arXiv subjects

D. B. Palakkatharappil

Publications and source records attributed to D. B. Palakkatharappil.

6 recordsLinked to original sources

arXiv2026

Luminaries in the Sky: The TESS LEGACY sample of bright stars. II. In-depth seismic characterisation of 32 naked-eye stars in the PLATO LOP fields

The NASA TESS mission is conducting a nearly full-sky survey, enabling the photometric characterisation of millions of stars. The forthcoming ESA PLATO mission 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 for the first time for 26 of them. We used three independent seismic pipelines, one of which is similar to the algorithms used in the official PLATO pipeline. Statistical criterion were applied to identify the optimal combination of data calibration, observing cadence, and fitting pipeline for each star. For all stars, we derived large and small separations, the asymptotic phase term, radial mode amplitudes, and mean linewidths per order. Comparisons reveal consistent trends in the seismic parameters, confirming the robustness of our analysis. In SGs, mixed-mode identification differs in the three pipelines, revealing extraction inconsistencies requiring longer datasets to improve our mode identifications. 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.

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arXiv2026

Characterizing six seismic solar analogs observed by Kepler, K2, and HERMES

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 in 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 \textit{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 HERMES spectrograph 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~$\mathrm{M}_\odot$, radii between 0.95 and 1.08~$\mathrm{R}_\odot$, and ages from about 1.8 to 9.1~Gyr, with typical systematic uncertainties of $\pm$ 0.02~$\mathrm{M}_\odot$, $\pm$ 0.01~$\mathrm{R}_\odot$, and $\pm$ 0.7~Gyr, respectively. One star, EPIC~206064678, exhibits properties very similar to those of the Sun, with $M = 1.016 \pm 0.033\,\mathrm{M}_\odot$, $R = 0.990 \pm 0.011\,\mathrm{R}_\odot$, and an age of $5.40 \pm 0.12$\,Gyr. It can therefore be considered a close solar twin, although it is slightly older and more metal-rich ($0.25 \pm 0.07$\,dex). Four targets display binarity signatures and all exhibit very low chromospheric activity. This work broadens the sample of well-characterized seismic solar analogs towards a larger sample of metallicities and ages, providing new references for comparative stellar studies and future asteroseismic investigations.

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arXiv2026

Dynamical mass of a solar-like oscillator at the main-sequence turnoff from Gaia astrometry & ground-based spectroscopy

Asteroseismology is widely used for precise determining of masses of solar-like oscillating stars by performing individual-frequency modeling or applying homological scaling relations. However, these methods lack dynamical validation on the main sequence 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 KIC693187 as the first SB2, hosting a solar-like oscillating post-main-sequence star with dynamical masses. By combining Gaia astrometry with spectroscopic obtained with the Las Cumbres Observatory network (LCO), we find $M_1^\mathrm{dyn}$=0.99$\pm$0.05$M_\odot$ and $M_2^\mathrm{dyn}$=0.89$\pm$0.04$M_\odot$ for the primary and secondary, respectively. Asteroseismic parameters were extracted from photometry of the NASA \Kepler satellite. The mass from individual frequency modeling is $M_1^\mathrm{IF}$=0.92$\pm$0.01$M_\odot$. Taking into account the systematic uncertainty of 0.04$M_\odot$ for best fit models from individual frequency fitting, we find an agreement within 1.2$σ$. From scaling relations we obtain a mass range of 0.93 to 0.98$M_\odot$ by using the observed large frequency separations (\dnu) in the scaling relations for the primary. By using standard corrections for departures from the asymptotic regime of \dnu, we obtained a mass range of 0.83 to 1.03$M_\odot$. The upper ends of both ranges agree well with the dynamical mass of the primary. This approach provides the first empirical validation for main-sequence solar-like oscillators and opens a new window for validating asteroseismology. Through a dedicatded program targeting astrometric SB2 binary systems, ESA's PLATO space mission will provide will enlarge the benchmark sample substantially.

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arXiv2025

Beyond the Nyquist frequency: Asteroseismic catalog of undersampled Kepler late subgiants and early red giants

Subgiants and early red giants are crucial for studying the first dredge-up, a key evolutionary phase where the convective envelope deepens, mixing previously interior-processed material and bringing it to the surface. Yet, very few have been seismically characterized with Kepler because their oscillation frequencies are close to the 30 minute sampling frequency of the mission. We developed a new method as part of the new PyA2Z code to identify super-Nyquist oscillators and infer their global seismic parameters, $ν_\mathrm{max}$ and large separation, $Δν$. Applying PyA2Z to 2 065 Kepler targets, we seismically characterize 285 super-Nyquist and 168 close-to-Nyquist stars with masses from 0.8 to 1.6 M$_\odot$. In combination with APOGEE spectroscopy, Gaia spectro-photometry, and stellar models, we derive stellar ages for the sample. There is good agreement between the predicted and actual positions of stars on the HR diagram (luminosity vs. effective temperature) as a function of mass and composition. While the timing of dredge-up is consistent with predictions, the magnitude and mass dependence show discrepancies with models, possibly due to uncertainties in model physics or calibration issues in observed abundance scales.

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arXiv2024

Measuring stellar surface rotation and activity with the PLATO mission -- I. Strategy and application to simulated light curves

The Planetary Transits and Oscillations of stars mission (PLATO) will allow us to measure surface rotation and monitor photometric activity of tens of thousands of main sequence solar-type and subgiant stars. This paper is the first of a series dedicated to the preparation of the analysis of stellar surface rotation and photospheric activity with the near-future PLATO data. We describe in this work the strategy that will be implemented in the PLATO pipeline to measure stellar surface rotation, photometric activity, and long-term modulations. The algorithms are applied on both noise-free and noisy simulations of solar-type stars, which include activity cycles, latitudinal differential rotation, and spot evolution. PLATO simulated systematics are included in the noisy light curves. We show that surface rotation periods can be recovered with confidence for most of the stars with only six months of observations and that the {recovery rate} of the analysis significantly improves as additional observations are collected. This means that the first PLATO data release will already provide a substantial set of measurements for this quantity, with a significant refinement on their quality as the instrument obtains longer light curves. Measuring the Schwabe-like magnetic activity cycle during the mission will require that the same field be observed over a significant timescale (more than four years). Nevertheless, PLATO will provide a vast and robust sample of solar-type stars with constraints on the activity-cycle length. Such a sample is lacking from previous missions dedicated to space photometry.

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arXiv2023

Asteroseismic age constraints on the open cluster NGC 2477 using oscillating stars identified with TESS FFI

The ages of pulsating stars in clusters can be determined by isochrone fitting and it can be further improved by asteroseismic modelling. We analyse the intermediate-age open cluster NGC2477, known to suffer from differential extinction, to explore if asteroseismology and clusters characteristics can help understand the metallicity, extinction and result in better age determinations than isochrone-fitting alone. We combine a multitude of recent observations from Gaia, high-resolution spectroscopy, and extinction maps to analyse the cluster and then search for and detect variability in the member stars using TESS FFI data. To interpret all of these data, we used stellar structure, evolution and oscillation codes. We performed an isochrone fitting to the cluster using publically-available isochrones, which provides a cluster age of between 0.6 to 1.1 Ga. Then using TESS Full-frame images, we analysed the time dimension of the members of this cluster. We created optimised pixel light curves using the ${\tt tessipack}$ package which allows us to consider possible contamination by nearby stars. Using these light curves, we identified many interesting levels of variability of stars in this cluster, including binaries and oscillating stars. For the asteroseismic analysis, we selected a few uncontaminated A--F type oscillating stars and used the MESA and GYRE codes to interpret the frequency signals. By comparing the theoretical and the observed spectra, we identified frequency separations, $Δν$, for four stars. Then using the identified $Δν$ and imposing that the best matched theoretical models have the same age, metallicity and background extinction, we constrained the cluster's age to 1.0 $\pm$ 0.1 Ga. We conclude that using the TESS FFI data, we can identify oscillating stars in clusters, which allows us to better refine their ages.

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