Timothy Van Reeth

Context. Isochrone fitting is a classical method used to determine the ages of open clusters. However, the derived ages depend strongly on the chosen input physics of the stellar models used to deduce the isochrones. Additionally, it remains an open question whether the isochronal ages based on stellar surface parameters are consistent with the asteroseismic ages derived from stellar interior properties. Aims. Our aim was to investigate how isochrones based on different input physics and computed for a variety of initial conditions affect the age dating of the open cluster NGC 6866. Using the four-year light curves of its members assembled by the Kepler mission, we further compared these results with asteroseismically derived ages. Methods. We extracted 180 cluster members by applying a clustering algorithm to Gaia DR3 data, and subsequently developed an isochrone-cloud fitting method that simultaneously accounts for the range of free parameters occurring in the input physics. Synthetic colour–magnitude diagrams (CMDs) with various initial rotation rates were generated and compared with the observations to determine the best-fitting rotation distribution. Variable stars were then identified among the cluster members. For the 19 gravity-mode (g-mode) pulsators, we performed modelling using a dedicated grid of rotating stellar models. This modelling was constrained by stellar surface parameters, the measured asymptotic gravity-mode (g-mode) period spacing Π ₀ , and the near-core rotation rate. Two approaches were considered: modelling the pulsators individually and under the assumption that they share a common age. Results. We found discrepant age estimates from PARSEC and MIST isochrones, which yielded 690 ⁺¹⁴⁰ ₋₃₀ Myr and 467 ⁺⁷⁰ ₋₅₀ Myr, respectively. The isochrone-cloud fitting indicates that NGC 6866 has an initial rotation distribution peaking at v / v _crit = 0.6, which is a discrepancy with a factor of about two compared to the asteroseismic values. The asteroseismic modelling of the g-mode pulsators revealed agreement between seismic and isochronal masses, whereas the derived ages differ substantially due to the difference in internal mixing. When the g-mode pulsators were modelled under the assumption of one shared cluster age, we obtained a value of 579 ⁺⁵⁴ ₋₈₂ Myr, in agreement with the PARSEC-based isochronal age. Conclusions. We conclude that using different input physics and various initial conditions impacts the age-dating results of open clusters. Our findings point to the need for more carefully calibrated evolutionary models. The seismic content of NGC 6866 makes it a suitable middle-aged cluster to achieve such calibrations.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Context. The sparsely sampled time-series photometry from Gaia Data Release 3 (DR3) led to the discovery of more than 100 000 main-sequence non-radial pulsators. The majority of these were further scrutinised by uninterrupted high-cadence space photometry assembled by the Transiting Exoplanet Survey Satellite (TESS). Aims. We combined Gaia DR3 and TESS photometric light curves to estimate the internal physical properties of 2497 gravity-mode pulsators. We performed asteroseismic analyses with two major aims: (1) to measure the near-core rotation frequency and its evolution during the main sequence and (2) to estimate the mass, radius, evolutionary stage, and convective core mass from stellar modelling. Methods. We relied on asteroseismic properties of Kepler γ Doradus and slowly pulsating B stars to derive the cyclic near-core rotation frequency, f_rot, of the Gaia-discovered pulsators from their dominant prograde dipole gravito-inertial pulsation mode. Further, we investigated the impact of adding f_rot as an extra asteroseismic observable apart from the luminosity and effective temperature on the outcome of grid-based modelling from rotating stellar models. Results. We offer a recipe based on linear regression to deduce f_rot from the dominant gravito-inertial mode frequency. It is applicable to prograde dipole modes with an amplitude above 4 mmag and occurring in the sub-inertial regime. By applying it to 2497 pulsators with such a mode, we have increased the sample of intermediate-mass dwarfs with such an asteroseismic observable by a factor of four. We used the estimate of f_rot to deduce spin parameters between two and six, while the sample’s near-core rotation rates range from 0.7% to 25% of the critical Keplerian rate. We used f_rot, along with the Gaia effective temperature and luminosity to deduce the (convective core) mass, radius, and evolutionary stage from grid modelling based on rotating stellar models. We derived a decline of f_rot with a factor of two during the main-sequence evolution for this population of field stars, which covers a mass range from 1.3 M_⊙ to 7 M_⊙. We found observational evidence for an increase in the radial order of excited gravity modes as the stars evolve. For 969 pulsators, we derived an upper limit of the radial differential rotation between the convective core boundary and the surface from Gaia’s vbroad measurement and found values up to 5.4. Conclusions. Our recipe to deduce the near-core rotation frequency from the dominant prograde dipole gravito-inertial mode detected in the independent Gaia and TESS light curves is easy to use, facilitates applications to large samples of pulsators, and allows to map their angular momentum and evolutionary stage in the Milky Way.

Context. With the PLAnetary Transits and Oscillation of stars (PLATO) space mission set for launch in December 2026 by the European Space Agency (ESA), a new photometric legacy and a future of new scientific discoveries await the community. By exploring scientific topics outside of the core science program, the PLATO complementary science program (PLATO-CS) provides a unique opportunity to maximise the scientific yield of the mission. Aims. In this work, we investigate PLATO’s potential for observing pulsating stars across the Hertzsprung–Russell diagram (HRD). This search is distinct from the core science program. Here, we present a PLATO mock asteroseismic catalogue (MOCKA) of intermediate to massive stars as a benchmark to highlight the asteroseismic yield of PLATO-CS in a quantitative way. MOCKA includes simulations of β Cephei, slowly pulsating B (SPB), δ Scuti, γ Doradus, RR Lyrae, Cepheid, hot sub-dwarf, and white dwarf stars. In particular, main sequence gravity (g) mode pulsators are of interest, as some of these stars form an important foundation for the scientific calibration of PLATO. Their pulsation modes primarily probe the radiative region near the convective core boundary, making them unique stellar laboratories for studying the deep internal structure of stars. Methods. MOCKA is based on a magnitude-limited (G ≲ 17) Gaia catalogue. It is a product of realistic end-to-end PlatoSim simulations of stars for the first PLATO pointing field in the southern hemisphere, which will be observed for a minimum duration of two years. Comprising a state-of-the-art hare-and-hound detection exercise, the simulations of this project explore the impact of spacecraft systematics and stellar contamination on the on-board PLATO light curves. Results. We demonstrate, for the first time, PLATO’s ability to detect and recover the oscillation modes for main sequence g-mode pulsators. We show that an abundant spectrum of frequencies is achievable across a wide range of magnitudes and co-pointing PLATO cameras. Within the magnitude-limited regimes simulated in this work (G ≲ 14 for γ Doradus stars and G ≲ 16 for SPB stars), the dominant g-mode frequency was recovered in more than 95% of cases. Furthermore, we find that an increased spacecraft noise budget impacts the recovery of g modes more than the stellar contamination by variable stars. Conclusions. MOCKA helps improve our understanding of the limits of the PLATO mission, as well as to highlight the opportunities to push astrophysics beyond current stellar models. All the data products of this paper are made available to the community for further exploration. The key data products of MOCKA can be found include the magnitude-limited Gaia catalogue of the first PLATO pointing field, together with fully reduced light curves from multi-camera observations for each pulsation class.

Context. KIC 4150611 is a high-order (seventh-order) multiple composed of a triple system with: a F1V primary (Aa), which is eclipsed on a 94.2 d period by a tight binary composed of two K/M dwarfs (Ab1 and Ab2) that also eclipse each other; an eccentric, eclipsing binary composed of two G stars (Ba and Bb); and another faint eclipsing binary composed of two stars of unknown spectral type (Ca and Cb). In addition to its many eclipses, the system is an triple-lined spectroscopic multiple (Aa, Ba, and Bb) and the primary (Aa) is a hybrid pulsator that exhibits high amplitude pressure and gravity modes (g-modes). Furthermore, its g-modes are arrayed in a period-spacing pattern, which greatly assists with mode identification and asteroseismic modelling. In aggregate, this richness in physics offers an excellent opportunity to obtain a precise physical characterisation for some of the stars in this system. Aims. In this work we estimate the stellar parameters of the primary (Aa) by performing asteroseismic analysis on its period-spacing pattern. Methods. We used the C-3PO neural network to perform asteroseismic modelling of the g-mode period-spacing pattern of Aa, examining the interplay of this information with external constraints from spectroscopy (T_eff and log(g)) and eclipse modelling (R). To estimate the level of uncertainty due to different frequency extraction and pattern identification processes, we considered four different variations of the period-spacing patterns. To better understand the correlations between and the uncertainty structure of our parameter estimates, we also employed a classical, parameter-based Markov chain Monte Carlo (MCMC) grid search on four different stellar grids. Results. The externally constrained model that best fits the period-spacing pattern arrives at estimates of the stellar properties for Aa of M = 1.51 ± 0.05 M_⊙, X_c = 0.43 ± 0.04, R = 1.66 ± 0.1 R_⊙, f_ov = 0.010, Ω_c = 1.58 ± 0.01 d⁻¹ with rigid rotation to within the measurement errors, log(T_eff) = 3.856 ± 0.008 dex, log(g) = 4.18 ± 0.04 dex, and log(L) = 0.809 ± 0.005 dex, which agree well with previous measurements from eclipse modelling, spectroscopy, and the Gaia DR3 luminosity. Conclusions. We find that the near-core properties of the best-fitting asteroseismic models are consistent with external constraints from eclipse modelling and spectroscopy. For stellar properties not related to the near-core region, external constraints on the asteroseismic best-fitting models are informative. Aa appears to be a typical example of a γ Dor star, fitting well within existing populations. We find that Aa is quasi-rigidly rotating to within the uncertainties, and note that the asteroseismic age estimate for Aa (1100 ± 100 Myr) is considerably older than the young age (35 Myr) implied by previous isochrone fits to the B binary in the literature. Our MCMC parameter-based grid search agrees well with our pattern-modelling approach. Improved future modelling could come from detailed coverage of metallicity effects and a careful treatment of envelope physics.

Context. Massive main-sequence stars have convective cores and radiative envelopes, but can also have sub-surface convection zones caused by partial ionisation zones. However, the convective properties of such regions strongly depend on opacity and therefore a star’s metallicity. Non-rotating 1D evolution models of main-sequence stars between 7 ≤ M ≤ 40 M_⊙ and the metallicity of the Small Magellanic Cloud (SMC) galaxy suggest tenuous (if any) sub-surface convection zones when using the Rayleigh number as a criterion for convection owing to their substantially lower metallicity compared to Galactic massive stars. Aims. We test whether massive stars of different metallicities both inside and outside of asteroseismically calibrated stability windows for sub-surface convection exhibit different properties in stochastic low-frequency (SLF) variability. Thus, we aim to constrain the metallicity dependence of the physical mechanism responsible for SLF variability commonly found in light curves of massive stars. Methods. We extracted customised light curves from the ongoing NASA Transiting Exoplanet Survey Satellite (TESS) mission for a sample of massive stars using an effective point spread function (ePSF) method, and compared their morphologies in terms of characteristic frequency, ν_char, and amplitude using a Gaussian process (GP) regression methodology. Results. We demonstrate that the properties of SLF variability observed in time series photometry of massive stars are generally consistent across the metallicity range from the Milky Way down to the SMC galaxy, for stars both inside and outside of the sub-surface stability windows based on the Rayleigh number as a criterion for convection. Conclusions. We conclude that non-rotating 1D stellar structure models of sub-surface convection cannot alone be used to explain the mechanism giving rise to SLF variability in light curves of massive stars. Additionally, the similar properties of SLF variability across a wide range of metallicity values, which follow the same trends in mass and age in the Hertzsprung–Russell (HR) diagram at both high and low metallicity, support a transition in the dominant mechanism causing SLF variability from younger to more evolved stars. Specifically, core-excited internal gravity waves (IGWs) are favoured for younger stars lacking sub-surface convection zones, especially at low metallicity, and sub-surface convection zones are favoured for more evolved massive stars.

Context. Gravito-inertial asteroseismology saw its birth from the 4-year-long light curves of rotating main-sequence stars assembled by the Kepler space telescope. High-precision measurements of internal rotation and mixing are available for about 600 stars of intermediate mass so far that are used to challenge the state-of-the-art stellar structure and evolution models. Aims. Our aim is to prepare for future large ensemble modelling of gravity-mode pulsators by relying on a new sample of such stars recently discovered from the third Data Release of the Gaia space mission and confirmed by space photometry from the TESS mission. This sample of potential asteroseismic targets is about 23 times larger than the Kepler sample. Methods. We use the effective temperature and luminosity inferred from Gaia to deduce evolutionary masses, convective core masses, radii, and ages for ∼14 000 gravity-mode pulsators classified as such from their nominal TESS light curves. We do so by constructing two dedicated grids of evolutionary models for rotating stars with input physics from the asteroseismic calibrations of Keplerγ Dor pulsators. These two grids consider the distribution of initial rotation velocities at the zero-age main sequence deduced from gravito-inertial asteroseismology, for two extreme values found for the metallicity of γ Dor stars deduced from spectroscopy ([M/H]=0.0 and −0.5). Results. We find the new gravity-mode pulsators to cover an extended observational instability region covering masses from about 1.3 M_⊙ to about 9 M_⊙. We provide their mass-luminosity and mass-radius relations, as well as convective core masses. Our results suggest that oscillations excited by the opacity mechanism occur uninterruptedly for the mass range above about 2 M_⊙, where stars have a radiative envelope aside from thin convection zones in their excitation layers. Conclusions. Our evolutionary parameters for the sample of Gaia-discovered gravity-mode pulsators with confirmed modes by TESS offer a fruitful starting point for future TESS ensemble asteroseismology once a sufficient number of modes is identified in terms of the geometrical wave numbers and overtone for each of the pulsators.

Context. KIC 4150611 is a high-order multiple composed of a triple system. It comprises: (1) a F1V primary (Aa) that is eclipsed on a 94.2d period by a tight 1.52d binary composed of two dim K/M dwarfs (Ab1 and Ab2), which also eclipse each other; (2) an 8.65d eccentric, eclipsing binary composed of two G stars (Ba and Bb); and (3) another faint eclipsing binary composed of two stars of unknown spectral type (Ca and Cb). In addition to its many eclipses, the system is an SB3 spectroscopic multiple (Aa, Ba, and Bb), and the primary (Aa) is a hybrid pulsator that exhibits high amplitude pressure and gravity modes. In aggregate, this richness in physics offers an excellent opportunity to obtain a precise physical characterisation of some of the stars in this system. Aims. In this work we aim to characterise the F1V primary by modelling its complex eclipse geometry and disentangled stellar spectra in preparation for follow-up work that will focus on its pulsations. Methods. We employed a novel photometric analysis of the complicated eclipse geometry of Aa to obtain the orbital and stellar properties of the triple. We acquired 51 TRES spectra at the Fred L. Whipple Observatory, calculating radial velocities and orbital elements of Aa (SB1) and the B binary (SB2). These spectra and radial velocities were used to perform spectral disentangling for Aa, Ba, and Bb. Spectral modelling was applied to the disentangled spectrum of Aa to obtain atmospheric properties. Results. From our eclipse modelling we obtain precise stellar properties of the triple, including the mass ratios (M_Aa/(M_Ab1 + M_Ab2) = 3.61 ± 0.01, M_Ab1/M_Ab2 = 1.113 ± 0.001), the separation ratio (a_Aab/a_Ab1Ab2 = 21.81 ± 0.01), orbital periods (P_Aab = 94.29486 ± 0.00008d, P_Ab1Ab2 = 1.522248 ± 0.000001d), and stellar radii (R_Aa = 1.64 ± 0.06 R_⊙, R_Ab1 = 0.42 ± 0.01 R_⊙, R_Ab2 = 0.38 ± 0.01 R_⊙). Via radial velocity fitting and spectral disentangling, we find orbital elements for Aa, Ba, and Bb that are in excellent agreement with each other and with previous results in the literature. Spectral modelling on the disentangled spectrum of Aa provides constraints on the effective temperature (T_eff = 7280 ± 70 K), surface gravity (log(g) = 4.14 ± 0.18 dex), micro-turbulent velocity (v_micro = 3.61 ± 0.19 km s⁻¹), rotation velocity (v sin i = 127 ± 4 km s⁻¹), and metallicity ([M/H] = − 0.23 ± 0.06) that are also in good agreement with previous spectral modelling. Particular attention is paid to the light fraction of Aa, which our spectroscopic analysis determines to be between 0.92 and 0.94, while our eclipse modelling prefers a lower light fraction of 0.84 ± 0.03, similar to the previous literature value of 0.85. However, the eclipse models are still able to obtain an excellent fit to the solution when constrained to light fractions between 0.92 and 0.96, while our spectroscopic analysis proves to be far more sensitive to the light fraction, leading us to conclude that the higher light fraction from spectroscopy is likely the correct solution.

Context.Asteroseismic modelling of isolated stars presents significant challenges due to the difficulty in accurately determining stellar parameters, particularly the stellar age. These challenges can be overcome by observing stars in open clusters whose coeval members share an initial chemical composition. The light curves from the all-sky survey by the Transiting Exoplanet Survey Satellite (TESS) allow us to investigate and analyse stellar variations in clusters with an unprecedented level of detail for the first time.Aims.We aim to detect gravity-mode oscillations in the early-type main-sequence members of the young open cluster NGC 2516 to deduce their internal rotation rates.Methods.We selected the 301 member stars with no more than mild contamination as our sample. We analysed the full-frame image light curves, which provide nearly continuous observations in the first and third years of TESS monitoring. We also collected high-resolution spectra using the Fiber-fed Extended Range Optical Spectrograph for theg-mode pulsators, with the aim of assessing theGaiaeffective temperatures and gravities and preparing for future seismic modelling.Results.By fitting the theoretical isochrones to the colour-magnitude diagram of a cluster, we determined an age of 102 ± 15 Myr and inferred that the extinction at 550 nm (A₀) is 0.53 ± 0.04 mag. We identified 147 stars with surface-brightness modulations: 24 with gravity (g-)mode pulsations (γDoradus or slowly pulsating B-type stars) and 35 with pressure (p-)mode pulsations (δSct stars). When sorted by colour index, the amplitude spectra of theδSct stars show a distinct ordering and reveal a discernible frequency-temperature relationship. The near-core rotation rates, measured from period spacing patterns in two slowly pulsating B-type (SPB) stars and nineγDor stars, reach up to 3 d⁻¹. This is at the high end of the values found fromKeplerdata of field stars of similar variability type. TheγDor stars of NGC 2516 have internal rotation rates as high as 50% of their critical value, whereas the SPB stars exhibit rotation rates close to their critical rate. Although the B-type stars are rotating rapidly, we did not find long-term brightness and colour variations in the mid-infrared, which suggests that there are no disc or shell formation events in our sample. We also discussed the results of our spectroscopic observations for theg-mode pulsators.