Marco Montalto

Context . Spectroscopic monitoring surveys have shown that the magnetic cycle alters the photospheric structure of the Sun, perturbing the formation of molecular lines. In parallel, the study of globular clusters (GCs) has remained centred on the multiple stellar populations (MPs) conundrum, a phenomenon defined – most notably – by the anti-correlation between CH and CN molecular bands, serving to distinguish between first-population (1P) and second-population (2P) stars. Aims . Since these molecular transitions are intrinsically sensitive to photospheric conditions, we investigated whether stellar magnetic activity can induce spectroscopic variations consistent with the signatures traditionally utilised to disentangle 1P and 2P populations. Methods . We bridged the stellar and solar domains by applying standard spectroscopic index analyses – routinely used to identify MPs in clusters – to a decade-long dataset of disc-integrated high-resolution solar spectra. We systematically monitored the behaviour of the CH and CN molecular bands as a function of the solar magnetic cycle. These observational results are further complemented by first-order magnetohydrodynamic (MHD) simulations to evaluate how these signatures scale with increased magnetic activity. Results . We demonstrate that the disc-integrated solar spectrum exhibits a distinct CH/CN anti-correlation driven by the magnetic cycle. Our simulations indicate that these variations in molecular line formation scale significantly with increased magnetic filling factors, rendering them, in principle, capable of reaching amplitudes comparable to the spectroscopic spreads observed between 1P and 2P stars in GCs. Conclusions . While the broader implications of our discovery of a CH/CN anti-correlation in the single-star Sun remain to be fully elucidated, our results suggest that the spectroscopic and photometric anomalies defining the MP conundrum may be physically linked to a differential spectral response to varying degrees of surface magnetic coverage. The emerging mechanism depends on the intrinsic magnetic state of the star – distinguishing relatively active from relatively quiescent stars – rather than the existence of a magnetic cycle itself. Surface magnetism offers a novel physical framework that may provide new insights into the origin of MPs and clearly warrants future exploration.

The PLanetary Transits and Oscillations of stars (PLATO) mission will begin its four-year nominal mission in early 2027 by monitoring its Long-duration Observation Phase field at South (LOPS2) for at least two years continuously. The primary aim of PLATO is a very ambitious and challenging one: the discovery of Earth-like planets in the habitable zone of nearby and bright solar analogs. To this purpose, the PLATO Mission Consortium, through its Ground-based Observing Program, will perform the follow-up needed to confirm part of the candidate planets photometrically detected by PLATO and measure their masses through radial velocity curves. For the LOPS2, the Ground-based Observing Program is committed (as part of the PLATO mission) to follow up the candidate exoplanets discovered orbiting the 15 000 high-quality target subset of the PLATO Input Catalog (PIC) known as the Prime Sample. The Prime Sample will be made public nine months before launch in the context of the first Guest Observer call for proposals to be issued by the European Space Agency. Here, we present the quantitative metrics and thresholds defined to select and prioritize the Prime Sample. Our method is perfectly general and suitable to rank any list of stars surveyed for transiting planets. We also describe the astrophysical properties of the LOPS2 Prime Sample, both in a statistical sense and for some specific targets of interest.

Context . The aim of the European Space Agency’s PLAnetary Transits and Oscillations of Stars (PLATO) mission is to detect planets orbiting around dwarfs and subgiant stars with spectral type F5 or later, including M dwarfs. The PLATO Input Catalogue (PIC) contains all PLATO targets available for observation by the PLATO nominal science. The latest version, PIC2.1.0.1, focuses on the Southern PLATO field, named LOPS2, selected as the first long observation field, and includes the P4 sample, one of the four target samples outlined in the Science Requirement Document. P4 includes the M dwarfs with magnitudes brighter than V=16 located within LOPS2. Aims . A characterisation of the M dwarfs in the PIC is essential for assessing their potentiality to host exoplanets, and eventually for estimating the hosted planet(s) properties. The purpose of this paper is to describe how we selected the P4 M dwarf targets, and obtained their fundamental parameters and properties. Methods . Measuring stellar parameters is a challenging task. Interferometry provides direct estimates of radii, whereas alternative approaches relying on theoretical assumptions are still affected by significant uncertainties. In this work, we introduce the P4 sample and detail the methodologies, all based on photometric criteria, adopted for the measurement of their stellar parameters. Results . Based on a statistical analysis of the P4 sample, we assess both the photometric and volume completeness, and classify the stellar populations according to their Galactic spatial-velocity components. The adopted stellar parameters are validated by comparison with independent methods from the literature used to estimate stellar radii. Conclusions . We conclude that the P4 sample is compliant with the PLATO science requirements. Being magnitude limited, its volume completeness decreases going towards distances larger than 30 pc, where late-type targets are progressively less covered. The observed large spread in the colour-magnitude diagram is likely due to the combination of several effects such as metallicity, age, binarity, and activity. The strategy we adopted for deriving stellar parameters provides results consistent with those obtained in the literature with different and independent methods.

PLAnetary Transits and Oscillations of stars (PLATO) is an ESA M-class mission to be launched by the end of 2026 to discover and characterize transiting planets around bright and nearby stars, and in particular habitable rocky planets hosted by solar-like stars. Over the mission lifetime, an average of 8% of the science data rate will be allocated to Guest Observer programs selected by ESA through public calls. Hence, it is essential for the community to know in advance where the observing fields will be located. In a previous paper, we identified two preliminary long-pointing fields (LOPN1 and LOPS1) for PLATO, respectively in the northern and southern hemispheres. Here we present LOPS2, a slightly adjusted version of the southern field that has recently been selected by the PLATO Science Working Team as the first field to be observed by PLATO for at least two continuous years, following the scientific requirements. In this paper, we describe the astrophysical content of LOPS2 in detail, including known planetary systems, bright stars, variables, binary stars, star clusters, and synergies with other current and future facilities.

The DIAmante Transiting Exoplanet Sky Survey (TESS) AutoRegressive Planet Search (DTARPS) project, using novel statistical methods, has identified several hundred candidates for transiting planetary systems obtained from 0.9 million full-frame Image light curves obtained in the TESS Year 1 southern-hemisphere survey. Ten lines of evidence including limited reconnaissance spectroscopy indicate that approximately half are true planets rather than false positives. Here various population properties of these candidates are examined. Half of the DTARPS-S candidates are hot Neptunes, populating the “Neptune desert” found in Kepler-planet samples. The DTARPS-S samples also identify dozens of ultrashort-period planets with orbital periods down to 5 hr, high-priority systems for atmospheric transmission spectroscopy, and planets orbiting low-mass M stars. DTARPS-S methodology is sufficiently well characterized at each step so that preliminary planet occurrence rates can be estimated. Except for the increase in hot Neptunes, DTARPS-S planet occurrence rates are consistent with Kepler rates. Overall, DTARPS-S provides one of the most reliable and useful catalogs of TESS exoplanet candidates that can be used to to improve our understanding of various exoplanetary populations and astrophysical processes.

Recent observations of giant planets have revealed unexpected bulk densities. Hot Jupiters, in particular, appear larger than expected for their masses compared to planetary evolution models, while warm Jupiters seem denser than expected. These differences are often attributed to the influence of the stellar incident flux, but it has been unclear if they also result from different planet formation processes, and if there is a trend linking the planetary density to the chemical composition of the host star. In this work, we present the confirmation of three giant planets in orbit around solar analogue stars. TOI-2714 b (P ≃ 2.5 d, R_p ≃ 1.22 R_J, M_p = 0.72 M_J) and TOI-2981 b (P ≃ 3.6 d, R_P ≃ 1.2 R_J, M_P = 2 M_J) are hot Jupiters on nearly circular orbits, while TOI-4914 b (P ≃ 10.6 d, R_P ≃ 1.15 R_J, M_p = 0.72 M_J) is a warm Jupiter with a significant eccentricity (e = 0.41 ± 0.02) that orbits a star more metal-poor ([Fe/H] = −0.13) than most of the stars known to host giant planets. Similarly, TOI-2981 b orbits a metal-poor star ([Fe/H] = −0.11), while TOI-2714 b orbits a metal-rich star ([Fe/H] = 0.30). Our radial velocity follow-up with the HARPS spectrograph allows us to detect their Keplerian signals at high significance (7, 30, and 23σ, respectively) and to place a strong constraint on the eccentricity of TOI-4914 b (18σ). TOI-4914 b, with its large radius (R_p ≃ 1.15 R_J) and low insolation flux (F_⋆ < 2 × 108 erg s⁻¹ cm⁻²), appears to be more inflated than what is supported by current theoretical models for giant planets. Moreover, it does not conform to the previously noted trend that warm giant planets orbiting metal-poor stars have low eccentricities. This study thus provides insights into the diverse orbital characteristics and formation processes of giant exoplanets, in particular the role of stellar metallicity in the evolution of planetary systems.

We report the discovery and characterization of a transiting sub-Saturn exoplanet TOI-6651b using PARAS-2 spectroscopic observations. The host, TOI-6651 (m_V ≈ 10.2), is a sub-giant, metal-rich G-type star with [Fe / H] = 0.225_−0.045^0.044[Fe/H] = 0.225_−0.045^+0.044, T_eff = 5940 ± 110 K, and log g = 4.087_−0.032^+0.035. Joint fitting of the radial velocities from PARAS-2 spectrograph and transit photometric data from Transiting Exoplanet Survey Satellite (TESS) reveals a planetary mass of 61.0_−7.9^+7.6 M_⊕ and radius of 5.09_−0.26^+0.27 R_⊕, in a 5.056973_−0.000018^+0.000016 day orbit with an eccentricity of 0.091_−0.062^+0.096. TOI-6651b has a bulk density of 2.52_−0.44^+0.52 g cm⁻³, positioning it among the select few known dense sub-Saturns and making it notably the densest detected with TESS. TOI-6651b is consistent with the positive correlation between planet mass and the host star’s metallicity. We find that a considerable portion ≈87% of the planet’s mass consists of dense materials such as rock and iron in the core, while the remaining mass comprises a low-density envelope of H/He. TOI-6651b lies at the edge of the Neptunian desert, which will be crucial for understanding the factors shaping the desert boundaries. The existence of TOI-6651b challenges conventional planet formation theories and could be a result of merging events or significant atmospheric mass loss through tidal heating, highlighting the complex interplay of dynamical processes and atmospheric evolution in the formation of massive dense sub-Saturns.

Aims. We report the confirmation of a new transiting exoplanet orbiting the star TOI-5076. Methods. We present our vetting procedure and follow-up observations which led to the confirmation of the exoplanet TOI-5076b. In particular, we employed high-precision TESS photometry, high-angular-resolution imaging from several telescopes, and high-precision radial velocities from HARPS-N. Results. From the HARPS-N spectroscopy, we determined the spectroscopic parameters of the host star: T_eff = (5070±143) K, log 𝑔 = (4.6±0.3), [Fe/H] = (+0.20±0.08), and [α/Fe] = 0.05±0.06. The transiting planet is a warm sub-Neptune with a mass m_p = (16±2) M_⊙, a radius r_p =(3.2±0.l) R_⊙ yielding a density ρ_p = (2.8±0.5) g cm⁻³. It revolves around its star approximately every 23.445 days. Conclusions. The host star is a metal-rich, K2V dwarf, located at about 82 pc from the Sun with a radius of R_⋆ = (0.78±0.01) R_⊙ and a mass of M_⋆ = (0.80±0.07) M_⊙. It forms a common proper motion pair with an M-dwarf companion star located at a projected separation of 2178 au. The chemical analysis of the host-star and the Galactic-space velocities indicate that TOI-5076 belongs to the old population of thin-to-thick-disk transition stars. The density of TOI-5076b suggests the presence of a large fraction by volume of volatiles overlying a massive core. We found that a circular orbit solution is marginally favored with respect to an eccentric orbit solution for TOI-5076b.

Nearly one million light curves from the TESS Year 1 southern hemisphere extracted from Full Field Images with the DIAmante pipeline are processed through the AutoRegressive Planet Search statistical procedure. ARIMA models remove lingering autocorrelated noise, the Transit Comb Filter identifies the strongest periodic signal in the light curve, and a Random Forest machine-learning classifier is trained and applied to identify the best potential candidates. Classifier training sets are based on injections of planetary transit signals, eclipsing binaries, and other variable stars. The optimized classifier has a True Positive Rate of 92.5% and a False Positive Rate of 0.43% from the labeled training set. The result of this DIAmante TESS autoregressive planet search of the southern ecliptic hemisphere analysis is a list of 7377 potential exoplanet candidates. The classifier had a 64% recall rate for previously confirmed exoplanets and a 78% negative recall rate for known False Positives. The completeness map of the injected planetary signals shows high recall rates for planets with 8–30R _⊕ radii and periods 0.6–13 days and poor completeness for planets with radii R _⊕ or periods <1 day. The list has many False Alarms and False Positives that need to be culled with multifaceted vetting operations (Paper II).