J. Montalbán

Kepler Detected Gravity-Mode Period Spacings in a Red Giant Star

Asteroseismology Delivers Using asteroseismology—the study of stellar oscillations, it is possible to probe the interior of stars and to derive stellar parameters, such as mass and radius (see the Perspective by Montgomery). Based on asteroseismic data from the NASA Kepler mission, Chaplin et al. (p. 213) detected solarlike oscillations in 500 solartype stars in our Galaxy. The distribution of the radii of these stars matches that expected from stellar evolution theory, but the distribution in mass does not, which challenges our knowledge of star formation rates, the mass of forming stars, and the models of the stars themselves. Derekas et al. (p. 216) report the detection of a triple-star system comprising a red giant star and two red dwarfs. The red giant star, instead of the expected solarlike oscillations, shows evidence for tidally induced oscillations driven by the orbital motion of the red dwarf pair. Finally, Beck et al. (p. 205) describe unusual oscillations from a red giant star that may elucidate characteristics of its core. Asteroseismic observations with the Kepler satellite probed the deep interior of an evolved star. Stellar interiors are inaccessible through direct observations. For this reason, helioseismologists made use of the Sun’s acoustic oscillation modes to tune models of its structure. The quest to detect modes that probe the solar core has been ongoing for decades. We report the detection of mixed modes penetrating all the way to the core of an evolved star from 320 days of observations with the Kepler satellite. The period spacings of these mixed modes are directly dependent on the density gradient between the core region and the convective envelope.

Theoretical amplitudes and lifetimes of non-radial solar-like oscillations in red giants

Context. Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal structure of these stars? Aims. We investigate under what physical circumstances non-radial modes could be observable in red giants; what would be their amplitudes, lifetimes and heights in the power spectrum (PS)? Methods. Using a non-radial non-adiabatic pulsation code including a non-local time-dependent treatment of convection, we compute the theoretical lifetimes of radial and non-radial modes in several red giant models. Next, using a stochastic excitation model, we compute the amplitudes of these modes and their heights in the PS. Results. Distinct cases appear. Case A corresponds to subgiants and stars at the bottom of the ascending giant branch. Our results show that the lifetimes of the modes are mainly proportional to the inertia I, which is modulated by the mode trapping. The predicted amplitudes are lower for non-radial modes. But the height of the peaks in the PS are of the same order for radial and non-radial modes as long as they can be resolved. The resulting frequency spectrum is complex. Case B corresponds to intermediate models in the red giant branch. In these models, the radiative damping becomes high enough to destroy the non-radial modes trapped in the core. Hence, only modes trapped in the envelope have significant heights in the PS and could be observed. The resulting frequency spectrum of detectable modes is regular for � = 0 and 2, but a little more complex for � = 1 modes because of less efficient trapping. Case C corresponds to models of even higher luminosity. In these models the radiative damping of non-radial modes is even larger than in the previous case and only radial and non-radial modes completely trapped in the envelope could be observed. The frequency pattern is very regular for these stars. The comparison between the predictions for radial and non-radial modes is very different if we consider the heights in the PS instead of the amplitudes. This is important as the heights (not the amplitudes) are used as detection criterion.

Mixed modes in red-giant stars observed with CoRoT

Context. The CoRoT mission has provided thousands of red-giant light curves. The analysis of their solar-like oscillations allows us to characterize their stellar properties. Aims. Up to now, the global seismic parameters of the pressure modes have been unable to distinguish red-clump giants from members of the red-giant branch. As recently done with Kepler red giants, we intend to analyze and use the so-called mixed modes to determine the evolutionary status of the red giants observed with CoRoT. We also aim at deriving different seismic characteristics depending on evolution. Methods. The complete identification of the pressure eigenmodes provided by the red-giant universal oscillation pattern allows us to aim at the mixed modes surrounding the l = 1 expected eigenfrequencies. A dedicated method based on the envelope autocorrelation function is proposed to analyze their period separation. Results. We have identified the mixed-mode signature separation thanks to their pattern that is compatible with the asymptotic law of gravity modes. We have shown that, independent of any modeling, the g-mode spacings help to distinguish the evolutionary status of a red-giant star. We then report the different seismic and fundamental properties of the stars, depending on their evolutionary status. In particular, we show that high-mass stars of the secondary clump present very specific seismic properties. We emphasize that stars belonging to the clump were affected by significant mass loss. We also note significant population and/or evolution differences in the different fields observed by CoRoT.

CLÉS, Code Liégeois d'Évolution Stellaire

Abstract CLÉS is an evolution code recently developed to produce stellar models meeting the specific requirements of studies in asteroseismology. It offers the users a lot of choices in the input physics they want in their models and its versatility allows them to tailor the code to their needs and implement easily new features. We describe the features implemented in the current version of the code and the techniques used to solve the equations of stellar structure and evolution. A brief account is given of the use of the program and of a solar calibration realized with it.

The Liège Oscillation code

Abstract The Liège Oscillation code can be used as a stand-alone program or as a library of subroutines that the user calls from a Fortran main program of his own to compute radial and nonradial adiabatic oscillations of stellar models. We describe the variables and the equations used by the program and the methods used to solve them. A brief account is given of the use and the output of the program.

An asteroseismic study of the β Cephei star θ Ophiuchi: constraints on global stellar parameters and core overshooting

We present a seismic study of the β Cephei star θ Ophiuchi. Our analysis is based on the observation of one radial mode, one rotationally split � = 1 triplet and three components of a rotationally split � = 2 quintuplet for which the m values were well identified by spectroscopy. We identify the radial mode as fundamental, the triplet as p1 and the quintuplet as g1. Our non-local thermodynamic equilibrium abundance analysis results in a metallicity and CNO abundances in full agreement with the most recent updated solar values. With X ∈ [0.71, 0.7211] and Z ∈ [0.009, 0.015], and using the Asplund et al. mixture but with a Ne abundance about 0.3 dex larger, the matching of the three independent modes enables us to deduce constrained ranges for the mass (M = 8.2 ± 0.3 M� ) and central hydrogen abundance (Xc = 0.38 ± 0.02) of θ Oph and to prove the occurrence of core overshooting (αov = 0.44 ± 0.07). We also derive an equatorial rotation velocity of 29 ± 7k m s −1 . Moreover, we show that the observed non-equidistance of the � = 1 triplet can be reproduced by the second-order effects of rotation. Finally, we show that the observed rotational splitting of two modes cannot rule out a rigid rotation model.

PROPERTIES OF 42 SOLAR-TYPE KEPLER TARGETS FROM THE ASTEROSEISMIC MODELING PORTAL

Recently the number of main-sequence and subgiant stars exhibiting solar-like oscillations that are resolved into individual mode frequencies has increased dramatically. While only a few such data sets were available for detailed modeling just a decade ago, the Kepler mission has produced suitable observations for hundreds of new targets. This rapid expansion in observational capacity has been accompanied by a shift in analysis and modeling strategies to yield uniform sets of derived stellar properties more quickly and easily. We use previously published asteroseismic and spectroscopic data sets to provide a uniform analysis of 42 solar-type Kepler targets from the Asteroseismic Modeling Portal. We find that fitting the individual frequencies typically doubles the precision of the asteroseismic radius, mass, and age compared to grid-based modeling of the global oscillation properties, and improves the precision of the radius and mass by about a factor of three over empirical scaling relations. We demonstrate the utility of the derived properties with several applications.

Kepler-91b: a planet at the end of its life Planet and giant host star properties via light-curve variations ,

Context. The evolution of planetary systems is intimately linked to the evolution of their host star. Our understanding of the whole planetary evolution process is based on the large planet diversity observed so far. To date, only few tens of planets have been discovered orbiting stars ascending the Red Giant Branch. Although several theories have been proposed, the question of how planets die remains open due to the small number statistics, making clear the need of enlarging the sample of planets around post-main sequence stars. Aims. In this work we study the giant star Kepler-91 (KIC 8219268) in order to determine the nature of a transiting companion. This system was detected by the Kepler Space Telescope, which identified small dims in its light cur ve with a period of 6.246580±0.000082 days. However, its planetary confirmation is needed due to th e large pixel size of the Kepler camera which can hide other stellar configurations able to mimic planet-like transit events. Methods. We analyse Kepler photometry to: 1) re-calculate transit parameters, 2) stud y the light-curve modulations, and 3) to perform an asteroseismic analysis (accurate stellar parameter det ermination) by identifying solar-like oscillations on the periodogram. We also used a high-resolution and high signal-to-noise ratio spec trum obtained with the Calar Alto Fiber-fed ´ Echelle spectrograph (CAFE) to measure stellar properties. Additionally, false-positiv e scenarios were rejected by obtaining high-resolution images with the AstraLux lucky-imaging camera on the 2.2 m telescope at the Calar Alto Observatory. Results. We confirm the planetary nature of the object transiting the s tar Kepler-91 by deriving a mass of Mp = 0.88 +0.17 −0.33 MJup and a planetary radius of Rp = 1.384 +0.011 −0.054 RJup. Asteroseismic analysis produces a stellar radius of R⋆ = 6.30± 0.16 R⊙ and a mass of M⋆ = 1.31± 0.10 M⊙. We find that its eccentric orbit ( e = 0.066 +0.013 −0.017 ) is just 1.32 +0.07 −0.22 R⋆ away from the stellar atmosphere at the pericenter. We also detected three small dims in the phase-folded light-curve. The combination of two of them agrees with the theoretical characteristics expected for secondary eclip se. Conclusions. Kepler-91b could be the previous stage of the planet engulfment, recently detected for BD+48 740. Our estimations show that Kepler-91b will be swallowed by its host star in less than 55 Myr. Among the confirmed planets around giant stars, this is the planetary-mass body closest to its host star. At pericen ter passage, the star subtends an angle of 48 ◦ , covering around 10% of the sky as seen from the planet. The planetary atmosphere seems to be inflated probably due to the high stellar irradiation.

Testing Convective-core Overshooting Using Period Spacings of Dipole Modes in Red Giants

Uncertainties on central mixing in main-sequence (MS) and core He-burning (He-B) phases affect key predictions of stellar evolution such as late evolutionary phases, chemical enrichment, ages, etc. We propose a test of the extension of extra-mixing in two relevant evolutionary phases based on period spacing (ΔP) of solar-like oscillating giants. From stellar models and their corresponding adiabatic frequencies (respectively, computed with ATON and LOSC codes), we provide the first predictions of the observable ΔP for stars in the red giant branch and in the red clump (RC). We find (1) a clear correlation between ΔP and the mass of the helium core (M He); the latter in intermediate-mass stars depends on the MS overshooting, and hence it can be used to set constraints on extra-mixing during MS when coupled with chemical composition; and (2) a linear dependence of the average value of the asymptotic period spacing (ΔP a ) on the size of the convective core during the He-B phase. A first comparison with the inferred asymptotic period spacing for Kepler RC stars also suggests the need for extra-mixing during this phase, as evinced from other observational facts.

SEISMIC DIAGNOSTICS OF RED GIANTS: FIRST COMPARISON WITH STELLAR MODELS

The clear detection with CoRoT and KEPLER of radial and non-radial solar-like oscillations in many red giants paves the way for seismic inferences on the structure of such stars. We present an overview of the properties of the adiabatic frequencies and frequency separations of radial and non-radial oscillation modes for an extended grid of models. We highlight how their detection allows a deeper insight into the internal structure and evolutionary state of red giants. In particular, we find that the properties of dipole modes constitute a promising seismic diagnostic tool of the evolutionary state of red giant stars. We compare our theoretical predictions with the first 34 days of KEPLER data and predict the frequency diagram expected for red giants in the CoRoT exofield in the galactic center direction.