The NASA space telescope Kepler has provided unprecedented time-series observations which have revolutionised the field of asteroseismology, i.e. the use of stellar oscillations to probe the interior of stars. The Kepler-data include observations of
stars in open clusters, which are particularly interesting for asteroseismology. One of the clusters observed with Kepler is NGC 6811, which is the target of the present paper. However, apart from high-precision time-series observations, sounding the interiors of stars in open clusters by means of asteroseismology also requires accurate and precise atmospheric parameters as well as cluster membership indicators for the individual stars. We use medium-resolution (R~25,000) spectroscopic observations, and three independent analysis methods, to derive effective temperatures, surface gravities, metallicities, projected rotational velocities and radial velocities, for 15 stars in the field of the open cluster NGC 6811. We discover two double-lined and three single-lined spectroscopic binaries. Eight stars are classified as either certain or very probable cluster members, and three stars are classified as non-members. For four stars, cluster membership could not been assessed. Five of the observed stars are G-type giants which are located in the colour-magnitude diagram in the region of the red clump of the cluster. Two of these stars are surely identified as red clump stars for the first time. For those five stars, we provide chemical abundances of 31 elements. The mean radial-velocity of NGC 6811 is found to be +6.68$pm$0.08 km s$^{-1}$ and the mean metallicity and overall abundance pattern are shown to be very close to solar with an exception of Ba which we find to be overabundant.
The asteroseismic and planetary studies, like all research related to stars, need precise and accurate stellar atmospheric parameters as input. We aim at deriving the effective temperature (Teff), the surface gravity (log g), the metallicity ([Fe/H])
, the projected rotational velocity (v sin i) and the MK type for 169 F, G, K, and M-type Kepler targets which were observed spectroscopically from the ground with five different instruments. We use two different spectroscopic methods to analyse 189 high-resolution, high-signal-to-noise spectra acquired for the 169 stars. For 67 stars, the spectroscopic atmospheric parameters are derived for the first time. KIC 9693187 and 11179629 are discovered to be double-lined spectroscopic binary systems. The results obtained for those stars for which independent determinations of the atmospheric parameters are available in the literature are used for a comparative analysis. As a result, we show that for solar-type stars the accuracy of present determinations of atmospheric parameters is +/- 150 K in Teff, +/- 0.15 dex in [Fe/H], and +/-? 0.3 dex in log g. Finally, we confirm that the curve-of-growth analysis and the method of spectral synthesis yield systematically different atmospheric parameters when they are applied to stars hotter than 6,000 K.
MOST time-series photometry of mu Eri, an SB1 eclipsing binary with a rapidly-rotating SPB primary, is reported and analyzed. The analysis yields a number of sinusoidal terms, mainly due to the intrinsic variation of the primary, and the eclipse ligh
t-curve. New radial-velocity observations are presented and used to compute parameters of a spectroscopic orbit. Frequency analysis of the radial-velocity residuals from the spectroscopic orbital solution fails to uncover periodic variations with amplitudes greater than 2 km/s. A Rossiter-McLaughlin anomaly is detected from observations covering ingress. From archival photometric indices and the revised Hipparcos parallax we derive the primarys effective temperature, surface gravity, bolometric correction, and the luminosity. An analysis of a high signal-to-noise spectrogram yields the effective temperature and surface gravity in good agreement with the photometric values. From the same spectrogram, we determine the abundance of He, C, N, O, Ne, Mg, Al, Si, P, S, Cl, and Fe. The eclipse light-curve is solved by means of EBOP. For a range of mass of the primary, a value of mean density, very nearly independent of assumed mass, is computed from the parameters of the system. Contrary to a recent report, this value is approximately equal to the mean density obtained from the stars effective temperature and luminosity. Despite limited frequency resolution of the MOST data, we were able to recover the closely-spaced SPB frequency quadruplet discovered from the ground in 2002-2004. The other two SPB terms seen from the ground were also recovered. Moreover, our analysis of the MOST data adds 15 low-amplitude SPB terms with frequencies ranging from 0.109 c/d to 2.786 c/d.
Stellar structure and evolution can be studied in great detail by asteroseismic methods, provided data of high precision are available. We determine the effective temperature (Teff), surface gravity (log g), metallicity, and the projected rotational
velocity (v sin i) of 44 Kepler asteroseismic targets using our high-resolution (R > 20,000) spectroscopic observations; these parameters will then be used to compute asteroseismic models of these stars and to interpret the Kepler light curves.We use the method of cross correlation to measure the radial velocity (RV) of our targets, while atmospheric parameters are derived using the ROTFIT code and spectral synthesis method. We discover three double-lined spectroscopic binaries, HIP 94924, HIP 95115, and HIP 97321 - for the last system, we provide the orbital solution, and we report two suspected single-lined spectroscopic binaries, HIP94112 and HIP 96062. For all stars from our sample we derive RV, v sin i, Teff, log g, and metallicity, and for six stars, we perform a detailed abundance analysis. A spectral classification is done for 33 targets. Finally, we show that the early-type star HIP 94472 is rotating slowly (v sin i = 13 kms/1) and we confirm its classification to the Am spectral type which makes it an interesting and promising target for asteroseismic modeling. The comparison of the results reported in this paper with the information in the Kepler Input Catalog (KIC) shows an urgent need for verification and refinement of the atmospheric parameters listed in the KIC. That refinement is crucial for making a full use of the data delivered by Kepler and can be achieved only by a detailed ground-based study.
We report on the ground-based follow-up program of spectroscopic and photometric observations of solar-like asteroseismic targets for the Kepler space mission. These stars constitute a large group of more than thousand objects which are the subject o
f an intensive study of the Kepler Asteroseismic Science Consortium Working Group 1 (KASC WG-1). The main goal of this coordinated research is the determination of the fundamental stellar atmospheric parameters, which are used for the computing of their asteroseismic models, as well as for the verification of the Kepler Input Catalogue (KIC).
We present results of our 5-years-long program of ground-based spectroscopic and photometric observations of individual Kepler asteroseismic targets and the open clusters NGC6866 and NGC6811 from the Kepler field of view.We determined the effective t
emperature, surface gravity, metallicity, the projected rotational velocity and the radial velocity of 119 Kepler asteroseismic targets for which we acquired high-resolution spectra. For many of these stars the derived atmospheric parameters agree with Teff, log g, and [Fe/H] from the Kepler Input Catalog (KIC) to within their error bars. Only for stars hotter than 7000K we notice significant differences between the effective temperature derived from spectroscopy and Teff given in the KIC. For 19 stars which we observed photoelectrically, we measured the interstellar reddening and we found it to be negligible. Finally, our discovery of the delta Sct and gamma Dor pulsating stars in the open cluster NGC6866 allowed us to discuss the frequency of the occurrence of gamma Dor stars in the open clusters of different age and metallicity and show that there are no correlations between these parameters.
We report the discovery of 19 variable stars and two blue-stragglers in the field of the open cluster NGC6866. Three of the variable stars we classify as delta Sct, two, as gamma Dor, four, as WUMa, two, as ellipsoidal variables, and one, as an eclip
sing binary. Seven stars show irregular variability. Two of the pulsators, a delta Sct star NGC 6866-29 and a gamma Dor star NGC6866-21, are multiperiodic. From an analysis of proper motions, we conclude that the delta Sct stars, one of the gamma Dor stars and both blue-stragglers are very probable members of the cluster. The position on the color-magnitude diagram of seven other variables suggests that they also belong to the cluster. The eclipsing binary, which we discover to be a new high-velocity star, and the seven irregular variables are non-members. Then, we discuss in detail the age and metallicity of open clusters that host gamma Dor stars and we show that none of these parameters is correlated with the number of gamma Dor stars in cluster.
