Maria Rosa Zapatero Osorio

Context. Magnetic activity in M dwarfs is commonly studied through chromospheric spectral indices. Deriving chromospheric fluxes in physical energy units provides a complementary characterization of chromospheric radiative losses and enables direct comparisons with other flux-calibrated diagnostics and stellar properties. Aims. We aim to derive chromospheric fluxes in physical energy units for 301 M dwarfs using the CaII infrared triplet (CaT) lines, as well as assess their consistency and their relation with other activity indicators and stellar rotation. Methods. We analyzed high-resolution CARMENES spectra and isolated the chromospheric contribution to the CaT lines through the spectral synthesis method. Based on nonlocal thermodynamic equilibrium (NLTE), this method enables the photospheric component to be consistently reproduced and subsequently subtracted. Results. We obtained homogeneously calibrated chromospheric fluxes, expressed in erg cm ⁻² s ⁻¹ , for the three CaT lines and show that systematic differences are present among the three indicators across different activity regimes and spectral types. We establish empirical relations between the CaT fluxes and measured R ′ _HK , enabling conversions between these two diagnostics. We find that our measured CaT chromospheric fluxes show strong correlations with other chromospheric indicators, such as H α and He I D ₃ equivalent widths, while weaker or no correlations are found with near-infrared and photospheric indices. The CaT fluxes also exhibit a strong correlation with the mean surface magnetic field strength. Finally, an analysis based on the Rossby number reveals the presence of the expected saturation regime for fast rotators, with the transition between the saturated and unsaturated regimes occurring at Ro ≃0.16. Conclusions. The CaT lines provide a robust chromospheric activity diagnostic in M dwarfs. They show internal consistency, correlation with other activity indicators in the optical wavelength range, and a solid connection to stellar magnetic field strength and rotation.

Ultracool dwarfs (UCDs) encompass the lowest-mass stars and brown dwarfs at the stellar–substellar boundary. These intrinsically faint objects are challenging to detect. However, the wide coverage and deep sensitivity of the Euclid survey will increase the number of confirmed and well-characterised UCDs by several orders of magnitude. We carried out a systematic study using Euclid Quick Data Release (Q1) low-resolution near-infrared (NIR) spectra to identify and characterise UCDs. A sample of previously known UCDs in Q1 was compiled to validate the analysis, which was then applied to newly identified UCD candidates in Euclid Deep Field North (EDF-N; 22.9 deg ² , down to J _E ≈ 24.5 mag). We confirm 33 new UCDs in EDF-N, spanning spectral types (SpTs) M7–T1 ( J _E = 17–21 mag) and distances of 50–600 pc. Six of them were also independently identified in other Q1 studies. We followed up on two of these objects with ground-based optical and NIR observations using the Gran Telescopio Canarias, providing an independent validation of the Euclid spectral classification and the ability of Q1 spectra to constrain the stellar parameters of UCDs. We determined the SpTs from Euclid Q1 spectra of 33 newly identified UCDs and 167 previously known UCDs by fitting spectral standards from the SpeX Prism Library Analysis Toolkit. The known UCD sample includes 103 objects in common with the Dark Energy Survey (M7–T0, J _E = 17–22 mag, and d = 50–600 pc), 6 objects from literature compilations of UCD sources (M8–T7, J _E = 18–20 mag, and d = 30–300 pc), and 58 in common with Gaia (M7–L1, J _E = 17–24 mag, and d = 70–300 pc). We derived independent stellar parameter estimates through atmospheric model fitting; the resulting effective temperatures are in good agreement with Euclid SpTs. Combined with complementary ground- and space-based data, these results demonstrate the potential for determining proper motions and identifying companions. Additionally, we investigated Gaia UCDs in Q1 by combining their mean XP spectra with Euclid NIR spectra, extending the wavelength coverage and enabling more comprehensive determinations of stellar parameters, including radii. To support broader classification efforts, we analysed the UCDs in colour–colour space and assessed their separation from quasar loci. We present a publicly available, machine-readable catalogue that includes Euclid observations of known and newly identified sources, with spectral classifications and stellar parameters. This work demonstrates Euclid ’s capability to identify and characterise the faintest stellar populations, establishing a foundation for large-scale UCD studies with forthcoming survey data.

Aims . We search for evidence of star–planet magnetic interactions in the nearby Proxima Centauri planetary system using high-quality, high-spectral-resolution optical observations spanning three years. Methods . We investigate the variability of photospheric and chromospheric features traced by oxides, H I , Na I , Fe I , He I , and Ca II using a differential spectroscopic method and narrowband light curves derived from the spectra. Results . We measure a photospheric stellar rotation period of 84.9 ± 0.6 d and a half-rotation period of 44.3 ± 0.2 d, consistent with previous studies. The stellar photospheric absorption lines show no variability correlated with the orbital periods of Proxima b or d, supporting their planetary nature. Using Fe I absorption and emission lines, we find that Proxima Centauri was flaring for 24.8 ± 4.7% of the observing time, with significant statistical evidence (≥99.8%) of flare events likely phase-locked to the inner Mars-mass planet Proxima d. Modeling the star–planet interaction via the helicity-driven reconnection mechanism with the Poynting flux formalism, we estimate a likely polar magnetic field of ∼16 G for Proxima d (assuming a Mars-sized radius), with a plausible range of 3–280 G accounting for radial and dipolar stellar magnetic field configurations, planetary radii comparable to Mars and Earth, and the observed range of stellar flare intensities. This represents the first such estimate for a terrestrial exoplanet. Evidence of a potential star–planet interaction with the outer, Earth-mass Proxima b arises not from phase-locked flare clustering, but from modulation of flare intensities. Applying a prewhitening analysis to the full time series of combined chromospheric H α , Na I D1 and D2, and Ca II H and K lines reveals peaks, in order, at half the stellar rotation period, Proxima b’s orbital period, the full stellar rotation, and Proxima d’s orbital period. All evidence suggests that both planets show magnetic interaction with their host star. Focusing on flaring epochs only, the periodogram of these chromospheric lines shows a peak consistent with the synodic period between half the stellar rotation and the mutual synodic period of Proxima b and d, implying prograde stellar rotation and planetary orbits. We also identify spectroscopic features at around 7794.0 and 7808.5 Å that behave differently from the rest of the spectrum.

Aims. This study focuses on the low-mass binary 2MASS J05082729-2101444 (2M0508-21), one of the few known radio-loud members of the complex periodic variable sample. Our aim is to use very long baseline interferometry to constrain the orbit. Methods. We observed the system with the Very Long Baseline Array (VLBA) in three epochs at a frequency of 4.85 GHz, which provides an angular resolution of about 3 mas. We combined the three radio astrometric observations, 119 RVs (60 VIS and 59 NIR) obtained with the CARMENES high-resolution spectrograph over a period of 8.1 years, and a relative astrometric measurement of an archival H -band Keck NIRC adaptive optics image to fit the orbital motion of the binary system. Results. The VLBA observations resolved the binary system and show emission from both stellar components, with similar fluxdensity levels (0.34-0.67 mJy), and showing slight temporal flux variations. The emission appears quiescent, with no significant circular polarization, and with no flare events. We obtained an orbital motion fit of the binary system, which shows an eccentric orbit ( e = 0.71), an orbital period of 2.19 yr, and a semimajor axis of 26.964 mas (1.3 au). Conclusions. The VLBA observations made it possible to resolve the binary system and identify both stars as radio-loud sources. The combined fit shows that 2M0508-21 is an M-dwarf binary with a total dynamical mass of 0.459 ± 0.007 M _⊙ , assuming Gaia ’s parallax. This mass is slightly higher than those estimated from the luminosity and theoretical evolutionary models. The upper limit of the circular polarization at 4.85 GHz (Gaia ’s parallax of the system. A complete characterization of the system will help improve evolutionary models for young objects at the substellar boundary.

We present a catalogue of 5306 new ultracool dwarf (UCD) candidates in the three Euclid Deep Fields in the Q1 data release. They range from late M to late T dwarfs, and include 1200 L and T dwarfs. A total of 546 objects have been spectroscopically confirmed, including 329 L dwarfs and 26 T dwarfs. We also provide empirical Euclid colours as a function of spectral type. Our UCD selection criteria are based only on colour ( I _E − Y _E > 2.5). The combined requirement for optical detection and a stringent signal-to-noise ratio threshold ensure a high purity of the sample, but at the expense of completeness, especially for T dwarfs. The detections range from magnitudes 19 and 24 in the near-infrared bands, and extend down to 26 in the optical band. We discuss Euclid ’s capability to identify UCD candidates based on its photometric passbands. The average surface density of detected UCDs on the sky is approximately 100 objects per deg ² , including 20 L and T dwarfs per deg ² . This leads to an expectation of at least 1.4 million UCDs in the final data release of the Euclid Wide Survey, including at least 300 000 L dwarfs, and more than 2600 T dwarfs, using the strict selection criteria from this work.

Theory predicts that a planet with a sufficiently strong magnetic field orbiting close to its host star could induce star-planet magnetic interactions. This is potentially observable as an optical or radio stellar activity signal synchronised with the planet’s orbital period. We analyze 18 years of high-resolution optical spectroscopy of GJ 436, a low mass star orbited by a Neptune-sized exoplanet on a polar eccentric orbit. Stellar activity indicators show enhancements at a period corresponding to the exoplanet orbit, modulated by stellar rotation, and the star’s 8-year magnetic cycle. We interpret this as a signal of star-planet magnetic interaction. Using a geometric model, we reproduce these periods if GJ 436 b has a magnetic field strength of 6 to 110 Gauss.

GJ 9827 is a compact system hosting three close-in planets in near-resonant orbits that span the radius gap, a deficit of close-in planets separating super-Earths and mini-Neptunes. We analysed transit observations from the ESPRESSO GTO program acquired on each planet and performed a joint Rossiter-McLaughlin (RM) analysis of the three planets using the ANTARESS workflow. We measured the sky-projected spin-orbit angle of each transiting planet ( λ _b = 9 ₋₆₄ ^+67° , λ _c = −21 ₋₄₄ ^+43° , and λ _d = −1 ± 26°) and found them, within uncertainties, to be consistent with an alignment with the stellar spin axis. These results, derived with the RM Revolutions technique, are fully compatible with a classical analysis of the RM anomaly. By combining information on the stellar radius and rotation period, we derived the true 3D spin-orbit angles. Our results indicate that the star is viewed equator-on, with 3D spin-orbit angles of Ψ _b = 54 ₋₃₈ ^+26° , Ψ _c = 46 ₋₃₀ ^+22° , and Ψ _d = 33 ₋₂₄ ^+16° . The 3D spin-orbit angles are broadly consistent with a coplanar system but remain weakly constrained, with posterior distributions peaking at moderate misalignments. An aligned architecture remains compatible at the 2 σ level. These findings support a scenario of smooth disc-driven migration for the three planets, potentially allowing for a moderate primordial misalignment between the protoplanetary disc and the stellar spin axis. Further observations are encouraged to confirm these conclusions.

HAT-P-26,b is a well-characterized Neptunian exoplanet ̊m p R_⊕) orbiting a K1V star. It lies within the Neptunian ridge (P_ ̊m orb days), following a moderately eccentric orbit (e and hosting a water-rich atmosphere. The possible presence of a stellar companion hinted at by near-infrared (NIR) spectral emission, along with candidate planetary companions inferred from transit timing variations, calls for further investigations of the system architecture. Hence, the HAT-P-26 system is a prominent target for investigating the atmospheric and dynamical evolution of exo-Neptunes and, in particular, via the study carried out by the Ancestry, Traits, and Relations of Exoplanets Inhabiting the Desert Edges and Savanna (ATREIDES) collaboration. We combined three transit observations of HAT-P-26,b using ESPRESSO on ESO's VLT to precisely characterize its orbital architecture and to investigate whether it has a misaligned orbit, as reported in cases of other evaporating and eccentric planets in the ridge. We employed the workflow to perform a careful and robust data reduction. We explored the system's orbital architecture using the in-built Rossiter-McLaughlin Revolutions (RMR) technique. ANTARESS HAT-P-26,b exhibits a polar orbit (projected obliquity angle of łambda = -78_ -13 ^ +13 ^ ̧irc and a 3D true obliquity angle of ψ = 85_ -5 ^ +5 ^̧irc). This places it alongside GJ,436,b, HAT-P-11,b, and GJ,3470,b as polar-orbit and eccentric planets lying within the ridge population. It has the lowest density (0.40 ) of these four planets, placing it near the density brink where full atmospheric erosion after early disk-driven migration or tidal disruption after high-eccentricity migration has been proposed to occur. Our dynamical analysis suggests that the observed polar orbit might either be a transient state arising from orbital-plane precession or a relic of high-eccentricity migration via Von Zeipel–Lidov–Kozai cycles driven by an inclined stellar companion. We note that in the latter case, the existence of planet,c would be strongly disfavored. g cm^ -3 HAT-P-26,b stands out as a special target in the Neptunian ridge, owing to its low density and polar orbit. The current dynamical analysis is limited by the uncertainties surrounding companion(s) in this system. As better constraints on the companion(s) become available, it will be particularly interesting to further explore the origin of this fluffy ridge Neptune through a joint analysis of migration, evaporation, tidal disruption, and inflation. Our results also demonstrate the importance of homogeneous data reduction when combining multi-epoch transit observations for accurate spin-orbit characterization.

Small planets are extremely common in the Galaxy, including planets with masses and radii between those of Earth and Neptune. Characterising the mass of such planets requires ultra-precise radial velocities. The ESPRESSO spectrograph was designed and built for this purpose. We present an overview of the ESPRESSO Guaranteed Time Observations transit follow-up sub-programme, which aimed to confirm and characterise small transiting planet candidates from the K2 and TESS space missions. We analysed the global stellar and planetary properties of the sample of 65 planets in 30 systems characterised by this sub-programme. This includes six systems presented in this paper, for which we either obtain only the upper mass limits or provide updates to previously published parameters. We also placed this sample in the context of the overall population of precisely characterised small planets. After separating the population into insolation regimes, we find a tentative mass threshold at for the rocky to volatile-rich composition transition in the medium-insolation regime, along with a population of likely stripped massive rocky planets in the high-insolation regime. Likewise, we find a correlation between planet mass and stellar metallicity, with more massive planets hosted by more metal-rich stars. We also explored the radius valley, finding that planets below the gap have a tighter mass distribution. We compared planetary masses with typical protoplanetary disk masses, drawing tentative conclusions about likely formation conditions. We also discuss the impact of our observing strategy on our results. The ESPRESSO transit follow-up sub-programme has been highly productive in characterising a diverse population of small planets, which has enabled us to identify population-level features. Likewise, the lessons learned from this sub-programme will be valuable for PLATO follow-up planning.