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

Frasca, A, Biazzo, K, Lanzafame, AC, Alcalá, JM, Brugaletta, E, Klutsch, A, Stelzer, B, Sacco, GG, Spina, L, Jeffries, RD, Montes, D, Alfaro, EJ, Barentsen, G, Bonito, R, Gameiro, JF, Lopez-Santiago, J, Pace, G, Pasquini, L, Prisinzano, L, Sousa, SG , Gilmore, G, Randich, S, Micela, G, Bragaglia, A, Flaccomio, E, Bayo, A, Costado, MT, Franciosini, E, Hill, V, Hourihane, A, Jofre', P, Lardo, C, Maiorca, E, Masseron, T, Morbidelli, L and Worley, CC (2015) The Gaia-ESO Survey: Chromospheric Emission, Accretion Properties, and Rotation in $γ$ Velorum and Chamaeleon I. Astronomy & Astrophysics, 575. ISSN 1432-0746

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.