arXiv subjects

S. Benatti

Publications and source records attributed to S. Benatti.

At least 37 records · Page 2Linked to original sources

arXiv2025

Revisiting the multi-planetary system of the nearby star HD 20794: Confirmation of a low-mass planet in the habitable zone of a nearby G-dwarf

Close-by Earth analogs and super-Earths are of primary importance because they will be preferential targets for the next generation of direct imaging instruments. Bright and close-by G-to-M type stars are preferential targets in radial velocity surveys to find Earth analogs. We present an analysis of the RV data of the star HD 20794, a target whose planetary system has been extensively debated in the literature. The broad time span of the observations makes it possible to find planets with signal semi-amplitudes below 1 m/s in the habitable zone. We monitored the system with ESPRESSO. We joined ESPRESSO data with the HARPS data, including archival data and new measurements from a recent program. We applied the post-processing pipeline YARARA to HARPS data to correct systematics, improve the quality of RV measurements, and mitigate the impact of stellar activity. Results. We confirm the presence of three planets, with periods of 18.3142 +/- 0.0022 d, 89.68 +/- 0.10 d, and 647.6 +/- 2.6 d, along with masses of 2.15 +/- 0.17 MEarth, 2.98 +/- 0.29 MEarth, and 5.82 +/- 0.57 MEarth respectively. For the outer planet, we find an eccentricity of 0.45 +/- 0.10, whereas the inner planets are compatible with circular orbits. The latter is likely to be a rocky planet in the habitable zone of HD 20794. From the analysis of activity indicators, we find evidence of a magnetic cycle with a period around 3000 d, along with evidence pointing to a rotation period around 39 d. We have determined the presence of a system of three planets orbiting the solar-type star HD 20794. This star is bright (V=4.34 mag) and close (d = 6.04 pc), and HD 20794 d resides in the stellar habitable zone, making this system a high-priority target for future atmospheric characterization with direct imaging facilities.

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arXiv2024

The GAPS Programme at TNG. LXV. Precise density measurement of TOI-1430 b, a young planet with an evaporating atmosphere

Small-sized exoplanets in tight orbits around young stars (10-1000 Myr) give us the opportunity to investigate the mechanisms that led to their formation, the evolution of their physical and orbital properties and, especially, of their atmospheres. Thanks to the all-sky survey carried out by TESS, many of these exoplanets have been discovered and have subsequently been characterized with dedicated follow-up observations. In the context of a collaboration among the GAPS, TKS and CPS teams, we measured with a high level of precision the mass and the radius of TOI-1430 b, a young (~700 Myr) exoplanet with an escaping He atmosphere orbiting the K-dwarf star HD 235088 (TOI-1430). By adopting appropriate stellar parameters, which were measured in this work, we were able to simultaneously model the signals due to strong stellar activity and the transiting planet TOI-1430 b in both photometric and spectroscopic series. This allowed us to measure the density of the planet with high precision, and reconstruct the evolution of its atmosphere. TOI-1430 is an active K-dwarf star born 700+/-150 Myr ago and rotates in ~12 d. It hosts a mini-Neptune whose orbital period is Pb=7.434133+/-0.000004 d. Thanks to long-term monitoring of this target performed with TESS, HARPS-N, HIRES, and APF, we estimated a radius Rb=1.98+/-0.07 $R_{\oplus}$, a mass Mb=4.2+/-0.8 $M_{\oplus}$, and thus a planetary density $ρ$b=0.5+/-0.1 $ρ_{\oplus}$. TOI-1430 b is hence a low-density mini-Neptune with an extended atmosphere, at the edge of the radius gap. Because this planet is known to have an evaporating atmosphere of He, we reconstructed its atmospheric history. Our analysis supports the scenario in which, shortly after its birth, TOI-1430 b may have been super-puffy, with a radius 5x-13x and a mass 1.5x-2x that of today; in ~200 Myr from now, TOI-1430 b should lose its envelope, showing its Earth-size core.

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arXiv2024

The GAPS programme at TNG -- LXIII. Photo-evaporating puzzle: Exploring the enigmatic nature of TOI-5398 b atmospheric signal

Atmospheric characterization is key to understanding exoplanetary systems, offering insights into the planets current and past conditions. By analyzing key lines like H alpha and the He I triplet, we can trace the evolution of planets through atmospheric photo-evaporation. While ultra-hot Jupiters have been the focus for years, attention is shifting toward smaller, colder planets, which are more challenging to study due to weaker signals, requiring more precise techniques. This study aims to characterize the atmosphere of TOI-5398 b, a warm Saturn with a 10.59-day orbit around a young (650 Myr) G-type star. The system also hosts a smaller inner planet, TOI-5398 c, with a 4.77-day orbit. Both planets are ideal for atmospheric studies due to their proximity to the host star, which drives strong photo-evaporation, especially in planet b, whose high transmission spectroscopy metric (288) makes it a prime target. We analyzed data from a transit observed with the HARPS-N and GIANO-B high-resolution spectrographs, using cross-correlation and single-line analysis to search for atomic species. During this observation, planet c was also transiting, so we investigated the source of the signals. Based on photo-evaporation models, we attribute the signal mainly to planet b, which is expected to lose more mass. We detected H alpha and He I triplets, key markers of photo-evaporation, corresponding to atmospheric heights of 2.33 Rp and 1.65 Rp, respectively. The ATES models supported our observations, predicting a similar He I absorption for planet b and suggesting an He/H ratio of 1/99. Additionally, we detected an Na I doublet via single-line analysis, though cross-correlation did not reveal other atomic species.

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arXiv2024

A sub-Earth-mass planet orbiting Barnard's star

Barnard's star is a primary target within the ESPRESSO guaranteed time observations (GTO) as it is the second closest neighbour to our Sun after the $α$ Centauri stellar system. We present here a large set of 156 ESPRESSO observations of Barnard's star carried out over four years with the goal of exploring periods of shorter than 50 days, thus including the habitable zone (HZ). Our analysis of ESPRESSO data using Gaussian process (GP) to model stellar activity suggests a long-term activity cycle at 3200d and confirms stellar activity due to rotation at 140d as the dominant source of radial velocity (RV) variations. These results are in agreement with findings based on publicly available HARPS, HARPS-N, and CARMENES data. ESPRESSO RVs do not support the existence of the previously reported candidate planet at 233d. After subtracting the GP model, ESPRESSO RVs reveal several short-period candidate planet signals at periods of 3.15d, 4.12d, 2.34d, and 6.74d. We confirm the 3.15d signal as a sub-Earth mass planet, with a semi-amplitude of $55 \pm 7$cm/s, leading to a planet minimum mass $m_p \sin i$ of $0.37 \pm 0.05$Mearth, which is about three times the mass of Mars. ESPRESSO RVs suggest the possible existence of a candidate system with four sub-Earth mass planets in circular orbits with semi-amplitudes from 20 to 47cm/s, thus corresponding to minimum masses in the range of 0.17-0.32Mearth. The sub-Earth mass planet at $3.1533 \pm 0.0006$d is in a close-to circular orbit with a semi-major axis of $0.0229 \pm 0.0003$AU, thus located inwards from the HZ of Barnard's star, with an equilibrium temperature of 400K. Additional ESPRESSO observations would be required to confirm that the other three candidate signals originate from a compact short-period planet system orbiting Barnard's star inwards from its HZ.

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arXiv2024

XUV irradiation of young planetary atmospheres. Results from a joint XMM-Newton and HST observation of HIP67522

The evaporation and the chemistry of the atmospheres of warm and hot planets are strongly determined by the high-energy irradiation they receive from their parent stars. This is more crucial among young extra-solar systems, due to the high activity of stars at early ages. In particular, the EUV part of the stellar spectra drives significant processes of photo-chemical interaction, but it is not directly measurable due to strong interstellar absorption and the lack of sufficiently sensitive instrumentation. An alternative approach is to derive synthetic spectra from the analysis of FUV and X-ray emission lines, that allow to estimate the missed flux in the EUV band. We performed joint and simultaneous spectroscopy of HIP 67522 with XMM-Newton and HST aimed at reconstructing the full high-energy spectrum of this 17 Myr old solar-type (G0) star, which is the youngest known transiting multi-planet system at present time. We performed a time-resolved spectral analysis of the observations, including quiescent emission and flaring variability. Then, we derived the Emission Measure Distribution (EMD) vs. temperature of the chromospheric and coronal plasma from the high-resolution spectra obtained in X-rays with RGS and in FUV with COS. We derived broad-band X-ray and EUV luminosities from the synthetic spectrum based on the EMD, that allowed us to test alternative EUV vs. X-ray scaling laws available in literature. We also employed the total XUV flux received by the inner planet of the system to estimate its instantaneous atmospheric mass loss rate. We confirm that HIP 67522 is a very active star with a hot corona, reaching plasma temperatures above 20 MK even in quiescent state. Its EUV/X-ray flux ratio falls in between the predictions of the two scaling laws we have tested, indicating an important spread in the stellar properties, that requires further investigation.

astro-ph.SR↗

arXiv2024

The GAPS programme at TNG LX Atmospheric characterisation of KELT-9 b via single-line analysis: Detection of six H I Balmer lines, Na I, Ca I, Ca II, Fe I, Fe II, Mg I, Ti II, Sc II, and Cr II

We analysed six primary transits of the ultra-hot Jupiter KELT-9,b obtained with the HARPS-N high-resolution spectrograph in the context of the Global Architecture of Planetary Systems (GAPS2) project, to characterise the atmosphere via single-line analysis. We extracted the transmission spectrum of each individual line by comparing the master out-of-transit spectrum with the in-transit spectra and computing the weighted average of the tomography in the planet reference frame. We corrected for the centre-to-limb variation and the Rossiter-McLaughlin effect by modelling the region of the star disc obscured by the planet during the transit and subtracting it from the master-out spectrum. We detected all six observable lines of the Balmer series within the HARPS-N wavelength range, from H$α$ to H$ζ$, with a significance exceeding 5$σ$. We focussed on metal species, detecting Na I, Ca I, Ca II, Fe I, Fe II, Mg I, Ti II, Sc II, and Cr II lines. This is the first detection in the atmosphere of an exoplanet of H$ε$ and H$ζ$ lines, as well as of individual lines of Sc II and Cr II. Our detections are supported by a comparison with published synthetic transmission spectra of KELT-9b obtained accounting for non-local thermodynamic equilibrium effects. The results underline the presence of a systematic blueshift due to night-side to day-side winds. The single-line analysis allowed us not only to assess the presence of atomic species in the atmosphere of KELT-9 b, but also to further characterise the local stratification of the atmosphere. Coupling the height distribution of the detected species with the velocity shift retrieved, we acknowledged the height distribution of night-side to day-side winds. Moreover, the study of the rotational broadening of different species supports the prediction of a tidally locked planet rotating as a rigid body.

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arXiv2024

The GAPS Programme at TNG. LXI. Atmospheric parameters and elemental abundances of TESS young exoplanet host stars

The study of exoplanets at different evolutionary stages can shed light on their formation, migration, and evolution. The determination of exoplanet properties depends on the properties of their host stars. It is therefore important to characterise the host stars for accurate knowledge on their planets. Our final goal is to derive, in a homogeneous and accurate way, the stellar atmospheric parameters and elemental abundances of ten young TESS transiting planet-hosting GK stars followed up with the HARPS-N at TNG spectrograph within the GAPS programme. We derived stellar kinematic properties, atmospheric parameters, and abundances of 18 elements. Lithium line measurements were used as approximate age estimations. We exploited chemical abundances and their ratios to derive information on planetary composition. Elemental abundances and kinematic properties are consistent with the nearby Galactic thin disk. All targets show C/O<0.8 and 1.0<Mg/Si<1.5, compatible with silicate mantles made of a mixture of pyroxene and olivine assemblages. The Fe/Mg ratios, with values of $\sim$0.7-1.0, show a propensity for the planets to have big (iron) cores. All stars hosting very low-mass planets show Mg/Si values consistent with the Earth values, thus demonstrating their similar mantle composition. Hot Jupiter host stars show a lower content of O/Si, which could be related to the lower presence of water content. We confirm a trend found in the literature between stellar [O/Fe] and total planetary mass, implying an important role of the O in shaping the mass fraction of heavy elements in stars and their disks. The detailed host star abundances provided can be employed for further studies on the composition of the planets within the current sample, when their atmospheres will be exploited.

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arXiv2024

EWOCS-II: X-ray properties of the Wolf-Rayet stars in the young Galactic super star cluster Westerlund 1

We present the most comprehensive and deepest X-ray study to date of the properties of the richest Wolf-Rayet (WR) population observed in a single stellar cluster, Westerlund 1 (Wd1). This work is based on 36 Chandra observations obtained from the "Extended Westerlund 1 and 2 Open Clusters Survey" (EWOCS) project, plus 8 archival Chandra observations. The overall exposure depth (~1.1 Ms) and baseline of the EWOCS observations extending over more than one year enable us to perform a detailed photometric, colour, and spectral analysis, as well as to search for short- and long-term periodicity. In X-rays, we detect 20 out of the 24 known Wolf-Rayet stars in Wd1 down to an observed luminosity of ~7$\times10^{29}$erg s$^{-1}$ (assuming a distance of 4.23 kpc to Wd1), with 8 WR stars being detected in X-rays for the first time. Nine stars show clear evidence of variability over the year-long baseline, with clear signs of periodicity. The X-ray colours and spectral analysis reveal that the vast majority of the WR stars are hard X-ray sources (kT$\geq$2.0keV). The Fe XXV emission line at ~6.7 keV, which commonly originates from the wind-wind collision zone in binary systems, is detected for the first time in the spectra of 17 WR stars in Wd1. In addition the ~6.4 keV fluorescent line is observed in the spectra of three stars, indicating that dense cold material coexists with the hot gas in these systems. Overall, our X-ray results alone suggest a very high binary fraction ($\geq$80%) for the WR star population in Wd1. When combining our results with properties of the WR population from other wavelengths, we estimate a binary fraction of $\geq$92%, which could even reach unity. This suggests that either all the most massive stars are found in binary systems within Wd1, or that binarity is essential for the formation of such a rich population of WR stars.

astro-ph.HE↗

arXiv2024

The GAPS Programme at TNG. LIX. A characterisation study of the $\sim$300 Myr old multi-planetary system orbiting the star BD+40 2790 (TOI-2076)

We collected more than 300 high-resolution spectra of the 300 Myr old star BD+40 2790 (TOI-2076) over ~3 years. This star hosts three transiting planets discovered by TESS, with orbital periods ~10, 21, and 35 days. BD+40 2790 shows an activity-induced scatter larger than 30 m/s in the radial velocities. We employed different methods to measure the stellar radial velocities and several models to filter out the dominant stellar activity signal, in order to bring to light the planet-induced signals which are expected to have semi-amplitudes one order of magnitude lower. We evaluated the mass loss rate of the planetary atmospheres using photoionization hydrodynamic modeling. The dynamical analysis confirms that the three sub-Neptune-sized companions (our radius measurements are $R_b$=2.54$\pm$0.04, $R_c$=3.35$\pm$0.05, and $R_d$=3.29$\pm$0.06 $R_{\rm Earth}$) have masses in the planetary regime. We derive 3$σ$ upper limits below or close to the mass of Neptune for all the planets: 11--12, 12--13.5, and 14--19 $M_{\rm Earth}$ for planet $b$, $c$, and $d$ respectively. In the case of planet $d$, we found promising clues that the mass could be between ~7 and 8 $M_{\rm Earth}$, with a significance level between 2.3--2.5$σ$ (at best). This result must be further investigated using other analysis methods or using high-precision near-IR spectrographs to collect new radial velocities, which could be less affected by stellar activity. Atmospheric photo-evaporation simulations predict that BD+40~2790 b is currently losing its H-He gaseous envelope, which will be completely lost at an age within 0.5--3 Gyr if its current mass is lower than 12 $M_{\rm Earth}$. BD+40 2790 c could have a lower bulk density than $b$, and it could retain its atmosphere up to an age of 5 Gyr. For the outermost planet $d$, we predict almost negligible evolution of its mass and radius induced by photo-evaporation.

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arXiv2024

Ariel stellar characterisation II. Chemical abundances of carbon, nitrogen, and oxygen for 181 planet-host FGK dwarf stars

One of the ultimate goals of the ESA Ariel space mission is to shed light on the formation pathways and evolution of planetary systems in the Solar neighbourhood. Such an endeavour is only possible by performing a large chemical survey of not only the planets, but also their host stars, inasmuch as stellar elemental abundances are the cipher key to decode the planetary compositional signatures. This work aims at providing homogeneous abundances of C, N, and O of a sample of 181 stars belonging to the Tier 1 of the Ariel Mission Candidate Sample. We applied the spectral synthesis and the equivalent width methods to a variety of atomic and molecular indicators (C I lines at 5052 and 5380.3 A, [O I] forbidden line at 6300.3 A, C_2 bands at 5128 and 5165 A, and CN band at 4215 A) using high-resolution and high S/N spectra collected with several spectrographs. We provide carbon abundances for 180 stars, nitrogen abundances for 105 stars, and oxygen abundances for 89 stars. We analyse the results in the light of the Galactic chemical evolution, and in terms of the planetary companions properties. Our sample basically follows the typical trends with metallicity expected for the [C/Fe], [N/Fe], and [O/Fe] abundance ratios. The fraction between C and O abundances, both yields of primary production, is consistent with a constant ratio as [O/H] increases, whereas the abundance of N tends to increase with the increasing of the O abundance, supporting the theoretical assumption of a secondary production of nitrogen. The [C/N], [C/O], and [N/O] ratios are also correlated with [Fe/H], which might introduce biases in the interpretation of the planetary compositions and formation histories if host stars of different metallicity are compared. We provide relations that can be used to qualitatively estimate whether the atmospheric composition of planets is enriched or not with respect to the host stars.

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arXiv2024

TOI-837 b: characterisation, formation and evolutionary history of an infant warm Saturn-mass planet

We aim to determine the fundamental properties of the $\sim$35 Myr old star TOI-837 and its close-in Saturn-sized planet, and to investigate the system's formation and evolutionary history. We analysed TESS photometry and HARPS spectroscopic data, measured stellar and planetary parameters, and characterised the stellar activity. We performed population synthesis simulations to track the formation history of TOI-837 $b$, and to reconstruct its possible internal structure. We investigated the planetary atmospheric evolution through photo-evaporation, and quantified the prospects for atmospheric characterisation with JWST. TOI-837 $b$ has radius and mass similar to those of Saturn ($r_b$=9.71$^{+0.93}_{-0.60}$ \rearth, $m_b$=116$^{+17}_{-18}$ M$_\odot$, and $ρ_b$=0.68$^{+0.20}_{-0.18}$ gcm$^{-3}$), on a primordial circular orbit. Population synthesis and early migration simulations suggest that the planet could have originated between 2-4 au, and have either a large and massive core, or a smaller Saturn-like core, depending on the opacity of the protoplanetary gas and on the growth rate of the core. We found that photo-evaporation produced negligible effects even at early ages (3-10 Myr). Transmission spectroscopy with JWST is very promising, and expected to provide constraints on atmospheric metallicity, abundance of H$_2$O, CO$_2$, CH$_4$ molecules, and to probe the presence of refractory elements. TOI-837 offers valuable prospects for follow-up observations, which are needed for a thorough characterisation. JWST will help to better constraining the formation and evolution history of the system, and understand whether TOI-837 $b$ is a Saturn-analogue.

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arXiv2024

Orbital obliquity of the young planet TOI-5398 b and the evolutionary history of the system

Multi-planet systems exhibit remarkable architectural diversity. However, short-period giant planets are typically isolated. Compact systems like TOI-5398, with an outer close-orbit giant and an inner small-size planet, are rare among systems containing short-period giants. TOI-5398's unusual architecture coupled with its young age (650 $\pm$ 150 Myr) make it a promising system for measuring the original obliquity between the orbital axis of the giant and the stellar spin axis in order to gain insight into its formation and orbital migration. We collected in-transit (plus suitable off-transit) observations of TOI-5398 b with HARPS-N at TNG on March 25, 2023, obtaining high-precision radial velocity time series that allowed us to measure the Rossiter-McLaughlin (RM) effect. By modelling the RM effect, we obtained a sky-projected obliquity of $λ= 3.0^{+6.8}_{-4.2}$ deg for TOI-5398 b, consistent with the planet being aligned. With knowledge of the stellar rotation period, we estimated the true 3D obliquity, finding $ψ= (13.2\pm8.2)$ deg. Based on theoretical considerations, the orientation we measure is unaffected by tidal effects, offering a direct diagnostic for understanding the formation path of this planetary system. The orbital characteristics of TOI-5398, with its compact architecture, eccentricity consistent with circular orbits, and hints of orbital alignment, appear more compatible with the disc-driven migration scenario. TOI-5398, with its relative youth (compared with similar compact systems) and exceptional suitability for transmission spectroscopy studies, presents an outstanding opportunity to establish a benchmark for exploring the disc-driven migration model.

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arXiv2024

The GAPS Programme at TNG. LIII. New insights on the peculiar XO-2 system

Planets in binary systems are a fascinating and yet poorly understood phenomenon. Since there are only a few known large-separation systems in which both components host planets, characterizing them is a key target for planetary science. In this paper, we aim to carry out an exhaustive analysis of the interesting XO-2 system, where one component appears to be a system with only one planet, while the other has at least three planets. Over the last 9 years, we have collected 39 spectra of XO-2N and 106 spectra of XO-2S with the High Accuracy Radial velocity Planet Searcher for the Northern emisphere (HARPS-N) in the framework of the Global Architecture of Planetary Systems project, from which we derived precise radial velocity and activity indicator measurements. Additional spectroscopic data from the High Resolution Echelle Spectrometer and from the High Dispersion Spectrograph, and the older HARPS-N data presented in previous papers, have also been used to increase the total time span. We also used photometric data from TESS to search for potential transits that have not been detected yet. For our analysis, we mainly used PyORBIT, an advanced Python tool for the Bayesian analysis of RVs, activity indicators, and light curves. We found evidence for an additional long-period planet around XO-2S and characterized the activity cycle likely responsible for the long-term RV trend noticed for XO-2N. The new candidate is an example of a Jovian analog with $m\sin i \sim 3.7$ M$_J$, $a \sim 5.5$ au, and $e = 0.09$. We also analyzed the stability and detection limits to get some hints about the possible presence of additional planets. Our results show that the planetary system of XO-2S is at least one order of magnitude more massive than that of XO-2N. The implications of these findings for the interpretation of the previously known abundance difference between components are also discussed.

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arXiv2023

The GAPS programme at TNG LII. Spot modeling of V1298 Tau using SpotCCF tool

The intrinsic variability due to the magnetic activity of young active stars is one of the main challenges in detecting and characterising exoplanets. We present a method able to model the stellar photosphere and its surface inhomogeneities (starspots) in young/active and fast-rotating stars, based on the cross-correlation function (CCF) technique, to extract information about the spot configuration of the star. Within the Global Architecture of Planetary Systems (GAPS) Project at the Telescopio Nazionale Galileo, we analysed more than 300 spectra of the young planet-hosting star V1298 Tau provided by HARPS-N high-resolution spectrograph. By applying the SpotCCF model to the CCFs we extracted the spot configuration (latitude, longitude and projected filling factor) of this star, and also provided the new RVs time series of this target. We find that the features identified in the CCF profiles of V1298 Tau are modulated by the stellar rotation, supporting our assumption that they are caused by starspots. The analysis suggests a differential rotation velocity of the star with lower rotation at higher latitudes. Also, we find that SpotCCF provides an improvement in RVs extraction with a significantly lower dispersion with respect to the commonly used pipelines, with consequent mitigation of the stellar activity contribution modulated with stellar rotation. A detection sensitivity test, by the direct injection of a planetary signal into the data, confirmed that the SpotCCF model improves the sensitivity and ability to recover planetary signals. Our method enables the modelling of the stellar photosphere, extracting the spot configuration of young/active and rapidly rotating stars. It also allows for the extraction of optimised RV time series, thereby enhancing our detection capabilities for new exoplanets and advancing our understanding of stellar activity.

astro-ph.SR↗

arXiv2023

The GAPS Programme at TNG L -- TOI-4515 b: An eccentric warm Jupiter orbiting a 1.2 Gyr-old G-star

Context. Different theories have been developed to explain the origins and properties of close-in giant planets, but none of them alone can explain all of the properties of the warm Jupiters (WJs, Porb = 10 - 200 days). One of the most intriguing characteristics of WJs is that they have a wide range of orbital eccentricities, challenging our understanding of their formation and evolution. Aims. The investigation of these systems is crucial in order to put constraints on formation and evolution theories. TESS is providing a significant sample of transiting WJs around stars bright enough to allow spectroscopic follow-up studies. Methods. We carried out a radial velocity (RV) follow-up study of the TESS candidate TOI-4515 b with the high-resolution spectrograph HARPS-N in the context of the GAPS project, the aim of which is to characterize young giant planets, and the TRES and FEROS spectrographs. We then performed a joint analysis of the HARPS-N, TRES, FEROS, and TESS data in order to fully characterize this planetary system. Results. We find that TOI-4515 b orbits a 1.2 Gyr-old G-star, has an orbital period of Pb = 15.266446 +- 0.000013 days, a mass of Mb = 2.01 +- 0.05 MJ, and a radius of Rb = 1.09 +- 0.04 RJ. We also find an eccentricity of e = 0.46 +- 0.01, placing this planet among the WJs with highly eccentric orbits. As no additional companion has been detected, this high eccentricity might be the consequence of past violent scattering events.

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arXiv2023

The GAPS programme at TNG XLIX. TOI-5398, the youngest compact multi-planet system composed of an inner sub-Neptune and an outer warm Saturn

Short-period giant planets are frequently found to be solitary compared to other classes of exoplanets. Small inner companions to giant planets with $P \lesssim$ 15 days are known only in five compact systems: WASP-47, Kepler-730, WASP-132, TOI-1130, and TOI-2000. Here, we report the confirmation of TOI-5398, the youngest compact multi-planet system composed of a hot sub-Neptune (TOI-5398 c, $P_{\rm c}$ = 4.77271 days) orbiting interior to a short-period Saturn (TOI-5398 b, $P_{\rm b}$ = 10.590547 days) planet, both transiting around a 650 $\pm$ 150 Myr G-type star. As part of the GAPS Young Object project, we confirmed and characterised this compact system, measuring the radius and mass of both planets, thus constraining their bulk composition. Using multidimensional Gaussian processes, we simultaneously modelled stellar activity and planetary signals from TESS Sector 48 light curve and our HARPS-N radial velocity time series. We have confirmed the planetary nature of both planets, TOI-5398 b and TOI-5398 c, alongside a precise estimation of stellar parameters. Through the use of astrometric, photometric, and spectroscopic observations, our findings indicate that TOI-5398 is a young, active G dwarf star (650 $\pm$ 150 Myr), with a rotational period of $P_{\rm rot}$ = 7.34 days. The transit photometry and radial velocity measurements enabled us to measure both the radius and mass of planets b, $R_b = 10.30\pm0.40 R_{\oplus}$, $M_b = 58.7\pm5.7 M_{\oplus}$, and c, $R_c = 3.52 \pm 0.19 R_{\oplus}$, $M_c = 11.8\pm4.8 M_{\oplus}$. TESS observed TOI-5398 during sector 48 and no further observations are planned in the current Extended Mission, making our ground-based light curves crucial for ephemeris improvement. With a Transmission Spectroscopy Metric value of around 300, TOI-5398 b is the most amenable warm giant (10 < $P$ < 100 days) for JWST atmospheric characterisation.

astro-ph.EP↗

arXiv2023

Photometric follow-up of the 20 Myr-old multi-planet host star V1298~Tau with CHEOPS and ground-based telescopes

V1298 Tau hosts at least four planets. Since its discovery, this system has been a target of intensive photometric and spectroscopic monitoring. The characterisation of its architecture and planets' fundamental properties turned out to be very challenging so far. The determination of the orbital ephemeris of the outermost planet V1298 Tau $e$ remains an open question. Only two transits have been detected so far by $Kepler/K2$ and TESS, allowing for a grid of reference periods to be tested with new observations, without excluding the possibility of transit timing variations. Observing a third transit would allow to better constrain the orbital period, and would also help determining an accurate radius of V1298 Tau $e$ because the former transits showed different depths. We observed V1298 Tau with the CHEOPS space telescope to search for a third transit of planet $e$ within observing windows that have been selected in order to test three of the shortest predicted orbital periods. We also collected ground-based observations to verify the result found with CHEOPS. We reanalysed $Kepler/K2$ and TESS light curves to test how the results derived from these data are affected by alternative photometric extraction and detrending methods. We report the detection with CHEOPS of a transit that could be attributed to V1298 Tau $e$. If so, that result implies that the orbital period calculated from fitting a linear ephemeris to the three available transits is close to $\sim45$ days. Results from the ground-based follow-up marginally support this possibility. We found that $\textit{i}$) the transit observed by CHEOPS has a longer duration compared to that of the transits observed by $Kepler/K2$ and TESS; $\textit{ii}$) the transit observed by TESS is $>30\%$ deeper than that of $Kepler/K2$ and CHEOPS, and deeper than the measurement previously reported in the literature, according to our reanalysis.

astro-ph.EP↗

arXiv2023

A compact multi-planet system transiting HIP 29442 (TOI-469) discovered by TESS and ESPRESSO. Radial velocities lead to the detection of transits with low signal-to-noise ratio

We followed-up with ESPRESSO the K0V star HIP 29442 (TOI-469), already known to host a validated sub-Neptune companion TOI-469.01. We aim to verify the planetary nature of TOI-469.01. We modelled radial velocity and photometric time series to measure the dynamical mass, radius, and ephemeris, and to characterise the internal structure and composition of TOI-469.01. We confirmed the planetary nature of TOI-469.01. Thanks to ESPRESSO we discovered two additional close-in companions. We also detected their low signal-to-noise transit signals in the TESS light curve. HIP 29442 is a compact multi-planet system, and the three planets have orbital periods $P_{\rm orb, b}=13.63083\pm0.00003$, $P_{\rm orb, c}=3.53796\pm0.00003$, and $P_{\rm orb, d}=6.42975^{+0.00009}_{-0.00010}$ days, and we measured their masses with high precision: $m_{\rm p, b}=9.6\pm0.8~M_{\oplus}$, $m_{\rm p, c}=4.5\pm0.3~M_{\oplus}$, and $m_{\rm p, d}=5.1\pm0.4~M_{\oplus}$. We measured radii and bulk densities of all the planets (the 3$σ$ confidence intervals are shown in parenthesis): $R_{\rm p, b}=3.48^{+0.07 (+0.19)}_{-0.08 (-0.28)} ~R_{\oplus}$ and $ρ_{\rm p, b}=1.3\pm0.2 (0.3) g~cm^{-3}$; $R_{\rm p, c}=1.58^{+0.10 (+0.30)}_{-0.11 (-0.34)}~R_{\oplus}$ and $ρ_{\rm p, c}=6.3^{+1.7 (+6.0)}_{-1.3 (-2.7)} g~cm^{-3}$; $R_{\rm p, d}=1.37\pm0.11^{(+0.32)}_{(-0.43)}~R_{\oplus}$ and $ρ_{\rm p, d}=11.0^{+3.4 (+21.0)}_{-2.4 (-6.3)} g~cm^{-3}$. We used the more conservative 3$σ$ confidence intervals for the radii as input to the interior structure modelling. We find that HIP 29442 $b$ appears as a typical sub-Neptune, likely surrounded by a gas layer of pure H-He with a mass of $0.27^{+0.24}_{-0.17} M_{\oplus}$ and a thickness of $1.4\pm0.5 R_{\oplus}$. For the innermost companions HIP 29442 $c$ HIP 29442 $d$, the model supports an Earth-like composition.

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