arXiv subjects

L. Fossati

Publications and source records attributed to L. Fossati.

At least 19 recordsLinked to original sources

arXiv2026

The GAPS Programme at TNG LXXXI. The puzzle of the HeI absorption signal in the atmosphere of V1298 Tau b with GIARPS@TNG

Atmospheric escape shapes close-in planets mostly in their first tens of Myr. The young V1298Tau system (10-30Myr) is a benchmark for investigating photoevaporation and planetary evolution at early stages. The metastable HeI near-infrared triplet is a key proxy for atmospheric escape. We aim to detect and characterise HeI in the extended atmosphere of V1298Taub. We analysed four transits over a 4-year baseline using GIARPS (GIANO-B + HARPS-N) at the Telescopio Nazionale Galileo. We re-derived the projected spin-orbit angle by modelling the Rossiter-McLaughlin effect. Using simultaneous optical and near-infrared high-resolution transmission spectroscopy of V1298Taub, we monitored the HeI triplet alongside key chromospheric indicators ($\log R'_{HK}$, H$α$, and NaI) to disentangle stellar activity from planetary evaporation. While the stellar spectra show significant night-to-night variability, transmission spectroscopy reveals persistent HeI absorption across all four epochs. This contrasts with H$α$ and NaI, observed over two nights with simultaneous HARPS-N monitoring, which fluctuate between emission and absorption due to stellar activity. The consistent presence of HeI over 4 years may suggest a planetary origin, despite stellar-driven depth modulation. ATES hydrodynamic simulations show that stellar XUV flux variations modulate the HeI absorption depth and equivalent width. Furthermore, stellar wind fluctuations likely drive the observed night-to-night variability of the exospheric structure. The detection of HeI across all epochs may support planetary evaporation, despite modulation by stellar activity. Our results underscore that disentangling a planetary signal from stellar contamination remains challenging for highly active hosts; securing the planetary nature of the HeI signature requires activity monitoring and full transit coverage to establish a robust baseline.

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arXiv2026

WALTzER: Wide-band Atmospheric Laboratory for Transiting Exoplanet Research

Understanding the origin and evolution of planetary atmospheres, and the connection between their lower and upper atmospheric layers, remains a fundamental challenge in exoplanet science. Addressing these questions requires observations spanning a broad wavelength range, from the ultraviolet to the near-infrared, together with the ability to simultaneously characterise planetary and stellar variability. The Wide-band Atmospheric Laboratory for Transiting Exoplanet Research (WALTzER) is an ESA F3 mission candidate designed to address this need through simultaneous low-resolution spectroscopy and high-precision photometry of transiting exoplanets. WALTzER covers simultaneously the near-ultraviolet (NUV: 240--325 nm) and visible (VIS: 450--820 nm) bands with two medium resolution spectrographs and the near-infrared (NIR: 900--1700 nm) range with a broadband photometer. We outline the main science drivers and present the mission concept and instrument design. WALTzER's broad, simultaneous wavelength coverage enables the detection and characterisation of the atmospheres of more than 30 gas-rich exoplanets and provides data relevant for a broad range of Solar System and Galactic astrophysics science cases. The payload consists of a 35 cm Cassegrain telescope feeding three co-aligned channels (i.e. NUV, VIS, NIR), sharing a common optical train to minimise instrumental systematics, while maximising observational efficiency.

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arXiv2026

The GAPS programme at TNG LXXXII. The first atmospheric investigation of the gas giant WASP-13b at high resolution with GIARPS

Low-density transiting gas giant exoplanets are key benchmarks for atmospheric studies. Despite their large-scale heights, many of them remain poorly characterised. We present the first atmospheric investigation at high-resolution of the inflated (R$_\mathrm{p}$$\sim$1.4 R$_{\mathrm{J}}$), hot Jupiter (T$\sim$1500$ \mathrm{K}$) WASP-13 b aiming at constraining its atmospheric composition and refine its orbital geometry. We analysed four transits observed with GIARPS at the TNG. Updated stellar and planetary parameters were re-derived from the most precise literature values, and a new Rossiter-McLaughlin analysis was performed to measure the projected spin-orbit angle ($λ$) and stellar rotational velocity ($v\sin{i_\star}$). Atmospheric signals were searched using high-resolution spectroscopy, both with cross-correlation and single-line analyses, targeting key atomic and molecular species (e.g. \ion{He}{I}, H$α$, \ion{Na}{I}, H$_2$O, CO). We find a well-aligned orbit ($λ$ = 2.7$\pm$1.1$^\circ$) and $v\sin{i_\star}$ = 4.5$\pm$0.1 km s$^{-1}$, improving upon previous constraints. No significant atmospheric absorption is detected in either VIS or NIR data. Injection-and-recovery tests exclude an isothermal atmosphere with metallicity [M/H]$\lesssim$10$\times$[M/H]$_\odot$ and simultaneously a grey cloud top pressure P$_{\rm cloud}\gtrsim10$ mbar, under the assumption of chemical equilibrium. WASP-13 b appears to belong to the class of low-density hot Jupiters with weak or featureless transmission spectra, most likely shaped by the presence of high-altitude clouds. Although a highly metal-enriched atmosphere cannot be fully excluded, current evidence disfavours this scenario. Further observations with next-generation facilities will be essential to characterise the atmospheric properties of this challenging target.

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arXiv2026

Searching for hot water world candidates with CHEOPS: II. A low-density super-Earth orbiting the K dwarf TOI-2211

Small exoplanets on close-in orbits are exposed to high levels of stellar irradiation, which can effectively remove low-mass primordial H/He envelopes through atmospheric escape. Under these conditions, planets whose masses and radii are incompatible with both bare rocky interiors and extended H-dominated atmospheres are therefore expected to host high mean molecular weight atmospheres, making them promising candidates for volatile-rich hot water worlds. We present the first detailed characterisation of the low-density super-Earth TOI-2211 b (P~3.09 d), a recently validated transiting planet orbiting a K type star (G mag: 9.1). Combining TESS and CHEOPS photometry with radial velocity measurements from HARPS, ESPRESSO, and KPF, we precisely constrain the planet's radius and mass to $1.457 \pm 0.047$ R$_\oplus$ and $2.69 \pm 0.56$ M$_\oplus$. We also identify an outer, non-transiting planetary candidate (P~15.6 d, Msin(i) $ = 8.53 \pm 1.04$ M$_\oplus$) in the radial velocity data. We investigate the internal structure of the transiting planet and assess the stability of the inferred volatile envelopes through a detailed evaporation analysis, thereby identifying TOI-2211 b as a promising hot water world candidate. TOI-2211 b therefore joins the small but growing sample of these close-in low-density super-Earths, which represent particularly interesting targets for testing current planet formation and evolution theories. Additionally, we investigate the full architecture of the planetary system using a generative model of planet formation and evolution, with the resulting posterior distributions showing a clear over-density consistent with the observed orbital period and minimal mass of the non-transiting planetary candidate identified in the radial velocity data.

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arXiv2026

The GAPS programme at TNG LXXIX. New mass constraints for the infant planets in the V1298 Tau system through a long-term radial velocity monitoring with HARPS-N

We aim to constrain the dynamical masses of the four confirmed transiting planets orbiting the infant ($20\pm10$ Myr) star V1298,Tau (orbital periods $P_b\sim$24.1 d, $P_c\sim$8.2 d, $P_d\sim$12.4 d, and $P_e\sim$48.7 d), through a new analysis of a homogeneous radial velocity dataset. This system is a key benchmark for testing formation and early-stage evolutionary models of planets. We analysed more than 380 high-resolution spectra collected with the HARPS-N spectrograph over a time span of 4.5,yr. To account for dominant stellar activity and transit-timing-derived planet-planet interactions, we analysed radial velocities extracted using two different approaches, and combined N-body simulations with multi-dimensional Gaussian process regression. We tested a set of spectroscopic and photometric activity diagnostics to mitigate the variability due to the stellar magnetic activity in the radial velocity time series. The mass determinations are sensitive to the choice of the radial velocity extraction method and activity diagnostics. For some specific models, we measured masses with a significance of 3.5--4$σ$ for planets b ($0.52^{+0.12}{-0.14}$, $0.65^{+0.21}{-0.18}$, $0.67\pm0.16$, and $0.78^{+0.19}{-0.20}$ $M_{\rm Jup}$) and e (0.40$^{+0.14}{-0.13}$, 0.66$^{+0.17}{-0.18}$, and $0.71\pm0.20$ $M_{\rm Jup}$). These values are compatible with masses of giant planets, and are notably higher than those recently derived from transit timing variations alone, which indicate that these are low-density ``super-puffy'' planets. Injection-and-recovery simulations moderately support this conclusion for planet b. For planets c and d, we primarily provide updated mass upper limits. Our analysis represents an exploratory study to address the complex issue of mass measurement in a multi-planetary system characterised by high levels of stellar activity.

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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 Hot Neptune Initiative (HONEI) III. An ultra-hot super Neptune orbiting the metal-rich subgiant star TOI-5646: the largest host for a desert dweller

We report the confirmation of the TESS transiting-planet candidate orbiting the metal-rich F8\,IV-V subgiant star TOI-5646 (V=11.38 mag; T=6136 K). The planetary nature of TOI-5646b was confirmed by means of HARPS-N follow-up radial velocity measurements. We determined that the planet has an orbital period of 2.4277 days and an orbital eccentricity compatible with zero. With a mass of 45.9 Earth masses, a radius of 6.6 Earth radii, and an equilibrium temperature of 1935 K, it is one of the hottest super-Neptune exoplanets ever discovered in the Neptune desert. Its density, 0.87 g cm^-3, is lower than that of most other desert dwellers, but still appears to follow the trend that the desert is populated by dense planets than those in the ridge and savanna. The metallicity of the parent star ([Fe/H]=0.32) follows the trend whereby the host stars of Neptune-sized exoplanets in the desert and the ridge are generally richer in metal than those in the savanna. The log{g} of the parent star (4.09) suggests that the host is an evolved star, making this a peculiar system: TOI-5646 is the largest star (1.8 Solar radii) ever discovered to host a Neptune-sized planet in the desert. Internal structure modelling revealed that TOI-5646b is highly enriched in metals, with an inferred bulk metallicity between 0.7 and 0.95. Atmospheric evolution simulations indicate that the planet has undergone significant mass loss (6-16%) through intense XUV irradiation from its host star. These results indicate a peculiar formation history (potentially involving early protoplanetary disk dispersal), a planetary collision, or a late tidal destruction event, to explain its current state as a low-density survivor in the Neptune desert.

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arXiv2026

Three new exoplanet systems from the Dispersed Matter Planet Project

We present a radial velocity analysis of three bright, low-activity stars identified by the Dispersed Matter Planet Project (DMPP). We use a Bayesian framework to compare purely Keplerian models with models incorporating stellar activity via a quasi-periodic Gaussian Process (GP). DMPP-7 (HD 118006) is a slightly evolved star that harbours a single 0.72 Saturn-mass giant ($m_\textrm{p}\sin i$ = 69 M$_\oplus$) with an orbital period of P = 4.93 d. A longer 21 d - 22 d period cannot be conclusively confirmed as a stellar rotation signature rather than a purely Keplerian signal. For HD 67200, which exhibits Ca ii H&K variability, a model with only a GP is strongly favoured over a purely dynamical model. The GP model shows moderate evidence for a single Keplerian with P = 2.67 d. For HD 2134, a 21 d - 32 d rotation period signal is associated with tentative FWHM variability. A model with a GP is not conclusively favoured, but all models considered show moderate evidence for an additional single Keplerian with P = 2.78 d. Despite our target selection favouring near edge-on orbital geometries, we find no evidence for transits in TESS photometry. DMPP-7 b lies at the transition between the high-radius population and the Neptunian ridge and savannah regions. Further observations are required to establish whether the coherent short-period HD 67200 and HD 2134 signals are stellar or dynamical in origin. If planetary, the signals correspond to minimum masses of $m_\textrm{p}\sin i$ = 2.07 M$_\oplus$ and $m_\textrm{p}\sin i$ = 2.86 M$_\oplus$

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arXiv2026

CHEOPS photometry from 2024 reveals a reversal in the transit-timing variations of AU Mic c

We present new CHEOPS transit observations of AU Mic b and AU Mic c obtained between June and September 2024, extending the baseline of transit-timing measurements of this young planetary system. For AU Mic b, the timing signal is well established, with a semi-amplitude (10 $\pm$ 3 min) and a characteristic modulation timescale (1168 $\pm$ 20 d) consistent with previous determinations. By contrast, the new CHEOPS data show that the large transit-timing deviation of AU Mic c reported previously was not sustained. After the steadily increasing timing trend observed in 2022 and 2023, the 2024 timings returned closer to the zero point of the observed-minus-calculated diagram, indicating a reversal of the previously reported behavior. For AU Mic c, both the transit-timing semi-amplitude (46 $\pm$ 26 min) and the characteristic modulation timescale (2150 $\pm$ 110 d) remain tentative. These results highlight the importance of continued long-term monitoring of the AU Mic system.

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arXiv2026

The multi-planet system TOI-5624: Four transiting sub-Neptunes with an outer companion revealed by transit-timing variations

Following the 2022 alert of a TESS object of interest transiting TOI-5624 (a G7 V star $\sim$100 pc away), a CHEOPS campaign in 2023 detected four planetary signals at $P_b\approx3.4$, $P_c\approx7.9$, $P_d\approx13.7$, and $P_e\approx21.5$ days, later confirmed by additional TESS and CHEOPS photometry in 2024-2025. After analysing the TESS & CHEOPS photometric data, we extracted and modelled the HARPS-N & SOPHIE RV time series using two independent methodologies both within an MCMC framework. We further integrated the N-body equations of motion, while simultaneously fitting the transit times and the detrended RVs, to dynamically characterise the system. We present the discovery of four transiting sub-Neptunes with radii of $R_b=2.314\pm0.035 R_{\oplus}$, $R_c=2.474\pm0.042 R_{\oplus}$, $R_d=3.584_{-0.050}^{+0.051} R_{\oplus}$, and $R_e=3.247_{-0.043}^{+0.042} R_{\oplus}$ and masses of $M_b=9.4\pm1.4 M_{\oplus}$, $M_c=4.8\pm1.9 M_{\oplus}$, $M_d=4.9\pm2.2 M_{\oplus}$, and $M_e=8.9_{-3.0}^{+2.9} M_{\oplus}$. Our photometric analysis reveals that the outermost transiting planet TOI-5624 e shows significant TTVs. We find a robust Keplerian signal in the RV time series close to the 2:1 period commensurability with TOI-5624 e, which explains the TTV pattern exhibited by TOI-5624 e according to our dynamical analysis. We label this non-transiting planet as TOI-5624 f and find its minimum mass to be $M_f\sin{i_f}=13.0\pm3.7 M_{\oplus}$. Among the known systems hosting more than four planets, the remarkable precision with which the radii have been measured (<1.7%) and the firm assessment (>3$σ$) of the mass for at least three planets has been previously reached only for TRAPPIST-1. Additional photometric observations will enable a better sample of the TTV modulation and a more robust dynamical determination of the masses.

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arXiv2026

Discovery and characterisation of two exoplanets orbiting the metal-poor, solar-type star TOI-5788 with TESS, CHEOPS, and HARPS-N

We present the discovery and characterisation of two transiting exoplanets orbiting the metal-poor, solar-type star TOI-5788. From our analysis of six \textit{TESS} sectors and a dedicated \textit{CHEOPS} programme, we identify an inner planet (TOI-5788~b; $P = 6.340758\pm0.000030\,\si{\day}$) with radius $1.528\pm0.075\,\mathrm{R_\oplus}$ and an outer planet (TOI-5788~c; $P = 16.213362\pm0.000026\,\si{\day}$) with radius $2.272\pm0.039\,\mathrm{R_\oplus}$. We obtained 125 radial-velocity spectra from HARPS-N and constrain the masses of TOI-5788~b and~c as $3.72\pm0.94\,\mathrm{M_\oplus}$ and $6.4\pm1.2\,\mathrm{M_\oplus}$, respectively. Although dynamical analyses indicate that a third planet could exist in a stable orbit between 8 and 14 days, we find no evidence of additional planets. Since the TOI-5788 system is one of the few systems with planets straddling the radius gap, and noting that there are even fewer such systems around metal poor stars, it is a promising system to constrain planet formation theories. We therefore model the interior structures of both planets. We find that TOI-5788~b is consistent with being a rocky planet with almost no envelope, or having an atmosphere of a high mean molecular weight. We find that TOI-5788~c is consistent with both gas-dwarf and water-world hypotheses of mini-Neptune formation. We model the atmospheric evolution history of both planets. Whilst both scenarios are consistent with the atmospheric evolution of TOI-5788~c, the gas-dwarf model is marginally preferred. The results of the atmospheric evolution analysis are not strongly dependent on stellar evolution. This makes the system a promising target to test internal structure and atmospheric evolution models.

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arXiv2026

Investigating Circumstellar Atomic Radiation-driven Dynamics

The interactions between stars and their orbiting planets, driven by forces such as stellar radiation and gravity, play an essential role in shaping exoplanetary atmospheres and gas-rich debris discs. One way to look into the composition of these environments is to observe how they can contaminate the stellar photospheres. For that, we examine how stellar radiation pressure and gravity influence atomic species and analyse their effects across various stellar effective temperatures. Using the radiative-to-gravitational force ratio, we determined the atomic movement direction and assessed the velocity boost imparted to neutral atoms escaping from exoplanet atmospheres or debris discs. Incorporating the solar far ultraviolet/extreme ultraviolet spectrum to address flux discrepancies in the {\sc{atlas9}} model, we find that radiation affects atoms differently according to their ionisation state, with highly ionised species less affected by stellar radiation. Our results conclude that the stars most suitable for observing stellar contamination are those between 6,500 and 8,000 K, with neutral noble gases and ionised iron-peak elements as the most likely contaminants.

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arXiv2025

Giant Outer Transiting Exoplanet Mass (GOTEM) Survey.VII. TOI-6041: a multi-planet system including a warm Neptune exhibiting strong TTVs

We present the characterization of the TOI-6041 system, a bright ($V = 9.84 \pm 0.03$) G7-type star hosting at least two planets. The inner planet, TOI-6041b, is a warm Neptune with a radius of $4.55^{+0.18}_{-0.17}\,R_\oplus$, initially identified as a single-transit event in \textit{TESS} photometry. Subsequent observations with \textit{TESS} and \textit{CHEOPS} revealed additional transits, enabling the determination of its $26.04945^{+0.00033}_{-0.00034}$~d orbital period and the detection of significant transit timing variations (TTVs), exhibiting a peak-to-peak amplitude of about 1~hour. Radial velocity (RV) measurements obtained with the APF spectrographs allow us to place a $3σ$ upper mass limit of $28.9\,M_\oplus$ on TOI-6041b. In addition, the RV data reveal a second companion, TOI-6041c, on an 88~d orbit, with a minimum mass of $0.25\,M_{\mathrm{Jup}}$. A preliminary TTV analysis suggests that the observed variations could be caused by gravitational perturbations from planet c; however, reproducing the observed amplitudes requires a relatively high eccentricity of about 0.3 for planet c. Our dynamical stability analysis indicates that such a configuration is dynamically viable and places a $1σ$ upper limit on the mass of TOI-6041c at $0.8\,M_{\mathrm{Jup}}$. An alternative is the presence of a third, low-mass planet located between planets b and c, or on an inner orbit relative to planet b -- particularly near a mean-motion resonance with planet b -- which could account for the observed variations. These findings remain tentative, and further RV and photometric observations are essential to better constrain the mass of planet b and to refine the TTV modeling, thereby improving our understanding of the system's dynamical architecture.

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arXiv2025

HARPS-N, TESS, and CHEOPS discover a transiting sub-Neptune and two outer companions around the bright solar analogue HD 85426

We provide a detailed characterisation of the planetary system orbiting HD 85426 (TOI-1774). This bright G-type star ($M_{\ast}$: 0.99 $\text{M}_{\odot}$; $R_{\ast}$: 1.13 $\text{R}_{\odot}$; age: 7.4 Gyr; V mag: 8.25) hosts a transiting sub-Neptune, HD 85426 b, with an orbital period of 16.71 days and a blackbody equilibrium temperature of $824^{+11}_{-11}$ K. By jointly analysing HARPS-N RVs, TESS, and CHEOPS photometric data and using two different stellar activity mitigation techniques, we constrain planet b's mass to $6.0^{+1.5}_{-1.6}$ $\text{M}_{\oplus}$ and $8.5^{+1.3}_{-1.4} $ $\text{M}_{\oplus}$, depending on the mitigation technique. We investigate the dependence of these results on the priors, data selection, and inclusion of other Keplerians in the modelling. Using this approach, we identify the presence of two non-transiting planetary companions with minimum masses near 10 $\text{M}_{\oplus}$ and orbital periods of 35.7 and 89 days. Additionally, we reject the initial hypothesis that the 35.7-day periodic signal was due to stellar activity. We also determine HD 85426 b's radius to be $2.78^{+0.05}_{-0.04}$ $\text{R}_{\oplus}$ and compute a transmission spectroscopy metric in the range of 82 to 115, making this planet a highly valuable target for atmospheric characterisation.

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arXiv2025

Mass determination of the three long-period Neptune- and sub-Neptune-sized planets transiting TOI-282

TOI-282 is a bright (V=9.38) F8 main-sequence star known to host three transiting long-period ($P_b$=22.9 d, $P_c$=56.0 d, and $P_d$=84.3 d) small ($R_p\approx$ 2-4 $R_{\oplus}$) planets. The orbital period ratio of the two outermost planets, namely TOI-282 c and d, is close to the 3:2 commensurability, suggesting that the planets might be trapped in a mean motion resonance. We combined space-borne photometry from the TESS telescope with high-precision HARPS and ESPRESSO Doppler measurements to refine orbital parameters, measure the planetary masses, and investigate the architecture and evolution of the system. We performed a Markov chain Monte Carlo joint analysis of the transit light curves and radial velocity time series, and carried out a dynamical analysis to model transit timing variations and Doppler measurements along with N-body integration. In agreement with previous results, we found that TOI-282 b, c, and d have radii of $R_b=2.69 \pm 0.23 \ R_{\oplus}$, $R_c=4.13^{+0.16}_{-0.14} \ R_{\oplus}$, and $R_d=3.11 \pm 0.15 \ R_{\oplus}$, respectively. We measured planetary masses of $M_b=6.2\pm1.6 \ M_{\oplus}$, $M_c=9.2\pm2.0 \ M_{\oplus}$, and $M_d=5.8^{+0.9}_{-1.1} \ M_{\oplus}$, which imply mean densities of $ρ_b=1.8^{+0.7}_{-0.6} \ \text{g cm}^{-3}$, $ρ_c=0.7 \pm 0.2 \ \text{g cm}^{-3}$, and $ρ_d=1.1^{+0.3}_{-0.2} \ \text{g cm}^{-3}$, respectively. The three planets may be water worlds, making TOI-282 an interesting system for future atmospheric follow-up observations with JWST and ELT.

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arXiv2025

A scaling relationship for non-thermal radio emission from ordered magnetospheres - II. Investigating the efficiency of relativistic electron production in magnetospheres of BA-type stars

Magnetic BA stars host dipole-like magnetospheres. When detected as radio sources, their luminosities correlate with the magnetic field and rotation. Rotation is crucial because the mechanism undergirding the relativistic electron production is powered by centrifugal breakouts. CBOs occur wherever magnetic tension does not balance centrifugal force; the resulting magnetic reconnection provides particle acceleration. To investigate how physical conditions at the site of the CBOs affect the efficiency of the acceleration mechanism, we broadly explore the parameter space governing radio emission by increasing the sample of radio-loud magnetic stars. High-sensitivity VLA observations of 32 stars were performed in the hope of identifying new centrifugal magnetospheres and associated CBOs. We calculated gyro-synchrotron spectra using 3D modeling of a dipole-shaped magnetosphere. We evaluated combinations of parameters. The number of relativistic electrons was constrained by the need to produce the emission level predicted by the scaling relationship for the radio emission from magnetic BA stars. About half of the observed stars were detected, with luminosities in agreement with the expected values, reinforcing the robust nature of the scaling relationship for CBO-powered radio emission. Comparing the competing centrifugal and magnetic effects on plasma locked in a rigidly rotating magnetosphere, we located the site of CBOs and inferred the local plasma density. We then estimated the efficiency of the acceleration mechanism needed to produce enough non-thermal electrons to support the radio emission level. Given a constant acceleration efficiency, relativistic electrons represent a fixed fraction of the local thermal plasma. Thus, dense magnetospheres host more energetic particles than less dense ones; consequently, with other parameters similar, they are intrinsically brighter radio sources.

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arXiv2025

Transit Timing Variations in HIP 41378: CHEOPS and TESS confirm a non-transiting sixth planet in the system

In multiple-planet systems, gravitational interactions of exoplanets could lead to transit timing variations (TTVs), whose amplitude becomes significantly enhanced when planets are in or near mean-motion resonances (MMRs). In cases where both TTVs and radial velocity (RV) measurements are available, combined analysis can break degeneracies and provide robust planetary and system characterization, even detecting non-transiting planets. In this context, HIP 41378 hosts five confirmed transiting planets with periods ranging from 15 to over 542 days, providing a unique dynamical laboratory for investigating wide multi-planet systems analogous to the Solar System. In this study, we present an intensive space-based photometric follow-up of HIP 41378, combining 15 new CHEOPS observations with eight TESS sectors, alongside data from K2, Spitzer, HST, and HARPS. We dynamically modeled the TTVs and RV signals of the two inner sub-Neptunes via N-body integration. These planets, HIP 41378 b ($P_{b}$ = 15.57 days) and HIP 41378 c ($P_{c}$ = 31.71 days), are close to ($Δ\sim1.8$ %) a 2:1 period commensurability. We report a clear detection of TTVs with amplitudes of 20 mins for planet b and greater than 3 hrs for planet c. We dynamically confirm the planetary nature of HIP 41378 g, a non-transiting planet with a period of about 64 days and a mass of about 7 $M_{\oplus}$, close to a 2:1 commensurability with planet c, suggesting a possible MMR chain in the inner system. Our precise determination of the masses, eccentricities, and radii of HIP 41378 b and c enabled us to investigate their possible volatile-rich compositions. Finally, by leveraging on the last TESS sectors we constrained the period of HIP 41378 d to three possible aliases ($P_{d} =$ 278, 371, and 1113 days) suggesting that the system could be placed in a double quasi resonant chain, highlighting its complex dynamical architecture.

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arXiv2025

A four-planet system orbiting the old thick disk star TOI-1203

TOI-1203 is a bright (V=8.6) G3 V star known to host a transiting warm sub-Neptune on a 25.5 d orbit. Here we report on an intensive high-precision radial velocity and photometric follow-up campaign carried out with the HARPS spectrograph and the CHEOPS space telescope. We found that TOI-1203 has an enhancement of $α$ elements relative to iron of [$α$/Fe]=$0.21\pm0.04$. With an age of $\sim$12.5 Gyr, TOI-1203 belongs to the old, $α$-element enhanced stellar population of the galactic thick disk. We spectroscopically confirmed the planetary nature of the 25.5 d sub-Neptune TOI-1203 d, measured its mass ($M_{d}=7.39\pm0.62~M_{\oplus}$) and refined its radius ($R_{d}=2.918_{-0.045}^{+0.046}~R_{\oplus}$). We discovered the presence of an additional transiting super-Earth on a 4.2 d orbit (TOI-1203 b) with a mass of $M_{b}=3.51_{-0.32}^{+0.33}~M_{\oplus}$ and a radius of $R_{b}=1.520_{-0.046}^{+0.045}~R_{\oplus}$. We also revealed the presence of two additional low-mass planets at 13.1 d and 204.6 d (TOI-1203 c and e), with minimum masses of $5.46_{-0.50}^{+0.51}~M_{\oplus}$ and $42.10_{-1.78}^{+1.83}~M_{\oplus}$. We found that the outer planet TOI-1203 e lies on an eccentric orbit with $e_{e}=0.152\pm0.029$. We performed a stability analysis of the system confirming that there are configurations consistent with the observed parameters that are dynamically stable over billion-year timescales. While analyzing the HARPS time series, we discovered that the FWHM of the HARPS cross-correlation function shows a significant long-period signal ($\sim$615 d) that has no counterpart in the radial velocity data or in the remaining HARPS ancillary time series. We significantly detected the same signal in the FWHM of the Th-Ar calibration lines used to compute the nightly wavelength solution, and attributed this systematic effect to a long-term variation of the HARPS instrumental profile.

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