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Verifying Asteroseismically Determined Parameters of Kepler Stars Using Hipparcos Parallaxes: Self-consistent Stellar Properties and Distances
Accurately determining the properties of stars is of prime importancefor characterizing stellar populations in our Galaxy. The field ofasteroseismology has been thought to be particularly successful in suchan endeavor for stars in different evolutionary stages. However, tofully exploit its potential, robust methods for estimating stellarparameters are required and independent verification of the results ismandatory. With this purpose, we present a new technique to obtainstellar properties by coupling asteroseismic analysis with the InfraRedFlux Method. By using two global seismic observables and multi-bandphotometry, the technique allows us to obtain masses, radii, effectivetemperatures, bolometric fluxes, and hence distances for field stars ina self-consistent manner. We apply our method to 22 solar-likeoscillators in the Kepler short-cadence sample, that have accurateHipparcos parallaxes. Our distance determinations agree to better than5%, while measurements of spectroscopic effective temperatures andinterferometric radii also validate our results. We briefly discuss thepotential of our technique for stellar population analysis and models ofGalactic Chemical Evolution.
| Oscillation mode frequencies of 61 main-sequence and subgiant stars observed by Kepler
Context. Solar-like oscillations have been observed by Kepler and CoRoTin several solar-type stars, thereby providing a way to probe the starsusing asteroseismology Aims: We provide the mode frequencies ofthe oscillations of various stars required to perform a comparison withthose obtained from stellar modelling. Methods: We used a timeseries of nine months of data for each star. The 61 stars observed werecategorised in three groups: simple, F-like, and mixed-mode. The simplegroup includes stars for which the identification of the mode degree isobvious. The F-like group includes stars for which the identification ofthe degree is ambiguous. The mixed-mode group includes evolved stars forwhich the modes do not follow the asymptotic relation of low-degreefrequencies. Following this categorisation, the power spectra of the 61main-sequence and subgiant stars were analysed using both maximumlikelihood estimators and Bayesian estimators, providing individual modecharacteristics such as frequencies, linewidths, and mode heights. Wedeveloped and describe a methodology for extracting a single set of modefrequencies from multiple sets derived by different methods andindividual scientists. We report on how one can assess the quality ofthe fitted parameters using the likelihood ratio test and the posteriorprobabilities. Results: We provide the mode frequencies of 61stars (with their 1-σ error bars), as well as their associatedéchelle diagrams.Appendices are available in electronic form at http://www.aanda.org
| Accurate fundamental parameters and detailed abundance patterns from spectroscopy of 93 solar-type Kepler targets
We present a detailed spectroscopic study of 93 solar-type stars thatare targets of the NASA/Kepler mission and provide detailed chemicalcomposition of each target. We find that the overall metallicity is wellrepresented by Fe lines. Relative abundances of light elements (CNO) andα elements are generally higher for low-metallicity stars. Ourspectroscopic analysis benefits from the accurately measured surfacegravity from the asteroseismic analysis of the Kepler light curves. Theaccuracy on the log g parameter is better than 0.03 dex and is heldfixed in the analysis. We compare our Teff determination witha recent colour calibration of VT-KS [TYCHO Vmagnitude minus Two Micron All Sky Survey (2MASS) KSmagnitude] and find very good agreement and a scatter of only 80 K,showing that for other nearby Kepler targets, this index can be used.The asteroseismic log g values agree very well with the classicaldetermination using Fe I-Fe II balance, although we find a smallsystematic offset of 0.08 dex (asteroseismic log g values are lower).The abundance patterns of metals, α elements and the lightelements (CNO) show that a simple scaling by [Fe/H] is adequate torepresent the metallicity of the stars, except for the stars withmetallicity below -0.3, where α-enhancement becomes important.However, this is only important for a very small fraction of the Keplersample. We therefore recommend that a simple scaling with [Fe/H] beemployed in the asteroseismic analyses of large ensembles of solar-typestars.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| Spectroscopic Study of Candidates for Kepler Asteroseismic Targets - Solar-Like Stars
We report spectroscopic observations of 23 candidates for Keplerasteroseismic targets and 10 other stars in the Kepler field. For allthese stars, we derive the radial velocities, effective temperature,surface gravity, metallicity, the projected rotational velocity, andestimate the MK type. HIP 97513 and HIP 92132 are classified assuspected new single-lined spectroscopic binaries. For 28 stars, theradial velocity is measured for the first time.
| Direct detection of exoplanet host star companion γ Cep B and revised masses for both stars and the sub-stellar object
Context: .The star γ Cep is known as a single-lined spectroscopictriple system at a distance of 13.8 pc, composed of a K1 III-IV primarystar with V = 3.2 mag, a stellar-mass companion in a 66-67 year orbit(Torres 2007, ApJ, 654, 1095), and a substellar companion withMp sin i = 1.7 M_Jup that is most likely a planet (Hatzes etal. 2003, ApJ, 599, 1383). Aims: .We aim to obtain a first directdetection of the stellar companion, to determine its current orbitalposition (for comparison with the spectroscopic and astrometric data),its infrared magnitude and, hence, mass. Methods: .We use theAdaptive Optics camera CIAO at the Japanese 8 m telescope Subaru onMauna Kea, Hawaii, with the semi-transparent coronograph to block mostof the light from the bright primary γ Cep A, and to detect at thesame time the faint companion B. In addition, we also used the IR cameraΩ Cass at the Calar Alto 3.5 m telescope, Spain, to image γCep A and B by adding up many very short integrations (without AO). Results: .γ Cep B is clearly detected on our CIAO and ΩCass images. We use a photometric standard star to determine themagnitude of B after PSF subtraction in the Subaru image, and themagnitude difference between A and B in the Calar Alto images, and findan average value of K = 7.3 ± 0.2 mag. The separations andposition angles between A and B are measured on 15 July 2006 and 11 and12 Sept. 2006, B is slightly south of west of A. Conclusions: .Bycombining the radial velocity, astrometric, and imaging data, we haverefined the binary orbit and determined the dynamical masses of the twostars in the γ Cep system, namely 1.40 ± 0.12Mȯ for the primary and 0.409 ± 0.018Mȯ for the secondary (consistent with being a M4 dwarf).We also determine the minimum mass of the sub-stellar companion to beMp sin i = 1.60 ± 0.13 M_Jup.
| Common proper motion stars in the AGK 3
A search was made of common-proper-motion (CPM) systems among AGK 3stars. The selection of physical systems was based upon the ratiobetween the angular separation (rho) and the proper motion (mu); the CPMstars found are presented in two tables. Table I lists systems withrho/mu less than 1000 years. It contains 326 entries, and the proportionof optical pairs is estimated to be 1 percent. Table II lists systemswith rho/mu in the range 1000 to 3500 years; it contains 113 systems,but only 60 percent of them are physical. Nevertheless, these systemsoften have separations larger than 10,000 AU and are the mostinteresting for the study of the tail of the distribution function ofthe semimajor axes.
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