Asteroseismology of Close Binary Stars: Tides and Mass Transfer

The study of stellar oscillations allows us to infer the properties of stellar interiors. Meanwhile, fundamental parameters such as mass and radius can be obtained by studying stars in binary systems. The synergy between binarity and asteroseismology can constrain the parameter space of stellar properties and facilitate the asteroseismic inference. On the other hand, binarity also introduces additional complexities such tides and mass transfer. From an observational perspective, we briefly review the recent advances in the study of tidal effects on stellar oscillations, focusing on upper main sequence stars (F-, A-, or OB- type). The effect can be roughly divided into two categories. The first one concerns the tidally excited oscillations (TEOs) in eccentric binaries where TEOs are mostly due to resonances between dynamical tides and gravity modes of the star. TEOs appear as orbital-harmonic oscillations on top of the eccentric ellipsoidal light curve variations (the “heartbeat” feature). The second category is regarding the self-excited oscillations perturbed by static tides in circularized and synchronized close binaries. It includes the tidal deformation of the propagation cavity and its effect on eigenfrequencies, eigenfunctions, and the pulsation alignment. We list binary systems that show these two types of tidal effect and summarize the orbital and pulsation observables. We also discuss the theoretical approaches used to model these tidal oscillations and relevant complications such as non-linear mode coupling and resonance locking. Further information can be extracted from the observations of these oscillations which will improve our understanding of tides. We also discuss the effect of mass transfer, the extreme result of tides, on stellar oscillations. We bring to the readers' attention: (1) oscillating stars undergoing mass accretion (A-, F-, and OB type pulsators and white dwarfs), for which the pulsation properties may be changed significantly by accretion; (2) post-mass transfer pulsators, which have undergone a stable or unstable Roche-Lobe overflow. These pulsators have great potential in probing detailed physical processes in stellar interiors and mass transfer, as well as in studying the binary star populations.

Multifrequency Gravitational Wave Background From Continuous Sources

Gravitational waves have been detected in the past few years from several transient events such as merging stellar mass black holes, binary neutron stars, etc. These waves have frequencies in a band ranging from a few hundred hertz to around a kilohertz to which LIGO type instruments are sensitive. LISA would be sensitive to much lower range of frequencies from SMBH mergers. Apart from these cataclysmic burst events, there are innumerable sources of radiation which are continuously emitting gravitational waves of all frequencies. These include a whole mass range of compact binary and isolated compact objects as well as close planetary stellar entities. In this work, quantitative estimates are made of the gravitational wave background produced in typical frequency ranges from such sources emitting over a Hubble time and the fluctuations in the h values measured in the usual devices. Also estimates are made of the high frequency thermal background gravitational radiation from hot stellar interiors and newly formed compact objects.

The MHD Plasma Flow Near the Heliopause Stagnation Region with A View On Kinetic Consistency

Most of the representative space plasma systems in our cosmic environment, - outside of stellar interiors, - like heliospheric, interstellar, or intergalactic plasmas etc., are collision-free or, at least, only weakly collision-determined systems. Nevertheless, these plasmas consist of at least two very different particle species, namely ions and electrons, i.e. particles with very disparate masses and opposite electric charges. If in these systems concerted fluid motions are arranged by electro-magnetic or gravitational forces or by inner forces like pressure gradients, then it must be asked how this combined electron-ion system finds its common internal dynamics. In most text book literature this problem is treated by considering the plasma as a mono-fluid system in which the massive protons and the nearly massless electrons are electrically closely bound together and move as an electrically neutral couple with an identical bulk velocity. Under these conditions the well-known Bernoulli law is derived for the standard MHD. If the electron pressure, however, does compete with the energy density of the ion bulk motion, then a two-fluid situation occurs, and the resulting bulk motion of the charge-neutral plasma needs to be determined on the basis of the kinetic conditions of the two different plasma fluids. In the following we shall exactly study this specific situation.

An improved parametric method for evaluating radiative accelerations in stellar interiors

ABSTRACT The single-valued parameter (SVP) method is a parametric method that offers the possibility of computing radiative accelerations in stellar interiors much faster than other methods. It has been implemented in a few stellar evolution numerical codes for about a decade. In this paper, we describe improvements we have recently brought in the process of preparing, from atomic/opacity data bases, the SVP tables that are needed to use the method, and their extension to a larger stellar mass domain (from 1 to 10 solar mass) on the main sequence. We discuss the validity domain of the method. We also present the website from where new tables and codes can be freely accessed and implemented in stellar evolution codes.