arXiv subjects

G. Guilluy

Publications and source records attributed to G. Guilluy.

At least 19 recordsLinked to original sources

arXiv2026

The GAPS programme at TNG LXXVIII. Phase-resolved detection of multiple atomic species in the atmosphere of KELT-20b/MASCARA-2b

Among the different classes of exoplanets, ultra-hot Jupiters represent prime laboratories for atmospheric characterisation. With an equilibrium temperature of ~2300 K, KELT-20b/MASCARA-2b (HD 185603 b) is a well-studied ultra-hot Jupiter, for which several atomic species have already been detected in both transmission and emission spectroscopy. Nevertheless, important open questions remain regarding the completeness of the chemical inventory and the interpretation of spurious aliasing signals that may mimic genuine atmospheric signatures. In addition, asymmetries in the detected signals across transits can encode valuable information on the spatial structure and dynamics of the planetary atmosphere. In this work, we analyse seven transits of KELT-20b/MASCARA-2b observed with the high-resolution spectrograph HARPS-N at the Telescopio Nazionale Galileo. We apply the cross-correlation function technique to a large set of atomic and ionic species, accounting for aliasing effects that can lead to false detections. We confirm previous detections of HI, NaI, MgI, CrI, FeI, and FeII, and tentatively detect CrII. We further report new detections of KI, CaI, VI, MnI, and BaII, and a tentative detection of PrI in the planetary atmosphere. By performing a phase-resolved analysis and splitting the transit into pre- and post-mid-transit phases, we investigate longitudinal variations in the detected signals. For several species, we observe a double-peaked structure in the Kp-Vsys maps, notably in CaI, VI, CrI, and FeI, consistent with absorption arising from two distinct atmospheric limbs of the planet.

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arXiv2026

The GAPS programme at TNG LXXV. Validating and confirming Gaia substellar astrometric candidates with HARPS-N

The astrometric measurements provided by the Gaia space mission represent a key advancement in the search and characterization of exoplanets, helping in particular to solve the mass degeneracy intrinsic to the radial velocity (RV) method. The fact that a fraction of astrophysical false positives contaminates the current catalog of astrometric candidate solutions requires an RV follow-up to validate and confirm such candidates. Within the GAPS programme, we have observed a selected sample of 14 stars having Gaia astrometric solutions compatible with the presence of a substellar companion. The immediate aim of this survey is to identify astrophysical false positives and provide the first RV validation and confirmation of the remaining candidates. We analysed data collected with the HARPS-N spectrograph to identify stellar binary systems from the spectral cross-correlation function profiles. The remaining astrometric candidates were characterized via Markov chain Monte Carlo analysis searching for the best-fit RV solution. Among the stars in our sample with astrometric candidate solutions, we identify 6 as originating from close binary companions mimicking the astrometric motion of distant substellar companions, from which we can estimate an updated value of $43_{-11}^{+13}\%$ for the binary contamination fraction in the Gaia DR3 catalog of astrometric candidates. We validate and confirm the remaining 8 solutions, corresponding to giant and brown dwarf companions with minimum masses between 8 and 62 $M_{\rm Jup}$ and semimajor axes between 0.76 and 1.42 au, providing the first RV characterization for 6 of these candidates and updated orbital solutions for 2 previously confirmed ones.

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arXiv2026

A tidally detached super Neptune on a strongly misaligned retrograde orbit

The obliquity between a planet's orbital axis and its host star's spin axis provides crucial insights into planetary formation and migration. Planets with scaled semi-major axes ($a/R_\star$) large enough to be unaffected by tidal alterations ("tidally detached"), offer a unique opportunity to study the original obliquity in which the system formed. We therefore observed TOI-1710 b ($a/R_\star \approx 36$) in-transit using HARPS-N + GIANO-B, collecting high-precision radial velocities to measure the Rossiter-McLaughlin (RM) effect. Spectral analysis of the H$\alpha$ and HeI triple lines was also pursued to evaluate atmospheric photoevaporation. Using our knowledge of the star rotation period ($21.5 \pm 0.2$ d), we estimated a true obliquity of $\psi = 149 ^{+11}_{-10}$ deg, which indicates a retrograde motion and places TOI-1710 b among the most misaligned systems -- and the only one known orbiting a cool star in retrograde motion. The strong misalignment favours a high-eccentricity migration (HEM) origin for this low-density super-Neptune planet in the savanna region, challenging previous findings that claimed a minor role of HEM in this period-radius(-density) domain. Moreover, the strong misalignment and lack of a detected close stellar companion suggests a purely planetary post-migration misalignment, likely due to planet-planet scattering followed by planet-planet Kozai-Lidov oscillations and tidal circularisation.

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arXiv2025

The GAPS Programme at TNG. LXIX.The Dayside of WASP-76b revealed by GIANO-B, HARPS-N and ESPRESSO: Evidence for Three-Dimensional Atmospheric Effects

The study of the atmosphere of ultra-hot Jupiters (UHJs) with equilibrium temperature $\geq$2000 K provides valuable insights into atmospheric physics under such extreme conditions. We aim to characterise the dayside thermal spectrum of the UHJ WASP-76b and investigate its properties. We analysed data gathered with three high-resolution spectrographs, specifically two nights with simultaneous observations of HARPS-N and GIANO-B, and four nights of publicly available ESPRESSO optical spectra. We observed the planet's dayside covering orbital phases between quadratures (0.25 < $\phi$ < 0.75). We performed a homogeneous analysis of the GIANO-B, HARPS-N and ESPRESSO data and co-added the signal of thousands of planetary lines through cross-correlation with simulated spectra of the planetary atmosphere. We report the detection of CO in the dayside atmosphere of WASP-76b with a signal-to-noise ratio (S/N) of 10.4 in the GIANO-B spectra. In addition, we detect Fe I in both the HARPS-N and ESPRESSO datasets, with S/N of 3.5 and 6.2, respectively. A signal from Fe I is also identified in one of the two GIANO-B observations, with a S/N of 4.0. Interestingly, a qualitatively similar pattern - with a weaker detection in one epoch compared to the other - is also observed in the two HARPS-N nights. The GIANO-B results are therefore consistent with those obtained with HARPS-N. Finally, we compared our strongest detections of CO (GIANO-B) and Fe I (ESPRESSO), with predictions from Global Circulation Models (GCMs). Both cross-correlation and likelihood analyses favour the GCM that includes atmospheric dynamics over a static (no-dynamics) model when applied to the ESPRESSO data. This study adds to the growing body of literature employing GCMs to interpret high-resolution spectroscopic measurements of exoplanet atmospheres.

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arXiv2025

The GAPS Programme at TNG LXVII. Detection of water and preliminary characterisation of the atmospheres of the two hot Jupiters KELT-8 b and KELT-23 Ab

Expanding the number of hot giant planets with atmospheric characterisation can improve our understanding of their atmospheres as well as their formation and evolution mechanisms. In this work, we use high-resolution spectroscopy in the near-infrared (NIR) to search for chemical signatures in the atmosphere of the two hot Jupiters KELT-8 b and KELT-23 Ab, and perform a first characterisation of their atmospheric properties. We measured the transmission spectrum of each target with the NIR high-resolution spectrograph GIANO-B at the TNG and searched for atmospheric signals by cross-correlating the data with synthetic transmission spectra. In order to characterise the chemical-physical properties of the two atmospheres, we ran two different atmospheric retrievals for each dataset: a retrieval assuming chemical equilibrium and a `free-chemistry' retrieval, in which the abundance of each molecule could vary freely. We detect H$_2$O in the atmospheres of KELT-8 b and KELT-23 Ab with an S/N = 6.6 and S/N = 4.2, respectively. The two retrievals indicate a water-rich atmosphere for both targets. For KELT-8 b, we determine a water volume mixing ratio of log$_{10}$(VMR$_{\rm H_2O})=-2.07^{+0.53}_{-0.72}$, a metallicity of [M/H] $=0.77^{+0.61}_{-0.89}$ dex, and a sub-solar C/O ratio (C/O $\leq0.30$, at $2\,\sigma$). For KELT-23 Ab, we find log$_{10}$(VMR$_{\rm H_2O})=-2.26^{+0.75}_{-1.24}$, [M/H] $=-0.42^{+1.56}_{-1.35}$ dex, and C/O ratio $\leq0.78$ (at $2\,\sigma$). Comparing these chemical properties with those of the host stars, we suggest that, for both planets, the accretion of gaseous material occurred within the H$_2$O snowline in a pebble-rich disk enriched in oxygen due to sublimation of water ice from the inward-drifting pebbles. In conclusion, we measure the atmospheric signals of KELT-8 b and KELT-23 Ab for the first time and place first constraints on their properties.

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arXiv2024

The GAPS programme at TNG LXVI. A homogeneous search for Na i and its possible variability in ten gas giant exoplanets

The neutral sodium resonance doublet (Na i D) has been detected in the upper atmosphere of several close-in gas giants, through high-resolution transmission spectroscopy. We aim to investigate whether its variability is linked to the planets' properties, the data quality, or the accuracy of the system parameters used. Using the public code SLOPpy, we extracted the transmission spectrum in the Na i D region of ten gas giants for which a large number of HARPS-N observations are available. We modelled the absorption signals found, performing an MCMC analysis, and converted the measured absorption depth to the corresponding atmospheric height over which most sodium absorption occurs. While two targets (GJ 436 b and KELT-7 b) show no Na i D feature, we found variability in the transmission spectrum of the other targets. Three of them (HD 209458 b, WASP-80 b, and WASP-127 b) present absorption on only some nights, while in the other five targets (HD 189733 b, KELT-9 b, KELT-20 b, WASP-69 b, and WASP-76 b), a significant absorption signal is present on most of the nights analysed. Except for WASP-69 b, the measured absorption depths lead to a ratio of the two Na I D depths that is compatible with or slightly larger than one. As was expected from literature, the relative atmospheric height follows an empirical exponential trend as a function of a scaled product of the planet's equilibrium temperature and surface gravity. We confirm the sodium detection on HD 189733 b, KELT-9 b, KELT-20 b, WASP-69 b, and WASP-76 b. The signal detected in WASP-127 b requires further observations for definitive confirmation. We exclude a planetary origin for the signals found on HD 209458 b and WASP-80 b. The sodium absorption variability does not appear to be related to planetary properties, but rather to data quality, sub-optimal data treatment, or stellar activity.

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arXiv2024

The GAPS programme at TNG LXIV: An inner eccentric sub-Neptune and an outer sub-Neptune-mass candidate around BD+00 444 (TOI-2443)

We examined in depth the star BD+00 444 (GJ 105.5, TOI-2443; V = 9.5 mag; d = 23.9 pc), with the aim of characterizing and confirming the planetary nature of its small companion, the planet candidate TOI-2443.01, which was discovered by TESS. We monitored BD+00 444 with the HARPS-N spectrograph for 1.5 years to search for planet-induced radial-velocity (RV) variations, and then analyzed the RV measurements jointly with TESS and ground-based photometry. We determined that the host is a quiet K5 V, and we revealed that the sub-Neptune BD+00 444 b has a radius of $R_b=2.36\pm0.05 R_{\oplus}$, a mass of $M_b=4.8\pm1.1 M_{\oplus}$ and, consequently, a rather low-density value of $\rho_b=2.00+0.49-0.45$ g cm-3, which makes it compatible with both an Earth-like rocky interior with a thin H-He atmosphere and a half-rocky, half-water composition with a small amount of H-He. Having an orbital period of about 15.67 days and an equilibrium temperature of about 519 K, BD+00 444 b has an estimated transmission spectroscopy metric of about 159, which makes it ideal for atmospheric follow-up with the JWST. Notably, it is the second most eccentric inner transiting planet, $e=0.302+0.051-0.035$, with a mass below 20 $M_{\oplus}$, among those with well-determined eccentricities. We estimated that tidal forces from the host star affect both planet b's rotation and eccentricity, and strong tidal dissipation may signal intense volcanic activity. Furthermore, our analysis suggests the presence of a sub-Neptune-mass planet candidate, BD+00 444 c, having an orbital period of $P=96.6\pm1.4$ days, and a minimum mass $M\sin{i}=9.3+1.8-2.0 M_{\oplus}$. With an equilibrium temperature of about 283 K, BD+00 444 c is right inside the habitable zone; however, this candidate necessitates further observations and stronger statistical evidence to be confirmed. [...]

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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

The GAPS programme at TNG: TBD. Studies of atmospheric FeII winds in ultra-hot Jupiters KELT-9b and KELT-20b using HARPS-N spectrograph

Ultra-hot Jupiters (UHJs) are gas giant planets orbiting close to their host star, with equilibrium temperatures exceeding 2000 K, and among the most studied planets in terms of their atmospheric composition. Thanks to a new generation of ultra-stable high-resolution spectrographs, it is possible to detect the signal from the individual lines of the species in the exoplanetary atmospheres. We employed two techniques in this study. First, we used transmission spectroscopy, which involved examining the spectra around single lines of FeII. Then we carried out a set of cross-correlation studies for two UHJs: KELT-9b and KELT-20b. Both planets orbit fast-rotating stars, which resulted in the detection of the strong Rossiter-McLaughlin (RM) effect and center-to-limb variations in the transmission spectrum. These effects had to be corrected to ensure a precise analysis. Using the transmission spectroscopy method, we detected 21 single lines of FeII in the atmosphere of KELT-9b. All of the detected lines are blue-shifted, suggesting strong day-to-night side atmospheric winds. The cross-correlation method leads to the detection of the blue-shifted signal with a signal-to-noise ratio (S/N) of 13.46. Our results are in agreement with models based on non-local thermodynamical equilibrium (NLTE) effects, with a mean micro-turbulence of nu_mic = 2.73 +/- 1.5 km/s and macro-turbulence of nu_mac = 8.22 +/- 3.85 km/s. In the atmosphere of KELT-20b, we detected 17 single lines of FeII. Considering different measurements of the systemic velocity of the system, we conclude that the existence of winds in the atmosphere of KELT-20b cannot be determined conclusively. The detected signal with the cross-correlation method presents a S/N of 11.51. The results are consistent with NLTE effects, including means of nu_mic = 3.04 +/- 0.35 km/s and nu_mac = 6.76 +/- 1.17 km/s.

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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$\alpha$ to H$\zeta$, with a significance exceeding 5$\sigma$. 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$\epsilon$ and H$\zeta$ 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: LIV. A HeI survey of close-in giant planets hosted by M-K dwarf stars with GIANO-B

Atmospheric escape plays a fundamental role in shaping the properties of exoplanets. The metastable near-infrared helium triplet at 1083.3 nm (HeI) is a powerful proxy of extended and evaporating atmospheres. We used the GIARPS (GIANO-B+HARPS-N) observing mode of the Telescopio Nazionale Galileo to search for HeI absorption in the upper atmosphere of five close-in giant planets hosted by the K and M dwarf stars of our sample, namely WASP-69b, WASP-107b, HAT-P-11b, GJ436b, and GJ3470b. We focused our analysis on the HeI triplet by performing high-resolution transmission spectroscopy. When nightly variability in the HeI absorption signal was identified, we investigated the potential influence of stellar magnetic activity by searching for variations in the H$α$. We spectrally resolve the HeI triplet and confirm the published detections for WASP-69b (3.91$\pm$0.22%, 17.6$σ$), WASP-107b (8.17$^{+0.80}_{-0.76}$%, 10.5$σ$), HAT-P-11b (1.36$\pm$0.17%, 8.0$σ$), and GJ3470b (1.75$^{+0.39}_{-0.36}$%, 4.7$σ$). We do not find evidence of extra absorption for GJ436b. We observe night-to-night variations in the HeI absorption signal for WASP-69b, associated with variability in H$α$, which likely indicates the influence of stellar activity. Additionally, we find that the HeI signal of GJ3470b originates from a single transit, thereby corroborating the discrepancies in the existing literature. An inspection of the H$α$ reveals an absorption signal during the same transit. By combining our findings with previous analyses of GIANO-B HeI measurements of planets around K dwarfs, we explore potential trends with planetary/stellar parameters that are thought to affect the HeI absorption. Our analysis is unable to identify clear patterns, emphasising the need for further measurements and the exploration of additional potential parameters that might influence HeI absorption.

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arXiv2024

The GAPS Programme at TNG. XXX: Characterization of the low-density gas giant HAT-P-67 b with GIARPS

HAT-P-67 b is one of the lowest-density gas giants known to date, making it an excellent target for atmospheric characterization through the transmission spectroscopy technique. In the framework of the GAPS large programme, we collected four transit events, with the aim of studying the exoplanet atmosphere and deriving the orbital projected obliquity. We exploited the high-precision GIARPS observing mode of the TNG, along with additional archival TESS photometry, to explore the activity level of the host star. We performed transmission spectroscopy, both in the VIS and in the nIR wavelength range, and analysed the RML effect both fitting the RVs and the Doppler shadow. Based on the TESS photometry, we redetermined the transit parameters of HAT-P-67 b. By modelling the RML effect, we derived a sky-projected obliquity of ($2.2\pm0.4$)° indicating an aligned planetary orbit. The chromospheric activity index $\log\,R^{\prime}_{\rm HK}$, the CCF profile, and the variability in the transmission spectrum of the H$α$ line suggest that the host star shows signatures of stellar activity and/or pulsations. We found no evidence of atomic or molecular species in the VIS transmission spectra, with the exception of pseudo-signals corresponding to Cr I, Fe I, H$α$, Na I, and Ti I. In the nIR range, we found an absorption signal of the He I triplet of 5.56$^{+0.29}_{-0.30}$%(19.0$σ$), corresponding to an effective planetary radius of $\sim$3$R_p$ (where $R_p\sim$2$R_J$) which extends beyond the planet's Roche Lobe radius. Owing to the stellar variability, together with the high uncertainty of the model, we could not confirm the planetary origin of the signals found in the optical transmission spectrum. On the other hand, we confirmed previous detections of the infrared He I triplet, providing a 19.0$σ$ detection. Our finding indicates that the planet's atmosphere is evaporating.

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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 LV. Multiple molecular species in the atmosphere of HAT-P-11 b and review of the HAT-P-11 planetary system

The atmospheric characterisation of hot and warm Neptune-size exoplanets is challenging due to their small radius and atmospheric scale height. The warm-Neptune HAT-P-11b is a remarkable target for such characterisation due to the large brightness of its host star (V=9.46 mag; H=7.13 mag). The aims of this work are to review the main physical and architectural properties of the HAT-P-11 planetary system, and to probe the presence of 8 molecular species in the atmosphere of HAT-P-11b at high spectral resolution in the near-infrared. The planetary system was reviewed by analysing transits and occultations of HAT-P-11b from the Kepler data set as well as HIRES at Keck archival radial-velocity (RV) data. We modelled the latter with Gaussian-process regression and a combined quasi-periodic and squared-exponential kernel to account for stellar variations on both (short-term) rotation and (long-term) activity-cycle timescales. In order to probe the atmospheric composition of HAT-P-11b, we observed 4 transits of this target with GIANO-B at TNG. We find that the long-period ($P\sim9.3$ years) RV signal previously attributed to planet HAT-P-11c is more likely due to the stellar magnetic activity cycle. Nonetheless, the Hipparcos-Gaia difference in the proper-motion anomaly suggests that an outer-bound companion might still exist. For HAT-P-11b, we measure a radius $R_{\rm p}=0.4466\pm0.0059\,R_{\rm J}$, a mass $M_{\rm p}=0.0787\pm0.0048\,M_{\rm J}$, and an eccentricity $e=0.2577^{+0.0033}_{-0.0025}$, in accordance with values in the literature. Probing its atmosphere, we detect $NH_3$ (S/N$=5.3$, significance$=5.0σ$) and confirm the presence of $H_2O$ (S/N$=5.1$, significance$=3.4σ$). We also tentatively detect the signal of $CO_2$ (S/N$=3.0$, significance$=3.2σ$) and $CH_4$ (S/N$=4.8$, significance$=2.6σ$), whose presence need to be confirmed by further observations.

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arXiv2023

A super-massive Neptune-sized planet

Neptune-sized planets exhibit a wide range of compositions and densities, depending onf cators related to their formation and evolution history, such as the distance from their host stars and atmospheric escape processes. They can vary from relatively low-density planets with thick hydrogen-helium atmospheres to higher-density planets with a substantial amount of water or a rocky interior with a thinner atmosphere, such as HD 95338 b, TOI-849 b and TOI-2196 b. The discovery of exoplanets in the hot-Neptune desert, a region close to the host stars with a deficit of Neptune-sized planets, provides insights into the formation and evolution of planetary systems, including the existence of this region itself. Here we show observations of the transiting planet TOI-1853 b, which has a radius of 3.46 +- 0.08 Earth radii and orbits a dwarf star every 1.24 days. This planet has a mass of 73.2 +- 2.7 Earth masses, almost twice that of any other Neptune-sized planet known so far, and a density of 9.7 +- 0.8 grams per cubic centimetre. These values place TOI-1853 b in the middle of the Neptunian desert and imply that heavy elements dominate its mass. The properties of TOI-1853 b present a puzzle for conventional theories of planetary formation and evolution, and could be the result of several proto-planet collisions or the final state of an initially high-eccentricity planet that migrated closer to its parent star.

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arXiv2023

DREAM: III.A helium survey in exoplanets on the edge of the hot Neptune desert with GIANO-B@TNG

The population of close-in exoplanets features a desert of hot Neptunes whose origin is uncertain. These planets may have lost their atmosphere, eroding into mini-Neptunes and super-Earths. Direct observations of evaporating atmospheres are essential to derive mass-loss estimates and constrain this scenario. The metastable 1083.3nm HeI triplet represents a powerful diagnostic of atmospheric evaporation since it traces the hot gas in extended exoplanet atmospheres, is observable from the ground, and is weakly affected by interstellar medium absorption. We conducted a uniform HeI transmission spectroscopy survey, focusing on 9 planets located at the edges of the Neptunian desert, aiming to gain insights into the role of photo-evaporation in its formation. We observed one transit per planet using the high-resolution, near-infrared spectrograph GIANO-B on the Telescopio Nazionale Galileo. We focused our analysis on the HeI triplet by computing high-resolution transmission spectra. We then employed the p-winds model to interpret the observed transmission spectra. We found no sign of planetary absorption in the HeI triplet in any of the investigated targets. We thus provided 3sigma upper-limit estimations on the thermosphere absorption, temperature, and mass loss, and combined them with past measurements to search for correlations with parameters thought to be drivers in the formation of the HeI triplet. Our results strengthen the importance of performing homogeneous surveys and analyses to bring clarification in the HeI detection and hence in the Neptunian desert origin. Our findings corroborate the literature expectations that the HeI absorption signal correlates with the stellar mass and the received XUV flux. However, these trends seem to disappear in terms of mass-loss rates; further studies are essential to shed light on this aspect and to understand better the photo-evaporation process.

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arXiv2023

The GAPS programme at TNG. XLV. HI Balmer lines transmission spectroscopy and NLTE atmospheric modelling of the ultra-hot Jupiter KELT-20b/MASCARA-2b

We aim at extracting the transmission spectrum of the HI Balmer lines of the ultra-hot Jupiter (UHJ) KELT-20b/MASCARA-2b from observations and to further compare the results with what obtained through forward modelling accounting for non-local thermodynamic equilibrium (NLTE) effects. We extract the line profiles from six transits obtained with the HARPS-N high-resolution spectrograph attached to the Telescopio Nazionale Galileo telescope. We compute the temperature-pressure (TP) profile employing the helios code in the lower atmosphere and the Cloudy NLTE code in the middle and upper atmosphere. We further use Cloudy to compute the theoretical planetary transmission spectrum in LTE and NLTE for comparison with observations. We detected the Halpha (0.79+/-0.03%; 1.25 Rp), Hbeta (0.52+/-0.03%; 1.17 Rp), and Hgamma (0.39+/-0.06%; 1.13 Rp) lines, while we detected the Hdelta line at almost 4 sigma (0.27+/-0.07%; 1.09 Rp). The models predict an isothermal temperature of about2200 K at pressures >10^-2 bar and of about 7700 K at pressures <10^-8 bar, with a roughly linear temperature rise in between. In the middle and upper atmosphere, the NLTE TP profile is up to about 3000 K hotter than in LTE. The synthetic transmission spectrum derived from the NLTE TP profile is in good agreement with the observed HI Balmer line profiles, validating our obtained atmospheric structure. Instead, the synthetic transmission spectrum derived from the LTE TP profile leads to significantly weaker absorption compared to the observations. Metals appear to be the primary agents leading to the temperature inversion in UHJs and the impact of NLTE effects on them increases the magnitude of the inversion. We find that the impact of NLTE effects on the TP profile of KELT-20b/MASCARA-2b is larger than for the hotter UHJ KELT-9b, and thus NLTE effects might be relevant also for planets cooler than KELT-20b/MASCARA-2b.

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arXiv2023

On the origin of the non-detection of metastable HeI in the upper atmosphere of the hot Jupiter WASP-80b

We aim to narrow down the origin of the non-detection of the metastable HeI triplet at about 10830 A obtained for the hot Jupiter WASP-80b. We measure the X-ray flux of WASP-80 from archival observations and use it as input to scaling relations accounting for the coronal [Fe/O] abundance ratio to infer the extreme-ultraviolet (EUV) flux in the 200-504 A range, which controls the formation of metastable HeI. We run three dimensional (magneto) hydrodynamic simulations of the expanding planetary upper atmosphere interacting with the stellar wind to study the impact on the HeI absorption of the stellar high-energy emission, the He/H abundance ratio, the stellar wind, and the possible presence of a planetary magnetic field up to 1 G. For a low stellar EUV emission, which is favoured by the measured logR'HK value, the HeI non-detection can be explained by a solar He/H abundance ratio in combination with a strong stellar wind, or by a sub-solar He/H abundance ratio, or by a combination of the two. For a high stellar EUV emission, the non-detection implies a sub-solar He/H abundance ratio. A planetary magnetic field is unlikely to be the cause of the non-detection. The low EUV stellar flux, driven by the low [Fe/O] coronal abundance, is the likely primary cause of the HeI non-detection. High-quality EUV spectra of nearby stars are urgently needed to improve the accuracy of high-energy emission estimates, which would then enable one to employ the observations to constrain the planetary He/H abundance ratio and the stellar wind strength. This would greatly enhance the information that can be extracted from HeI atmospheric characterisation observations.

astro-ph.EP↗