Joey S. G. Mombarg

The asteroseismology of gravity-mode (g-mode) pulsators covering BAF-type stars has shown that angular momentum is redistributed during the main sequence. The efficiency of the transport, however, remains largely uncalibrated. This aim of this paper is to exploit a sample of 2937 characterised g-mode pulsators (the largest sample to date) to place constraints on the efficiency of angular momentum transport by assuming an effective viscosity or an Eddy viscosity based on the Tayler--Spruit dynamo within a fully diffusive framework. We computed grids of rotating stellar evolution models that we then used to simulate a population of stars by sampling from these grids with prior distributions on the mass, age, and initial rotation rate. We then compared these simulated distributions of rotation frequencies and specific angular momentum (J/M) to those of the sample of observed stars. cm^2, s^ -1 or higher. Viscosities predicted by the Tayler--Spruit dynamo are in general far above this value and can explain the data as well. Future observational studies of main sequence g-mode pulsators are encouraged to measure core-to-surface rotation rates, particularly of B-type stars. For this work we have exploited the constraining potential of near-core rotation frequencies alone, while the contrast with the surface rotation would allow us to further unravel the mechanisms driving the transport.

Context. HR6819 is the first system with a puffed-up low-mass stripped star + a classical Be star whose nature has been confirmed by optical interferometry. The system exhibits the most extreme mass ratio (15.7 ± 1.1), the lowest stripped star mass (0.270 ± 0.056 M _⊙ ), and one of the shortest orbital periods (40.3266 ± 0.0016 days) compared to similar observed binaries. As a post mass transfer candidate, HR6819 offers a unique opportunity to test the physics of binary interaction, especially the fraction of mass accreted by the Be progenitor (the efficiency of the mass transfer) required to produce the system’s extreme mass ratio. Aims. This work aims to reconstruct the possible evolutionary history of HR6819 in the context of stable mass transfer via Roche lobe overflow. We want to explore how the tight constraints on the system’s total mass, mass ratio and orbital period are limiting the range of possible progenitors of the system. Methods. Based on analytical expectations for the orbital evolution, we build grids of MESA simulations designed to match the present-day orbital period and mass ratio of the system, with different mass transfer efficiencies from fully conservative to 50% efficient. Results. We show that evolution via stable mass transfer cannot explain the combined extreme current mass ratio and tight orbital period of the system. There is a limit on how extreme the post-mass transfer mass ratio of the progenitor binary can be at a fixed detachment period, and this limit is dependent on the efficiency of the mass transfer episode: the less efficient the mass transfer episode, the less extreme the mass ratio at detachment. Even in the case of fully conservative mass transfer, the most extreme mass ratio we can produce with binary evolution simulations is q ∼ 11.5 at P ∼ 40 days, which is significantly below the observed value. We also show that the reported luminosities for each component significantly exceed the value expected from their mass. In particular, based on simple stripped star models, we find that the luminosity of the bloated stripped star requires a star with a mass of ∼0.7 M _⊙ , which is over twice the measured value. Conclusions. Our work shows that the post-interaction properties of HR6819, especially its extreme mass ratio and orbital period, cannot be produced by stable mass transfer under standard assumptions.

Context. Asymmetries in the observed rotational splittings of a multiplet contain information about the star’s rotation profile and internal magnetic field. Moreover, the frequency regularities of multiplets can be used for mode identification. However, to exploit this information, highly accurate theoretical predictions are needed. Aims. We aim to quantify the difference in the predicted mode asymmetries between a 1D perturbative method and a 2D method that includes a 2D stellar structure model, which takes rotation into account. We then place these differences between 1D and 2D methods in the context of asteroseismic measurements of internal magnetic fields. We only focus on the asymmetries and not on possible additional frequency peaks that can arise when the magnetic and rotation axis are misaligned. Methods. We coupled the 1D pulsation codes GYRE and StORM to the 2D stellar structure code ESTER and compared the oscillation predictions with the results from the 2D TOP pulsation code. We focused on zero-age main-sequence models representative of rotating β Cephei pulsators spinning at up to 20 per cent of the critical Keplerian rotation rate. Specifically, we investigated low-radial-order gravity and pressure modes. Results. We find a generally good agreement between the oscillation frequencies resulting from the 1D and 2D pulsation codes. We report differences in predicted mode multiplet asymmetries of mostly below 0.06 d ⁻¹ . Since the magnetic asymmetries are small compared to the differences in the rotational asymmetries resulting from the 1D and 2D predictions, accurate measurements of the magnetic field are in most cases challenging. Conclusions. Differences in the predicted mode asymmetries of a rotating star between 1D perturbative methods and 2D non-perturbative methods can greatly hinder accurate measurements of internal magnetic fields in main-sequence pulsators with low-order modes. Nevertheless, reasonably accurate measurements could be possible with n _pg ≥ 2 modes if the internal rotation is roughly below 10 per cent of the Keplerian critical rotation frequency for (aligned) magnetic fields of the order of a few hundred kilogauss. While the differences between the 1D and 2D frequency predictions are mostly too large for internal magnetic field detections, the rotational asymmetries predicted by StORM are in general accurate enough for asteroseismic modelling of the stellar rotation in main-sequence stars with identified low-order modes.

Context. In the modern era of large-scale photometric time-domain surveys, relatively rare but information-rich eclipsing binary systems can be leveraged at a population level across the Hertzsprung-Russel diagram to improve our knowledge of stellar evolution. This high-precision photometry is also excellent for assessing and exploiting the asteroseismic properties of such stars and results in a powerful synergy that has great potential for shedding light on how stellar interiors and tides affect stellar evolution and mass transfer. Aims. In this work, we seek to characterise a large sample of 14 377 main sequence eclipsing binaries in terms of their stellar, astero-seismic, and orbital properties. Methods. We conducted manual vetting on a sub-set of 4000 targets from our full 14 377 target sample to identify targets with pressure or gravity modes. We inferred stellar properties including mass, the convective core mass, radius, and central H fraction for the primary using the Gaia Data Release 3 effective temperature and luminosity estimates and a grid of asteroseismically calibrated stellar models. We used surface brightness ratio and radius ratio estimates from previous eclipse analyses to study the effect of binarity on our results. Results. Through our manual vetting, we identified 751 candidate g-mode pulsators, 131 p-mode pulsators, and a further 48 hybrid pulsators. The inferred stellar properties of the hybrid and p-mode pulsators are highly correlated, while the orbital properties of the hybrid pulsators align best with the g-mode pulsators. The g-mode pulsators themselves show a distribution that peaks around the classical γ Dor instability region but extends continuously towards higher masses, with no detectable divide between the classical γ Dor and SPB instability regions. There is evidence at the population level for a heightened level of tidal efficiency in stars showing g-mode or hybrid variability. We corrected the primary mass inference for binarity based on eclipse measurements of the surface brightness and radius ratios, resulting in a relatively small shift towards lower masses. Conclusions. This work provides a working initial characterisation of this sample from which more detailed analyses folding in aster-oseismic information can be built. It also provides a foundational understanding of the limitations and capabilities of this kind of rapid, scalable analysis that will be highly relevant in planning the exploitation of future large-scale binary surveys.

Context. Although asteroseismology is regarded as the most powerful tool for probing stellar interiors, seismic modelling remains dependent on global stellar parameters such as temperature and luminosity. Stellar clusters offer direct measurements of these parameters by fitting a colour–magnitude diagram. This makes the application of asteroseismology in stellar clusters a valuable approach to advancing the entire field of stellar physics modelling. Aims. We aimed to develop seismic modelling for gravity-mode pulsators in the open cluster NGC 2516 to determine stellar ages and investigate internal mixing processes. Methods. We computed 1D stellar evolutionary models using the code called modules for experiments in stellar astrophysics ( MESA ), which incorporates rotation-induced transport processes. Exponential overshooting in the transition layers between convective and radiative regions was included, and rotation-induced mixing in the radiative envelope. Grids of evolutionary models that covered isochrone-derived mass ranges were computed. The models were evolved up to 300 Myr because of the young age of the cluster (∼100 Myr). Results. By fitting the frequencies of identified modes of four gravity-mode member pulsators simultaneously, we measured the seismic age of cluster NGC 2516 as 132 ± 8 Myr. This high-precision seismic age estimate deviates by 1 σ from the isochronal age derived from public MESA isochrones and stellar tracks ( MIST ) isochrones for rotating stars. Our findings show that seismic modelling strongly constrains core overshooting, but because the period spacing patterns are smooth, it provides weak constraints on mixing in the radiative envelope of young gravity-mode pulsators. The two most massive gravity-mode pulsators have MIST masses of ∼2.0 M _⊙ while their seismic masses are ∼1.75 M _⊙ . We constructed new asteroseismology-calibrated isochrones using input physics identical to that of our seismic model grid. While this resolved the age discrepancy, the mass discrepancy is only partially addressed. The remaining small but persisting mass discrepancy implies a mismatch between the physics in core to surface environments of 1D stellar models and the seismic observables probing these areas of fast-rotating stars.

Although asteroseismology is regarded as the most powerful tool for probing stellar interiors, seismic modelling remains dependent on global stellar parameters. Stellar clusters offer direct measurements of these parameters by fitting a CMD, making the application of asteroseismology in clusters a valuable approach to advancing stellar physics modelling. We aimed to develop seismic modelling for gravity-mode pulsators in the open cluster NGC 2516 to determine stellar ages. We computed 1D stellar models using MESA, incorporating rotation-induced transport processes. Exponential overshooting was included, as well as rotationally induced mixing in the radiative envelope. Grids of evolutionary models were computed covering isochrone-derived mass ranges. The models were evolved up to 300 Myr because of the cluster's young age (~100Myr). By fitting the frequencies of identified modes of four gravity-mode member pulsators simultaneously, we measure the seismic age of the cluster NGC 2516 as 132+-8Myr. This high-precision seismic age estimate deviates by 1sigma from the isochronal age derived from public MIST isochrones for rotating stars. Our findings show that seismic modelling strongly constrains core overshooting, but because the period spacing patterns are smooth, it provides weak constraints on mixing in the radiative envelopes. The two most massive gravity-mode pulsators have MIST masses ~2.0M_sun while their seismic masses are 1.75M_sun. We constructed new asteroseismology-calibrated isochrones using input physics identical to that of our seismic model grid. While this resolves the age discrepancy, the mass discrepancy is only partially addressed. The remaining small yet persisting mass discrepancy implies a mismatch between the physics in core to surface environments of 1D stellar models and the seismic observables probing those areas of fast-rotating stars.

Context. The rotational evolution of stars remains an open question in stellar physics because numerous phenomena contribute to the distribution of angular momentum. Aims. This paper aims to determine the timescale over which a rotating early-type star relaxes to a steady baroclinic state or, equivalently, the conditions under which its nuclear evolution is slow enough to allow the star's evolution to be modelled as a series of quasi-steady states. Methods. We investigate the damping timescale of baroclinic and viscous eigenmodes that are potentially excited by the continuous forcing of nuclear evolution. We first examine this with a spherical Boussinesq model. Since much of the dynamics is concentrated in the radiative envelope of the star, we then improve the realism of the modelling by using a polytropic model of the envelope that incorporates a realistic density profile. Results. The polytropic model of the envelope highlights the key role of the region at the core-envelope interface. The results of evolutionary models recently obtained with two-dimensional axisymmetric ESTER models appear to arise from the slow damping of viscous modes. Using a vanishing Prandtl number appears to be too strong an approximation to explain the models’ dynamics. Baroclinic modes, previously thought to be good candidates for this relaxation process, are found to be too rapidly damped. Conclusions. The dynamical response of rotating stars to the slow forcing of their nuclear evolution appears as a complex combination of non-oscillating eigenmodes. Simple Boussinesq approaches are not sufficiently realistic to explain this reality. This study underlines the key role of layers near the core-envelope interface in early-type stars as well as the importance of angular momentum transport mechanisms-here represented by viscosity-for early-type stars to reach critical rotation, which is presumably associated with the Be phenomenon.

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

The rotational evolution of stars is still an open question of stellar physics because of the numerous phenomena that can contribute to the distribution of angular momentum. This paper aims at determining the time scale over which a rotating early-type star relaxes to a steady baroclinic state or, equivalently, in which case its nuclear evolution is slow enough to let the evolution of the star be modelled by a series of quasi-steady states. We investigate the damping time scale of baroclinic and viscous eigenmodes that are potentially excited by the continuous forcing of nuclear evolution. We first investigate this problem with a spherical Boussinesq model. Since much of the dynamics is concentrated in the radiative envelope of the star, we then improve the realism of the modelling by using a polytropic model of the envelope that takes into account a realistic density profile. The polytropic model of the envelope underlines the key role of the region at the core-envelope interface. The results of evolutionary models recently obtained with two-dimensional axisymmetric ESTER models turn out to be a consequence of the slow damping of viscous modes. Using a vanishing Prandtl number appears to be too strong an approximation to explain the dynamics of the models. Baroclinic modes previously thought as good candidates of this relaxation process turn out to be too quickly damped. The dynamical response of rotating stars to the slow forcing of its nuclear evolution appears as a complex combination of non-oscillating eigenmodes. Simple Boussinesq approach are not realistic enough to explain this reality. The present work underlines the key role of layers near the core-envelope interface in an early-type star and also the key role of any angular momentum transport mechanism, here played by viscosity, for early-type stars to reach critical rotation, presumably associated with the Be phenomenon.

Context. Internal gravity waves (IGWs) have been shown to contribute to the transport of chemical elements in stars with a convective core and radiative envelope. Recent two-dimensional hydrodynamical simulations of convection in intermediate-mass stars have provided estimates of the chemical mixing efficiency of such waves. The chemical diffusion coefficient from IGW mixing is described by a constant A, times the squared wave velocity. However, the value of A remains unconstrained by such simulations. Aims. This work aims to investigate what values A can take in order to reproduce the observed nitrogen surface abundances of the most nitrogen-enriched massive stars. Furthermore, we discuss the prevalence of IGW mixing compared to rotational mixing. Methods. We provide an implementation of these (time-dependent) mixing profiles predicted from hydrodynamical simulations in the one-dimensional stellar evolution code MESA. We computed evolution tracks for stars between 3 and 30 M_⊙ with this new implementation for IGW mixing and studied the evolution for the surface abundances of isotopes involved in the CNO cycle, particularly the nitrogen-14 isotope. Results. We show that this one-dimensional framework that predicts the chemical diffusion coefficient from IGW mixing yields consistent morphologies of the mixing profile in comparison with hydrodynamical simulations. We find that the value of A must increase with mass in order to reproduce the most nitrogen-enriched stars. If we assume these calibrated values for A, mixing by IGWs is a potential mechanism to reproduce well-mixed stars without needing rapid rotation. Conclusions. We have provided observational limits on the efficiency of IGW mixing for future theoretical studies. Furthermore, future asteroseismic modelling efforts that take IGW mixing into account will be able to place additional constraints on the convective core mass, as our models predict that the convective core would be significantly more massive if IGW mixing is indeed efficient.