The Gaia-ESO Survey: Chromospheric Emission, Accretion Properties, and Rotation in ${\hat{I}}^3$ Velorum and Chamaeleon I

Abstract

We use the fundamental parameters delivered by the GES consortium in the first internal data release to select the members of ${\hat{I}}^3$ Vel and Cha I among the UVES and GIRAFFE spectroscopic observations. A total of 140 ${\hat{I}}^3$ Vel members and 74 Cha I members were studied. 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$\hat{I}\pm$ and H${\hat{I}}^2$ lines. The H$\hat{I}\pm$ line was also used for identifying accreting objects and for evaluating the mass accretion rate (${\dot{M}}_{\mathrm{acc}}$). The distribution of $v\sin i$ for the members of ${\hat{I}}^3$ 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. Only a handful of stars in ${\hat{I}}^3$ Vel display signatures of accretion, while many more accretors were detected in the younger Cha~I. Accreting and active stars occupy two different regions in a $T_{\mathrm{eff}}$-flux diagram and we propose a criterion for distinguishing them. We derive ${\dot{M}}_{\mathrm{acc}}$ in the ranges $10^{-11}$-$10^{-9}{M}_{\odot }$yr${}^{-1}$ and $10^{-10}$-$10^{-7}{M}_{\odot }$yr${}^{-1}$ for ${\hat{I}}^3$ Vel and Cha I accretors, respectively. We find less scatter in the ${\dot{M}}_{\mathrm{acc}}-M_{\star }$ relation derived through the H$\hat{I}\pm$ EWs, when compared to the H$\hat{I}\pm$ $10\%W$ diagnostics, in agreement with other authors.