Oscillating red-giant stars in eccentric binary systems

The unparalleled photometric data obtained by NASAs Kepler Space Telescope has led to improved understanding of red-giant stars and binary stars. We discuss the characterization of known eccentric system, containing a solar-like oscillating red-giant primary component. We also report several new binary systems that are candidates for hosting an oscillating companion. A powerful approach to study binary stars is to combine asteroseimic techniques with light curve fitting. Seismology allows us to deduce the properties of red giants. In addition, by modeling the ellipsoidal modulations we can constrain the parameters of the binary system. An valuable independent source are ground-bases, high-resolution spectrographs.

Detection of solar-like oscillations in the bright red giant stars $gamma$ Psc and $theta^1$ Tau from a 190-day high-precision spectroscopic multisite campaign

Red giants are evolved stars which exhibit solar-like oscillations. Although a multitude of stars have been observed with space telescopes, only a handful of red-giant stars were targets of spectroscopic asteroseismic observing projects. We search for solar-like oscillations in the two bright red-giant stars $gamma$ Psc and $theta^1$ Tau from time series of ground-based spectroscopy and determine the frequency of the excess of oscillation power $ u_{max}$ and the mean large frequency separation $Delta u$ for both stars. The radial velocities of $gamma$ Psc and $theta^1$ Tau were monitored for 120 and 190 days, respectively. Nearly 9000 spectra were obtained. To reach the accurate radial velocities, we used simultaneous thorium-argon and iodine-cell calibration of our optical spectra. In addition to the spectroscopy, we acquired VLTI observations of $gamma$ Psc for an independent estimate of the radius. Also 22 days of observations of $theta^1$ Tau with the MOST-satellite were analysed. The frequency analysis of the radial velocity data of $gamma$ Psc revealed an excess of oscillation power around 32 $mu$Hz and a large frequency separation of 4.1$pm$0.1$mu$Hz. $theta^1$ Tau exhibits oscillation power around 90 $mu$Hz, with a large frequency separation of 6.9$pm$0.2$mu$Hz. Scaling relations indicate that $gamma$ Psc is a star of about $sim$1 M$_odot$ and $sim$10 R$_odot$. $theta^1$ Tau appears to be a massive star of about $sim$2.7 M$_odot$ and $sim$11 R$_odot$. The radial velocities of both stars were found to be modulated on time scales much longer than the oscillation periods. While the mass of $theta^1$ Tau is in agreement with results from dynamical parallaxes, we find a lower mass for $gamma$ Psc than what is given in the literature. The long periodic variability agrees with the expected time scales of rotational modulation.

Pulsating red giant stars in eccentric binary systems discovered from Kepler space-based photometry

The unparalleled photometric data obtained by NASAs Kepler space telescope led to an improved understanding of red giant stars and binary stars. Seismology allows us to constrain the properties of red giants. In addition to eclipsing binaries, eccentric non-eclipsing binaries, exhibiting ellipsoidal modulations, have been detected with Kepler. We aim to study the properties of eccentric binary systems containing a red giant star and derive the parameters of the primary giant component. We apply asteroseismic techniques to determine masses and radii of the primary component of each system. For a selected target, light and radial velocity curve modelling techniques are applied to extract the parameters of the system. The effects of stellar on the binary system are studied. The paper presents the asteroseismic analysis of 18 pulsating red giants in eccentric binary systems, for which masses and radii were constrained. The orbital periods of these systems range from 20 to 440days. From radial velocity measurements we find eccentricities between e=0.2 to 0.76. As a case study we present a detailed analysis of KIC5006817. From seismology we constrain the rotational period of the envelope to be at least 165 d, roughly twice the orbital period. The stellar core rotates 13 times faster than the surface. From the spectrum and radial velocities we expect that the Doppler beaming signal should have a maximum amplitude of 300ppm in the light curve. Through binary modelling, we determine the mass of the secondary component to be 0.29$pm$0.03,$M_odot$. For KIC5006817 we exclude pseudo-synchronous rotation of the red giant with the orbit. The comparison of the results from seismology and modelling of the light curve shows a possible alignment of the rotational and orbital axis at the 2$sigma$ level. Red giant eccentric systems could be progenitors of cataclysmic variables and hot subdwarf B stars.

Fast core rotation in red-giant stars revealed by gravity-dominated mixed modes

When the core hydrogen is exhausted during stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant, in which convection occupies a large fraction of the star. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes, and indirect evidence supports this. Information about the angular momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the stellar interior. Here, we report the detection of non-rigid rotation in the interiors of red-giant stars by exploiting the rotational frequency splitting of recently detected mixed modes. We demonstrate an increasing rotation rate from the surface of the star to the stellar core. Comparing with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior.