In addition to their ubiquity, cool dwarfs (Teff < 4700 K) are optimal and primary targets for transit and radial velocity surveys of planets beyond our solar system. The incidence rate of planets around cool dwarfs therefore dominates the general incidence rates of planets around main sequence stars. Moreover, because of the tendency of cool dwarfs to host exoplanets that are the targets of atmospheric characterization studies, these stars themselves are main candidates for complementary chemical characterization
A planet and its host star are believed to originate from the same collapsing molecular cloud, and there is a chemical correlation between these two components. The comparison between the chemical abundances of planets and their hosts could then help to elucidate the formation mechanism and chemical evolution of planetary systems. To this end, the chemical abundance measurements of both planets and their parent stars are required. Given its unprecedented near and mid-IR sensitivity, JWST is measuring exoplanet atmospheric and/or surface composition with high precision. However, a large fraction of cool dwarfs with confirmed exoplanet(s) do not have uniformly measured chemical abundances, making direct comparisons with the JWST-derived planetary abundances impossible. The dominant molecular spectral lines make the atmospheric modeling and spectroscopic analysis of cool dwarfs difficult. Due to the substantial line blending and crowding, the methods, that are readily used for more massive dwarfs and giants, cannot be applied to the spectra of low-mass dwarfs.
The main goal of this splinter session is to discuss two subjects. Any oral talk covering at least one of these two topics can be suitable for our splinter session:
Several spectral synthesis codes have been used to generate synthetic spectra of cool dwarfs. However, the accuracy of these spectra mainly depends on the model atmospheres and atomic/molecular line data used in these synthesis codes. Regardless of remarkable progress that has been made in modeling the atmospheres of cool dwarfs, there are still deficiencies in these models, leading to inconsistencies between the observed and synthetic spectra. Furthermore, although many atomic/molecular line lists have been updated in the last decade, the insufficiency of line data is also a major source of errors in synthesizing cool-dwarf spectra. In this session, we tend to explore the reliability of important molecular line lists, the ability to model atomic lines especially if influenced by Zeeman splitting or NLTE effects, and the most applicable atmospheric models.
Studies have shown that the location of planet formation relative to ice lines in protoplanetary disk can be estimated by the comparison between the abundance ratio of volatile elements, like C/O, of a planet and its host star. A planet with a sub-stellar value C/O ratio is likely to have a formation location within the H2O ice line, and a planet with a super-stellar C/O ratio is likely to have a formation location beyond the H2O ice line and has then migrated inwards to its current region. The overabundance of alkali metals such as Na and K in the atmospheres of some hot gas giants relative to the host stars is also thought to be a result of planet formation exterior to the H2O ice line followed by inward migration. The abundance ratio C/O can be used to constrain planetary minerology as well; the stellar C/O can determine if the planetary composition is dominated by carbides (high C/O values) or silicates (low C/O values). In low C/O regimes, the type and distribution of silicates is governed by the abundance ratio of refractory elements such as Mg/Si. The abundances of hosts can also be utilized to break the degeneracy effect in chemical abundance measurement of planets using their measured mass and radius as well as planetary interior models. We aim to search for chemical links between planets and their parent stars and talk about the most recent studies in this regard.
Although some progress has been made in understanding star-planet chemical connections, most studies have been focused on more massive dwarfs. The inclusion and investigation of numerous planetary systems around cool dwarfs could improve the previous findings and lead to much more robust relations between the properties of planets and their host stars. As a result, the detailed abundance measurements of cool dwarfs are of great importance in exoplanetary studies, which can provide fundamental insights into planetary formation as well as the interplay between the initial composition and present-day chemistry of planetary systems.
While the major focus of this splinter is to link the chemistry between planets and their parent stars, fascinating discussions regarding other forms of star-planet connections such as magnetic field interaction between the two components and the effect of stellar flares on the condition of planets are also welcome.