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The core values of red giant stars The core rotation rate of a star, a key indicator of its evolutionary state, cannot be measured directly because the core is inaccessible to direct observation. This paper presents a method for calculating core rotation in an evolved star. The Fourier spectra of brightness variations of four stars derived from Kepler spacecraft data were used to measure the rotational frequency splitting of the recently identified 'mixed modes' caused by rotation in red giant stars. The results suggest that the core of a red giant rotates at least ten times faster than the surface. 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. Convection takes place over much of the star’s radius. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes; indirect evidence supports this 1 , 2 . 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 an increasing rotation rate from the surface of the star to the stellar core in the interiors of red giants, obtained using the rotational frequency splitting of recently detected ‘mixed modes’ 3 , 4 . By comparison 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 1 , 5 , 6 .

APOKASC is a collaboration between APOGEE's spectroscopic survey in the near IR at Apache Point (SDSS) and Kepler Asteroseismic Science Consortium (KASC) for the study of red-giant stars. The aim of the collaboration is to take advantage of the good spectroscopic data of APOGEE together with the excellent KASC asteroseismic data from the Kepler mission to derive reliable and precise masses and ages for a large number of red-giant stars by combining data from the two sources. We report here on the progress to date on the just over 6600 stars being considered.

Stars hosting hot Jupiters are often observed to have high obliquities, whereas stars with multiple coplanar planets have been seen to have low obliquities. This has been interpreted as evidence that hot-Jupiter formation is linked to dynamical disruption, as opposed to planet migration through a protoplanetary disk. We used asteroseismology to measure a large obliquity for Kepler-56, a red giant star hosting two transiting coplanar planets. These observations show that spin-orbit misalignments are not confined to hot-Jupiter systems. Misalignments in a broader class of systems had been predicted as a consequence of torques from wide-orbiting companions, and indeed radial velocity measurements revealed a third companion in a wide orbit in the Kepler-56 system.

From its surface properties it can be difficult to determine whether a red-giant star is in its heliumcore-burning phase or only burning hydrogen in a shell around an inert helium core. Stars in either of these stages can have similar effective temperatures, radii and hence luminosities, i.e. they can be located at the same position in the Hertzsprung-Russell diagram. Asteroseismology – the study of the internal structure of stars through their global oscillations – can provide the necessary additional constraints to determine the evolutionary states of red-giant stars. Here, we present a method that uses grid-based modelling based on global asteroseismic properties (vmax, frequency of maximum oscillation power; and Δv, frequency spacing between modes of the same degree and consecutive radial orders) as well as effective temperature and metallicity to determine the evolutionary phases. This method is applicable even to timeseries data of limited length, although with a small fraction of miss-classifications.

The frequencies of oscillation modes in stars contain valueable information about the stellar properties. In red giants the frequency spectrum also contains mixed modes, with both pressure (p) and gravity (g) as restoring force, which are key to understanding the physical conditions in the stellar core. We observe a high fraction of red giants in binary systems, for which g-dominated mixed modes are not pronounced. This trend leads us to investigate whether this is specific for binary systems or a more general feature. We do so by comparing the fraction of stars with only p-dominated mixed modes in binaries and in a larger set of stars from the APOKASC sample. We find only p-dominated mixed modes in about 50% of red giants in detached eclipsing binaries compared to about 4% in the large sample. This could indicate that this phenomenon is tightly related to binarity and that the binary fraction in the APOKASC sample is about 8%.

Kepler-based asteroseismology sorts red giants NASA's Kepler mission has been remarkably productive in its primary role, that of discovering and characterizing extrasolar planets. It does this indirectly, by monitoring the brightness of many thousands of main sequence stars in search of periodic fluctuations caused by planets crossing the face of the stars. But the high-precision photometry involved is also ideal for studying the stars themselves. Bedding et al . have used Kepler data to probe the internal structure of red giants. Their detailed measurements of the gravity modes in the cores of these stars allow them to distinguish between those burning hydrogen in a shell around a relatively inactive core and those burning helium in the core. Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell. Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion. However, it is difficult to distinguish between the two groups. Asteroseismology offers a way forward. This study reports observations of gravity-mode period spacings in red giants using high precision photometry obtained by the Kepler spacecraft. It is found that the stars fall into two clear groups, making it possible to distinguish unambiguously between hydrogen-shell-burning stars and those that are also burning helium. Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell 1 , 2 . Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion 3 . Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface before helium ignition and the amount of internal mixing from rotation and other processes 4 . Progress is hampered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology offers a way forward, being a powerful tool for probing the internal structures of stars using their natural oscillation frequencies 5 . Here we report observations of gravity-mode period spacings in red giants 6 that permit a distinction between evolutionary stages to be made. We use high-precision photometry obtained by the Kepler spacecraft over more than a year to measure oscillations in several hundred red giants. We find many stars whose dipole modes show sequences with approximately regular period spacings. These stars fall into two clear groups, allowing us to distinguish unambiguously between hydrogen-shell-burning stars (period spacing mostly ∼50 seconds) and those that are also burning helium (period spacing ∼100 to 300 seconds).

Asteroseismology involves probing the interiors of stars and quantifying their global properties, such as radius and age, through observations of normal modes of oscillation. The technical requirements for conducting asteroseismology include ultrahigh precision measured in photometry in parts per million, as well as nearly continuous time series over weeks to years, and cadences rapid enough to sample oscillations with periods as short as a few minutes. We report on results from the first 43 days of observations, in which the unique capabilities ofKeplerin providing a revolutionary advance in asteroseismology are already well in evidence. TheKeplerasteroseismology program holds intrinsic importance in supporting the core planetary search program through greatly enhanced knowledge of host star properties, and extends well beyond this to rich applications in stellar astrophysics.

Rotation is thought to drive cyclic magnetic activity in the Sun and Sun-like stars. Stellar dynamos, however, are poorly understood owing to the scarcity of observations of rotation and magnetic fields in stars. Here, inferences are drawn on the internal rotation of a distant Sun-like star by studying its global modes of oscillation. We report asteroseismic constraints imposed on the rotation rate and the inclination of the spin axis of the Sun-like star HD 52265, a principal target observed by the CoRoT satellite that is known to host a planetary companion. These seismic inferences are remarkably consistent with an independent spectroscopic observation (rotational line broadening) and with the observed rotation period of star spots. Furthermore, asteroseismology constrains the mass of exoplanet HD 52265b. Under the standard assumption that the stellar spin axis and the axis of the planetary orbit coincide, the minimum spectroscopic mass of the planet can be converted into a true mass of [Formula], which implies that it is a planet, not a brown dwarf.

Issue Title: The physics of solar and solar-like oscillations; guest editors T. Bedding and J. Leibacher The Birmingham Solar-Oscillations Network (BiSON) has acquired high-precision solar mean magnetic field (SMMF) data on a 40-s cadence for a decade. We present attempts to compare such data from recent years with the occurrence of coronal mass ejections (CMEs) as recorded by LASCO, using correlation techniques applied to measurements from different BiSON instruments to maximise the sensitivity to CME-related SMMF responses. SMMF measurements were recorded at the time of occurrence of several hundred CMEs. No CME event shows a convincing response in our SMMF data at short periods setting a threshold amplitude of 12 mG. By averaging data sets we are able to set lower thresholds, which depend somewhat on the distribution of response strengths. A brief summary of the very first results of this study is also given in Chaplin et al.[PUBLICATION ABSTRACT]

The Kepler mission will provide large separations for many stars. One of the tasks of Kepler Asteroseismic Consortium is to determine radii of the observed stars from the large separations and other catalogued \"classical\" data such as effective temperature, metallicities, brightness, distance etc. We present the results of a detailed analysis of errors in the radius estimates caused by errors in the input parameters. This exercise enables us to determine which parameters will benefit from follow-up observations of the interesting cases.