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

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 premain-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 Project and 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 premain-sequence population in the γ Vel sky region, in agreement with evidence obtained from the lithium depletion pattern of the same stars.

The Gaia-ESO Survey: Age spread in the star forming region NGC 6530 from the HR diagram and gravity indicators

“…The homogenized values constitute the official data set, the most recent internally distributed through the data release GESiDR5, as approved by the Working Group 15 (WG15). It includes the radial velocities (RVs), rotational velocities v sini, effective temperatures, gravities, γ indices (Damiani et al, 2014), lithium equivalent width (EW(Li)) and several parameters of the Hα line, as for example the Full Width at Zero Intensity (FWZI). Effective temperatures and gravities derived by the Palermo GES node were used for the objects not included in the WG15 recommended data set.…”

“…As in the case of the Carina Nebula (Damiani et al, 2017b), several spectra in our sample present unexpected features that are not considered in the WG15 standard procedure, but that are instead dealt with in the procedure adopted in the WG12 dedicated to PMS stars. In this case, RVs are provided by the Catania , and Palermo GES nodes (Damiani et al, 2014). For this reason, in general, we adopted the official WG15 RV values, but in the cases of spectra with features peculiar to young stars (e.g.…”

“…Finally, the γ index defined by Damiani et al (2014), being an indicator of the stellar gravity, is a very useful criterion to select true negatives. In fact, Damiani et al (2014) showed that for late spectral type stars, the γ index of giants is significantly larger than that of PMS stars, that is only slightly larger that that of MS stars. For this reason, the γ index can be used to select giants and then, to discard them as true negatives from the sample of YSOs.…”

The Gaia-ESO Survey: asymmetric expansion of the Lagoon Nebula cluster NGC 6530 from GES and Gaia DR2

“…One of the main sources of contamination in a sample of candidate members selected from the CMD is background giants. These can be identified and removed based on their low surface gravity using the gravity index γ (Damiani et al 2014) 5 . Figure 3 shows γ as a function of the effective temperature for GES targets observed towards NGC 6530.…”

“…Figure 3 shows γ as a function of the effective temperature for GES targets observed towards NGC 6530. Giant stars occupy the upper-right part of this plot, with γ > 1 and T ef f < 5600 K (Damiani et al 2014). There are 1064 stars with γ and T ef f measurements in our sample, 233 of which meet these criteria and are classified as giants.…”