Explore the vast range of titles available.

We examine the reasons for discrepancies between two alternative approaches to modeling small-amplitude tides in binary systems. The 'direct solution' (DS) approach solves the governing differential equations and boundary conditions directly, while the 'modal decomposition' (MD) approach relies on a normal-mode expansion. Applied to a model for the primary star in the heartbeat system KOI-54, the two approaches predict quite different behavior of the secular tidal torque. The MD approach exhibits the pseudosynchronization phenomenon, where the torque due to the equilibrium tide changes sign at a single, well-defined and theoretically predicted stellar rotation rate. The DS approach instead shows 'blurred' pseudosynchronization, where positive and negative torques intermingle over a range of rotation rates. We trace a major source of these differences to an incorrect damping coefficient in the profile functions describing the frequency dependence of the MD expansion coefficients. With this error corrected some differences between the approaches remain; however, both are in agreement that pseudosynchronization is blurred in the KOI-54 system. Our findings generalize to any type of star for which the tidal damping depends explicitly or implicitly on the forcing frequency.

The Of?p category was introduced more than 40 years ago to gather several Galactic stars with some odd properties. Since 2000, spectropolarimetry, high-resolution spectroscopy, long-term photometry, and X-ray observations have revealed their nature: magnetic oblique rotators - they all have magnetic fields that confine their winds. Several Of?p stars have now been detected in the Magellanic Clouds, likely the prototypes of magnetic massive stars at low metallicity. This contribution will present the most recent photometric, spectroscopic, and spectropolarimetric data, along with the first modeling of these objects.

Kawaler et al. (1985) present a variational expression for the eigenfrequencies associated with stellar oscillations. We highlight and correct a typographical error in the weight functions appearing in these expressions, and validate the correction numerically.

A subpopulation (~10%) of hot, luminous, massive stars have been revealed through spectropolarimetry to harbor strong (hundreds to tens of thousand Gauss), steady, large-scale (often significantly dipolar) magnetic fields. This review focuses on the role of such fields in channeling and trapping the radiatively driven wind of massive stars, including both in the strongly perturbed outflow from open field regions, and the wind-fed “magnetospheres” that develop from closed magnetic loops. For B-type stars with weak winds and moderately fast rotation, one finds “centrifugal magnetospheres”, in which rotational support allows magnetically trapped wind to accumulate to a large density, with quite distinctive observational signatures, e.g. in Balmer line emission. In contrast, more luminous O-type stars have generally been spun down by magnetic braking from angular momentum loss in their much stronger winds. The lack of centrifugal support means their closed loops form a “dynamical magnetosphere”, with trapped material falling back to the star on a dynamical timescale; nonetheless, the much stronger wind feeding leads to a circumstellar density that is still high enough to give substantial Balmer emission. Overall, this review describes MHD simulations and semi-analytic dynamical methods for modeling the magnetospheres, the magnetically channeled wind outflows, and the associated spin-down of these magnetic massive stars.

Laplace's tidal equations govern the angular dependence of oscillations in stars when uniform rotation is treated within the so-called traditional approximation. Using a perturbation expansion approach, I derive improved expressions for the eigenvalue associated with these equations, valid in the asymptotic limit of large spin parameter \\(q\\). These expressions have a relative accuracy of order \\(q^{-3}\\) for gravito-inertial modes, and \\(q^{-1}\\) for Rossby and Kelvin modes; the corresponding absolute accuracy is of order \\(q^{-1}\\) for all three mode types. I validate my analysis against numerical calculations, and demonstrate how it can be applied to derive formulae for the periods and eigenfunctions of Rossby modes.

The contour method is a new approach to calculating the non-adiabatic pulsation frequencies of stars. These frequencies can be found by solving for the complex roots of a characteristic equation constructed from the linear non-adiabatic stellar pulsation equations. A complex-root solver requires an initial trial frequency for each non adiabatic root. A standard method for obtaining initial trial frequencies is to use a star's adiabatic pulsation frequencies, but this method can fail to converge to non-adiabatic roots, especially as the growth and/or damping rate of the pulsations becomes large. The contour method provides an alternative way for obtaining initial trial frequencies that robustly converges to non-adiabatic roots, even for stellar models with extremely non-adiabatic pulsations and thus large growth/damping rates. We describe the contour method implemented in the GYRE stellar pulsation code and use it to calculate the non-adiabatic pulsation frequencies of \\(10\\,\\rm{M_{\\odot}}\\) and \\(20\\,\\rm{M_{\\odot}}\\) \\(\\beta\\) Cephei star models, and of a \\(0.9\\,\\rm{M_{\\odot}}\\) extreme helium star model.

The magnetic activity of solar-type and low-mass stars is a well known source of coronal X-ray emission. At the other end of the main sequence, X-rays emission is instead associated with the powerful, radiatively driven winds of massive stars. Indeed, the intrinsically unstable line-driving mechanism of OB star winds gives rise to shock-heated, soft emission (~0.5 keV) distributed throughout the wind. Recently, the latest generation of spectropolarimetric instrumentation has uncovered a population of massive OB-stars hosting strong, organized magnetic fields. The magnetic characteristics of these stars are similar to the apparently fossil magnetic fields of the chemically peculiar ApBp stars. Magnetic channeling of these OB stars' strong winds leads to the formation of large-scale shock-heated magnetospheres, which can modify UV resonance lines, create complex distributions of cooled Halpha emitting material, and radiate hard (~2-5 keV) X-rays. This presentation summarizes our coordinated observational and modelling efforts to characterize the manifestation of these magnetospheres in the X-ray domain, providing an important contrast between the emission originating in shocks associated with the large-scale fossil fields of massive stars, and the X-rays associated with the activity of complex, dynamo-generated fields in lower-mass stars.

The blue-straggler binary WOCS 5379 is a member of the old (6-7 Gyr) open cluster NGC 188. WOCS 5379 comprises a blue straggler star with a white dwarf companion in a 120-day eccentric orbit. Combined with the orbital period, this helium white dwarf is evidence of previous mass transfer by a red giant. Detailed models of the system evolution from a progenitor main-sequence binary, including mass transfer, are made using the Modules for Experiments in Stellar Astrophysics (MESA). Both of the progenitor stars are evolved in the simulation. WOCS 5379 is well reproduced with a primary star of initial mass 1.19 \\(M_{\\odot}\\), whose core becomes the white dwarf. The secondary star initially is 1.01 \\(M_{\\odot}\\). 300 Myr ago, the secondary finished receiving mass from the donor, having moved beyond the NGC 188 turnoff as a 1.20 \\(M_{\\odot}\\) blue straggler. The successful model has a mass transfer efficiency of 22\\%. This non-conservative mass transfer is key to expanding the orbit fast enough to permit stable mass transfer. Even so, the mass transfer begins with a short unstable phase, during which half of the accreted mass is transferred. With increasing mass, the secondary evolves from a radiative core to a convective core. The final blue straggler interior is remarkably similar to a 2.1 Gyr-old 1.21 \\(M_{\\odot}\\) main-sequence star at the same location in the HR diagram. The white dwarf effective temperature is also reproduced, but the modeled white dwarf mass of 0.33 \\(M_{\\odot}\\) is smaller than the measured mass of 0.42 \\(M_{\\odot}\\).

We present 18 years of OGLE photometry together with spectra obtained over 12 years, revealing that the early Oe star AzV 493 shows strong photometric (Delta I < 1.2 mag) and spectroscopic variability with a dominant, 14.6-year pattern and ~40-day oscillations. We estimate stellar parameters T_eff = 42000 K, log L/L_sun = 5.83 +/- 0.15, M/M_sun = 50 +/- 9, and vsini = 370 +/- 40 km/s. Direct spectroscopic evidence shows episodes of both gas ejection and infall. There is no X-ray detection, and it is likely a runaway star. AzV 493 may have an unseen companion on a highly eccentric (e > 0.93) orbit. We propose that close interaction at periastron excites ejection of the decretion disk, whose variable emission-line spectrum suggests separate inner and outer components, with an optically thick outer component obscuring both the stellar photosphere and the emission-line spectrum of the inner disk at early phases in the photometric cycle. It is plausible that AzV 493's mass and rotation have been enhanced by binary interaction followed by the core-collapse supernova explosion of the companion, which now could be either a black hole or neutron star. This system in the Small Magellanic Cloud can potentially shed light on OBe decretion disk formation and evolution, massive binary evolution, and compact binary progenitors.

The Tayler instability (TI) is a non-axisymmetric linear instability of an axisymmetric toroidal magnetic field in magneto-hydrostatic equilibrium (MHSE). Spruit (1999, 2002) has proposed that in a differentially rotating radiative region of a star, the TI drives a dynamo which generates magnetic fields that can efficiently transport angular momentum; a parameterized version of this dynamo has been implemented in stellar structure and evolution codes and shown to be important for determining interior spin. Numerical simulations, however, have yet to definitively demonstrate the operation of the dynamo. A criterion for the MHSE to develop the TI was derived using fully-compressible magneto-hydrodynamics, while numerical simulations of dynamical processes in stars frequently use an anelastic approximation. This motivates us to derive a new anelastic Tayler instability (anTI) criterion. We find that some MHSE configurations unstable in the fully-compressible case, become stable in the anelastic case. We find and characterize the unstable modes of a simple family of cylindrical MHSE configurations using numerical calculations, and discuss the implications for fully non-linear anelastic simulations.