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ABSTRACT We revisit the tidally excited oscillations (TEOs) in the A-type main-sequence eccentric binary KOI-54, the prototype of heartbeat stars. Although the linear tidal response of the star is a series of orbital-harmonic frequencies which are not stellar eigenfrequencies, we show that the non-linearly excited non-orbital-harmonic TEOs are eigenmodes. By carefully choosing the modes which satisfy the mode-coupling selection rules, a period spacing (ΔP) pattern of quadrupole gravity modes (ΔP ≈ 2520–2535 s) can be discerned in the Fourier spectrum, with a detection significance level of $$99.9{{\ \rm per\ cent}}$$. The inferred period spacing value agrees remarkably well with the theoretical l = 2, m = 0 g modes from a stellar model with the measured mass, radius, and effective temperature. We also find that the two largest-amplitude TEOs at N = 90, 91 harmonics are very close to resonance with l = 2, m = 0 eigenmodes, and likely come from different stars. Previous works on tidal oscillations primarily focus on the modelling of TEO amplitudes and phases, the high sensitivity of TEO amplitude to the frequency detuning (tidal forcing frequency minus the closest stellar eigenfrequency) requires extremely dense grids of stellar models and prevents us from constraining the stellar physical parameters easily. This work, however, opens the window of real tidal asteroseismology by using the eigenfrequencies of the star inferred from the non-linear TEOs and possibly very-close-to-resonance linear TEOs. Our seismic modelling of these identified eigen g-modes shows that the best-matching stellar models have (M ≈ 2.20, 2.35 M⊙) and super-solar metallicity, in good agreement with previous measurements. 

ABSTRACT The first bright objects to form in the Universe might not have been ‘ordinary’ fusion-powered stars, but ‘dark stars’ (DSs) powered by the annihilation of dark matter (DM) in the form of weakly interacting massive particles (WIMPs). If discovered, DSs can provide a unique laboratory to test DM models. DSs are born with a mass of the order of M⊙ and may grow to a few million solar masses; in this work we investigate the properties of early DSs with masses up to $$\sim \! 1000 \, \mathrm{ M}_\odot$$, fueled by WIMPS weighing 100 GeV. We improve the previous implementation of the DM energy source into the stellar evolution code mesa. We show that the growth of DSs is not limited by astrophysical effects: DSs up to $$\sim \!1000 \, \mathrm{ M}_{\odot }$$ exhibit no dynamical instabilities; DSs are not subject to mass-loss driven by super-Eddington winds. We test the assumption of previous work that the injected energy per WIMP annihilation is constant throughout the star; relaxing this assumption does not change the properties of the DSs. Furthermore, we study DS pulsations, for the first time investigating non-adiabatic pulsation modes, using the linear pulsation code gyre. We find that acoustic modes in DSs of masses smaller than $$\sim \! 200 \, \mathrm{ M}_\odot$$ are excited by the κ − γ and γ mechanism in layers where hydrogen or helium is (partially) ionized. Moreover, we show that the mass-loss rates potentially induced by pulsations are negligible compared to the accretion rates. 

ABSTRACT Strongly magnetic B-type stars with moderately rapid rotation form ‘centrifugal magnetospheres’ (CMs) from the magnetic trapping of stellar wind material in a region above the Kepler co-rotation radius. A long-standing question is whether the eventual loss of such trapped material occurs from gradual drift and/or diffusive leakage, or through sporadic ‘centrifugal breakout’ (CBO) events, wherein magnetic tension can no longer contain the built-up mass. We argue here that recent empirical results for Balmer-α emission from such B-star CMs strongly favour the CBO mechanism. Most notably, the fact that the onset of such emission depends mainly on the field strength at the Kepler radius, and is largely independent of the stellar luminosity, strongly disfavours any drift/diffusion process, for which the net mass balance would depend on the luminosity-dependent wind feeding rate. In contrast, we show that in a CBO model, the maximum confined mass in the magnetosphere is independent of this wind feeding rate and has a dependence on field strength and Kepler radius that naturally explains the empirical scalings for the onset of H α emission, its associated equivalent width, and even its line profile shapes. However, the general lack of observed Balmer emission in late-B and A-type stars could still be attributed to a residual level of diffusive or drift leakage that does not allow their much weaker winds to fill their CMs to the breakout level needed for such emission; alternatively, this might result from a transition to a metal–ion wind that lacks the requisite hydrogen.