Raphael A. García

Context. Asteroseismology provides precise stellar parameters, which are particularly useful for benchmark stars with accurate independent measurements. However, most benchmark solar-like oscillators are red giants. Aims. We present a detailed analysis of the asteroseismic main-sequence benchmark star χ Dra A in the Transiting Exoplanet Survey Satellite (TESS) northern continuous viewing zone. We aim to derive its stellar mass and radius from asteroseismic modelling of individual mode frequencies and test these derived values against an independent dynamical mass and interferometric radius measurements. Methods. We determined the dynamical mass of χ Dra using 618 radial velocity measurements obtained with the SONG telescope at Tenerife. Our results were derived via the spectral disentanglement method, and 53 relative astrometric measurements. From the disentangled spectra, we also determined the spectroscopic parameters of χ Dra A. With the PAVO beam combiner at CHARA, we obtained the interferometric radius of χ Dra A. We determined asteroseismic parameters from 16 sectors of 20 sec cadence TESS photometry. Results. We determine T _eff = 6277 ± 30 K, [Fe/H] = −0.51 ± 0.03 dex, and [ α /Fe] = 0.08 ± 0.03 dex from the spectroscopic analysis. For the dynamical fit, we obtain a mass of M _A = 1.028 ± 0.004 M _⊙ for χ Dra A and M _B = 0.735 ± 0.003 M _⊙ for χ Dra B. Combining the derived interferometric angular diameter with dynamical parallax yields an interferometric radius of R _A = 1.159 ^+0.029 _−0.028 R _⊙ , the first interferometric radius of χ Dra A. In the TESS power spectrum, we identify 38 individual oscillating modes. Using these modes, we modelled the star with nine independent pipelines to test the resulting model mass and radius against our independently calculated mass and radius. All models yield masses slightly lower than the dynamical mass. Using asteroseismic scaling relations, we find that scaling masses from corrected scaling relations best reproduce the dynamical mass. Conclusions. The combination of spectroscopy, interferometry, and asteroseismology yields precise results for the main component of the χ Dra system, making it one of the best-characterised main-sequence solar-like oscillators.

Context. Solar analogs are stars whose fundamental properties closely match those of the Sun and, as such, they serve as useful laboratories for tests of stellar structure and evolution, particularly in the context of the upcoming PLATO mission. Aims. Our study is focussed on six seismic solar analogs observed by Kepler and K2, which have been characterised in detail in a former analysis. We investigate the impact of the model physical ingredients on these six targets to evaluate how well we can satisfy the PLATO precision requirements for stellar parameters with the data quality that is currently available. Methods. We used the stellar mass, age, and chemical composition obtained with the Forward and Inverse COmbination (FICO) procedure, based on OPAL opacities and a combination of NACRE II and Solar Fusion II (SFII) nuclear reaction rates, as our reference values. The efficiency of macroscopic transport was calibrated on lithium abundances derived from the analysis of HERMES spectra. Next, we carried out extensive tests of the physical ingredients with the help of a Levenberg-Marquardt local minimisation method, using both classical (luminosity, mean density, metallicity, and effective temperature) and seismic constraints (frequency separation ratios). In particular, we investigated the impact induced by the use of OPLIB opacities and SFIII nuclear reaction rates on the stellar fundamental properties, as well as the use of various seismic constraints. Results. We quantified the potential differences that changes in the modelling input physics might lead to and compared their extent with the requirements expected for the PLATO mission (15% in mass, 2% in radius, and 10% in age), providing crucial constraints on the accuracy and robustness with respect to systematics of the determination of fundamental properties for PLATO. For our sample of solar analogs, we were able to accurately retrieve the stellar masses within 2.5%. As for the stellar ages, we quantified the systematics of the order of 10–14% at most, but there are also very promising cases for which deviations remain within 4% depending on the stars and constraints considered.

Context. The Stellar Observations Network Group (SONG) network has operated for more than a decade, providing long-baseline high-cadence spectroscopic observations of bright stars and the Sun. The observations from 2014 through 2025 constitute a substantial archive of high-resolution spectra and precise radial velocities suitable for a broad range of time-domain stellar astrophysics. Aims. We present an overview of the current status, instrumentation, and scientific capabilities of the SONG network and describe the scope and accessibility of the SONG data archive (SODA). We further illustrate the breadth of science enabled by SONG observations, including asteroseismology, stellar variability studies, binary-star characterisation, and exoplanet research. Methods. We summarise the operational status and observing strategies of the SONG facilities, describe the available data products and archive infrastructure, and outline procedures for accessing archival observations and proposing new observations within the SONG community framework. Results. The SODA archive currently contains more than 580, 000 spectra of 3091 stars obtained with SONG using either iodine-cell or thorium–argon wavelength calibration. The archive spans more than a decade of observations and includes extensive high-cadence time-series data for bright targets spanning a wide range of stellar types and variability classes. Access to the archive is available to members of the SONG community, which remains open to new participants who agree to follow the community policies. Conclusions. The SONG archive has developed into a major long-baseline resource for stellar spectroscopy and radial velocity time-series analysis. Continued expansion of the archive, together with coordinated observations obtained contemporaneously with TESS and future PLATO observations, is expected to enable new studies of stellar oscillations, variability, and exoplanet host stars through combined radial velocity and photometric analyses.

For low-mass stars ( M < 1.4 M _⊙ ), the connection between stellar rotation and magnetic activity governs stellar spin-down, shapes the environments of their exoplanets, and provides an age-diagnostic via magneto-gyro-chronology. Recently, unexpected phenomena known as the intermediate rotation period gap and the rotational stalling have been discovered. These effects are likely due to internal angular momentum redistribution, marking departures from a smooth spin-down evolutionary scenario. These rotational features have been shown to cause enhanced magnetic activity on the photosphere, as measured by the photometric index from light curves (⟨ S _ph ⟩) in both cluster and field stars. However, their influence on other magnetic activity proxies, and particularly in field stars, remains poorly understood. In this work, we study the impact of the intermediate-period gap on chromospheric magnetic activity as traced by the Ca II infrared triplet (IRT) index. We target the stars observed by the Kepler mission, as this is the largest and most reliable sample of field stars with measured rotation periods that are sensitive to the gap. We calculated the Ca II IRT index for the Kepler stars using the spectroscopic information from the Gaia mission data release three (DR3). We studied the rotation-activity relation as a function of location on the Hertzsprung-Russell (HR) diagram and spectral type, finding that K dwarfs are more active than G dwarfs, which in turn are more active than F dwarfs. For main-sequence stars, we find that chromospheric magnetic activity is also enhanced after the intermediate-period gap, mirroring its effect on the photospheric ⟨ S _ph ⟩ index. Our work reveals that the intermediate-period gap marks a genuine transition in the stellar magnetic behavior, not only at the photosphere, but also at the chromosphere. This highlights the need to account for its signatures across activity proxies, as well as its impact on exoplanet habitability and the age-rotation-activity relation.

Context . Thanks to helioseismology, the rotation profile of the Sun has been measured with great precision down to 20% of its total radius. This rotation profile is used as a calibration to infer the rotation of other stars as well as a test of angular momentum transport theory in stellar interiors. However, the deepest 20% of the layers remain out of reach of current observations, preventing astronomers from discriminating between currently competing angular momentum transport mechanisms. Aims . The main obstacle is that no global oscillation modes sensitive to rotation ( ℓ > 0) reaching the solar core have been detected yet, as non-radial p modes cannot reach it and g modes are evanescent at the surface and still elude detection. In this work, we propose and examine a new method to constrain the rotation of the core of the Sun, which does not require direct observation of solar g modes. Methods . Our method was based on a recent prediction that g modes in the radiative interior couple with f modes in the outer parts of the star. These mixed f – g are at the same time sensitive to the rotation of the core and able to reach the surface. These modes can be used together to build average inversion kernels and perform an inversion of the rotation of the solar core. Results . We find that the oscillation spectrum of the Sun should present six mixed f – g modes that can be used to measure the rotation rate of the Sun at r = 0.07 and 0.2 R _⊙ . We estimate that the uncertainty on the measurements should be small enough to distinguish between competing scenarios of angular momentum transport in the Sun.

Aims. K 1-6 has long been classified as a planetary nebula (PN) hosting a binary central star; however, it has remained poorly studied due to its faintness. The central star exhibits pronounced photometric variability whose origin is still unclear. We present a comprehensive characterisation of the K 1-6 system, including the physical properties of its stellar components and the nature of the surrounding nebulosity. Methods. We conducted a multi-wavelength analysis combining optical and UV spectroscopy obtained with the Gran Telescopio Canarias, the Telescopio Nazionale Galileo, the Nordic Optical Telescope, and the Hubble Space Telescope. We also present long-term multi-band ground- and space-based photometry, including high-cadence data from the Transiting Exoplanet Survey Satellite, narrowband imaging, and the latest astrometric constraints from Gaia . Results. Our results show that the nebula is not a remnant PN, but instead consists of interstellar medium photoionised by a hot white dwarf, which is relatively evolved. It has a cooling age of 1–2 Myr, implying that any original PN has long since dissipated. We further find that the central object is a hierarchical triple system, comprising an inner binary with an orbital period likely of the order of thousands of days and a distant tertiary companion orbiting on a timescale of tens of thousands of years. The optically dominant cool component of the inner binary is an inflated K-type star displaying extreme magnetic activity, including large-amplitude variability and flaring. Its properties resemble those of BY Dra-type binaries and Abell 35-type systems, and are difficult to reconcile with single-star evolution, pointing instead to a history of binary interaction.

Context. Asteroseismic modelling will be a key component of upcoming missions like PLATO, CubeSpec, and Roman . Magnetic activity effects in solar-type stars are often assumed to be negligible, with their influence absorbed into surface corrections, yet recent studies show that they can substantially bias seismic inferences, particularly age estimates, regardless of modelling strategy or surface treatment. Aims. We quantified how magnetic activity effects in the Sun-as-a-star observations are smoothed by temporal averaging by analysing 182.5-, 365-, 730-, and 1460-day time series from the BiSON network and the GOLF instrument. Methods. For each baseline, we analysed overlapping segments to track the apparent temporal evolution of the seismic age and its correlation with the 10.7 cm radio flux. We then estimated the associated systematic uncertainty using two metrics from the literature. We finally compared results across baselines to evaluate how the observing window shapes activity-induced biases. Results. Across all 28 tested configurations, solar-cycle signatures persist even in 1460-day windows. Under these conditions, activity-driven systematics reach ∼1–2.5% for fits to individual frequencies and ∼2.5–4.5% when using frequency separation ratios. With a two-year baseline, the corresponding biases reach ∼2–3% and ∼3.5–5.5%. The suppression of magnetic activity effects with increasing baseline is non-monotonic: 1- and 4-year windows reduce biases far more effectively than shorter baselines in most cases, whereas 730-day windows provide only limited improvement over 365-day ones. Improvements arise from enhanced frequency determination (dominant at 365 days) and from an increasingly efficient temporal averaging of the activity cycle (dominant at 1460 days). In contrast, 730 days is an intermediate regime: frequency accuracy has already plateaued, and the observing window remains too short to smooth out cycle-related variability. On average, we find that magnetic activity effects decrease by 24%, 12%, 30%, and 38% when transitioning from 182.5 to 365 days, 365 to 730 days, 730 to 1460 days, and 365 to 1460 days for frequency-based fits; the corresponding improvements for ratio-based fits are 13%, 14%, 20%, and 31%. Conclusions. These results indicate that a continuous single-field four-year PLATO observing programme would provide the most effective suppression of magnetic-activity biases for solar analogues, whereas a 2+2-year strategy (in two distinct fields) is significantly more sensitive to magnetic effects, with limited gains between 365- and 730-day windows.

Close-in substellar companions experience strong tidal and magnetic interactions with their host stars and are therefore subject to fast orbital evolution. Moreover, due to the strong irradiation of their surface, they exhibit orbital phase modulations of significant amplitudes, allowing their atmospheric radiative properties to be characterised . In particular, this means that companions can be detected and characterised even in the absence of transits. The solar-type pulsator KIC 9139163 exhibits, in its light curve, a stable 0.6-day modulation for which the best explanation is the presence of a close-in non-transiting companion that we therefore attempt to characterise. We combined and TESS photometric data with spectroscopic observations obtained with the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N). We validated the KIC 9139163 surface stellar rotation period and we used the HARPS-N spectra to refine the stellar modelling. The analysis of the radial velocities obtained with HARPS-N provides a companion mass M_p _⊕. Kepler i = 7.3 ± 1.4 , M Using a joint grid fitting of the and TESS phase curves with an ultra-hot exoplanet model, we inferred a planetary radius of 2.43 ± 0.14 , Kepler R _⊕, which, combined with the measured mass and retrieved inclination, implies a bulk density consistent with a hot water-rich world. This places the non-transiting companion candidate of KIC 9139163 within the Neptunian desert, a regime where planets are expected either to have lost their primordial hydrogen or helium envelopes or to harbour metal-enriched atmospheres. We further detected significant variations in amplitude between the and TESS phase curves, obtained six years apart, as well as a secular increase in amplitude over the baseline. Our joint fit favours a model with two distinct longitudinal cloud offsets over a single-offset scenario. Both datasets indicate a moderate-to-high geometric albedo and low-to-moderate heat redistribution. The opposite phase offsets observed in the and TESS datasets suggest a time-variable longitudinal brightness distribution, potentially driven by evolving cloud properties or changes in scattering behaviour. Kepler Kepler Kepler Nevertheless, given the larger noise level and contamination affecting the TESS photometry, this interpretation remains tentative. These results point towards a dynamic and evolving reflective component in the atmosphere of the candidate planet. While making KIC 9139163 an interesting candidate for future ground follow-ups, it also suggests that searching for other non-transiting planets around fast stellar rotators in space-borne photometric surveys might provide new insights into the physics of the Neptunian planets located in the desert.

Solar analogs–stars that closely match the fundamental properties of the Sun–provide key benchmarks for testing stellar structure and evolution across different ages and activity levels. Their detailed characterization helps place the Sun into context within the broader population of solar-like stars. This study presents the characterization of six seismic solar analogs observed by the NASA Kepler and K2 missions. Combining asteroseismic constraints from space-based photometry with high-resolution spectroscopy and Gaia astrometry, we derived their fundamental parameters and assessed their resemblance to the Sun. Global seismic properties and individual oscillation modes were extracted from the photometric light curves, while atmospheric parameters were obtained from data collected by the High-Efficiency and High-Resolution Mercator Echelle Spectrograph (HERMES) at the Mercator telescope. Stellar modeling using seven independent stellar evolution codes yielded consistent masses, radii, and ages. These stars have masses between 0.91 and 1.04 M _⊙ , radii between 0.95 and 1.08 R _⊙ , and ages from about 1.8–9.1 Gyr, with typical systematic uncertainties of ±0.02 M _⊙ , ±0.01 R _⊙ , and ±0.7 Gyr, respectively. One star, EPIC 206064678, exhibits properties very similar to those of the Sun, with M = 1.016 ± 0.033 M _⊙ , R = 0.990 ± 0.011 R _⊙ , and an age of 5.40 ± 0.12 Gyr. It can therefore be considered a close solar twin, although it is slightly older and more metal-rich (0.25 ± 0.07 dex). Four targets display binarity signatures and all exhibit very low chromospheric activity. This work broadens the sample of well-characterized seismic solar analogs toward a larger sample of metallicities and ages, providing new references for comparative stellar studies and future asteroseismic investigations.

The Transiting Exoplanet Survey Satellite (TESS) is conducting a nearly full-sky survey, enabling the photometric characterisation of millions of stars. The forthcoming PLAnetary Transits and Oscillations of stars mission (PLATO) will provide long-duration, high-precision photometry of tens of thousands of bright stars to be characterised through asteroseismology. The TESS Luminaries Sample is a catalogue of 196 bright naked-eye (V < 6) main-sequence (MS) and sub-giant (SG) stars exhibiting solar-like oscillations. Among them, the subset located within the PLATO long-duration observation phase (LOP) fields constitutes an exceptional set of targets that will be observable by PLATO from the earliest phases of the mission, making them ideal calibrators during commissioning and the first months of science operations. This paper aims to provide an in-depth asteroseismic characterisation of 32 Luminaries stars that fall within the PLATO LOP fields of view. Individual mode parameters were extracted using three independent seismic pipelines, one of which was similar to the algorithms used in the official PLATO pipeline. The Peirce criterion and a Z-score were applied to identify the optimal combination of data calibration, observing cadence, and fitting pipeline for each star. We analysed 32 MS and SG stars up to TESS Sector 88, with individual mode frequencies derived for the first time for 26 of them. For all stars, we derived large and small separations, the asymptotic phase term, ɛ, radial mode amplitudes, and mean linewidths per order. Comparisons with previous observations reveal consistent trends in the seismic parameters, confirming the robustness of our analysis based on TESS data. In SGs, mixed-mode identification differs in the three pipelines, revealing extraction inconsistencies requiring longer datasets to improve our mode identifications. These results demonstrate the capability of TESS to deliver high-quality asteroseismic observations for MS and SG stars. The Luminaries stars located in the PLATO LOP fields constitute a unique sample that will play a crucial role in validating, calibrating, and optimising PLATO’s seismic performance from the earliest stages of the mission. Kepler