The Gaia-ESO Survey: Chromospheric Emission, Accretion Properties, and Rotation in $γ$ Velorum and Chamaeleon I

A. Frasca, K. Biazzo, A. C. Lanzafame, J. M. Alcalá, E. Brugaletta, A. Klutsch, B. Stelzer, G. G. Sacco, L. Spina, R. D. Jeffries, D. Montes, E. J. Alfaro, G. Barentsen, R. Bonito, J. F. Gameiro, J. Lopez-Santiago, G. Pace, L. Pasquini, L. Prisinzano, S. G. Sousa, G. Gilmore, S. Randich, G. Micela, A. Bragaglia, E. Flaccomio, A. Bayo, M. T. Costado, E. Franciosini, V. Hill, A. Hourihane, P. Jofre', C. Lardo, E. Maiorca, T. Masseron, L. Morbidelli, C. C. Worley

Published 2014-12-12Version 1

We use the fundamental parameters 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. 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 H$\alpha$ line was also used for identifying accreting objects and for evaluating the mass accretion rate ($\dot M_{\rm acc}$). The distribution of $v\sin i$ for the members of $\gamma$ 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 $\gamma$ 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 $T_{\rm eff}$-flux diagram and we propose a criterion for distinguishing them. We derive $\dot M_{\rm acc}$ in the ranges $10^{-11}$-$10^{-9} M_\odot$yr$^{-1}$ and $10^{-10}$-$10^{-7} M_\odot$yr$^{-1}$ for $\gamma$ Vel and Cha I accretors, respectively. We find less scatter in the $\dot M_{\rm acc}-M_\star$ relation derived through the H$\alpha$ EWs, when compared to the H$\alpha$ $10\%W$ diagnostics, in agreement with other authors.