ROTATION, ACTIVITY AND LITHIUM IN NGC-6475

Radial and rotational velocities, chromospheric activity and lithium a bundances are presented for an X-ray-selected sample of stars in the y oung (220 Myr) open cluster NGC 6475. Low-mass members of the cluster have been identified on the basis of photometric and spectroscopic cri teria. The observations show that the rapid spin-down seen amongst sol ar-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 km s(-1). Peak rotation rates for G stars are 12-14 km s(-1) and are robust to uncertain incli nation angles, because the magnetic activity of these stars is less th an the saturation value observed for fast rotators (v sin i > 15 km s( -1)) in the Pleiades. Two mid-K stars are found with v sin i similar t o 25 km s(-1) and with saturated magnetic activity levels, indicating that spindown time-scales are mass dependent, increasing from similar to 20 Myr for early-G stars to > 75 Myr for mid-K stars. We have compa red our rotation data, for the most rapidly rotating G and K stars in NGC 6475 and in young open clusters, with published rotational evoluti on models incorporating magnetic dynamo saturation at high rotation ra tes (>Omega(sat)). Models with interior angular momentum transport are unable to simultaneously fit the rapidly rotating stars of the alpha Per, Pleiades and NGC 6475 clusters. Likely solutions are a plausible increase in the age of the Pleiades to 100 Myr, the incorporation of c entrifugal wind driving in the angular momentum loss treatment, or ad hoc differences in initial conditions. If an increase in the age of th e Pleiades is assumed, we find that a mass dependent Omega(sat) is req uired, such that dynamo saturation occurs for G and K stars at a Rossb y number of -0.4. This value is incompatible with the faster saturatio n thresholds inferred from coronal X-rays, but Omega(sat) can be incre ased 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 mom ent of inertia at early ages. All the cool stars exhibit Li I 6708-Ang strom absorption features. When compared to Li in the Pleiades and Hya des, it is evident that the G and K stars in the Hyades have experienc ed substantial Li depletion whilst on the main sequence, between 220 a nd 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 obj ects. 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 dif ference 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 synthesis estimate of [Fe/H] = + 0.110 +/- 0 .034 is comparable to the Pleiades and Hyades values.