Gaia-ESO Survey: Empirical classification of VLT/Giraffe stellar spectra in the wavelength range 6440–6810 Å in the γ Velorum cluster, and calibration of spectral indices^⋆,⋆⋆

¹ INAF – Osservatorio Astronomico di Palermo G.S.Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: damiani@astropa.inaf.it
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴ Centre for Astrophysics Research, STRI, University of Hertfordshire, College Lane Campus, Hatfield AL10 9AB, UK
⁵ Astrophysics Group, Research Institute for the Environment, Physical Sciences and Applied Mathematics, Keele University, Keele, Staffordshire ST5 5BG, UK
⁶ Royal Observatory of Belgium, 3 avenue circulaire, 1180 Brussels, Belgium
⁷ Instituto de Astrofísica de Andalucía, Camino Bajo de Huétor 50, 18008 Granada, Spain
⁸ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
⁹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁰ Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Università di Catania, via S. Sofia 78, 95123 Catania, Italy
¹¹ ASI Science Data Center, via del Politecnico SNC, 00133 Roma, Italy
¹² Laboratoire Lagrange (UMR 7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
¹³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹⁴ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland

Received: 20 December 2013
Accepted: 14 March 2014

Abstract

We present a study of spectral diagnostics available from optical spectra with R = 17 000 obtained with the VLT/Giraffe HR15n setup, using observations from the Gaia-ESO Survey, on the γ Vel young cluster, with the purpose of classifying these stars and finding their fundamental parameters. We define several spectroscopic indices, sampling the amplitude of TiO bands, the Hα line core and wings, and temperature- and gravity-sensitive sets of lines, each useful as a T_eff or log g indicator over a limited range of stellar spectral types. Hα line indices are also useful as chromospheric activity or accretion indicators. Furthermore, we use all indices to define additional global T_eff- and log g-sensitive indices τ and γ, valid for the entire range of types in the observed sample. We find a clear difference between gravity indices of main-sequence and pre-main-sequence stars, as well as a much larger difference between these and giant stars. The potentially great usefulness of the (γ,τ) diagram as a distance-independent age measurement tool for young clusters is discussed. We discuss the effect on the defined indices of classical T Tauri star veiling, which is however detected in only a few stars in the present sample. Then, we present tests and calibrations of these indices, on the basis of both photometry and literature reference spectra, from the UVES Paranal Observatory Projectand the ELODIE 3.1 Library. The known properties of these stars, spanning a wide range of stellar parameters, enable us to obtain a good understanding of the performances of our new spectral indices. For non-peculiar stars with known temperature, gravity, and metallicity, we are able to calibrate quantitatively our indices, and derive stellar parameters for a wide range of stellar types. To this aim, a new composite index is defined, providing a good metallicity indicator. The ability of our indices to select peculiar, or otherwise rare classes of stars is also established. For pre-main-sequence stars outside the parameter range of the ELODIE dataset, index calibration relies on model isochrones. We check our calibrations against current Gaia-ESO UVES results, plus a number of Survey benchmark stars, and also against Gaia-ESO observations of young clusters, which contribute to establishing the good performance of our method across a wide range of stellar parameters. Our gravity determination for late-type PMS stars is found to be accurate enough to let us obtain gravity-based age estimates for PMS clusters. Finally, our gravity determinations support the existence of an older pre-main-sequence population in the γ Vel sky region, in agreement with evidence obtained from the lithium depletion pattern of the same stars.