Xavier Delfosse

Context. Exoplanetary magnetic fields govern atmospheric escape and circulation but have eluded detection in close-in planets. In the ultra-hot Jupiter KELT-9b, atmospheric dynamo is predicted to generate atmospheric fields of 400−4000 G. Aims. We searched for the circular polarization in atomic lines that a magnetic field would imprint, through the Zeeman effect, on the transmission spectrum of KELT-9b. Methods. We observed three transits with SPIRou at the CFHT, on 07 September 2025 and 03 and 06 July 2026, and applied least squares deconvolution to 1398 atomic lines to map the transit-averaged Stokes V and null spectra against the orbital semi-amplitude, K _p , and systemic velocity, v _sys . Results. The 2025 transit shows a Stokes V signature of V / I _c = 9.3 × 10 ⁻⁵ at K _p = 219 ± 11 km s ⁻¹ , v = −34.0 ± 1.2 km s ⁻¹ , close to the expected planetary velocity. Its false-alarm probability is 1.9 × 10 ⁻⁴ , and thus it is formally a marginal detection. Both 2026 transits are of comparable sensitivity but neither exhibits a signal at that location, setting 3 σ limits of 4.9 and 6.0 × 10 ⁻⁵ ; the 2025 amplitude would have been recovered at 4.9 σ and 4.0 σ . Conclusions. The 2025 signature is robust to the tests we applied to it but qualifies as a marginal detection at best. Spurious signals of comparable strengths occur elsewhere in our Stokes V and null maps, and we cannot rule out that our marginal detection is excess noise. It is not reproduced ten months later.

The TRAPPIST-1 system is well known for its seven transiting Earth-sized exoplanets. It has been extensively studied and characterized, notably with transit timing variations (TTVs), to precisely measure the mass of the planets. Using near-infrared spectroscopic observations obtained as part of the SPIRou Legacy Survey and the NIRPS Guaranteed Time Observation programs, we aimed to verify those values through radial velocity (RV) measurements of the system. Our RV analysis reveals that the current data do not have the precision required to individually detect the TRAPPIST-1 planets. However, we confidently detect (Δ ln 𝒵 = 7.53, 1860:1 odds) the combined RV signature of the planets by using the relative masses derived from the TTV analysis, with TRAPPIST-1 b as a proxy for the whole system. For the first time, the RV signal of the TRAPPIST-1 system is recovered: we find a RV semi-amplitude of K _b = 3.65 _−0.83 ^+0.78 m s ⁻¹ . This corresponds to a planetary mass of M _{p , b} = 1.31 ± 0.29 M _⊕ and demonstrates that the RV measurements are consistent with the TTV model ( M _{p , b; TTV} = 1.374 ± 0.069 M _⊕ ). Additionally, the NIRPS RVs constrain the presence of giant planets beyond the snow line, excluding Saturn-mass planets out to 2.7-yr orbits and Neptune-mass objects out to 20 d. Through RV, we determined the stellar activity period to be of 3.22 _−0.20 ^+0.22 d. Its agreement with photometric measurements (K2 and TESS) confirms stellar rotation as the origin of the ~3.3-d periodicity observed for TRAPPIST-1. We further investigated stellar activity with SPIRou polarimetric measurements, and placed an upper limit on the longitudinal field (| B _l | < 40 G, 3 σ ). This limit is compatible with a weak multipolar large-scale magnetic geometry, as observed in some of the later-type rapidly rotating M dwarfs.

Context. Ground-based spectrographs operating in the near-infrared (NIR) regime are hampered by various absorption and emission features of Earth’s atmosphere. While considerable attention has been paid to mitigating telluric absorption, correcting telluric emission features remains non-trivial and can significantly affect the observation of faint targets. Aims. We aim to develop and implement automated algorithms for sky background emission correction in the context of high-resolution spectroscopy. These empirical-based algorithms have been officially integrated into both NIRPS data reduction pipelines: NIRPS DRS and APERO DRS. The NIRPS DRS algorithm is additionally functional at the telescope site, enabling quick, real-time telluric-corrected data processing. Designed for flexibility, these techniques can be incorporated into the reduction workflow of any high-resolution spectrograph to improve the radial velocity (RV) performance. Methods. In our approach, a reference sky spectrum is first created by deep-stacking NIRPS sky calibration frames on a common wavelength grid and calculating the weighted median flux per pixel, separately for both the high-accuracy and high-efficiency instrument modes. This process is repeated for all spectral orders and for both the object and sky-calibration fibres: fibre A and fibre B, respectively. In this reference sky spectrum, the sky emission lines can be identified and used to construct a static library. During the data reduction process, the emission features in the library are individually scaled in terms of flux using two distinct techniques, each specific to the two DRS pipelines. Finally, they are locally subtracted from the science observations to minimise their noise contribution to the final spectrum Results. We find that the correction algorithms significantly improve the RV measurements obtained using both the cross-correlation function and line-by-line techniques, enabling NIRPS to achieve submetre-per-second precision in the NIR. The techniques have been successfully validated and demonstrated on both bright (Proxima Centauri) and faint (TOI-406, TOI-3494, TOI-4552) targets, as well as in both high-efficiency and high-accuracy instrument modes. Conclusions. We present the first attempts at empirically subtracting emission line features from high-resolution spectra. These algorithms can be applied independently of the instrumentation and implemented into the data reduction workflow of any high-resolution instrument with minimal modifications required.

The angle between stellar spin axis and planetary orbital plane can provide key insights into the formation and dynamical evolution of planetary systems. In particular, this measurement in multi-planet systems can be used to further discriminate between different competing migration scenarios. We present six transit observations of the sub-Neptune GJ 3090 b obtained with NIRPS and HARPS. GJ 3090 b is the inner planet of a confirmed multi-planet system orbiting an M dwarf (M2). Using high spectral resolution and high temporal cadence spectroscopic observations, we analyzed the Rossiter-McLaughlin (RM) effect induced by GJ 3090 b to determine its orbital obliquity. Through the RM revolutions technique, we find that the planet is on a retrograde orbit with a derived 3D obliquity of ψ = 136 ⁺²⁴ ₋₁₈ deg. We find no evidence of massive outer planetary or wide stellar binary companions, which disfavors scenarios involving gravitational perturbations from a massive body and instead points toward a primordial misalignment of the protoplanetary disk. Our results establish GJ 3090 b as the first planet on a retrograde orbit discovered around an M dwarf and the first highly misaligned confirmed multi-planet system without a known massive companion. We further propose late secondary disk accretion around GJ 3090, in which the disk is not expected to be aligned with the stellar spin axis, followed by disk-driven migration as the most likely mechanisms to explain the observed architecture. This work also illustrates the capability of the RM revolutions technique when applied to near-infrared data to probe orbital architectures of the smallest planets around M dwarfs that have remained mostly inaccessible.

We present a comprehensive reanalysis of two star-planet systems: TOI-4438, an M3.5V star hosting a mini-Neptune in a 7.4-day orbit, and TOI-442, an M1V star with a 4-day period planet located within the hot Neptune desert. Both systems were originally identified as transiting planet candidates by the Transiting Exoplanet Survey Satellite (TESS) and subsequently validated through the radial velocity (RV) method. Our work incorporates new TESS transit data and high-resolution spectroscopy from the SPIRou near-infrared spectropolarimeter. We detected a persistent and relatively strong Zeeman signature in TOI-442, while TOI-4438 exhibits weaker and intermittent magnetic activity, and we inferred the stellar rotation periods of both stars from the variability of the longitudinal magnetic field. We jointly fit photometry and RV models for each system. For TOI-4438 b we combined archival CARMENES data with 81 SPIRou observations and five TESS sectors. This yielded a refined planetary mass of M _p = 4.11 ^+0.40 _−0.38 M _⊕ and a radius of R _p = 2.40 ^+0.09 _−0.10 R _⊕ , consistent with the previous estimate within 1.1 σ while improving by 53% the precision on the mass and 22% on the radius. For TOI-442 b, we added 29 SPIRou RV measurements to an extensive archival dataset, significantly extending the temporal baseline. Incorporating this with a new TESS sector, we tighten the constraints on the planetary mass to M _p = 28.38 ^+0.77 _−0.73 M _⊕ and radius to R _p = 4.25 ^+0.10 _−0.08 R _⊕ , which agrees with the previous values within 1.5 σ and improves the precisions by 46 and 67%, respectively. We found no clear signs of additional planets in the currently available RV data, although we detected a single-transit event in the TOI-4438 light curve. We compared various RV models and found that those accounting for stellar variability-induced signals yield improved constraints on the planetary parameters.

Context . Condensates are ubiquitous to all Solar System planets with significant ( > 10 mbar) atmospheres. The same is true for most exoplanets with characterised atmospheres, with even ultra-hot exoplanets being able to form clouds on their cooler nightsides. One high-interest condensate is titanium, a highly refractory element that as an oxide (TiO) is a potent optical absorber long believed to be a driver of thermal inversions in highly irradiated exoplanet atmospheres. Observations have shown that some ultra-hot Jupiters have strong thermal inversions despite being significantly Ti-depleted, likely due to cold trapping, raising doubts about whether TiO is the main cause of their inverted temperature structures. Aims . Our aim was to retrieve the titanium-to-iron ratio of the dayside atmosphere of the ultra-hot Jupiter WASP-189b to determine whether the full titanium budget is accounted for in the gas phase. Methods . We analysed thermal emission observations of WASP-189b taken with the HARPS and NIRPS spectrographs using different atmospheric retrieval prescriptions to infer the planet’s atmospheric thermal structure and composition. Results . We observed Fe and Ti signals in cross-correlation and measured a sub-solar Ti/Fe ratio for WASP-189b using both free and chemical equilibrium retrieval approaches. We find the Ti/Fe of the planetary atmosphere to be between 28% and 81% (1- σ bounds) that of the stellar value. Conclusions . The slight underabundance of Ti with respect to Fe on the dayside atmosphere of WASP-189b suggests that some titanium is missing from the gas phase, potentially due to a partial nightside cold trap. In the context of the ultra-hot Jupiter population, the onset of titanium in exoplanetary atmospheres appears to occur progressively, coinciding not with when thermal inversions begin but rather when the vapourisation threshold of titanium is reached on the nightside.

Context . LHS 3844 b (TOI-136b) is an ultra short-period, Earth-size exoplanet detected by TESS. It is one of the most favourable objects for atmospheric characterisation and study of its surface with the James Webb Space Telescope. However, the dynamical mass of this planet has not yet been measured. Aims . We aim to determine the mass of LHS 3844 b using high-precision radial velocity (RV) measurements and assess the robustness of the inferred signal across different noise and orbital modelling assumptions. Methods . We analyse 25 ESPRESSO RV observations within a fully Bayesian framework. We explore 15 competing RV models that differ in their treatment of correlated stellar variability (through different Gaussian process (GP) kernels) and long-term drifts. Marginal likelihoods are computed for all models and used for Bayesian model comparison and evidence-weighted parameter estimation. Results . The RV planetary signal is robustly detected across all models, and the inferred semi-amplitude remains stable under all tested noise and drift prescriptions. From the evidence-weighted posterior samples we derive a planetary mass of 2.27 ± 0.23 M _⊕ and a bulk density of 5.67 ± 0.65 gcm ⁻³ , consistent with a predominantly rocky composition. Model comparison favours GP kernels including periodic or quasi-periodic components associated with stellar rotation and disfavours models with additional long-term drifts. Using interior-structure inference, we find that the core mass fraction is comparable to (or slightly smaller than) Earth's and only trace amounts of water are permitted, supporting a dry, terrestrial interior. We also investigate a tentative additional signal near ~6.9 days, but Bayesian model comparison does not provide conclusive support for its planetary interpretation. Conclusions . We report the first dynamical mass measurement of LHS 3844 b, confirming it as a dense, terrestrial ultra-short-period planet. With its exceptionally well-constrained bulk properties and extensive JWST programme, LHS 3844 b remains a benchmark target for studies of the atmospheres and surfaces of rocky exoplanets.

Context . Detecting and characterizing the atmosphere of rocky planets in the habitable zone is a key objective of exoplanet research in the era of JWST and the upcoming E-ELT. Spectroscopy of a planetary transit is the primary method of obtaining a transmission spectrum of an exoplanetary atmosphere. A lunar eclipse is a natural proxy for an Earth transit, offering a unique opportunity to study Earth’s atmosphere as if it were an exoplanet. Aims . This work aims to derive the empirical, wavelength-dependent effective height of Earth’s atmosphere in the near-infrared, spanning the Y to K bands. Methods . During a lunar eclipse, the penumbra – the region where the Sun is partially occulted by the Earth – contains the signature of the transmission spectrum of the atmosphere along Earth’s limb. The high-resolution (R=70 000) SPIRou spectrograph at CFHT was used to collect spectra during and after the November 19, 2021 eclipse. Two distinct methods, using either one or two penumbral spectra, were employed to derive the effective height of the atmosphere. Each method was evaluated in the context of distinct astrometric and spectrophotometric approaches. Results . The derived effective height spans 964 to 2498 nm and shows the absorption bands of H ₂ O, O ₂ , CH ₄ , and CO ₂ peaking at up to 29.6 km at a spectral resolution of R=70 000, including a 12.7 km offset defining a reference altitude taken from a transit model. The uncertainty assessed from individual penumbra observations and from uncertainties on telluric lines correction and Moon reflectance is found to be 3 σ ≈ 3.6 km. Heights are up to 25.1 km at a convolved resolution of R=1000. Conclusions . The measured heights are in good agreement with theoretical predictions. The high-resolution wavelength-dependent effective heights are made publicly available to the community.

Context. M dwarfs are the most abundant stars in the Galaxy and exhibit diverse magnetic behaviours. While understanding their large-scale magnetic fields is essential for investigating stellar dynamos and assessing the impact of magnetic activity on planetary environments, their magnetic properties and long-term variability remain poorly characterised. Aims. Our aim was to characterise the large-scale magnetic fields of six M dwarfs in the southern PLATO field, with rotation periods ranging from approximately 1 to 17 days and masses between 0.26 and 0.64 M _⊙ . Five of these stars are partially convective, while one is fully convective. These targets extend the mass–rotation diagram into previously unsampled regions. Methods. We analysed TESS light curves to determine accurate rotation periods and optimise phase coverage for our spectropolarimetric observations. SPIRou data were reduced to obtain least-squares deconvolution (LSD) profiles and longitudinal field measurements, while synthetic spectra fitting yielded small-scale field strengths. We then applied ZDI to reconstruct the large-scale magnetic topologies of the six targets. Results. We report a wide diversity of magnetic topologies among the six M dwarfs, with three main results: (1) Rapidly rotating ( P _rot < 2 d) early M dwarfs can generate dipole-dominated magnetic fields of moderate intensity, similar to those of less massive mid-M dwarfs; (2) Rapidly rotating mid-M dwarfs can generate non-axisymmetric large-scale magnetic fields featuring a significant toroidal component; (3) We report a moderately rotating ( P _rot ∼ 17 d) early M dwarf featuring a surprisingly weak large-scale magnetic field. Conclusions. Our findings further highlight the diversity of magnetic field configurations among M dwarfs, including in previously unexplored regions of parameter space. Long-term monitoring of our sample is crucial in order to distinguish persistent features from variability-driven excursions and to characterise the long-term evolution of their surface magnetic fields. Complementary PLATO photometry, including flare and spot-induced variability analyses, will be essential to link surface activity with magnetic properties.

Context . Following the 2022 alert of a TESS object of interest transiting TOI-5624 (a G7 V star ∼ 100 pc away), a CHEOPS campaign in 2023 detected four planetary signals at P _b ≈ 3.4, P _c ≈ 7.9, P _d ≈ 13.7, and P _e ≈ 21.5 days. These signals were later confirmed by additional TESS and CHEOPS photometry in 2024-2025. Aims . By using TESS and CHEOPS photometry, along with HARPS-N and SOPHIE high-resolution spectra, we determined the planet properties and performed a dynamical analysis of the system. Methods . After analysing the photometric data, we extracted and modelled the radial velocity (RV) time series using two independent methodologies, both within a Markov chain Monte Carlo 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. Results . We present the discovery of four transiting sub-Neptunes with radii of R _b = 2.314 ± 0.035 R _⊕ , R _c = 2.474 ± 0.042 R _⊕ , R _d = 3.584 _−0.050 ^+0.051 R _⊕ , and R _e = 3.247 _−0.043 ^+0.042 R _⊕ , along with masses of M _b = 9.4 ± 1.4 M _⊕ , M _c = 4.8 ± 1.9 M _⊕ , Md = 4.9 ± 2.2 M _⊕ , and M _e = 8.9 _−3.0 ^+2.9 M _⊕ . Our photometric analysis revealed that the outermost transiting planet TOI-5624e shows significant transit-timing variations (TTVs). Indeed, we found 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 labelled this non-transiting planet as TOI-5624 f and determined its minimum mass to be M _f sin i _f = 13.0 ± 3.7 M _⊕ . Conclusions . Among the known systems hosting more than four planets, the remarkable precision with which the radii have been measured (3σ) of the mass for at least three planets has previously been 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.