arXiv subjects

Dario J. Fritzewski

Publications and source records attributed to Dario J. Fritzewski.

9 recordsLinked to original sources

arXiv2026

Open Clusters as Laboratories for Cool Star Evolution: Highlights from the Cool Stars 23 Splinter Session

Open clusters remain among the most powerful laboratories for calibrating how fundamental stellar properties such as rotation, magnetic activity, surface chemistry, and internal structure evolve over the pre-main-sequence to main-sequence and post-main-sequence stellar lifetime. Because cluster members share a common age, galactic environment, and initial composition, they provide empirical anchors upon which age-dating techniques such as gyrochronology, activity-age relations, and chemical clocks are built. This paper summarizes the Cool Stars 23 splinter session - Open Clusters as Laboratories for Cool Star Evolution, held on 15 June 2026 in Tokyo, Japan. The session comprised one invited review, ten contributed talks, and ten poster pop-up presentations, organized around three themes: the evolution of rotation, magnetic activity, and chemical abundances (Li-depletion, [C/N], [Y/Mg], and neutron-capture elements). We close with a summary of the open questions identified in the panel discussion and the observational and theoretical work that new surveys (e.g. Gaia DR4, 4MOST, WEAVE) and missions (e.g. PLATO, Roman) will enable over the coming years. Together, these efforts will help assess the current state of the field and shape its future directions.

astro-ph.SR↗

arXiv2026

Asteroseismic forward modelling of 36 $β$ Cep pulsators and inferences on their internal differential rotation

Asteroseismic observations of the interior rotation of main sequence stars have shown that angular momentum transport is much more efficient than expected. Which transport mechanisms are responsible for this is still unclear. Detections of radial differential rotation provide valuable constraints on these transport mechanisms. Differential rotation has been detected in several massive main sequence $β$ Cep pulsators, even though fewer than ten $β$ Cep stars have been asteroseismically modelled in detail so far. We aim to expand the sample of asteroseismically forward modelled $β$ Cep pulsators and exploit their potential to constrain angular momentum transport mechanisms. To that end, we seek to constrain their rotation profiles. We searched for rotational splitting of non-radial modes in a large $β$ Cep sample with identified mode degrees. These were subjected to a novel forward modelling approach involving a 6-dimensional parameter space, which consistently accounts for second-order rotation effects using the state-of-the-art StORM oscillation code. We successfully modelled 36 $β$ Cep stars and constrained crucial parameters such as their initial mass, internal rotation frequency, convective core mass, and age. Like in intermediate-mass stars, the internal rotation rate globally decreases in $β$ Cep stars as they evolve along the main sequence. Radial differential rotation is constrained in 17 $β$ Cep stars. We detect statistically significant deviations from quasi-rigid rotation in 13 stars. Our constraints on eight of these 13 stars indicate the rotation rate decreases with radius while it increases in three other stars, and two display non-monotonic rotation profiles. We affirm that radial differential rotation is common in $β$ Cep stars. Moreover, our constrained rotation profiles suggest that $β$ Cep rotation profiles may be non-monotonic.

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arXiv2026

Gravity-mode main-sequence pulsators in the open clusters NGC 3532 and NGC 2516: Instability strip, near-core rotation, and internal structure

Context. Studying pulsating stars in clusters opens a new window onto stellar physics. Gravity-mode (g-mode) pulsators in open clusters allow us to measure their near-core rotation rates and, together with cluster age and mass constraints, test angular momentum transport in stars above about 1.5 solar masses. Aims. We aim to detect g-mode pulsations in member stars of the young open cluster NGC 3532, about 300 Myr old, and to measure their near-core rotation rates and internal properties. Methods. We used TESS photometry to extract light curves of NGC 3532 members. The observed g-mode period spacings allow us to measure near-core rotation rates and the asymptotic period spacing, Pi0. We also fitted isochrones to the colour-magnitude diagrams of NGC 3532 and NGC 2516 to refine the cluster ages and obtain stellar masses. Results. We constrain the observed instability region of young gamma Doradus members from a blue edge at about 7760 K to a red edge at about 7070 K, while some hotter g-mode pulsators are also seen. The near-core rotation rates show a rotation-mass relation similar to that in NGC 2516: below 1.6 solar masses, the rotation rate increases with mass, whereas above 1.6 solar masses the NGC 3532 stars reach a plateau at about 2.8 d^-1. This mass may mark a threshold between different rotational spin-down mechanisms. Existing evolutionary models with angular momentum transport imply that these g-mode pulsators were born rotating above 55 percent of the critical value. Our simplified models, assuming spherical symmetry, angular momentum conservation, and rigid internal rotation, suggest that either mass-dependent initial rotation rates are required, or that minor angular momentum loss still operates above 1.6 solar masses. Finally, our Pi0 measurements reveal a discrepancy with theoretical predictions for some pulsators, as also found in the younger cluster NGC 2516.

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arXiv2026

Variability classification of TESS targets in LOPS2, the first long-term pointing field of PLATO. Version 1 of the public variability catalogue

The PLAnetary Transits and Oscillations of stars (PLATO) mission is expected to launch in January 2027. A total of 8\% of its data rate will be dedicated to complementary science targets selected from approved Guest Observer proposals. We seek to provide an open-source catalogue of variable stars in PLATO's first long-term observing field, LOPS2. We want to use existing observations from the Transiting Exoplanet Survey Satellite (TESS), which has observed many stars in LOPS2. We classified 38 million calibrated aperture light curves from the TESS-Gaia Light Curve pipeline (TGLC, $G\lesssim17$) for 6 million unique sources in LOPS2 with two machine learning frameworks -- a deep neural network and a feature-based gradient-boosted decision-tree ensemble. We combined their predictions to create this first version of the LOPS2 variability catalogue, performed manual vetting of a sub-sample classified light curves, and a statistical analysis of the results to validate our methodology and to assess the variability properties and parameters of the stars in the catalogue. Our classification resulted in the identification of approximately 72% of the light curves having dominant instrument- or pipeline-induced signal, with the remaining 28% representing 3.6 million individual candidate variable stars, including pulsating, rotating, and eclipsing stars. Candidate pulsators exhibit varied behaviour in terms of their frequencies, amplitudes, rotation, and fundamental parameters. To ensure purity of the samples, filtering on colour, luminosity, the dominant frequency and its amplitude, and presence of close neighbours is helpful. We provide the first version of our PLATO LOPS2 variability catalogue to the community for further study and scrutiny. It is to date one of the largest catalogues of variable stars from an automated classification pipeline.

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arXiv2026

Isochrone-cloud fitting and asteroseismology of the Kepler open cluster NGC6866

We investigate how isochrones computed with different input physics and initial conditions affect the age dating of the open cluster NGC 6866, and compare the results with asteroseismic ages derived from Kepler photometry. Using Gaia DR3 data, we identified 180 cluster members with a clustering algorithm. We then developed an isochrone-cloud fitting method that accounts for a range of free parameters in the input physics. Variable stars were subsequently identified among the cluster members. For 19 g-mode pulsators, we carried out modelling with a dedicated grid of rotating stellar models, constrained by spectroscopic and photometric parameters, the asymptotic gravity-mode period spacing, and the near-core rotation rate. We considered two cases: modelling each pulsator individually and modelling them under the assumption of a common cluster age. PARSEC and MIST isochrones yield discrepant ages of 690 and 467 Myr, respectively. The isochrone-cloud fit indicates an initial critical rotation distribution peaking at 0.6, about a factor of two higher than inferred from asteroseismology. The seismic modelling shows agreement between seismic and isochronal masses, but substantial differences in the derived ages due to differences in internal mixing. When the g-mode pulsators are modelled with a shared cluster age, we obtain 759 Myr, consistent with the PARSEC isochronal age. We conclude that age dating of open clusters is sensitive to the adopted input physics and initial conditions, highlighting the need for better calibrated stellar evolutionary models.

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arXiv2025

Automated all-sky detection of γ Doradus / δ Scuti hybrids in TESS data from positive unlabelled (PU) learning

The Transiting Exoplanet Survey Satellite (TESS) mission has observed hundreds of millions of stars, substantially contributing to the available pool of high-precision photometric space data. Among them are the relatively rare $γ$ Doradus / $δ$ Scuti ($γ$ Dor / $δ$ Sct) hybrid pulsators, which have been previously studied using Kepler data. These stars are perfect laboratories to probe both inner and outer interior stellar layers thanks to them exhibiting both pressure and gravity modes. We seek to classify an all-sky sample of AF stars observed by TESS to find previously undiscovered hybrid pulsators and supply them in a catalogue of candidates. We also aim to compare the light curves produced with the TESS-Gaia Light Curve (TGLC) pipeline, currently underused in variability studies, with other publicly available light curves. We compared dominant and secondary frequencies of confirmed hybrid pulsators in Kepler, extended mission Quick Look Pipeline (QLP) data, and nominal and extended mission TGLC data. We then used a feature-based positive unlabelled (PU) learning classifier to search for new hybrid pulsators amongst TESS AF stars and investigated the properties of the detected populations. We find that the variability of confirmed hybrids in TGLC agrees well with the one occurring in QLP light curves and has a high recovery rate of \kepler-extracted frequencies. Our `smart binning' method allows for robust extraction of hybrids from large unlabelled datasets, with an average out-of-bag prediction for test set hybrids at 93.04\%. The analysis of dominant frequencies in high-probability candidates shows that we find more pressure-mode dominant hybrids. Our catalogue includes 62,026 new candidate light curves from the nominal and extended TESS missions, with individual probabilities of being a hybrid in each available sector.

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arXiv2025

A star-by-star correspondence between X-ray activity and rotation in the young open cluster NGC 2516 with eROSITA

The coronal soft X-ray emission of cool stars, especially when taken in combination with their measured rotation periods, offers insights into their levels of magnetic activity and related transitions. We study the X-ray properties of low-mass members of the open cluster NGC 2516 to explicate their detailed dependencies on mass and rotation. We analysed the pointed SRG/eROSITA satellite observations of NGC 2516 obtained during the calibration and performance verification phase of the mission. We found 1561 X-ray sources within the field of view and related 1007 of them to their optical stellar counterparts, including 655 members of NGC 2516 (433 with rotation periods). We combined these detections with auxiliary optical data to facilitate their interpretation. Furthermore, we extracted X-ray spectra for all sources and fit two-component APEC models to them. To aid the analysis, we grouped stars with similar mass and rotational properties together, which allowed us to investigate the influence of rotation on various X-ray properties. The colour-activity diagram of NGC 2516 displays a general increase in the fractional X-ray luminosity with spectral type change from F through G and K to M-type. However, the behaviour of K-type stars, representing the ones that best sample the fast-to-slow rotational transition, is more complex, with both increased and decreased X-ray emission relative to G-type stars for fast and slow rotators, respectively. The rotation-activity diagram is analogous, with an identifiable desaturated group of X-ray emitters that corresponds to stars in the rotational gap between the fast and slow rotator sequences. We prefer to describe the normalised X-ray emission for all cluster stars as declining logarithmically with Rossby number over those using broken power laws. Coronal temperatures appear to be largely independent of mass or rotation. (abridged)

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arXiv2025

Stationary quasi-periodic pulsations in 20-second cadence TESS flares

Context. Quasi-periodic pulsations (QPPs) are an inherent feature of solar and stellar flares. However, the mechanism behind them is debated hence it is necessary to further study them to obtain a complete picture of flares and their contribution to coronal heating. Aims. We analyze 20-second cadence TESS light curves from sectors 27 to 80 to detect stellar flares and QPPs. Methods. Stellar flare detection was carried out using an automated detection routine based on autoregressive integrated moving average models. QPPs were detected using a Fourier model comparison test (AFINO). Results. We detected 3878 flares across 1285 flaring stars. Notably, 61.2% of flares had a duration of less than 10 min. 61 QPPs were detected across 57 stars significantly expanding the current stellar QPP catalog. The detected periods of the QPPs were in the range of 42 to 193 seconds. In the diagram showing QPP periods against the flare duration a branch emerges. It shows a positive correlation with the flare duration, meaning longer duration flares host longer period QPPs. Conclusion. Our study detected short-period and sub-minute QPPs in stellar flares that have rarely been explored in other works. We find similar scaling laws between solar and stellar QPPs which indicates that QPPs in stellar flares might be analogous to the ones in solar flares as both show evidence of scaling with flare duration.

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arXiv2024

Asteroseismology of the young open cluster NGC 2516 I: Photometric and spectroscopic observations

Asteroseismic modelling of isolated star presents significant challenges due to the difficulty in accurately determining stellar parameters, particularly the stellar age. These challenges can be overcomed by observing stars in open clusters, whose coeval members share an initial chemical composition. The light curves by TESS allow us to investigate and analyse stellar variations in clusters with an unprecedented level. We aim to detect gravity-mode oscillations in the early-type main-sequence members of the young open cluster NGC 2516. We selected the 301 member stars as our sample and analysed the TESS FFI light curves. We also collected high-resolution spectra using the FEROS for the g-mode pulsators. By fitting the theoretical isochrones to the colour-magnitude diagram (CMD) of a cluster, we determined an age of 102 $\pm$ 15 Myr and inferred the extinction at 550 nm ($A_0$) is 0.53 $\pm$ 0.04 mag. We identified 147 stars with surface brightness modulations, 24 with g-mode pulsations ($γ$ Doradus or Slowly Pulsating B stars), and 35 with p-mode pulsations ($δ$ Sct stars). When sorted by colour index, the amplitude spectra of the $δ$ Sct stars show a distinct ordering and reveal a discernible frequency-temperature relationship. The near-core rotation rates, measured from period spacing patterns in two SPB and nine $γ$ Dor stars, reach up to 3/d . This is at the high end of the values found from Kepler data of field stars of similar variability type. The $γ$ Dor stars have internal rotation rates as high as 50% of their critical value, whereas the SPB stars exhibit rotation rates close to their critical rate. We did not find long-term brightness and colour variations in the mid-infrared, which suggests that there are no disk or shell formation events in our sample. We also discussed the results of our spectroscopic observations for the g-mode pulsators.

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