Empirical classification of VLT/Giraffe stellar spectra in the wavelength range 6440-6810 A in the gamma Vel cluster, and calibration of spectral indices

TL;DR

This paper develops and calibrates spectral indices from VLT/Giraffe spectra to determine stellar parameters and distinguish between different stellar evolutionary stages in the gamma Vel cluster, aiding in age and population analysis.

Contribution

It introduces new spectral indices and calibration methods for stellar parameters, enabling age estimation and population differentiation in young clusters.

Findings

Indices effectively differentiate main-sequence and pre-main-sequence stars.

Calibrated indices provide reliable temperature and gravity estimates.

Support for an older pre-main-sequence population in gamma Vel.

Abstract

We study spectral diagnostics available from optical spectra with R=17000 obtained with the VLT/Giraffe HR15n setup, using observations from the Gaia-ESO Survey, on the gamma Vel young cluster, in order to determine the fundamental parameters of these stars. We define a set of spectroscopic indices, sampling TiO bands, H-alpha core and wings, and many temperature- and gravity-sensitive lines. Combined indices tau (gamma) are also defined as Teff (log g) indicators over a wide spectral-type range. H-alpha emission-line indices are also chromospheric activity or accretion indicators. A metallicity-sensitive index is also defined. These indices enable us to find a clear difference between gravities of main-sequence and pre-main-sequence stars (as well as giant stars): the (gamma,tau) diagram is thus argued to be a promising distance-independent age measurement tool for young clusters. Our indices were quantitatively calibrated by means of photometry and literature reference spectra (from UVES-POP and ELODIE 3.1 Libraries, and other datasets), over a wide range of stellar parameters. The ability of our indices to select peculiar, or other rare classes of stars is also established. Finally, our gravity determinations support the existence of an older pre-main-sequence population in the gamma Vel sky region, in agreement with the lithium depletion pattern of those stars (abridged).