William J. Chaplin

Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars

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. 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. 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. Here we report observations of gravity-mode period spacings in red giants 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).

Evolutionary influences on the structure of red-giant acoustic oscillation spectra from 600d of Kepler observations

Context. The Kepler space mission is reaching continuous observing times long enough to also start studying the fine structure of the observed pressure-mode spectra. Aims. In this paper, we aim to study the signature of stellar evolut ion on the radial and pressure-dominated l = 2 modes in an ensemble of red giants that show solar-type oscillations. Methods. We use established methods to automatically identify the mode degree of l = 0 and 2 modes and measure the large (�νc) and small (δν02) frequency separation around the central radial mode. We then determine the phase shiftǫc of the central radial mode, i.e. the linear offset in the asymptotic fit to the acoustic modes. Furthermore w e measure the individual frequencies of radial modes and investigate their average curvature. Results. We find that ǫc is significantly di fferent for red giants at a given �νc but which burn only H in a shell (RGB) than those that have already ignited core He burning. Even though not directly probing the stellar core the pair of local seismic observa bles (�νc,ǫc) can be used as an evolutionary stage discriminator that turn ed out to be as reliable as the period spacing of the mixed dipole modes. We find a tight correlation between ǫc and �νc for RGB stars and unlike less evolved stars we find no indicati on thatǫc depends on other properties of the star. It appears that the difference inǫc between the two populations becomes smaller and eventually indistinguishable if we use an average of several radial orders, instead of a loc al, i.e. only around the central radial mode, large separati on to determine the phase shift. This indicates that the information on the e volutionary stage is encoded locally, more precisely in the shape of the radial mode sequence. This shape turns out to be approximately symmetric around the central radial mode for RGB stars but asymmetric for core He burning stars. We computed radial mode frequencies for a sequence of red-giant models and find them to qualitat ively confirm our findings. We also find that, at least in our models, t he local �ν is an at least as good and mostly better proxy for both the asymptotic spacing and the large separation scaled from the model density than the average �ν. Finally, we investigate the signature of the evolutionary stage onδν02 and quantify the mass dependency of this seismic parameter.

The Kepler-454 System: A Small, Not-rocky Inner Planet, a Jovian World, and a Distant Companion

Kepler-454 (KOI-273) is a relatively bright (V = 11.69 mag), Sun-like star that hosts a transiting planet candidate in a 10.6 day orbit. From spectroscopy, we estimate the stellar temperature to be 5687 ± 50 K, its metallicity to be [m/H] = 0.32 ± 0.08, and the projected rotational velocity to be v sin i 10 years and mass >12.1 MJ. The 12 exoplanets with radii <2.7 R_⊕ and precise mass measurements appear to fall into two populations, with those <1.6 R_⊕ following an Earth-like composition curve and larger planets requiring a significant fraction of volatiles. With a density of 2.76 ± 0.73 g cm^(−3), Kepler-454b lies near the mass transition between these two populations and requires the presence of volatiles and/or H/He gas.

Preparation of Kepler light curves for asteroseismic analyses

The Kepler mission is providing photometric data of exquisite quality for the asteroseismic study of different classes of pulsating stars. These analyses place particular demands on the pre-processing of the data, over a range of timescales from minutes to months. Here, we describe processing procedures developed by the Kepler Asteroseismic Science Consortium (KASC) to prepare light curves that are optimized for the asteroseismic study of solar-like oscillating stars in which outliers, jumps and drifts are corrected.

The First APOKASC Catalog of Kepler Dwarf and Subgiant Stars

(Abridged) We present the first APOKASC catalog of spectroscopic and asteroseismic data for 415 dwarfs and subgiants. Asteroseismic data have been obtained by Kepler in short cadence. The spectroscopic parameters are based on spectra taken as part of APOGEE and correspond to DR13 of SDSS. We analyze our data using two Teff scales, the spectroscopic values from DR13 and those derived from SDSS griz photometry. We use the differences in our results arising from these choices as a test of systematic Teff, and find that they can lead to significant differences in the derived stellar properties. Determinations of surface gravity (

Helioseismology - a clear view of the interior

\log{g}

Stellar Surface Magneto-Convection as a Source of Astrophysical Noise II. Center-to-Limb Parameterisation of Absorption Line Profiles and Comparison to Observations

), mean density (

THE K2 GALACTIC ARCHAEOLOGY PROGRAM DATA RELEASE I: ASTEROSEISMIC RESULTS FROM CAMPAIGN 1

\rho

Observational Upper Limits for Low-Degree Solar g-modes

), radius (

High-frequency interference peaks in BiSON data

R