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Abstract We present the Rossiter–McLaughlin measurement of the sub-Neptune TOI-1759A b with MAROON-X. A joint analysis with MuSCAT3 photometry and nine additional TESS transits produces a sky-projected obliquity of ∣λ∣ = 4° ± 18°. We also derive a true obliquity ofψ= 24° ± 12° making this planet consistent with full alignment albeit to <1σ. With a period of 18.85 days and ana/R_*of 40, TOI-1759A b is the longest period single sub-Neptune to have a measured obliquity. It joins a growing number of smaller planets which have had this measurement made and, along with K2-25 b, is the only single, aligned sub-Neptune known to date. We also provide an overview of the emerging distribution of obliquity measurements for planets withR< 8R_⊕. We find that these types of planets tend toward alignment, especially the sub-Neptunes and super-Earths, implying a dynamically cool formation history. The majority of misaligned planets in this category have 4 <R≤ 8R_⊕and are more likely to be isolated than planets rather than in compact systems. We find this result to be significant at the 3σlevel, consistent with previous studies. In addition, we conduct injection and recovery testing on available archival radial velocity data to put limits on the presence of massive companions in these systems. Current archival data is insufficient for most systems to have detected a giant planet. 

Abstract The chemical abundance measurements of host stars and their substellar companions provide a powerful tool to trace the formation mechanism of the planetary systems. We present a detailed high-resolution spectroscopic analysis of a young M-type star, DH Tau A, which is located in the Taurus molecular cloud belonging to the Taurus-Auriga star-forming region. This star is host to a low-mass companion, DH Tau b, and both the star and the companion are still in their accreting phase. We apply our technique to measure the abundances of carbon and oxygen using carbon- and oxygen-bearing molecules, such as CO and OH, respectively. We determine a near-solar carbon-to-oxygen abundance ratio of C/O = 0.555 ± 0.063 for the host star DH Tau A. We compare this stellar abundance ratio with that of the companion from our previous study ( $\mathrm{C}/\mathrm{O}={0.54}_{-0.05}^{+0.06}$), which also has a near-solar value. This confirms the chemical homogeneity in the DH Tau system, which suggests a formation scenario for the companion consistent with a direct and relatively fast gravitational collapse rather than a slow core accretion process. 

Abstract We present an in-depth, high-resolution spectroscopic analysis of the M dwarf K2-18, which hosts a sub-Neptune exoplanet in its habitable zone. We show our technique to accurately normalize the observed spectrum, which is crucial for a proper spectral fitting. We also introduce a new automatic, line-by-line, model-fitting code, AutoSpecFit, which performs an iterativeχ²minimization process to measure individual elemental abundances of cool dwarfs. We apply this code to the star K2-18, and measure the abundance of 10 elements: C, O, Na, Mg, Al, K, Ca, Sc, Ti, and Fe. We find these abundances to be moderately supersolar, except for Fe, with a slightly subsolar abundance. The accuracy of the inferred abundances is limited by the systematic errors due to uncertain stellar parameters. We also derive the abundance ratios associated with several planet-building elements such as Al/Mg, Ca/Mg, Fe/Mg, and (a solar-like) C/O = 0.568 ± 0.026, which can be used to constrain the chemical composition and the formation location of the exoplanet. On the other hand, the planet K2-18 b has attracted considerable interest, given the JWST measurements of its atmospheric composition. Early JWST studies reveal an unusual chemistry for the atmosphere of this planet, which is unlikely to be driven by formation in a disk of unusual composition. The comparison between the chemical abundances of K2-18 b from future JWST analyses and those of the host star can provide fundamental insights into the formation of this planetary system.