Exoplanets and Stellar Physics

Abstract:

Thedistribution of close-in exoplanets is shaped by a complex interplay betweenatmospheric and dynamical processes. The Desert, Ridge, and Savanna(respectively a lack, overoccurence, and mild deficit of Neptunes withincreasing periods) illustrate the sensitivity of these worlds to suchprocesses, making them ideal targets to disentangle their roles. Determininghow many Neptunes are brought close-in by early disk-driven migration (DDM;expected to maintain primordial spin-orbit alignment) or late high-eccentricitytidal migration (HEM; expected to generate large misalignments) is essential tounderstanding how much atmosphere they lost. In this paper, we propose aunified view of the exo-Neptunian landscape to guide its exploration andspeculate that the Ridge is a hot spot for evolutionary processes. Low-densityNeptunes would mainly undergo DDM, becoming fully eroded at shorter periodsthan the Ridge. This is in contrast to denser Neptunes, which would be broughtto the Ridge and Desert by HEM. We embark on this exploration via the ATREIDES(Ancestry, Traits, and Relations of Exoplanets Inhabiting the Desert Edges andSavanna) collaboration, which relies on spectroscopic and photometricobservations of ~60 close-in Neptunes, their reduction with robust pipelines,and their interpretation through internal structure, atmospheric, andevolutionary models. We carried out a systematic Rossiter-McLaughlin censuswith VLT/ESPRESSO to measure the distribution of 3D spin-orbit angles,correlate its shape with the system properties (orbit, density, evaporation),and thus relate the fraction of aligned-misaligned Neptunian systems to DDM,HEM, and atmospheric erosion. The first ATREIDES target, TOI-421 c, lies in theSavanna with a neighboring sub-Neptune TOI-421 b. We measured for the firsttime their 3D spin-orbit angles (ψb = 57−15+11∘; ψc = 44.9−4.1+4.4∘). Together with the eccentricity and possibly large mutualinclination of their orbits, this hints at a chaotic dynamical origin thatcould result from DDM followed by HEM. Our program will provide the communitywith a wealth of constraints for formation and evolution models, and we welcomecollaborations that will contribute to pushing our understanding of theexo-Neptunian landscape forward.

Abstract:

High-resolution spectroscopy (HRS) of exoplanet atmospheres has successfully detected many chemical species and is quickly moving toward detailed characterization of the chemical abundances and dynamics. HRS is highly sensitive to the line shape and position; thus, it can detect three-dimensional (3D) effects such as winds, rotation, and spatial variation of atmospheric conditions. At the same time, retrieval frameworks are increasingly deployed to constrain chemical abundances, pressure–temperature (P–T) structures, orbital parameters, and rotational broadening. To explore the multidimensional parameter space, we need computationally fast models, which are consequently mostly one-dimensional (1D). However, this approach risks introducing interpretation bias since the planet’s true nature is 3D. We investigate the robustness of this methodology at high spectral resolution by running 1D retrievals on simulated observations in emission within an observational framework using 3D global circulation models of the quintessential HJ WASP-76 b. We find that the retrieval broadly recovers conditions present in the atmosphere, but that the retrieved P–T and chemical profiles are not a homogeneous average of all spatial and phase-dependent information. Instead, they are most sensitive to spatial regions with large thermal gradients, which do not necessarily coincide with the strongest emitting regions. Our results further suggest that the choice of parameterization for the P–T and chemical profiles, as well as Doppler offsets among opacity sources, impact the retrieval results. These factors should be carefully considered in future retrieval analyses.

Abstract:

Abstract Doppler Imaging (DI) is a well-established technique to map a physical field at a stellar surface from a time series of high-resolution spectra. In this proof-of-concept study, we aim to show that traditional DI algorithms, originally designed for rapidly-rotating stars, have also the ability to model the activity of Sun-like stars, when observed with new-generation highly-stable spectrographs, and search for low-mass planets around them. We used DI to retrieve the relative brightness distribution at the surface of the Sun from radial velocity (RV) observations collected by HARPS-N between 2022 and 2024. The brightness maps obtained with DI have a typical angular resolution of ~36○ and are a good match to low-resolution disc-resolved Dopplergrams of the Sun at epochs when the absolute, disc-integrated RV exceeds ~2 m s−1. The RV residuals after DI correction exhibit a dispersion of about 0.6 m s−1, comparable with existing state-of-the-art activity correction techniques. Using planet injection-recovery tests, we also show that DI can be a powerful tool for blind planet searches, so long as the orbital period is larger than ~100 days (i.e. 3 to 4 stellar rotation periods), and that it yields planetary mass estimates with an accuracy comparable to, for example, multi-dimensional Gaussian process regression. Finally, we highlight some limitations of traditional DI algorithms, which should be addressed to make DI a reliable alternative to state-of-the-art RV-based planet search techniques.