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Str ¨Omgren Survey for Asteroseismology and Galactic The Astrophysical Journal, 787:110 (25pp), 2014 June 1 doi:10.1088/0004-637X/787/2/110 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. STROMGREN¨ SURVEY FOR ASTEROSEISMOLOGY AND GALACTIC ARCHAEOLOGY: LET THE SAGA BEGIN∗ L. Casagrande1,14, V. Silva Aguirre2, D. Stello3, D. Huber4,5, A. M. Serenelli6, S. Cassisi7, A. Dotter1, A. P. Milone1, S. Hodgkin8,A.F.Marino1, M. N. Lund2, A. Pietrinferni7,M.Asplund1, S. Feltzing9, C. Flynn10, F. Grundahl2,P.E.Nissen2,R.Schonrich¨ 11,12,15, K. J. Schlesinger1, and W. Wang13 1 Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, The Australian National University, Canberra, ACT 2611, Australia; [email protected] 2 Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark 3 Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, Sydney, NSW 2006, Australia 4 NASA Ames Research Center, Moffett Field, CA 94035, USA 5 SETI Institute, 189 Bernardo Avenue, Mountain View, CA 94043, USA 6 Institute of Space Sciences (IEEC-CSIC), Campus UAB, Fac. Ciencies,´ Torre C5 parell 2, E-08193 Bellaterra, Spain 7 INAF-Osservatorio Astronomico di Collurania, via Maggini, I-64100 Teramo, Italy 8 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK 9 Lund Observatory, Department of Astronomy and Theoretical Physics, P.O. Box 43, SE-22100 Lund, Sweden 10 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia 11 Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210-1173, USA 12 Rudolf-Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, UK 13 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China Received 2013 December 20; accepted 2014 March 8; published 2014 May 9 ABSTRACT Asteroseismology has the capability of precisely determining stellar properties that would otherwise be inaccessible, such as radii, masses, and thus ages of stars. When coupling this information with classical determinations of stellar parameters, such as metallicities, effective temperatures, and angular diameters, powerful new diagnostics for Galactic studies can be obtained. The ongoing Stromgren¨ survey for Asteroseismology and Galactic Archaeology has the goal of transforming the Kepler field into a new benchmark for Galactic studies, similar to the solar neighborhood. Here we present the first results from a stripe centered at a Galactic longitude of 74◦ and covering latitude from about 8◦ to 20◦, which includes almost 1000 K giants with seismic information and the benchmark open cluster NGC 6819. We describe the coupling of classical and seismic parameters, the accuracy as well as the caveats of the derived effective temperatures, metallicities, distances, surface gravities, masses, and radii. Confidence in the achieved precision is corroborated by the detection of the first and secondary clumps in a population of field stars with a ratio of 2 to 1 and by the negligible scatter in the seismic distances among NGC 6819 member stars. An assessment of the reliability of stellar parameters in the Kepler Input Catalog is also performed, and the impact of our results for population studies in the Milky Way is discussed, along with the importance of an all-sky Stromgren¨ survey. Key words: Galaxy: stellar content – stars: abundances – stars: distances – stars: fundamental parameters – stars: oscillations – surveys – techniques: photometric Online-only material: color figures, machine-readable table 1. INTRODUCTION main constraint for Galactic chemo(dynamical) models (e.g., Cescutti et al. 2007; Schonrich¨ & Binney 2009; Kobayashi & The study of a sizeable number of individual stars in the Nakasato 2011; Minchev et al. 2013; Wang & Zhao 2013) and Milky Way enables us to directly access different phases of provide important clues to understanding the main processes at its formation and evolution in a fashion that is still hardly play in galaxy formation and evolution (e.g., Pilkington et al. achievable for external galaxies. For obvious reasons, stars in 2012; Bird et al. 2013). the vicinity of the Sun have been preferred targets for this This sort of study is now fostered by a number of spectro- purpose, from the very first (e.g., Gliese 1957; Wallerstein 1962; scopic and photometric surveys, targeting different and fainter van den Bergh 1962; Eggen et al. 1962) to the most recent components of the Milky Way (e.g., RAdial Velocity Experi- photometric and spectroscopic investigations of the Milky Way, ment (RAVE): Steinmetz et al. 2006; Sloan Digital Sky Survey for which Hipparcos astrometric distances are also available (SDSS)–Sloan Extension for Galactic Understanding and Ex- out to 100 pc (e.g., Nordstrom¨ et al. 2004; Reddy et al. 2006; ploration (SEGUE): Yanny et al. 2009; SDSS–APOGEE: Ahn Feltzing & Bensby 2008; Casagrande et al. 2011; Adibekyan et al. 2014; Vista Variables in the Via Lactea (VVV): Minniti et al. 2013; Bensby et al. 2014). Properties of stars in the solar et al. 2010; Gaia-ESO:Gilmore et al. 2012; GALactic Archaeol- neighborhood, in particular ages and metallicities, are still the ogy with HERMES (GALAH): Freeman & HERMES/GALAH Team 2014), although astrometric distances for the targets in ∗ Based on observations made with the Isaac Newton Telescope operated on these surveys will not be available until the Gaia spacecraft re- the island of La Palma by the Isaac Newton Group in the Spanish Observatorio leases its data (Lindegren & Perryman 1996; Perryman et al. del Roque de los Muchachos of the Instituto de Astrof´ısica de Canarias. 2001). A common feature of all past and current stellar surveys 14 Stromlo Fellow. 15 Hubble Fellow. is that while it is relatively straightforward to derive some sort of 1 The Astrophysical Journal, 787:110 (25pp), 2014 June 1 Casagrande et al. information on the chemical composition of the targets observed tions and was designed to provide reliable stellar parameters, (and in many cases even detailed abundances), that is not the in particular metallicities (Stromgren¨ 1963, 1987). Thus, even case when it comes to masses, radii, distances, and, in particular, compared to multifiber spectroscopy, wide-field Stromgren¨ ages. Even when accurate astrometric distances are available to imaging is very efficient. It also has the advantage that no prese- allow comparison of stars with isochrones, the derived ages are lection is made on targets: all stars that fall in a given brightness still highly uncertain, and statistical techniques are required to regime across the instrument field of view will be observed. avoid biases (e.g., Pont & Eyer 2004; Jørgensen & Lindegren This greatly simplifies the selection function, which in our case 2005). Furthermore, isochrone dating is meaningful only for is essentially dictated by the Kepler satellite (e.g., Batalha et al. stars in the turnoff and subgiant phase, where stars of different 2010;Farmeretal.2013). Further, relatively faint magnitudes ages are clearly separated on the H-R diagram. This is in con- can be probed at high precision even on a small-class telescope, trast, for example, to stars on the red giant branch (RGB), where making it possible to readily derive metallicities over a large isochrones with vastly different ages can fit equally well obser- magnitude range. The Kepler seismic sample presented in this vational constraints such as effective temperatures, metallicities, work can thus be used as reference, e.g., for assessing the ac- and surface gravities within their errors (see, e.g., Soderblom curacy at which stellar parameters can be obtained for stars 2010 for a review). Thus, alternative ways to precisely determine having only photometric measurements, such as the planet host masses and radii of stars are the only way forward. stars we observe in SAGA. At the same time, the sensitivity By measuring oscillation frequencies in stars, asteroseismol- of Stromgren¨ colors to metallicity, coupled with seismic ages, ogy allows us to determine fundamental physical quantities, in helps immensely in calibrating other metallicity and age-dating particular masses and radii, that otherwise would be inaccessi- techniques, thus creating powerful new tools to study more re- ble in single field stars and that can be used to obtain informa- mote Galactic populations than previously possible. Bearing in tion on stellar distances and ages (see, e.g., Chaplin & Miglio mind the expected precision at which Gaia will deliver astro- 2013 for a review). Individual frequencies can yield an accuracy physical parameters for stars of different brightness (Liu et al. of just a few percent on those parameters, but their exploita- 2012), the above science case highlights the importance of hav- tion is more demanding, both observationally and theoretically ing an all-sky Stromgren¨ survey going fainter than any of the (Silva Aguirre et al. 2013). Global asteroseismic observables current large spectroscopic surveys (Wang et al. 2014). (see Section 3.5), on the contrary, are easier to detect and an- The SAGA survey also represents a natural extension of the alyze yet are able to provide the aforementioned parameters Geneva–Copenhagen Survey (GCS; Nordstrom¨ et al. 2004), the for a large number of stars with an accuracy that is still much onlyall-skyStromgren¨ survey currently available, and a bench- better than achievable by isochrone fitting in the traditional mark for Galactic studies. Similar to our latest revision of the sense. Thanks to spaceborne missions such as CoRoT (Baglin & GCS (Casagrande et al. 2011), we combine Stromgren¨ metallici- Fridlund 2006) and Kepler (Gilliland et al. 2010), average os- ties with broadband photometry to obtain effective temperatures cillation frequencies are now robustly detected in more than (Teff) and metallicities ([Fe/H]) for all our targets via the infrared 500 main-sequence and subgiant stars and over 13,000 giants flux method (IRFM). This facilitates the task of placing SAGA (e.g., De Ridder et al.
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