The [Y/Mg] clock works for evolved solar metallicity stars

Previously [Y/Mg] has been proven to be an age indicator for solar twins. Here, we investigate if this relation also holds for helium-core-burning stars of solar metallicity. High resolution and high signal-to-noise ratio (S/N) spectroscopic data of stars in the helium-core-burning phase have been obtained with the FIES spectrograph on the NOT 2.56m telescope and the HIRES spectrograph on the Keck I 10 m telescope. They have been analyzed to determine the chemical abundances of four open clusters with close to solar metallicity; NGC 6811, NGC 6819, M67 and NGC 188. The abundances are derived from equivalent widths of spectral lines using ATLAS9 model atmospheres with parameters determined from the excitation and ionization balance of Fe lines. Results from asteroseismology and binary studies were used as priors on the atmospheric parameters, where especially the $\log g$ is determined to much higher precision than what is possible with spectroscopy. It is confirmed that the four open clusters are close to solar metallicity and they follow the [Y/Mg] vs. age trend previously found for solar twins. The [Y/Mg] vs. age clock also works for giant stars in the helium-core burning phase, which vastly increases the possibilities to estimate the age of stars not only in the solar neighborhood, but in large parts of the Galaxy, due to the brighter nature of evolved stars compared to dwarfs.Comment: 5 pages, 3 figures, accepted for publication as a Letter to A&

Old puzzle, new insights: a lithium rich giant quietly burning helium in its core

About 1% of giant stars have been shown to have large surface Li abundances, which is unexpected according to standard stellar evolution models. Several scenarios for lithium production have been proposed, but it is still unclear why these Li-rich giants exist. A missing piece in this puzzle is the knowledge of the exact stage of evolution of these stars. Using low-and-high-resolution spectroscopic observations, we have undertaken a survey of lithium-rich giants in the Kepler field. In this letter, we report the finding of the first confirmed Li-rich core-helium-burning giant, as revealed by asteroseismic analysis. The evolutionary timescales constrained by its mass suggest that Li-production most likely took place through non-canonical mixing at the RGB-tip, possibly during the helium flash.Comment: 16 pages, 4 figures, 1 table, accepted in ApJ Letter

Oscillations in the Sun with SONG: Setting the scale for asteroseismic investigations

Context. We present the first high-cadence multi-wavelength radial-velocity observations of the Sun-as-a-star, carried out during 57 consecutive days using the stellar \'echelle spectrograph at the Hertzsprung SONG Telescope operating at the Teide Observatory. Aims. The aim was to produce a high-quality data set and reference values for the global helioseismic parameters {\nu_{max}}, and {\Delta \nu} of the solar p-modes using the SONG instrument. The obtained data set or the inferred values should then be used when the scaling relations are applied to other stars showing solar-like oscillations which are observed with SONG or similar instruments. Methods. We used different approaches to analyse the power spectrum of the time series to determine {\nu_{max}}; simple Gaussian fitting and heavy smoothing of the power spectrum. {\Delta\nu} was determined using the method of autocorrelation of the power spectrum. The amplitude per radial mode was determined using the method described in Kjeldsen et al. (2008). Results. We found the following values for the solar oscillations using the SONG spectrograph: {\nu_{max}} = 3141 {\pm} 12 {\mu}Hz, {\Delta\nu} = 134.98 {\pm} 0.04 {\mu}Hz and an average amplitude of the strongest radial modes of 16.6 {\pm} 0.4 cm/s. These values are consistent with previous measurements with other techniques.Comment: 5 pages, 5 figures, letter accepted for A&

What asteroseismology can do for exoplanets

We describe three useful applications of asteroseismology in the context of exoplanet science: (1) the detailed characterisation of exoplanet host stars; (2) the measurement of stellar inclinations; and (3) the determination of orbital eccentricity from transit duration making use of asteroseismic stellar densities. We do so using the example system Kepler-410 (Van Eylen et al. 2014). This is one of the brightest (V = 9.4) Kepler exoplanet host stars, containing a small (2.8 Rearth) transiting planet in a long orbit (17.8 days), and one or more additional non-transiting planets as indicated by transit timing variations. The validation of Kepler-410 (KOI-42) was complicated due to the presence of a companion star, and the planetary nature of the system was confirmed after analyzing a Spitzer transit observation as well as ground-based follow-up observations.Comment: 4 pages, Proceedings of the CoRoT Symposium 3 / Kepler KASC-7 joint meeting, Toulouse, 7-11 July 2014. To be published by EPJ Web of Conference

Observing the changing surface structures of the active K giant sigma Gem with SONG

Aims: We aim to study the spot evolution and differential rotation in the magnetically active cool K-type giant star sigma Gem from broadband photometry and continuous spectroscopic observations that span 150 nights. Methods: We use high-resolution, high signal-to-noise ratio spectra obtained with the Hertzsprung SONG telescope to reconstruct surface (photospheric) temperature maps with Doppler imaging techniques. The 303 observations span 150 nights and allow for a detailed analysis of the spot evolution and surface differential rotation. The Doppler imaging results are compared to simultaneous broadband photometry from the Tennessee State University T3 0.4 m Automated Photometric Telescope. The activity from the stellar chromosphere, which is higher in the stellar atmosphere, is also studied using SONG observations of Balmer H alpha line profiles and correlated with the photospheric activity. Results: The temperature maps obtained during eight consecutive stellar rotations show mainly high-latitude or polar spots, with the main spot concentrations above latitude 45 deg. The spots concentrate around phase 0.25 near the beginning of our observations and around phase 0.75 towards the end. The photometric observations confirm a small jump in spot phases that occurred in February 2016. The cross-correlation of the temperature maps reveals rather strong solar-like differential rotation, giving a relative surface differential rotation coefficient of $\alpha$ = 0.10 +/- 0.02. There is a weak correlation between the locations of starspots and enhanced emission in the chromosphere at some epochs.Comment: 19 pages, 10 figures, accepted for publication in Astronomy & Astrophysics, in the published version the Tables B1 and B2 will only be available via CD

Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission

Asteroseismic constraints on K giants make it possible to infer radii, masses and ages of tens of thousands of field stars. Tests against independent estimates of these properties are however scarce, especially in the metal-poor regime. Here, we report the detection of solar-like oscillations in 8 stars belonging to the red-giant branch and red-horizontal branch of the globular cluster M4. The detections were made in photometric observations from the K2 Mission during its Campaign 2. Making use of independent constraints on the distance, we estimate masses of the 8 stars by utilising different combinations of seismic and non-seismic inputs. When introducing a correction to the Delta nu scaling relation as suggested by stellar models, for RGB stars we find excellent agreement with the expected masses from isochrone fitting, and with a distance modulus derived using independent methods. The offset with respect to independent masses is lower, or comparable with, the uncertainties on the average RGB mass (4-10%, depending on the combination of constraints used). Our results lend confidence to asteroseismic masses in the metal poor regime. We note that a larger sample will be needed to allow more stringent tests to be made of systematic uncertainties in all the observables (both seismic and non-seismic), and to explore the properties of RHB stars, and of different populations in the cluster.Comment: 6 pages, 3 figures, accepted for publication in MNRA

Asteroseismology of the Hyades red giant and planet host epsilon Tauri

Asteroseismic analysis of solar-like stars allows us to determine physical parameters such as stellar mass, with a higher precision compared to most other methods. Even in a well-studied cluster such as the Hyades, the masses of the red giant stars are not well known, and previous mass estimates are based on model calculations (isochrones). The four known red giants in the Hyades are assumed to be clump (core-helium-burning) stars based on their positions in colour-magnitude diagrams, however asteroseismology offers an opportunity to test this assumption. Using asteroseismic techniques combined with other methods, we aim to derive physical parameters and the evolutionary stage for the planet hosting star epsilon Tau, which is one of the four red giants located in the Hyades. We analysed time-series data from both ground and space to perform the asteroseismic analysis. By combining high signal-to-noise (S/N) radial-velocity data from the ground-based SONG network with continuous space-based data from the revised Kepler mission K2, we derive and characterize 27 individual oscillation modes for epsilon Tau, along with global oscillation parameters such as the large frequency separation and the ratio between the amplitude of the oscillations measured in radial velocity and intensity as a function of frequency. The latter has been measured previously for only two stars, the Sun and Procyon. Combining the seismic analysis with interferometric and spectroscopic measurements, we derive physical parameters for epsilon Tau, and discuss its evolutionary status.Comment: 13 pages, 13 figures, 4 tables, accepted for publication in Astronomy & Astrophysic

The Transiting System GJ1214: High-Precision Defocused Transit Observations and a Search for Evidence of Transit Timing Variation

Aims: We present 11 high-precision photometric transit observations of the transiting super-Earth planet GJ1214b. Combining these data with observations from other authors, we investigate the ephemeris for possible signs of transit timing variations (TTVs) using a Bayesian approach. Methods: The observations were obtained using telescope-defocusing techniques, and achieve a high precision with random errors in the photometry as low as 1mmag per point. To investigate the possibility of TTVs in the light curve, we calculate the overall probability of a TTV signal using Bayesian methods. Results: The observations are used to determine the photometric parameters and the physical properties of the GJ1214 system. Our results are in good agreement with published values. Individual times of mid-transit are measured with uncertainties as low as 10s, allowing us to reduce the uncertainty in the orbital period by a factor of two. Conclusions: A Bayesian analysis reveals that it is highly improbable that the observed transit times is explained by TTV, when compared with the simpler alternative of a linear ephemeris.Comment: Submitted to A&

Spectroscopic survey of Kepler stars – II. FIES/NOT observations of A- and F-type stars

We have analysed high-resolution spectra of 28 A and 22 F stars in the Kepler field, observed using the Fibre-Fed Échelle Spectrograph at the Nordic Optical Telescope. We provide spectral types, atmospheric parameters and chemical abundances for 50 stars. Balmer, Fe I and Fe II lines were used to derive effective temperatures, surface gravities and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The stars analysed include chemically peculiar stars of the Am and λ Boo types, as well as stars with approximately solar chemical abundances. The wide distribution of projected rotational velocity, vsin i, is typical for A and F stars. The microturbulence velocities obtained are typical for stars in the observed temperature and surface gravity ranges. Moreover, we affirm the results of Niemczura et al. that Am stars do not have systematically higher microturbulent velocities than normal stars of the same temperature