Diego Bossini

Context . Open clusters are excellent tracers of the Galactic thin disk, providing insights into its formation and evolution. Precise cluster age determination is essential for studying cluster evolution as well as the evolution of the disk as a whole. Aims . With this work, our aim was to produce an updated catalog of cluster parameters (age, extinction, distance) by constraining cluster ages via newly available spectroscopic measurements of metallicity. Methods . Using high-quality astrometry from Gaia Data Release 3, we derived new cluster memberships via a machine learning-based clustering algorithm (AstroLink). We compiled metallicity measurements from multiple ground-based spectroscopic surveys (Gaia-ESO, GALAH DR3, APOGEE-OCCAM) and Gaia GSP-Spec into a uniform scale. The ages of 320 clusters were determined using a machine learning-accelerated approximate Bayesian isochrone-fitting code (based on ASteCA), adopting a multiband approach that incorporates Gaia XP and synthetic photometry. Results . This work resulted in a robust sample of open clusters with precisely determined parameters and uncertainties. In addition, this catalog may provide a foundation for machine learning models to infer parameters for a larger sample of objects without spectroscopic data, which is to be explored in future work.

Context . The characterisation of exoplanetary systems depends on the accurate determination of host star parameters. The Ariel mission will probe the atmospheres of a statistically significant sample of exoplanets, and so requires a precise characterisation of the stellar properties well before its launch in 2031. The homogeneous determination of stellar parameters for Ariel will enable both the optimisation of the final target list and set roots for a reliable interpretation of the formation and evolution of planetary systems. Such a homogeneous characterisation has thus far only been carried out for the cool ( T _eff ≲ 7000 K) host stars among the Ariel target candidates. Aims . We present a uniform determination of fundamental stellar parameters for 18 hot stars ( T _eff ≳ 7000 K) in the Tier 1 candidate list of the Ariel mission candidate sample. Methods . We adopted an iterative spectro-trigonometric approach optimised for high-temperature stars ( T _eff ≳ 7000 K). Highresolution spectra were analysed using the ZEEMAN code with χ ² minimisation, combining model fits to metal and Balmer lines. Surface gravity was refined using photometry-based radii and masses from stellar evolutionary tracks. Results . We derived effective temperatures, surface gravities, projected rotational velocities, microturbulent velocities, overall metal-licities, iron abundances, stellar masses, and radii for our sample of 18 hot stars. Our results were validated against a set of benchmark stars previously presented in the literature, confirming that our methods yield results consistent with previous studies. This ensures an internally consistent parameter scale and maintains continuity across the transition from cool to hot stars within the Ariel sample. Conclusions . The derived parameters provide an internally consistent basis for studying the link between stellar properties and planetary characteristics in intermediate-mass stars (1.5 < M < 2.32 M _⊙ ). Building on our previous work on FGK host stars, we show that correlations between stellar mass, metallicity, and planetary radii also extend to early-type stars, and stellar properties influence the architecture of multi-planet systems.

NGC 2509 is a distant (≈2.5 kpc) and little-studied open cluster located in the third Galactic quadrant. It is a moderately old cluster, whose age has not yet been precisely determined. The main-sequence stars in NGC 2509 follow a narrow distribution in the colormagnitude diagram, unlike other clusters of similar age. In addition, its chemical composition has never been investigated. To address these issues and characterize the cluster we performed moderate- and high-resolution spectroscopy with FLAMES@VLT of 132 stars, both dwarfs and giants, which represents a significant fraction (≈73%) of likely members. We provide atmospheric stellar parameters and, for the first time, chemical abundances for 21 species with atomic numbers up to 60. In our analysis we followed two different methodologies, both of which will be used for the incoming WEAVE stellar surveys. We find an average radial velocity for NGC 2509 of 58.6±1.3km s ⁻¹ and a mild supersolar metallicity ([Fe/H]≈0.1 dex). This value is slightly higher than expected according to its galactocentric distance, but still compatible with the Galactic gradient. From the lithium content of the dwarfs and the isochrone-fitting method we obtain an age for NGC 2509 of 1.26±0.3Gyr. The reddening across the cluster field is negligible ( A _V = 0.25 ± 0.02 mag). The cluster peculiar main sequence turnoff is due to a narrow distribution of the rotational velocities peaking at v sin i ≈ 80 km s ⁻¹ , with little dispersion. The chemical pattern of NGC 2509 follows the Galactic trends shown by other open clusters in the Galactic thin disk.

Context. Carbon stars on the asymptotic giant branch are major contributors to the dust enrichment of galaxies, with gas mass-loss rate values up to Ṁ ≈ 10 ⁻⁴ M _⊙ yr ⁻¹ . They represent the final evolutionary stage of low- and intermediate-mass stars, during which recurrent dredge-up episodes enrich their atmospheres with carbon and trigger the formation of dust. Through their intense winds, they inject large amounts of newly formed carbonaceous dust into the interstellar medium, playing a central role in the chemical evolution of galaxies. Their stellar and dust properties have been studied for decades, with a particular focus on the carbon stars in the Magellanic Clouds (MCs). Aims. Our aim is to homogeneously analyse the Gaia DR3 Golden Sample of Carbon Stars through the spectral energy distribution (SED) fitting. Our focus is on sources belonging to the Milky Way (MW) and the MCs. Methods. Our dataset consists of 14 747 stars with complete multi-band photometry from Gaia , 2MASS, and WISE, combined with recent distance and extinction estimates. For a subsample of Mira variables made of 2494 stars, we also modelled multi-band light curves to obtain accurate mean magnitudes. Stellar and circumstellar properties were derived by fitting the observations with a large grid of synthetic models computed with the DUSTY radiative transfer code, using COMARCS model atmospheres as input. For each target, we determined stellar and dust parameters such as the effective temperature, optical depth, and gas mass-loss rate. Results. The resulting distributions reveal typical effective temperatures of around 3150 K. Mass-loss rates range from 10 ⁻¹¹ to 10 ⁻⁴ M _⊙ yr ⁻¹ . The average dust temperature at the inner bound of the dust shell is about T _d = 1000 K. We also observe a correlation between photometric variability amplitude and mass-loss rate. Conclusions. This homogeneous framework provides a unified view of carbon stars across environments spanning a wide range of metallicities, supported by strong statistical coverage. Our results show that some of the physical properties of carbon stars exhibit a dependence on the galactic environment. However, these dependences do not necessarily reflect intrinsic metallicity effects, but are influenced by differences in luminosity distributions and by the selection biases affecting the available samples. The use of Gaia and WISE introduces combined selection effects that are significant, limiting the detection of both the most dust-enshrouded objects and the less luminous sources in the Magellanic Clouds. While this limits the completeness of the comparison, the observed trends remain statistically robust within the selected samples.

Context. Wide binaries offer a unique opportunity to test gravity in the low-acceleration regime, where deviations from Newtonian dynamics may appear. Aims. We used high-resolution VLT–ESPRESSO archival spectra to study 26 wide binaries with projected separations > 13 000 AU. By combining precise radial velocities with Gaia proper motions and parallaxes, we tested whether these systems are consistent with Newtonian gravity in the low-acceleration regime. Methods. We used multiple radial-velocity measurements and stellar parameters to remove systems affected by unresolved triple or chance alignments as well as young systems. For the remaining binaries, we combined radial velocities (corrected for convective shift and gravitational redshift) with Gaia proper motions, parallaxes, and positions in a bid to find bound Newtonian orbital solutions. Results. Of the 26 initial systems, 14 were discarded: 12 due to radial-velocity variability indicating unresolved close binaries, 1 that hosts a faint Gaia companion, and 1 that is too young. Of the remaining 12, 9 can be fitted with a bound orbital solution, while the velocity differences of the other 3 are too large to be reconciled with any bound Newtonian orbit. Conclusions. For the three systems that cannot be fitted with a bound orbit, repeated radial-velocity observations allowed us to confidently exclude, with one possible exception, unresolved triple stellar companions or massive close-in planets as causes. Given their likely large 3D separations, these binaries may have been dynamically perturbed or disrupted by stellar encounters or Galactic tides, and may no longer be gravitationally bound. This highlights how utmost caution must be applied when studying wide binaries as isolated systems.

Aims. We analyse ESPRESSO data for the stars HD 10700 (τ Ceti), HD 20794 (e Eridani), HD 102365, and HD 304636 acquired via its Guaranteed Time Observations (GTO) programme. We characterise the stars’ radial velocity (RV) signals down to a precision of 10 cm/s on timescales ranging from minutes to planetary periods falling within the host’s habitable zone (HZ). We study the RV signature of pulsation, granulation, and stellar activity, inferring the potential presence of planets around these stars. Thus, we outline the population of planets that while undetectable remain compatible with the available data. Methods. We derived the stellar parameters through different methods for a complete characterisation of the star. We used these parameters to model the effects of stellar pulsations on intra-night RV variations and of stellar activity on nightly averaged values. The RVs were derived both with the cross-correlation method and template matching, as well as over the blue and red ESPRESSO detectors independently to identify colour-dependent parasitic effects of an instrumental or stellar nature. The study of RVs was complemented by an investigation of stellar activity indicators using photospheric information and chromospheric indexes. Results. A simple model of stellar pulsations successfully reproduced the intra-night RV scatter of HD 10700 down to a few cm/s. For HD 102365 and HD 20794, an additional source of scatter at the level of several 10 cm/s remains necessary to explain the data. A kima analysis was used to evaluate the number of planets supported by the nightly averaged time series of each of these three stars, under the assumption that a quasi-periodic Gaussian process (GP) regression is able to model the activity signal. While a frequency analysis of HD 10700 RVs is able to identify a periodic signal at 20 d, when it is modelled along with the activity signal the signal is formally non-significant. Moreover, its physical origin remains uncertain due to the similarity with the first harmonic of the stellar rotation. ESPRESSO data on their own do not provide conclusive evidence to support the existence of planets around HD 10700, HD 102365, or HD 304636. In addition, the comparison of RVs with the contemporaneous indicators displays a strong correlation for HD 102365. The direct interpretation is that half of the RV variance on this star is directly attributed to activity. Conclusions. ESPRESSO is shown to reach an on-sky RV precision of better than 10 cm/s on short timescales (⊕ for periods of up to 100 d, and a mass of 2–5 M_⊕ for the star’s HZ.

Context. Binary stars and products of binary evolution contribute significantly to shaping stellar populations, yet they are often neglected due to the difficulty involved in identifying them. With asteroseismology providing precise stellar parameters, we have the opportunity to better characterize binaries and their products, as well as to refine our understanding of their role in shaping Galactic populations. Aims. We estimate the occurrence rates, mass distributions, and evolutionary states of asteroseismic binaries that exhibit solar-like oscillations from both components, as well as those of products of binary interactions with detectable solar-like oscillations. Additionally, we explore the effects of mass accretion or loss on the relation between the “apparent age” and metallicity. Mehods. We simulated 121 deg² of Kepler’s field of view using the TRILEGAL population synthesis code, adopting the Eggleton and Moe & Di Stefano distributions of initial binary parameters. We also generated an additional simulation with non-interacting binaries for comparison. Results. We find that asteroseismic binaries require an initial mass ratio that is close to 1 and that even small mass transfer events can prevent the detection of oscillations from both components. The noninteracting case yields the highest fraction of asteroseismic binaries for red giant stars with detectable oscillations (0.46%), while Eggleton yields the lowest (0.06%). Asteroseismic binaries composed of two red clump stars are not expected at separations smaller than 500 R_⊙ due to the interplay of stellar evolution and binary interactions. The simulation carried out with the Moe & Di Stefano distribution suggests that ∼1% of Kepler’s field red giants with detectable oscillations have undergone significant mass accretion or loss, appearing rejuvenated or prematurely aged. They could also potentially affect Galactic age-metallicity relations, although the occurrence and properties of these populations strongly depend on the assumed initial binary parameters. Conclusions. Comparing the predicted and observed asteroseismic binaries, as well as over- and under-massive stars, offers a way to constrain key binary evolution assumptions, such as the initial binary fraction and period distribution, and to reduce uncertainties in mass-transfer modeling.

Context. The next mission dedicated to the study of planetary atmospheres is the Ariel space mission, planned for launch in 2029, which will observe a variety of planetary systems belonging to different classes around stars with spectral types from M to A. To optimise the scientific outcome of the mission, such stars need to be homogeneously characterised beforehand. Aims. In this work, we focus on a methodology based on spectral synthesis for the characterisation of FGK-type stars from the Ariel Tier 1 mission candidate sample (MCS) that exhibit fast rotation. In addition, we analyse 108 slow-rotating FGK-type stars, with either new observations or archival spectra available, consistently as in our previous work using the equivalent width (EW) analysis. Methods. To ensure consistency between our methods, we re-analysed a sample of FGK-type stars with the spectral synthesis method and compared it to our previous work. The results of our analysis show excellent agreement with the previous set of derived parameters. Results. We provide homogeneous effective temperature, surface gravity, metallicity, projected rotational velocity, and stellar mass for a sample of 36 fast rotators with the spectral synthesis technique, and we include 108 FGK-type dwarfs with the EW analysis. An additional 25 stars were analysed with the spectral synthesis method because their EW analysis did not converge on the final parameters. We computed their orbital parameters establishing whether they belong to the Galactic thin or thick discs. With the current set of stellar parameters, we almost double the analysed hosts in the Ariel MCS to 353 stars in total. Conclusions. Using our homogeneous set of stellar parameters, we studied the correlations between stellar and planetary properties for the Ariel MCS analysed so far. We confirmed a close relationship between stellar mass (up to 1.8 M_⊙) and giant planet radius, with more inflated planets at lower metallicity. We confirm that giant planets are more frequent around more metal-rich stars that belong to the thin disc, while lower-mass planets are also found in more metal-poor environments, and are more frequent than giant planets in the thick disc as also seen in other works in the literature.

Context. Different stellar populations may be identified through differences in chemical, kinematic, and chronological properties, suggesting the interplay of various physical mechanisms that led to their origin and subsequent evolution. As such, the identification of stellar populations is key for gaining an insight into the evolutionary history of the Milky Way. This task is complicated by the fact that stellar populations share a significant overlap in their chrono-chemo-kinematic properties, hindering efforts to identify and define stellar populations. Aims. Our goal is to offer a novel and effective methodology that can provide a deeper insight into the nonlinear and nonparametric properties of the multidimensional physical parameters that define stellar populations. Methods. For this purpose, we explore the ability of manifold learning to differentiate stellar populations with minimal assumptions about their number and nature. Manifold learning is an unsupervised machine learning technique that seeks to intelligently identify and disentangle manifolds hidden within the input data. To test this method, we make use of Gaia DR3-like synthetic stellar samples generated from the FIRE-2 cosmological simulations. These represent red-giant stars constrained by asteroseismic data from TESS. Results. We reduced the 5D input chrono-chemo-kinematic parameter space into 2D latent space embeddings generated by manifold learning. We then study these embeddings to assess how accurately they represent the original data and whether they contain meaningful information that can be used to discern stellar populations. Conclusions. We conclude that manifold learning possesses promising abilities to differentiate stellar populations when considering realistic observational constraints.

Context. One of the goals of the ESPRESSO guaranteed time observations (GTOs) at the ESO 8.2 m telescope is to follow up on candidate planets from transit surveys such as the TESS mission. High-precision radial velocities are required to characterize small exoplanets. Aims. We intend to confirm the existence of a transiting super-Earth around the bright (V=9.74) K0-type star TOI-512 (TIC 119292328) and provide a characterization. Methods. Combining photometric data from TESS and 37 high-resolution spectroscopic observations from ESPRESSO in a joint Markov chain Monte Carlo analysis, we determined the planetary parameters of TOI-512b and characterized its internal structure. Results. We find that TOI-512b is a super-Earth, with a radius of 1.54 ± 0.10 R_⊕ and mass of 3.57_−0.55^+0.53 M_⊕, on a 7.19_{−6.1·10⁻⁵}+7.10⁻⁵ day orbit. This corresponds to a bulk density of 5.62_−1.28^+1.59 g cm⁻³. Our interior structure analysis presents a small inner core representing 0.13_−0.11^+0.13 of the solid mass fraction for the planet, surrounded by a mantle with a mass fraction of 0.69_−0.22^+0.20, and an upper limit of the water layer of 0.16. The gas mass below 10^−8.93 indicates a very small amount of gas on the planet. We find no evidence of the second candidate found by the TESS pipeline, TOI-512.02, neither in TESS photometry, nor in the ESPRESSO radial velocities. The low stellar activity makes it an interesting transmission spectroscopy candidate for future-generation instruments.