Guillem Anglada-Escudé

Context . Despite recent advances in the precision of high-resolution spectrographs, the detection of Earth-like exoplanets is still limited by the effects of stellar activity, which introduce radial velocity variations at the metre-per-second level or larger. Aims . We present a framework to disentangle stellar effects from planetary signals by exploiting high-order distortions of the cross-correlation function (CCF; a measure of the average spectral line profile), thus moving beyond the commonly applied Gaussian fit approximation. Methods . We decomposed the CCF using a Gram–Schmidt orthogonal basis function, enabling the separation of pure line shifts from line-shape distortions. To model activity-induced contributions to the radial velocities, we have developed a time-aware convolutional attention network dubbed CANSTAR. This network was trained on synthetic line-shape distortion coefficients produced with the realistic stellar simulator StarSim to learn the temporal evolution of stellar activity features. Results . We validated our framework using HARPS and CARMENES observations of two active stars, ϵ Eridani and TZ Arietis. The network effectively mitigates stellar activity, reducing the radial velocity RMS to 52.5% and 62.4% of the uncorrected variability, respectively. This correction enables a more precise determination of the orbital parameters of TZ Arietis b compared to a Gaussian process regression. Conclusions . Our results demonstrate that neural networks that incorporate the temporal context can outperform state-of-the-art methods in complex activity regimes. Future improvements on StarSim that will allow us to train CANSTAR on 3D magnetohydrodynamic spectra and more complex instrumental modelling are expected to bridge the performance gap between synthetic and real data, offering a robust pathway towards detecting Earth-mass planets around Sun-like stars.

The radii of small exoplanets form two populations, super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This feature could be produced by the removal of atmospheres by stellar or internal heating or by the lack of an initial envelope. We used transit photometry and radial velocity measurements to detect and characterize four exoplanets orbiting LHS 1903, a red dwarf star in the Milky Way’s thick disk. These four planets have orbital periods ranging from 2.2 to 29.3 days and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, whereas LHS 1903 c and LHS 1903 d have extended atmospheres. The most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating that it formed from gas-depleted material.

Exoplanets that reside close to their host stars, and therefore receive substantial amounts of X-ray and ultraviolet radiation, are prone to suffer from strong atmospheric escape. This can lead to the creation of an envelope of escaping gas along the planet’s orbital trajectory, often referred to as a tail. When transiting in front of their host star, these tails can not only produce larger depths in the transit light curves but also introduce significant asymmetries between ingress and egress. Using the publicly available software Harmonica, we present a method to model the light curves of transiting planets surrounded by extended envelopes of escaping gas and subsequently infer the shape and size of the latter. We apply this method to the JWST NIRISS/SOSS observations of HAT-P-18 b, which show pronounced helium tail features in the spectroscopic light curve of the metastable helium triplet at 10830 Å. Our model reveals that, in order to fit the observed light curve of HAT-P-18 b, the planet must possess a trailing helium tail of 15.7 9 − 1.05 + 1.14 planetary radii. We carry out injection-recovery tests to validate the effectiveness of the proposed methodology. We demonstrate that, with sufficient precision, we would be able to fit a multilayer envelope to the data, which would provide insight into the relative radial variations in the opacity profile.

Context. Surveys for exoplanets indicate that the occurrence rate of gas giant planets orbiting late-type stars in orbits with periods shorter than 1000 days is lower than in the case of Sun-like stars. This is in agreement with planet formation models based on the core or pebble accretion paradigm. Aims. The CARMENES exoplanet survey has been conducting radial-velocity observations of several targets that show long-period trends or modulations that are consistent with the presence of giant planets at large orbital separations. Methods. We present an analysis of five such systems that were monitored with the CARMENES spectrograph, as well as with the IRD spectrograph. In addition, we used archival data to improve the orbital parameters of the planetary systems. Results. We improve the parameters of three previously known planets orbiting the M dwarfs GJ 317, GJ 463, and GJ 3512. We also determine the orbital parameters and minimum mass of the planet GJ 3512 c, for which only lower limits had been given previously. Furthermore, we present the discovery of two new giant planets orbiting the stars GJ 9733 and GJ 508.2, although for the second one only lower limits to the orbital properties can be determined. Conclusions. The new planet discoveries add to the short list of known giant planets orbiting M-dwarf stars with subsolar metallicity at long orbital periods above 2000 days. These results reveal that giant planets appear to form more frequently in wide orbits than in close-in orbits around low-mass and lower metallicity stars.

The ultraviolet and optical wavelength ranges have proven to be a key addition to infrared observations of exoplanet atmospheres because they offer unique insights into the properties of clouds and hazes and are sensitive to signatures of disequilibrium chemistry. Here, we present the 0.2–0.8 μm transmission spectrum of the T _eq = 2000 K Jupiter KELT-7b, acquired with Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3)/UVIS G280 as part of the HUSTLE Treasury program. We combined this new spectrum with the previously published HST WFC3/IR G141 (1.1–1.7 μm) spectrum and Spitzer photometric points at 3.6 and 4.5 μm, to reveal a generally featureless transmission spectrum between 0.2 and 1.7 μm, with a slight downward slope toward bluer wavelengths, and an asymmetric water feature in the 1.1–1.7 μm band. Retrieval models conclude that the 0.2–1.7 μm spectrum is primarily explained by a high H⁻ abundance (∼10⁻⁵), significantly above the equilibrium chemistry prediction (∼10⁻¹²), suggesting disequilibrium in KELT-7b’s upper atmosphere. Our retrievals also suggest the presence of bright inhomogeneities in the stellar surface, and tentative evidence of CO₂ at the Spitzer wavelengths. We demonstrate that with the UV–optical coverage provided by WFC3 UVIS/G280, we are able to confirm the presence and constrain the abundance of H⁻, and obtain evidence for bright stellar inhomogeneities that would have been overlooked using infrared data alone. Observations redward of 1 μm with JWST should be able to further constrain the abundance of H⁻, as well as confirm the presence of CO₂ inferred by the two Spitzer data points.

The ultraviolet and optical wavelength ranges have proven to be a key addition to infrared observations of exoplanet atmospheres, as they offer unique insights into the properties of clouds and hazes and are sensitive to signatures of disequilibrium chemistry. Here we present the 0.2-0.8 $μ$m transmission spectrum of the Teq = 2000 K Jupiter KELT-7b, acquired with HST WFC3/UVIS G280 as part of the HUSTLE Treasury program. We combined this new spectrum with the previously published HST WFC3/IR G141 (1.1-1.7 $μ$m) spectrum and Spitzer photometric points at 3.6$μ$m and 4.5$μ$m, to reveal a generally featureless transmission spectrum between 0.2 and 1.7 $μ$m, with a slight downward slope towards bluer wavelengths, and a asymmetric water feature in the 1.1-1.7 $μ$m band. Retrieval models conclude that the 0.2 - 1.7$μ$m spectrum is primarily explained by a high H- abundance ($\sim 10^{-5}$), significantly above the equilibrium chemistry prediction ($\sim 10^{-12}$), suggesting disequilibrium in KELT-7b's upper atmosphere. Our retrievals also suggest the presence of bright inhomogeneities in the stellar surface, and tentative evidence of CO2 at the Spitzer wavelengths. We demonstrate that with the UV-optical coverage provided by WFC3 UVIS/G280, we are able to confirm the presence and constrain the abundance of H-, and obtain evidence for bright stellar inhomogeneities that would have been overlooked using infrared data alone. Observations redward of 1$μ$m with JWST should be able to further constrain the abundance of H-, as well as confirm the presence of CO2 inferred by the two Spitzer datapoints.

The research is of high quality. Both the general astrophysical introduction and the methodology sections are very clear and well-structured. The results read well and are clearly presented. The work is well-framed within the state-of-the-art global...

Scallop-shell stars, a recently discovered class of young M dwarfs, show complex optical light curves that are characterized by periodic dips as well as other features that are stable over tens to hundreds of rotation cycles. The origin of these features is not well-understood. 2MASS J05082729−2101444 is a ∼25 Myr old scallop-shell star that was identified using TESS data; it has a photometric period of 6.73 h that has been attributed to rotation. Of the ∼50 recently confirmed scallop-shell stars, it is one of the few detected at radio frequencies between 1 and 8 GHz. We observed this rare system with the upgraded Giant Meterwave Radio Telescope at 575–720 MHz, covering 88% of the photometric period in each of the two observations scheduled almost a month apart in 2023. We detected approximately millijansky emission from the target in both epochs, with a significant circular polarization fraction: |V/I|∼20 − 50%. The 3.5-minute phase-folded light curves show unique variability in circular polarization. We detected an approximately hour-long helicity reversal during both epochs, and the reversals had similar amplitudes, lengths, and (possibly) occured at similar phases. These results suggest two emission components: The first is a persistent, moderately polarized component possibly ascribable to gyro-synchrotron emission driven by centrifugal breakout events. The second is a highly polarized, short burst-like component that is likely due to an electron cyclotron maser (ECM); it is indicative of auroral emission and is potentially responsible for the helicity reversal. To explain this, we discuss the different origins of the plasma responsible for the radio emission, including the possibility that the occulting material is acting as a plasma source. Future coordinated multifrequency radio and optical observations can further constrain the underlying scenario, as well as the magnetic geometry of the system, if we assume an ECM-like auroral emission.