Valentina Vaulato

Context . The carbon-to-oxygen (C/O) ratio of a giant planet’s atmosphere has long been theorised to hold compositional information that can be traced back to its formation history. Typically, the C/O ratio of an exoplanetary atmosphere is inferred from abundance measurements of major C- and O-bearing molecules such as CO, CO ₂ , H ₂ O, and OH. However, some exoplanets have such elevated temperatures that molecules can be nearly completely thermally dissociated at the pressure levels that observations probe, making it difficult to measure their C/O ratio via these traditional tracers. Aims . Here, rather than using molecules, we aim to retrieve the C/O ratio of KELT-9b (T _eq = 4000 K) directly from atomic C and O, which are detected in its atmosphere. Methods . We analysed two transits of KELT-9b observed with the MAROON-X high-resolution spectrograph, finding absorption cross-correlation signals from atomic C and O as well as refractory metals. From this, we inferred the vertical temperature structure, the relative proportions of volatile and refractory species, and the C/O ratio of KELT-9b using a 1D local thermodynamic equilibrium atmospheric retrieval framework applied only to spectral regions mostly unaffected by non-local thermodynamic equilibrium effects. Results . We measure the atomic C/O ratio of the atmosphere of KELT-9b to be 0.20 _−0.07 ^+0.13 , which is slightly lower than the stellar value of 0.38 ± 0.15 and significantly below the solar value of 0.59 ± 0.07. We otherwise confirm previous investigations of the terminator region, finding KELT-9b’s atmosphere to be thermally inverted and slightly metal-rich. We measure the volatile-to-refractory ratio, a proxy for the ice-to-rock ratio, to be 1.24 _−0.78 ^+1.93 × solar ([M _vol /M _ref ] = 0.09 _−0.20 ^+0.19 ), which is consistent with both KELT-9 and the Sun. Conclusions . While there are caveats to interpreting observations of such an extreme planet, our results demonstrate that atomic species can be used to measure C/O ratios of exoplanetary atmospheres in high-temperature regimes where molecules are thermally dissociated.

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.

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.

Ultrahot Jupiters (UHJs) are exceptional laboratories for studying planetary atmospheres under extreme irradiation conditions. With close-in tidally locked orbits, these planets can have daysides hot enough for metals to be significantly ionized while still maintaining nightsides cold enough for refractory species to potentially condense. We present an analysis of the UHJ HAT-P-70b taken with the MAROON-X high-resolution spectrograph. Using cross correlations, we detect 14 neutral and singly ionized species, including Fe I , Fe II , Ti I , Ca I , Ca II , Cr I , Na I , V I , Mn I , Ni I , Mg I , Ba II , O I , and Sr I , with tentative evidence for H I , Co I , and K I . The absorption signals exhibit blueshifts on the order of a few kilometers per second, consistent with day-to-night winds. We further constrain relative abundances with atmospheric retrievals and demonstrate that some inferred elemental abundance ratios depend strongly on modeling assumptions. In particular, we show that a well-mixed retrieval approach neglecting ionization can strongly bias highly ionizable elements such as Ca and Ti. Accounting for the effects of equilibrium chemistry and thermal ionization generally results in inferred elemental abundance ratios that are closer to expectations for a solar-like composition, although not in all cases. Interestingly, we find a distinct nickel enrichment on HAT-P-70b, adding to the growing number of UHJ studies where the Ni abundance is seemingly enhanced. Our results underline the importance of considering physical and chemical atmospheric processes such as ionization when interpreting high-resolution transmission spectra of UHJs.

Tracing the compositional link between terrestrial super-Earths and their host stars provides clues about their dominant formation pathway. By constraining the stellar abundances of refractory elements, we can predict the core mass fractions (CMFs) of their super-Earths. The level of agreement between this prediction and the planetary CMF derived from their masses and radii can reveal past formation processes, such as mantle stripping and water-rich formation plus sequestration in the planet core. We present the first results from the Near Infrared Planet Searcher (NIRPS) GTO CMF subprogram: an intensive radial velocity campaign to refine masses and compute host stellar abundances of three hot super-Earths around M dwarfs (GJ1132 b, GJ1252 b, and LTT 3780 b). We calculated masses of 1.69 ± 0.15 M _⊕ , 1.54 ± 0.18 M _⊕ , and 2.34 ± 0.10 M _⊕ respectively. We measured the CMFs of these and six further hot super-Earths with masses already available in the literature to a precision of 10–15%. We compared them to CMF predictions made from measuring the Fe, Mg, and Si abundances of their host stars measured from the NIRPS spectra. The CMFs of these planets are smaller than expected from their host stellar abundances to a statistically significant degree. This discrepancy is suggestive of significant reservoirs of water, and while these planets are too hot to harbor surface water, they likely have interior water mass fractions of ~1%.

Context . Hot Jupiters are ideal natural laboratories to investigate atmospheric composition and dynamics. However, high-resolution transmission spectroscopy is currently limited by our capability of removing planet-occulted line-distortion (POLD) contamination from the signal. Aims . In this paper, we aim to characterize the transmission spectrum of WASP-31 b from two and a half transits observed with the ESPRESSO spectrograph at the VLT. Methods . The Rossiter-McLaughlin (RM) signature was analyzed using the RM “revolutions” method. Before extracting the transmission spectrum of the planet, we corrected the dataset for telluric lines using molecfit and further modeled the POLD deformations using EvE . Results . We confirm the planet low sky-projected spin-orbit angle from previous studies and further refine its value to λ = −0.09 _−0.32 ^+0.31 deg. We do not detect any species (including previously detected species such as K or CrH) in the planetary atmosphere. In most cases the non-detections are due to the strong POLDs contamination or lack of observable lines in the ESPRESSO wavelength range, and so previous detections cannot be ruled out. Conclusions . Planet-occulted line-distortion contamination continues to be the main limitation of high-resolution transmission spectroscopy for species present in both the star and the planet, hindering atmospheric detections even with state-of-the-art models, in particular for planets with a low sky-projected spin-orbit angle. Developing advanced techniques to isolate planetary signatures is of utmost importance in the advent of ELT-like observations.

Context . Gas giant planets orbiting low-mass stars ( T _eff ≲ 4600 K) are uncommon outcomes of planet formation. Increasing the sample of well-characterised giants around early M dwarfs will enable population-level studies of their properties, offering valuable insights into their formation and evolutionary histories. Aims . We aim to confirm and characterise giant exoplanets transiting M dwarfs identified by the TESS mission. To this end, we have started the Gas giAnts Transiting 1Ow-mass Stars (GATOS) programme within the NIRPS guaranteed time observations (GTO). Methods . High-resolution spectroscopic data were obtained in the optical and near-infrared (nIR), combining HARPS and NIRPS. We derived radial velocities (RVs) via the cross-correlation function and implemented a novel post-processing procedure to further mitigate telluric contamination in the nIR. The resulting RVs were jointly fit with TESS and ground-based photometry to derive the orbital and physical parameters of the systems. Results . We present the GATOS programme and its first results. We confirm two gas giants transiting the low-mass stars TOI-3288 A (K9V, T _eff = 3933 ± 48 K) and TOI-4666 (M2.5V, T _eff = 3512 ± 36 K). TOI-3288 A hosts a hot Jupiter with a mass of 2.11 ± 0.08 M _Jup and a radius of 1.00 ± 0.03 R _Jup , with an orbital period of 1.43 days ( T _eq = 1059 ± 20 K). TOI-4666 hosts a 0.70 ± 0.06 M _Jup warm Jupiter ( T _eq = 713 ± 14 K) with a radius of 1.11 ± 0.04 R _Jup , with an orbital period of 2.91 days. At a population level, we identify a decrease in planetary mass with spectral type, whereby late M dwarfs host less massive giant planets than early M dwarfs. More massive gas giants that deviate from this trend are preferentially hosted by more metal-rich stars. Furthermore, we find an increased binarity fraction among low-mass stars hosting gas giants, which may play a role in enhancing giant planet formation around low-mass stars. Conclusions . These mass characterisations contribute to the growing catalogue of well-defined giant exoplanets around low-mass stars. The observed population trends agree with theoretical predictions, whereby higher metallicity can compensate for lower disc masses, and wide binary systems may influence planet formation and migration through Kozai–Lidov cycles or disc instabilities.

We present an updated characterization of the planetary system orbiting the nearby M2 dwarf GJ 3090 (TOI-177; d =22 pc), based on new high-precision radial velocity (RV) observations from NIRPS and HARPS. With an orbital period of 2.85 d, the transiting sub-Neptune GJ 3090 b has a mass we refine to 4.52 ± 0.47 M _⊕ , which, combined with our derived radius of 2.18 ± 0.06 R _⊕ , yields a density of 2.40 _−0.30 ^+0.33 g∉cm ⁻³ . The combined interior structure and atmospheric constraints indicate that GJ 3090 b is a compelling water-world candidate, with a volatile-rich envelope in which water likely represents a significant fraction. We also confirm the presence of a second planet, GJ 3090 c, a sub-Neptune with a 15.9 d orbit and a minimum mass of 10.0 ± 1.3 M _⊕ , which does not transit. Despite its proximity to the star’s 18 d rotation period, our joint analysis using a multidimensional Gaussian process (GP) model that incorporates TESS photometry and differential stellar temperature measurements distinguishes this planetary signal from activity-induced variability. In addition, we place new constraints on a non-transiting planet candidate with a period of 12.7 d, suggested in earlier RV analyses. This candidate remains a compelling target for future monitoring. These results highlight the crucial role of multidimensional GP modelling in disentangling planetary signals from stellar activity, enabling the detection of a planet near the stellar rotation period that could have remained undetected with traditional approaches.

Ultra-hot gas giants such as WASP-121b provide unique laboratories for exploring atmospheric chemistry and dynamics under extreme irradiation conditions. Uncovering their chemical composition and atmospheric circulation is critical for tracing planet formation pathways. Here, we present a comprehensive atmospheric characterisation of WASP-121b using high-resolution transit spectroscopy across the optical to infrared with HARPS, NIRPS, and CRIRES+ spanning nine transit events. These observations are complemented with five TESS photometric sectors, two EulerCam light curves simultaneous to the HARPS and NIRPS transits, and an extensive radial velocity dataset in order to refine WASP-121b's orbital parameters. A cross-correlation analysis detected iron (Fe), carbon monoxide (CO) and vanadium (V) absorption signals with SNR of 5.8, 5.0, and 4.7, respectively. Our retrieval analysis constrains the water (H ₂ O) abundance to −6.52 _−0.68 ^+0.49 dex, although its absorption signal is effectively muted by the hydride (H ⁻ ) continuum. We constrained the relative abundances of the volatile and refractory elements - which represents a crucial diagnostic of atmospheric chemistry, evolution, and planet formation pathways. The retrieved abundance ratios are broadly consistent with expected values of a solar composition atmosphere in chemical equilibrium, likely indicating minimal disequilibrium chemistry alterations at the probed pressures (∼10 ⁻⁴ −10 ⁻³ bar). We update the orbital parameters of WASP-121b with its largest radial velocity dataset to date. By comparing orbital velocities derived from both the radial velocity analysis and the atmospheric retrieval, we determined a non-zero velocity offset caused by atmospheric circulation, ΔK _p = −15 ± 3 km s ⁻¹ (assuming M _⋆ = 1.38 ± 0.02 M _⊙ ), consistent with predictions from either drag-free or weak-drag 3D global circulation models, while we caution the non-negligible dependence on the assumed stellar mass. These results place new constraints on the thermal structure, dynamics, and chemical inventory of WASP-121b, highlighting the power of multi-wavelength high-resolution spectroscopy to probe exoplanetary atmospheres.