RAVE stars in K2

I. Improving RAVE red giants spectroscopy using asteroseismology from K2 Campaign 1^⋆

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: valentini@astro.ulg.ac.be
² School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
³ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
⁴ LESIA, Observatoire de Paris, PSL Research University, CNRS, Université Pierre et Marie Curie, Université Paris Diderot, 92195 Meudon, France
⁵ Osservatorio Astronomico di Padova, INAF, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ Dipartimento Fisica e Astronomia, Universitá di Padova, 35122 Padova, Italy
⁷ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12−14, 69120 Heidelberg, Germany
⁸ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
⁹ Osservatorio Astronomico di Padova, INAF, 36012 Asiago (VI), Italy
¹⁰ Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France
¹¹ Research School for Astronomy and Astrophysics, Mount Stromlo Observatory, The Australian National University, ACT 2611, Australia
¹² E. A. Milne Centre for Astrophysics, University of Hull, Hull, HU6 7RX, UK
¹³ Institute of Astronomy Cambridge University, Madingley Road Cambridge CB3 0HA, UK
¹⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁵ Lund Observatory, Box 43, 221 00 Lund, Sweden
¹⁶ Senior CIFAR Fellow, Dept. of Physics and Astronomy, University of Victoria, Victoria, BC, V8P 5C2, Canada
¹⁷ Department of Physics, The University of Hong Kong, Hong Kong, PR China
¹⁸ Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia
¹⁹ Centre for Astronomy, Astrophysics and Astrophotonics, Macquarie University, Sydney, NSW 2109, Australia
²⁰ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, RH5 6NT, UK
²¹ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
²² Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
²³ Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles St, Baltimore, MD 21218, USA
²⁴ Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia

Received: 12 September 2016
Accepted: 15 December 2016

Abstract

We present a set of 87 RAVE stars with detected solar like oscillations, observed during Campaign 1 of the K2 mission (RAVE K2-C1 sample). This data set provides a useful benchmark for testing the gravities provided in RAVE data release 4 (DR4), and is key for the calibration of the RAVE data release 5 (DR5). The RAVE survey collected medium-resolution spectra (R = 7500) centred in the Ca II triplet(8600 Å) wavelength interval, which although being very useful for determining radial velocity and metallicity, even at low S/N, is known be affected by a log (g)-T_eff degeneracy. This degeneracy is the cause of the large spread in the RAVE DR4 gravities for giants. The understanding of the trends and offsets that affects RAVE atmospheric parameters, and in particular log (g), is a crucial step in obtaining not only improved abundance measurements, but also improved distances and ages. In the present work, we use two different pipelines, GAUFRE and Sp_Ace, to determine atmospheric parameters and abundances by fixing log (g) to the seismic one. Our strategy ensures highly consistent values among all stellar parameters, leading to more accurate chemical abundances. A comparison of the chemical abundances obtained here with and without the use of seismic log (g) information has shown that an underestimated (overestimated) gravity leads to an underestimated (overestimated) elemental abundance (e.g. [Mg/H] is underestimated by ~0.25 dex when the gravity is underestimated by 0.5 dex). We then perform a comparison between the seismic gravities and the spectroscopic gravities presented in the RAVE DR4 catalogue, extracting a calibration for log (g) of RAVE giants in the colour interval 0.50 < (J−K_S) < 0.85. Finally, we show a comparison of the distances, temperatures, extinctions (and ages) derived here for our RAVE K2-C1 sample with those derived in RAVE DR4 and DR5. DR5 performs better than DR4 thanks to the seismic calibration, although discrepancies can still be important for objects for which the difference between DR4/DR5 and seismic gravities differ by more than ~0.5 dex. The method illustrated in this work will be used for analysing RAVE targets present in the other K2 campaigns, in the framework of Galactic Archaeology investigations.