arXiv subjects

G. Canocchi

Publications and source records attributed to G. Canocchi.

5 recordsLinked to original sources

arXiv2026

3D NLTE Sodium abundances in late-type stars. Abundance corrections and synthetic spectra

Neutral sodium is an important tracer of the Galactic chemical evolution, a powerful diagnostic of different stellar populations, and the subject of detailed studies of exoplanet atmospheres via transmission spectroscopy. This work aims to study and quantify the errors in stellar analyses of Na I lines caused by the use of one-dimensional (1D) hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE). We studied the line formation of nine Na I lines in FGK dwarfs and giants via, for the first time, 3D non-LTE (NLTE) radiative transfer post-processing with the code Balder on 3D radiation hydrodynamic stellar atmospheres from the Stagger grid spanning Teff= 4000 to 6500 K, log g = 1.5 to 5.0, and [Fe/H]=-4 to +0.5. We find that the 3D NLTE abundance corrections relative to 1D LTE tend to be negative, and more positive than the corresponding 1D NLTE corrections. This reflects more efficient overionisation in the steeper temperature gradient of the 3D models. The corrections are typically less severe than -0.1 dex for weak lines, but become much larger for saturated lines in low-gravity giants (log g < 2.0), even reaching -0.7 dex. However, for the D resonance lines, the 3D NLTE corrections relative to 1D LTE become slightly positive at the lowest metallicities in our grid, typically around +0.05 dex at [Fe/H]=-4. We make our 3D NLTE grid, together with interpolation routines based on radial basis functions and fully connected feedforward neural networks, publicly available. This will enable more accurate determination of sodium abundances in present and forthcoming stellar spectroscopic surveys, particularly for metal-poor stars, as well as a better characterisation of the Na I D lines in exoplanet atmospheres.

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arXiv2025

The peculiar composition of the Sun is not related to giant planets

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 which host planets of masses > 0.01 MJup. We derive abundances for 19 elements, and apply corrections to 14 of them for systematic errors associated with one dimensional (1D) model atmospheres, or 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 giants planets detected, that increases after applying 3D and non-LTE corrections. Our overall results suggests that the peculiar composition of the Sun is primarily related to Galactic chemical evolution rather than the presence of giant planets.

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arXiv2024

Probing Na in giant exoplanets with ESPRESSO and 3D NLTE stellar spectra

Neutral sodium was the first atom detected in an exoplanetary atmosphere via transmission spectroscopy and remains the most frequently detected species due to its strong doublet in the optical. However, the center-to-limb variation (CLV) of these lines in the host star can bias the Na detection.When combined with the Rossiter-McLaughlin (RM) effect, the CLV can mimic or obscure planetary absorption features. This work investigates the impact of 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 characterization of exoplanet atmospheres. We produced a grid of 3D NLTE synthetic spectra for Na I for FGK stars 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 giant exoplanets, to correct for the CLV+RM effect in their transmission spectra. We used ESPRESSO archival observations. Our work confirms the Na detections in three systems, namely WASP-52b, WASP-76b, and WASP-127b, improving the accuracy of the measured absorption depth. Furthermore, we find that 3D NLTE stellar models can explain the spectral features in HD 209458b without the need for any planetary absorption. In the grid of 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. With the continuous improvement of instrumentation, it is crucial to use the most accurate stellar models to correct for the CLV+RM effect in high-resolution transmission spectra, to achieve the best possible characterization of exoplanet atmospheres. We make our grid of 3D NLTE spectra for Na publicly available.

astro-ph.EP↗

arXiv2023

3D non-LTE modeling of the stellar center-to-limb variation for transmission spectroscopy studies

Context. Transmission spectroscopy is one of the most powerful techniques to characterize transiting exoplanets since it allows to measure the abundance of the atomic and molecular species in the planetary atmosphere. However, the stellar lines can 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 non-local thermodynamic equilibrium (non-LTE) line formation are required for an accurate modeling of the stellar CLV of the Na I D$_1$ and K I resonance lines on transmission spectra. Methods. We model the CLV of the Na I D$_1$ and K I resonance lines in the Sun with 3D non-LTE radiative transfer. The synthetic spectra are 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 D$_1$ 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. The simulated CLV curve for a Jupiter-Sun system inferred with a 3D non-LTE analysis shows significant differences from that obtained from a 1D atmosphere. The latter tends to overestimate the amplitude of the transmission curve by a factor that is of the same order of magnitude as a planetary absorption depth (up to 0.2 %). Conclusions. In order to correctly characterize exoplanetary atmospheres, 3D non-LTE synthetic spectra should be used to estimate the stellar CLV effect in transmission spectra of solar-like planet hosts. The work will be extended to other lines and FGK-type stars, allowing synthetic high-resolution spectra to mitigate the stellar contamination of low-resolution planetary spectra, e.g. those from JWST.

astro-ph.SR↗

arXiv2023

Discovering planets with PLATO: Comparison of algorithms for stellar activity filtering

Context. To date, stellar activity is one of the main limitations in detecting small exoplanets via transit photometry. Since this activity is enhanced in young stars, traditional filtering algorithms may severely under-perform in detecting such exoplanets. 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 (LCs) 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. We include in the comparison also the two best-performing algorithms implemented in Wotan, namely the Tukey's biweight and the Huber Spline. Methods. 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. The tests were carried out over 100 simulated LCs of both quiet and active solar-like stars that will be observed by the ESA space telescope PLATO. Results. We found that N&L is the best choice in many cases, since it misses the lowest number of transits. However, it under-performs if 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. For LCs with a large number of data, the combined results of YSD and Huber Spline yield the highest recovery percentage. Filtering algorithms allow us to get a very precise estimate of the orbital period and the mid-transit time of the detected planets, while the planet-to-star radius is under-estimated most of the time, especially in the case of grazing transits or eccentric orbits. A refined filtering taking into account the presence of the planet is compulsory for a proper planetary characterization.

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