity to previously identified younger (80–110 Myr) associations, Crius 221 and Theia 424/Volans-Carina, motivating further work to map out the group boundaries. In addition to a planet candidate detected by the TESS pipeline and alerted as a TESS object of interest, TOI-1224 b, we identify a second planet, TOI-1224 c, using custom search tools optimized for young stars (NotchandLOCoR). We find that the planets are 2.10 ± 0.09R_⊕and 2.88 ± 0.10R_⊕and orbit their host star every 4.18 and 17.95 days, respectively. With their bright (K= 9.1 mag), small (R_*= 0.44R_⊙), and cool (T_eff= 3326 K) host star, these planets represent excellent candidates for atmospheric characterization with JWST. 

Abstract Young eclipsing binaries (EBs) are powerful probes of early stellar evolution. Current models are unable to simultaneously reproduce the measured and derived properties that are accessible for EB systems (e.g., mass, radius, temperature, and luminosity). In this study we add a benchmark EB to the pre-main-sequence population with our characterization of TOI 450 (TIC 77951245). Using Gaia astrometry to identify its comoving, coeval companions, we confirm TOI 450 is a member of the ∼40 Myr Columba association. This eccentric (e= 0.2969), equal-mass (q= 1.000) system provides only one grazing eclipse. Despite this, our analysis achieves the precision of a double-eclipsing system by leveraging information in our high-resolution spectra to place priors on the surface-brightness and radius ratios. We also introduce a framework to include the effect of star spots on the observed eclipse depths. Multicolor eclipse light curves play a critical role in breaking degeneracies between the effects of star spots and limb-darkening. Including star spots reduces the derived radii by ∼2% from a unspotted model (>2σ) and inflates the formal uncertainty in accordance with our lack of knowledge regarding the starspot orientation. We derive masses of 0.1768( ± 0.0004) and 0.1767( ± 0.0003)M_⊙, and radii of 0.345(±0.006) and 0.346(±0.006)R_⊙for the primary and secondary, respectively. We compare these measurements to multiple stellar evolution isochones, finding good agreement with the association age. The MESA MIST and SPOTS (f_s= 0.17) isochrones perform the best across our comparisons, but detailed agreement depends heavily on the quantities being compared. 

Abstract The evolution of magnetism in late-type dwarfs remains murky, as we can only weakly predict levels of activity for M dwarfs of a given mass and age. We report results from our spectroscopic survey of M dwarfs in the Southern Continuous Viewing Zone (CVZ) of the Transiting Exoplanet Survey Satellite (TESS). As the TESS CVZs overlap with those of the James Webb Space Telescope, our targets constitute a legacy sample for studies of nearby M dwarfs. For 122 stars, we obtained at least one R≈ 2000 optical spectrum with which we measure chromospheric Hαemission, a proxy for magnetic field strength. The fraction of active stars is consistent with what is expected for field M dwarfs; as in previous studies, we find that late-type M dwarfs remain active for longer than their early-type counterparts. While the TESS light curves for ≈20% of our targets show modulations consistent with rotation, TESS systematics are not well enough understood for confident measurements of rotation periods (P_rot) longer than half the length of an observing sector. We report periods for 12 stars for which we measure P_rot ≲ 15 days or find confirmation for the TESS-derived P_rot in the literature. Our sample of 21 P_rot, which includes periods from the literature, is consistent with our targets being spun-down field stars. Finally, we examine the Hα-to-bolometric luminosity distribution for our sample. Two stars are rotating fast enough to be magnetically saturated, but are not, hinting at the possibility that fast rotators may appear inactive in Hα. 

Abstract Although all-sky surveys have led to the discovery of dozens of young planets, little is known about their atmospheres. Here, we present multiwavelength transit data for the super-Neptune sized exoplanet, K2-33b—the youngest (∼10 Myr) transiting exoplanet to date. We combined photometric observations of K2-33 covering a total of 33 transits spanning >2 yr, taken from K2, MEarth, the Hubble Space Telescope (HST), and Spitzer. The transit photometry spanned from the optical to the near-infrared (0.6–4.5μm), enabling us to construct a transmission spectrum of the planet. We find that the optical transit depths are nearly a factor of 2 deeper than those from the near-infrared. This difference holds across multiple data sets taken over years, ruling out issues of data analysis and unconstrained systematics. Surface inhomogeneities on the young star can reproduce some of the difference, but required spot coverage fractions (>60%) are ruled out by the observed stellar spectrum (<20%). We find a better fit to the transmission spectrum using photochemical hazes, which were predicted to be strong in young, moderate-temperature, and large-radius planets like K2-33b. A tholin haze with CO as the dominant gaseous carbon carrier in the atmosphere can reasonably reproduce the data with small or no stellar surface inhomogeneities, consistent with the stellar spectrum. The HST data quality is insufficient for the detection of any molecular features. More observations would be required to fully characterize the hazes and spot properties and confirm the presence of CO suggested by current data.