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The giant planet-metallicity correlation revealed that planetary formation depends on the stellar properties. There is growing evidence that it is also valid for smaller hot planets, but it is not clear whether elements other than iron also influence the properties of planetary systems. To investigate this, we determined the abundances of 13 chemical elements (Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni and Cu) for a sample of 561 Kepler exoplanet-hosting stars using high-resolution Keck/HIRES spectra. We find that stars in systems having only large or hot planets are enriched in some elements relative to those having only small or warm planets, respectively, with this signature being related to the underlying stellar metallicity. This Kepler sample is composed of stars belonging to the Galactic low- and high-\\(\\alpha\\) sequences, corresponding to the chemical thin and thick disks. Our results reveal that stars enhanced in \\(\\alpha\\)-elements may facilitate the formation of large planets in metal-poor environments although the iron abundance is still a limiting factor. We also investigated chemical abundances as a function of elemental condensation temperatures and found that there is a diversity of slopes regardless of the exoplanetary systems hosted by the star. We confirmed that the Sun is depleted in refractory elements relative to the solar twins in our sample, all of which host a diversity of exoplanets, suggesting that this depletion is caused by processes not related to planet formation.

Effective temperatures, surface gravities, and iron abundances were derived for 109 stars observed by the K2 mission using equivalent width measurements of Fe I and Fe II lines. Calculations were carried out in LTE using Kurucz model atmospheres. Stellar masses and radii were derived by combining the stellar parameters with Gaia DR3 parallaxes, V-magnitudes, and isochrones. The derived stellar and planetary radii have median internal precision of 1.8%, and 2.3%, respectively. The radius gap near \\(\\rm R_{planet}\\sim 1.9 R_\\oplus\\) was detected in this K2 sample. Chromospheric activity was measured from the Ca II H and K lines using the Values of \\(\\log R^\\prime_{\\rm HK}\\) were investigated as a function of stellar rotational period (P\\(_{rot}\\)) and we found that chromospheric activity decreases with increasing P\\(_{rot}\\), although there is a large scatter in \\(\\log R^\\prime_{\\rm HK}\\) (\\(\\sim\\)0.5) for a given P\\(_{rot}\\). Activity levels in this sample reveal a paucity of F & G dwarfs with intermediate activity levels (Vaughan-Preston gap). The effect that stellar activity might have on the derivation of stellar parameters was investigated by including magnetically-sensitive Fe I lines in the analysis and we find no significant differences between parameters with and without magnetically-sensitive lines, although the more active stars (\\(\\log R^\\prime _{\\rm HK}>-5.0\\)) exhibit a larger scatter in the differences in \\(T_{\\rm eff}\\) and [Fe/H].

We present independent and self-consistent metallicities for a sample of 807 planet-hosting stars from the California-Kepler Survey from an LTE spectroscopic analysis using a selected sample of Fe I and Fe II lines. Correlations between host-star metallicities, planet radii, and planetary architecture (orbital periods - warm or hot - and multiplicity - single or multiple), were investigated using non-parametric statistical tests. In addition to confirming previous results from the literature, e.g., that overall host star metallicity distributions differ between hot and warm planetary systems of all types, we report on a new finding that when comparing the median metallicities of hot versus warm systems, the difference for multiple Super-Earths is considerably larger when compared to that difference in single Super-Earths. The metallicity CDFs of hot single Super-Earths versus warm single Super-Earths indicate different parent stellar populations, while for Sub-Neptunes this is not the case. The transition radius between Sub-Neptunes and Sub-Saturns was examined by comparing the APOGEE metallicity distribution for the Milky Way thin disk in the solar neighborhood with metallicity distributions of host stars segregated based upon the largest known planet in their system. These comparisons reveal increasingly different metallicity distributions as the radius of the largest planet in the systems increases, with the parent stellar metallicities becoming significantly different for R\\(_{p}>\\) 2.7 R\\(_{\\oplus}\\). The behavior of the p-values as a function of planet radius undergoes a large slope change at R\\(_{p}\\) = 4.4 \\(\\pm\\) 0.5 R\\(_{\\oplus}\\), indicating the radius boundary between small and large planets.

The physical properties of transiting exoplanets are connected with the physical properties of their host stars. We present a homogeneous spectroscopic analysis based on spectra of FGK-type stars observed with the Hydra spectrograph on the WIYN telescope. We derived effective temperatures, surface gravities, and metallicities, for 81 stars observed by K2 and 33 from Kepler 1. We constructed an Fe I and II line list that is adequate for the analysis of R\\(\\sim\\)18,000 spectra covering 6050-6350 Å and adopted the spectroscopic technique based on equivalent width measurements. The calculations were done in LTE using Kurucz model atmospheres and the qoyllur-quipu (q\\(^2\\)) package. We validated our methodology via analysis of a benchmark solar twin and solar proxies, which are used as the solar reference. We estimated the effects that including Zeeman sensitive Fe I lines have on the derived stellar parameters for young and possibly active stars in our sample and found it not to be significant. Stellar masses and radii were derived by combining the stellar parameters with Gaia EDR3 and V magnitudes and isochrones. The measured stellar radii have 4.2\\% median internal precision, leading to a median internal uncertainty of 4.4\\% in the derived planetary radii. With our sample of 83 confirmed planets orbiting K2 host stars, the radius gap near R\\(_{planet}1.9R{_\\plus}\\) is detected, in agreement with previous findings. Relations between the planetary radius, orbital period and metallicity are explored and these also confirm previous findings for Kepler 1 systems.

The Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope has provided an abundance of exoplanet spectra over the years. These spectra have enabled analysis studies using atmospheric retrievals to constrain the properties of these objects. However, follow-up observations from the James Webb Space Telescope have called into question some of the results from these older datasets, and highlighted the need to properly understand the degeneracies associated with retrievals of WFC3 spectra. In this study, we perform atmospheric retrievals of 38 transmission spectra from WFC3 and use model comparison to determine the complexity required to fit the data. We explore the effect of retrieving system parameters such as the stellar radius and planet's surface gravity, and thoroughly investigate the degeneracies between individual model parameters -- specifically the temperature, abundance of water, and cloud-top level. We focus on three case studies (HD 209458b, WASP-12b, and WASP-39b) in an attempt to diagnose some of the issues with these retrievals, in particular the low retrieved temperatures when compared to the equilibrium values. Our study advocates for the careful consideration of parameter degeneracies when interpreting retrieval results, as well as the importance of wider wavelength coverage to break these degeneracies, in agreement with previous studies. The combination of data from multiple instruments, as well as analysis from multiple data reductions and retrieval codes, will allow us to robustly characterise the atmosphere of these exoplanets.

We present a homogeneous spectroscopic analysis of confirmed K2 mission exoplanet-hosting stars, comprising 301 targets with high-resolution optical spectra from HIRES and TRES taken from ExoFOP. We derived effective temperatures, surface gravities, and iron and magnesium abundances in LTE by measuring the equivalent widths of Fe I, Fe II, and Mg I lines. Three estimates of stellar masses and radii were obtained via Stefan-Boltzmann and isochrone methods using the codes PARAM and isochrones. These were used to derive exoplanetary radii reaching internal precisions of 2.5%, 2.6%, and 6.6%, respectively, and the radius gap being consistently detected near 1.9 R\\(_{\\oplus}\\). We measured chromospheric activity from the Ca II H & K and H\\(\\alpha\\) lines. Within the low-activity range (\\(\\log R^{\\prime}_{HK} < -4.75\\)), stellar activity appears to decrease with increasing planetary radius from super-Earths, sub-Neptunes, sub-Saturns, into the Jupiter regime. According to the [Mg/Fe] measurements, most of our K2 planet hosts belong to the Galactic thin disk, but our sample has a population from the thick disk (high-alpha sequence). Most stars show consistent chemo-dynamical behavior. We find that the [Mg/Fe] ratios are indistinguishable between systems containing Large or Small exoplanets, as well as Single- or Multi-exoplanetary systems. Both the [Fe/H] and [Mg/H] distributions reveal that stars hosting large planets are more iron- and magnesium-enhanced than those having only small planets, further confirming the link between stellar abundances and exoplanetary size, but no significant differences are found between the Single- versus Multi-exoplanetary systems.

We present results from a quantitative spectroscopic analysis conducted on archival Keck/HIRES high-resolution spectra from the California-\\(Kepler\\) Survey (CKS) sample of transiting planetary host stars identified from the \\(Kepler\\) mission. The spectroscopic analysis was based on a carefully selected set of Fe I and Fe II lines, resulting in precise values for the stellar parameters of effective temperature (T\\(_{\\rm eff}\\)) and surface gravity (log \\(g\\)). Combining the stellar parameters with \\(Gaia\\) DR2 parallaxes and precise distances, we derived both stellar and planetary radii for our sample, with a median internal uncertainty of 2.8\\(\\%\\) in the stellar radii and 3.7\\(\\%\\) in the planetary radii. An investigation into the distribution of planetary radii confirmed the bimodal nature of this distribution for the small radius planets found in previous studies, with peaks at: \\(\\sim\\)1.47 \\(\\pm\\) 0.05 R\\(_{\\oplus}\\) and \\(\\sim\\)2.72 \\(\\pm\\) 0.10 R\\(_{\\oplus}\\), with a gap at \\(\\sim\\) 1.9R\\(_{\\oplus}\\). Previous studies that modeled planetary formation that is dominated by photo-evaporation predicted this bimodal radii distribution and the presence of a radius gap, or photo-evaporation valley. Our results are in overall agreement with these models. The high internal precision achieved here in the derived planetary radii clearly reveal the presence of a slope in the photo-evaporation valley for the CKS sample, indicating that the position of the radius gap decreases with orbital period; this decrease was fit by a power law of the form R\\(_{pl}\\) \\(\\propto\\) P\\(^{-0.11}\\), which is consistent with photo-evaporation and Earth-like core composition models of planet formation.

The Homogeneous Study of Transiting Systems (HoSTS) will derive a consistent and homogeneous set of both the stellar and planetary physical properties for a large sample of bright transiting planetary systems with confirmed planetary masses and measured radii. Our resulting catalogs of the fundamental properties of these bright planets and their host stars will enable us to explore empirical correlations that will lead to a better understanding of planetary formation and evolution. We present our pilot study of the planet-hosting star WASP-13, and the framework of our project which will allow for the identification of true relationships among the physical properties of the systems from any systematics.

Stars and their associated planets originate from the same cloud of gas and dust, making a star's elemental composition a valuable indicator for indirectly studying planetary compositions. While the connection between a star's iron (Fe) abundance and the presence of giant exoplanets is established (e.g. Gonzalez 1997; Fischer & Valenti 2005), the relationship with small planets remains unclear. The elements Mg, Si, and Fe are important in forming small planets. Employing machine learning algorithms like XGBoost, trained on the abundances (e.g., the Hypatia Catalog, Hinkel et al. 2014) of known exoplanet-hosting stars (NASA Exoplanet Archive), allows us to determine significant \"features\" (abundances or molar ratios) that may indicate the presence of small planets. We test on three groups of exoplanets: (a) all small, R\\(_{P}\\) \\(<\\) 3.5 \\(R_{\\oplus}\\), (b) sub-Neptunes, 2.0 \\(R_{\\oplus}\\) \\(<\\) R\\(_{P}\\) \\(<\\) 3.5 \\(R_{\\oplus}\\), and (c) super-Earths, 1.0 \\(R_{\\oplus}\\) \\(<\\) R\\(_{P}\\) \\(<\\) 2.0 \\(R_{\\oplus}\\) -- each subdivided into 7 ensembles to test different combinations of features. We created a list of stars with \\(\\geq90\\%\\) probability of hosting small planets across all ensembles and experiments (\"overlap stars\"). We found abundance trends for stars hosting small planets, possibly indicating star-planet chemical interplay during formation. We also found that Na and V are key features regardless of planetary radii. We expect our results to underscore the importance of elements in exoplanet formation and machine learning's role in target selection for future NASA missions: e.g., the James Webb Space Telescope (JWST), Nancy Grace Roman Space Telescope (NGRST), Habitable Worlds Observatory (HWO) -- all of which are aimed at small planet detection.

Exoplanet surveys of evolved stars have provided increasing evidence that the formation of giant planets depends not only on stellar metallicity ([Fe/H]), but also the mass (\\(M_\\star\\)). However, measuring accurate masses for subgiants and giants is far more challenging than it is for their main-sequence counterparts, which has led to recent concerns regarding the veracity of the correlation between stellar mass and planet occurrence. In order to address these concerns we use HIRES spectra to perform a spectroscopic analysis on an sample of 245 subgiants and derive new atmospheric and physical parameters. We also calculate the space velocities of this sample in a homogeneous manner for the first time. When reddening corrections are considered in the calculations of stellar masses and a -0.12 M\\(_{\\odot}\\) offset is applied to the results, the masses of the subgiants are consistent with their space velocity distributions, contrary to claims in the literature. Similarly, our measurements of their rotational velocities provide additional confirmation that the masses of subgiants with \\(M_\\star \\geq 1.6\\) M\\(_{\\odot}\\) (the \"Retired A Stars\") have not been overestimated in previous analyses. Using these new results for our sample of evolved stars, together with an updated sample of FGKM dwarfs, we confirm that giant planet occurrence increases with both stellar mass and metallicity up to 2.0 M\\(_{\\odot}\\). We show that the probability of formation of a giant planet is approximately a one-to-one function of the total amount of metals in the protoplanetary disk \\(M_\\star 10^{[Fe/H]}\\). This correlation provides additional support for the core accretion mechanism of planet formation.