Rotation, activity and lithium in NGC 6475

Abstract

Radial and rotational velocities, chromospheric activity and lithium abundances are presented for an X-ray-selected sample of stars in the young (220 Myr) open cluster NGC 6475. Low-mass members of the cluster have been identified on the basis of photometric and spectroscopic criteria. The observations show that the rapid spindown seen amongst solar-type stars in the Pleiades is incomplete at 220 Myr, as there are F, G and K stars in NGC 6475 with v sin i > 10 kIn S-1. Peak rotation rates for G stars are 12-14 kIn S-1 and are robust to uncertain inclination angles, because the magnetic activity of these stars is less than the saturation value observed for fast rotators (v sin i > 15 kIn S-1) in the Pleiades. Two mid-K stars are found with v sin i ,25 kIn S-1 and with saturated magnetic activity levels, indicating that spindown time-scales are mass dependent, increasing from ' 20 Myr for early-G stars to > 75 Myr for mid-K stars. We have compared our rotation data, for the most rapidly rotating G and K stars in NGC 6475 and in young open clusters, with published rotational evolution models incorporating magnetic dynamo saturation at high rotation rates (> !lsat). Models with interior angular momentum transport are unable to simultaneously fit the rapidly rotating stars of the r:x Per, Pleiades and NGC 6475 clusters. Likely solutions are a plausible increase in the age of the Pleiades to 100 Myr, the incorporation of centrifugal wind driving in the angular momentum loss treatment, or ad hoc differences in initial conditions. If an increase in the age of the Pleiades is assumed, we find that a mass dependent !lsat is required, such that dynamo saturation occurs for G and K stars at a Rossby number of ' 0.4. This value is incompatible with the faster saturation thresholds inferred from coronal X-rays, but !lsat can be increased if solid body rotation is enforced or centrifugal wind driving is included. Comparisons with such models are favourable for G stars but less so for K stars. This may result from neglecting the evolving moment of inertia at early ages. All the cool stars exhibit LiI 6708-A absorption features. When compared to Li in the Pleiades and Hyades, it is evident that the G and K stars in the Hyades have experienced substantial Li depletion whilst on the main sequence, between 220 and 600 Myr, and probably at earlier times on the main sequence as well. There is marginal scatter of about 0.1 dex in the Li abundances of G and early K stars, possibly increasing to 1.0 dex in a few cooler objects. The data are insufficient to allow us to decide whether there is a mechanism by which to eliminate the scatter in Li abundances of the Pleiades K stars before they reach the Hyades age, or whether the difference reflects different cluster initial conditions. Interpretation is complicated by the X-ray selection bias, favouring the observation of the more rapidly rotating, Li-rich stars. Metallicity may also play a role, but our spectral *E-mai1: djj@st-and.ac.uk(DJJ);rdj@astro.keele.ac.uk (RDJ) tPresent address: School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS.