Savita Mathur

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.

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.

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.

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

The ∼200,000 stars observed by the Kepler mission have provided unprecedented constraints across astrophysics. With the advent of modern spectroscopic and photometric surveys, new limits in stellar characterizations are within reach. In this work, we report a compilation of atmospheric parameters ( T _eff , log ( g ) , and [M/H]) for the Kepler stars by crossmatching with several spectroscopic and spectro-photometric surveys. We use these to calculate bolometric corrections, which combined with color–magnitude diagram information from Gaia yield self-consistent luminosities on a survey-by-survey basis. These properties will aid in future explorations of Kepler data toward new astrophysical insights. We make our catalog publicly available online in Zenodo (doi: 10.5281/zenodo.18620911 ).

Asteroseismology is widely used for the precise mass determination of solar-like oscillating stars, based on individual frequency modeling or homological scaling relations. However, these methods have not been dynamically validated on the main sequence (MS) due to the absence of eclipsing double-lined binary system (SB2) as benchmark objects. By providing the orbital inclination, astrometric binary systems from ESA Gaia DR3 offer an abundant alternative for eclipsing systems. We present KIC 9693187 as the first SB2 hosting a solar-like oscillating post-MS star with dynamical masses. By combining Gaia astrometry with spectroscopic data obtained with the Las Cumbres Observatory network (LCO), we found M ₁ ^dyn = 0.99 ± 0.05 M _⊙ and M ₂ ^dyn = 0.89 ± 0.04 M _⊙ for the primary and secondary, respectively. The asteroseismic parameters were extracted from photometry of the NASA Kepler satellite. The mass from individual frequency modeling is M ₁ ^IF = 0.92 ± 0.01 M _⊙ . Taking into account the systematic uncertainty of 0.04 M _⊙ for best-fit models from individual frequency fitting, we found an agreement within 1.2 σ . From the scaling relations, we obtained a mass range of 0.93 to 0.98 M _⊙ by using the observed large frequency separations (Δ ν ) in the scaling relations for the primary. By using standard corrections for departures from the asymptotic regime of Δ ν , we obtained a mass range of 0.83 to 1.03 M _⊙ . The upper ends of both ranges agree well with the dynamical mass of the primary. This approach provides the first empirical validation for MS solar-like oscillators and opens a new window for validating the asteroseismology. Through a dedicated program targeting astrometric SB2 binary systems, ESA’s PLATO space mission will effectively enlarge the benchmark sample to a considerable extent.