With ground- and space-based telescopes, astronomers have now obtained high-resolution spectra for millions of stars and discovered thousands of new worlds — exoplanets — beyond our Solar System. Astrophysics is entering a new era in which elemental abundances can be measured with unprecedented precision, not only in stars but also in the atmospheres of their planets. These chemical fingerprints provide crucial clues to stellar evolution, Galactic chemical enrichment, and the formation and composition of planetary systems. However, interpreting stellar and planetary spectra remains challenging, as the simplifying assumptions traditionally adopted in spectral modeling often introduce systematic biases.

Neutral sodium (Na I) plays a particularly central role in this context, serving as both a tracer of Galactic chemical evolution in late-type stars and a diagnostic of atmospheric processes in exoplanets. It also acts as a key indicator of distinct stellar populations, such as those in globular clusters, where characteristic abundance variations reveal multiple stellar generations. Classical hydrostatic one-dimensional (1D) models assuming local thermodynamic equilibrium (LTE) can systematically overestimate Na I abundances — by up to 0.5 dex in giant stars — because the simplifying approximations break down. In high-resolution transmission spectroscopy of exoplanets, additional stellar phenomena such as center-to-limb variations (CLV) and the Rossiter–McLaughlin signal during transits must also be taken into account. These stellar effects can mimic or obscure planetary absorption features, leading to false detections if not modeled correctly. Accurately treating such processes requires realistic three-dimensional (3D) radiation-hydrodynamic (RHD) stellar atmospheres combined with non-local thermodynamic equilibrium (non-LTE) radiative transfer.

In this thesis, I develop a state-of-the-art grid of 3D non-LTE synthetic spectra for Na I lines in FGK-type stars, based on the extended and refined Stagger-grid of RHD models. This grid enables more accurate sodium abundance determinations in large spectroscopic surveys — such as GALAH DR4, which recently published parameters and abundances for nearly one million stars — and improves the interpretation of high-resolution exoplanet spectra from instruments such as ESPRESSO on the VLT and the forthcoming ANDES spectrograph on the ELT.

The thesis demonstrates several applications of these 3D non-LTE models: (i) an analysis of spatially resolved solar spectra from the Swedish 1-m Solar Telescope (Paper I); (ii) atmospheric characterization of four giant exoplanets observed with ESPRESSO (Paper II); and (iii) a detailed investigation of Na I abundances across Galactic stellar populations using GALAH DR4 data (Paper III). Together, these studies show that 3D non-LTE modeling provides a unified and more physically accurate framework for interpreting sodium lines in both stellar and planetary contexts.

Helioseismology and solar modelling have enjoyed a golden era thanks to decades-long surveys from ground-based networks such as for example GONG, BiSON, IRIS and the SOHO and SDO space missions which have provided high-quality helioseismic observations that supplemented photometric, gravitational, size and shape, limb-darkening and spectroscopic constraints as well as measurements of neutrino fluxes. However, the success of solar models is also deeply rooted in progress in fundamental physics (equation of state of the solar plasma, high-quality atomic physics computations and opacities, description of convection and the role of macroscopic transport processes of angular momentum and chemicals, such as for example meridional circulation, internal gravity waves, shear-induced turbulence or even convection. In this paper, we briefly outline some key areas of research that deserve particular attention in solar modelling. We discuss the current uncertainties that need to be addressed, how these limit our predictions from solar models and their impact on stellar evolution in general. We outline potential strategies to mitigate them and how multidisciplinary approaches will be needed in the future to tackle them.

Highly differential spectroscopic studies have revealed that the Sun is deficient in refractory elements relative to solar twins. To investigate the role of giant planets on this signature, we present a high-precision abundance analysis of HARPS spectra for 50 F- and G-type stars spanning −0.4 ≲ [Fe/H] ≲ +0.5. There are 29 stars in the sample that host planets of masses ≳0.01 MJup.. We derived abundances for 19 elements and applied corrections to 14 of them for systematic errors associated with one dimensional (1D) model atmospheres, the assumption of local thermodynamic equilibrium (LTE), or both. We find that, among the solar twins in our sample, the Sun is Li poor in comparison to other stars at similar age, in agreement to previous studies. The sample shows a variety of trends in elemental abundances as a function of condensation temperature. We find a strong correlation in these trends with [Fe/H], with a marginally significant difference in the gradients for stars with and without giant planets detected, which increases after applying 3D and non-LTE corrections. Our overall results suggest that the peculiar composition of the Sun is primarily related to Galactic chemical evolution rather than the presence of giant planets.

Context. Transmission spectroscopy is one of the most powerful techniques used to characterize transiting exoplanets, since it allows for the abundance of the atomic and molecular species in the planetary atmosphere to be measured. However, stellar lines may bias the determination of such abundances if their center-to-limb variations (CLVs) are not properly accounted for.

Aims. This paper aims to show that three-dimensional (3D) radiation hydrodynamic models and the assumption of non-local ther-modynamic equilibrium (non-LTE) line formation are required for an accurate modeling of the stellar CLV of the Na I D₁ and K I resonance lines on transmission spectra.

Methods. We modeled the CLV of the Na I D₁ and K I resonance lines in the Sun with 3D non-LTE radiative transfer. The synthetic spectra were compared to solar observations with high spatial and spectral resolution, including new data collected with the CRISP instrument at the Swedish 1-m Solar Telescope between µ = 0.1 and µ = 1.0.

Results. Our 3D non-LTE modeling of the Na I D1 resonance line at 5896 Å and the K I 7699 Å resonance line in the Sun is in good agreement with the observed CLV in the solar spectrum. Moreover, the simulated CLV curve for a Jupiter-Sun system inferred with a 3D non-LTE analysis shows significant differences from the one obtained from a 1D atmosphere. The latter does indeed tend to overestimate the amplitude of the transmission curve by a factor that is on the same order of magnitude as a planetary absorption depth (i.e., up to 0.2%).

Conclusions. This work highlights the fact that to correctly characterize exoplanetary atmospheres, 3D non-LTE synthetic spectra ought to be used to estimate the stellar CLV effect in transmission spectra of solar-like planet hosts. Moreover, since different spectral lines show different CLV curves for the same geometry of the planet-star system, it is fundamental to model the CLV individually for each line of interest. The work will be extended to other lines and FGK-type stars, allowing for synthetic high-resolution spectra to mitigate the stellar contamination of low-resolution planetary spectra, for example, those drawn from JWST.

Context. Neutral sodium was the first atom that was detected in an exoplanetary atmosphere using the transmission spectroscopy technique. To date, it remains the most successfully detected species due to its strong doublet in the optical at 5890 A° and 5896 A°. However, the center-to-limb variation (CLV) of these lines in the host star can bias the Na I detection. When combined with the Rossiter-McLaughlin (RM) effect, the CLV can mimic or obscure a planetary absorption feature if it is not properly accounted for. Aims. This work aims to investigate the impact of three-dimensional (3D) radiation hydrodynamic stellar atmospheres and non-local thermodynamic equilibrium (NLTE) radiative transfer on the modeling of the CLV+RM effect in single-line transmission spectroscopy to improve the detection and characterization of exoplanet atmospheres. Methods. We produced a grid of 3D NLTE synthetic spectra for Na I for FGK-type dwarfs within the following parameter space: Teff = 4500-6500 K, log g = 4.0-5.0, and [Fe/H] = [-0.5, 0, 0.5]. This grid was then interpolated to match the stellar parameters of four stars hosting well-known giant exoplanets, generating stellar spectra to correct for the CLV+RM effect in their transmission spectra. We used archival observations taken with the high-resolution ESPRESSO spectrograph. Results. Our work confirms the Na I detections in three systems, namely WASP-52b, WASP-76b, and WASP-127b, also improving the accuracy of the measured absorption depth. Furthermore, we find that 3D NLTE stellar models can explain the spectral features in the transmission spectra of HD 209458b without the need for any planetary absorption. In the grid of stellar synthetic spectra, we observe that the CLV effect is stronger for stars with low Teff and high log g. However, the combined effect of CLV and RM is highly dependent on the orbital geometry of the planet-star system. Conclusions. With the continuous improvement of instrumentation, it is crucial to use the most accurate stellar models available to correct for the CLV+RM effect in high-resolution transmission spectra to achieve the best possible characterization of exoplanet atmospheres. This will be fundamental in preparation for instruments such as ANDES at the Extremely Large Telescope to fully exploit its capabilities in the near future. We make our grid of 3D NLTE synthetic spectra for Na I publicly available.

Context. To date, stellar activity is one of the main limitations in detecting small exoplanets via the transit photometry technique. Since this activity is enhanced in young stars, traditional filtering algorithms may severely underperform in attempting to detect such exoplanets, with shallow transits often obscured by the photometric modulation of the light curve.

Aims. This paper aims to compare the relative performances of four algorithms developed by independent research groups specifically for the filtering of activity in the light curves of young active stars, prior to the search for planetary transit signals: Notch and LOCoR (N&L), Young Stars Detrending (YSD), K2 Systematics Correction (K2SC), and VARLET. Our comparison also includes the two best-performing algorithms implemented in the Wōtan package: Tukey’s biweight and Huber spline algorithms.

Methods. For this purpose, we performed a series of injection-retrieval tests of planetary transits of different types, from Jupiter down to Earth-sized planets, moving both on circular and eccentric orbits. These experiments were carried out over a set of 100 realistically simulated light curves of both quiet and active solar-like stars (i.e., F and G types) that will be observed by the ESA Planetary Transits and Oscillations of stars (PLATO) space telescope, starting 2026.

Results. From the experiments for transit detections, we found that N&L is the best choice in many cases, since it misses the lowest number of transits. However, this algorithm is shown to underperform when the planetary orbital period closely matches the stellar rotation period, especially in the case of small planets for which the biweight and VARLET algorithms work better. Moreover, for light curves with a large number of data-points, the combined results of two algorithms, YSD and Huber spline, yield the highest recovery percentage. Filtering algorithms allow us to obtain a very precise estimate of the orbital period and the mid-transit time of the detected planets, while the planet-to-star radius is underestimated most of the time, especially in cases of grazing transits or eccentric orbits. A refined filtering that takes into account the presence of the planet is thus compulsory for proper planetary characterization analyses.