The Gaia-ESO Survey: Hydrogen lines in red giants directly trace stellar mass

¹ Max-Planck Institute for Astronomy, 69117 Heidelberg, Germany
e-mail: bergemann@mpia-hd.mpg.de
² Instituto de Ciencias del Espacio (ICE-CSIC/IEEC), Campus UAB, Carrer Can Magrans S/N, 08193 Bellaterra, Spain
³ Rudolf-Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, OX1 3NP, Oxford, UK
⁴ Lund Observatory, Box 43, 221 00 Lund, Sweden
⁵ Department of Physics and Astronomy, Division of Astronomy and Space Physics, Angstrom laboratory, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁶ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Gttingen, Germany
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA, Cambridge, UK
⁸ INAF−Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
⁹ Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, The Australian National University, ACT 2611, Australia
¹⁰ INAF−Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹¹ Laboratoire Lagrange, Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
¹² Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabianska 8, 87-100 Torun, Poland
¹³ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munckegade 120, 8000 Aarhus, Denmark
¹⁴ Institute of Theoretical Physics and Astronomy, Vilnius University, Goštauto 12, 01108 Vilnius, Lithuania
¹⁵ ASI Science Data Center, via del Politecnico SNC, 00133 Roma, Italy
¹⁶ Instituto de Fisica y Astronomia, Fac. de Ciencias, U de Valparaiso, 1111 Gran Bretana, Playa Ancha, Chile
¹⁷ INAF−Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy
¹⁸ Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya st. 48, 119017 Moscow, Russia

Received: 18 December 2015
Accepted: 16 June 2016

Abstract

Red giant stars are perhaps the most important type of stars for Galactic and extra-galactic archaeology: they are luminous, occur in all stellar populations, and their surface temperatures allow precise abundance determinations for many different chemical elements. Yet, the full star formation and enrichment history of a galaxy can be traced directly only if two key observables can be determined for large stellar samples: age and chemical composition. While spectroscopy is a powerful method to analyse the detailed abundances of stars, stellar ages are the missing link in the chain, since they are not a direct observable. However, spectroscopy should be able to estimate stellar masses, which for red giants directly infer ages provided their chemical composition is known. Here we establish a new empirical relation between the shape of the hydrogen line in the observed spectra of red giants and stellar mass determined from asteroseismology. The relation allows determining stellar masses and ages with an accuracy of 10−15%. The method can be used with confidence for stars in the following range of stellar parameters: 4000 < T_eff < 5000 K, 0.5 < log g< 3.5, −2.0 < [Fe/H] < 0.3, and luminosities log L/L_Sun < 2.5. Our analysis provides observational evidence that the H_α spectral characteristics of red giant stars are tightly correlated with their mass and therefore their age. We also show that the method samples well all stellar populations with ages above 1 Gyr. Targeting bright giants, the method allows obtaining simultaneous age and chemical abundance information far deeper than would be possible with asteroseismology, extending the possible survey volume to remote regions of the Milky Way and even to neighbouring galaxies such as Andromeda or the Magellanic Clouds even with current instrumentation, such as the VLT and Keck facilities.