The Gaia-ESO Survey: Chromospheric emission, accretion properties, and rotation in gamma Velorum and Chamaeleon*,**

Abstract

One of the goals of the Gaia-ESO Survey (GES), which is conducted with FLAMES at the VLT, is the census and the characterization of the low-mass members of very young clusters and associations. We conduct a comparative study of the main properties of the sources belonging to gamma Velorum (gamma Vel) and Chamaeleon I (Cha I) young associations, focusing on their rotation, chromospheric radiative losses, and accretion. Methods. We used the fundamental parameters (effective temperature, surface gravity, lithium abundance, and radial velocity) delivered by the GES consortium in the first internal data release to select the members of gamma Vel and Cha I among the UVES and GIRAFFE spectroscopic observations. A total of 140 gamma Vel members and 74 Cha I members were studied. The procedure adopted by the GES to derive stellar fundamental parameters also provided measures of the projected rotational velocity (v sin i). We calculated stellar luminosities through spectral energy distributions, while stellar masses were derived by comparison with evolutionary tracks. The spectral subtraction of low-activity and slowly rotating templates, which are rotationally broadened to match the v sin i of the targets, enabled us to measure the equivalent widths (EWs) and the fluxes in the H alpha and H beta lines. The Ha line was also used for identifying accreting objects, on the basis of its EW and the width at the 10% of the line peak (10% W), and for evaluating the mass accretion rate ((M)over dot(acc)). Results. The distribution of v sin i for the members of. Vel displays a peak at about 10 km s(-1) with a tail toward faster rotators. There is also some indication of a different v sin i distribution for the members of its two kinematical populations. Most of these stars have Ha fluxes corresponding to a saturated activity regime. We find a similar distribution, but with a narrower peak, for Cha I. Only a handful of stars in gamma Vel display signatures of accretion, while many more accretors were detected in the younger Cha I, where the highest Ha fluxes are mostly due to accretion, rather than to chromospheric activity. Accreting and active stars occupy two different regions in a T-eff-flux diagram and we propose a criterion for distinguishing them. We derive (M)over dot(acc) in the ranges 10(-11)-10(-9) M-circle dot yr(-1) and 10(-10)-10(-7) M-circle dot yr(-1) for gamma Vel and Cha I accretors, respectively. We find less scatter in the (M)over dot(acc)-M-star relation derived through the H alpha EWs, when compared to the Ha 10% W diagnostics, in agreement with other authors.