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1. On the Probability That a Rocky Planet’s Composition Reflects Its Host Star

   https://doi.org/10.3847/PSJ/abcaa8  

   Schulze, J. G. ; Wang 王, J. 吉. ; Johnson, J. A. ; Gaudi, B. S. ; Unterborn, C. T. ; Panero, W. R. ( June 2021 , The Planetary Science Journal)

   The bulk density of a planet, as measured by mass and radius, is a result of planet structure and composition. Relative proportions of iron core, rocky mantle, and gaseous envelopes are degenerate for a given density. This degeneracy is reduced for rocky planets without significant gaseous envelopes when the structure is assumed to be a differentiated iron core and rocky mantle, in which the core mass fraction (CMF) is a first-order description of a planet’s bulk composition. A rocky planet’s CMF may be derived both from bulk density and by assuming the planet reflects the host star’s major rock-building elemental abundances (Fe, Mg, and Si). Contrasting CMF measures, therefore, shed light on the outcome diversity of planet formation from processes including mantle stripping, out-gassing, and/or late-stage volatile delivery. We present a statistically rigorous analysis of the consistency of these two CMF measures accounting for observational uncertainties of planet mass and radius and host-star chemical abundances. We find that these two measures are unlikely to be resolvable as statistically different unless the bulk density CMF is at least 40% greater than or 50% less than the CMF as inferred from the host star. Applied to 11 probable rocky exoplanets, Kepler-107 c has a CMF as inferred from bulk density that is significantly greater than the inferred CMF from its host star (2σ) and is therefore likely an iron-enriched super-Mercury. K2-229b, previously described as a super-Mercury, however, does not meet the threshold for a super-Mercury at a 1σor 2σlevel.

    

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2. TOI-561 b: A Low-density Ultra-short-period “Rocky” Planet around a Metal-poor Star

   https://doi.org/10.3847/1538-3881/acad83  

   Brinkman, Casey L. ; Weiss, Lauren M. ; Dai, Fei ; Huber, Daniel ; Kite, Edwin S. ; Valencia, Diana ; Bean, Jacob L. ; Beard, Corey ; Behmard, Aida ; Blunt, Sarah ; et al ( February 2023 , The Astronomical Journal)

   TOI-561 is a galactic thick-disk star hosting an ultra-short-period (0.45-day-orbit) planet with a radius of 1.37R_⊕, making it one of the most metal-poor ([Fe/H] = −0.41) and oldest (≈10 Gyr) sites where an Earth-sized planet has been found. We present new simultaneous radial velocity (RV) measurements from Gemini-N/MAROON-X and Keck/HIRES, which we combined with literature RVs to derive a mass ofM_b= 2.24 ± 0.20M_⊕. We also used two new sectors of TESS photometry to improve the radius determination, findingR_b= 1.37 ± 0.04R_⊕and confirming that TOI-561 b is one of the lowest-density super-Earths measured to date (ρ_b= 4.8 ± 0.5 g cm⁻³). This density is consistent with an iron-poor rocky composition reflective of the host star’s iron and rock-building element abundances; however, it is also consistent with a low-density planet with a volatile envelope. The equilibrium temperature of the planet (∼2300 K) suggests that this envelope would likely be composed of high mean molecular weight species, such as water vapor, carbon dioxide, or silicate vapor, and is likely not primordial. We also demonstrate that the composition determination is sensitive to the choice of stellar parameters and that further measurements are needed to determine whether TOI-561 b is a bare rocky planet, a rocky planet with an optically thin atmosphere, or a rare example of a nonprimordial envelope on a planet with a radius smaller than 1.5R_⊕.

    

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3. A detailed analysis of the Gl 486 planetary system

   https://doi.org/10.1051/0004-6361/202243548  

   Caballero, J. A. ; González-Álvarez, E. ; Brady, M. ; Trifonov, T. ; Ellis, T. G. ; Dorn, C. ; Cifuentes, C. ; Molaverdikhani, K. ; Bean, J. L. ; Boyajian, T. ; et al ( September 2022 , Astronomy & Astrophysics)

   Context . The Gl 486 system consists of a very nearby, relatively bright, weakly active M3.5 V star at just 8 pc with a warm transiting rocky planet of about 1.3 R ⊕ and 3.0 M ⊕ . It is ideal for both transmission and emission spectroscopy and for testing interior models of telluric planets. Aims . To prepare for future studies, we aim to thoroughly characterise the planetary system with new accurate and precise data collected with state-of-the-art photometers from space and spectrometers and interferometers from the ground. Methods . We collected light curves of seven new transits observed with the CHEOPS space mission and new radial velocities obtained with MAROON-X at the 8.1 m Gemini North telescope and CARMENES at the 3.5 m Calar Alto telescope, together with previously published spectroscopic and photometric data from the two spectrographs and TESS. We also performed near-infrared interferometric observations with the CHARA Array and new photometric monitoring with a suite of smaller telescopes (AstroLAB, LCOGT, OSN, TJO). This extraordinary and rich data set was the input for our comprehensive analysis. Results . From interferometry, we measure a limb-darkened disc angular size of the star Gl 486 at θ LDD = 0.390 ± 0.018 mas. Together with a corrected Gaia EDR3 parallax, we obtain a stellar radius R * = 0.339 ± 0.015 R ⊕ . We also measure a stellar rotation period at P rot = 49.9 ± 5.5 days, an upper limit to its XUV (5-920 A) flux informed by new Hubble /STIS data, and, for the first time, a variety of element abundances (Fe, Mg, Si, V, Sr, Zr, Rb) and C/O ratio. Moreover, we imposed restrictive constraints on the presence of additional components, either stellar or sub-stellar, in the system. With the input stellar parameters and the radial-velocity and transit data, we determine the radius and mass of the planet Gl 486 b at R p = 1.343 −0.062 +0.063 R ⊕ and M p = 3.00 −0.12 +0.13 M ⊕ , with relative uncertainties of the planet radius and mass of 4.7% and 4.2%, respectively. From the planet parameters and the stellar element abundances, we infer the most probable models of planet internal structure and composition, which are consistent with a relatively small metallic core with respect to the Earth, a deep silicate mantle, and a thin volatile upper layer. With all these ingredients, we outline prospects for Gl 486 b atmospheric studies, especially with forthcoming James Webb Space Telescope ( Webb ) observations. 

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4. Updated Planetary Mass Constraints of the Young V1298 Tau System Using MAROON-X

   https://doi.org/10.3847/1538-3881/acc865  

   Sikora, James ; Rowe, Jason ; Barat, Saugata ; Bean, Jacob L. ; Brady, Madison ; Désert, Jean-Michel ; Feinstein, Adina D. ; Gilbert, Emily A. ; Henry, Gregory ; Kasper, David ; et al ( May 2023 , The Astronomical Journal)

   The early K-type T-Tauri star, V1298 Tau (V= 10 mag, age ≈ 20–30 Myr) hosts four transiting planets with radii ranging from 4.9 to 9.6R_⊕. The three inner planets have orbital periods of ≈8–24 days while the outer planet’s period is poorly constrained by single transits observed with K2 and the Transiting Exoplanet Survey Satellite (TESS). Planets b, c, and d are proto–sub-Neptunes that may be undergoing significant mass loss. Depending on the stellar activity and planet masses, they are expected to evolve into super-Earths/sub-Neptunes that bound the radius valley. Here we present results of a joint transit and radial velocity (RV) modeling analysis, which includes recently obtained TESS photometry and MAROON-X RV measurements. Assuming circular orbits, we obtain a low-significance (≈2σ) RV detection of planet c, implying a mass of${19.8}_{-8.9}^{+9.3}{M}_{\oplus }$and a conservative 2σupper limit of <39M_⊕. For planets b and d, we derive 2σupper limits ofM_b< 159M_⊕andM_d< 41M_⊕, respectively. For planet e, plausible discrete periods ofP_e> 55.4 days are ruled out at the 3σlevel while seven solutions with 43.3 <P_e/d< 55.4 are consistent with the most probable 46.768131 ± 000076 days solution within 3σ. Adopting the most probable solution yields a 2.6σRV detection with a mass of 0.66 ± 0.26M_Jup. Comparing the updated mass and radius constraints with planetary evolution and interior structure models shows that planets b, d, and e are consistent with predictions for young gas-rich planets and that planet c is consistent with having a water-rich core with a substantial (∼5% by mass) H₂envelope.

    

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5. Measuring Elemental Abundances of JWST Target Stars for Exoplanet Characterization. I. FGK Stars

   https://doi.org/10.3847/1538-3881/ac7de3  

   Kolecki, Jared R. ; Wang, Ji ( August 2022 , The Astronomical Journal)

   Abstract With the launch of the JWST, we will obtain more precise data for exoplanets than ever before. However, these data can only inform and revolutionize our understanding of exoplanets when placed in the larger context of planet–star formation. Therefore, gaining a deeper understanding of their host stars is equally important and synergistic with the upcoming JWST data. We present detailed chemical abundance profiles of 17 FGK stars that will be observed in exoplanet-focused Cycle 1 JWST observer programs. The elements analyzed (C, N, O, Na, Mg, Si, S, K, and Fe) were specifically chosen as being informative to the composition and formation of planets. Using archival high-resolution spectra from a variety of sources, we perform an LTE equivalent width analysis to derive these abundances. We look to literature sources to correct the abundances for non-LTE effects, especially for O, S, and K, where the corrections are large (often >0.2 dex). With these abundances and the ratios thereof, we will begin to paint clearer pictures of the planetary systems analyzed by this work. With our analysis, we can gain insight into the composition and extent of migration of Hot Jupiters, as well as the possibility of carbon-rich terrestrial worlds. 

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