BD-DEGREES-4409 - A RAPIDLY ROTATING, LOW-MASS MEMBER OF THE LOCAL ASSOCIATION(22)

We present spectroscopic and photometric observations of BD + 22-degre es-4409, a nearby, low-mass star which, on the basis of its Galactic s pace motions and high EUV-to-bolometric luminosity ratio, has been pre viously assigned membership of the young kinematic group known as the Local Association. Our observations show that BD + 22-degrees-4409 is a single, K5V-K7V, chromospherically active ultrafast rotator, with a probable period of 10.17 +/- 0.10 h and a projected equatorial velocit y of 69 +/- 1 km s-1. It has a comparatively high photospheric lithium abundance of N(Li) = 1.30 +/- 0.25, and this, combined with our photo metry and a trigonometric parallax, leads to the conclusion that the s tar is young, although probably no younger than 20-30 Myr, and is a pr ime Local Association candidate in all respects. Considerable variabil ity is seen in the rotationally broadened Halpha emission line, and ca n be interpreted in terms of transient flare activity at very high lat itudes or the combination of a rotationally modulated, high-latitude a ctive region and a rather smaller flare at an unconstrained latitude. In either case, the flare radiative losses in the Halpha line are at l east 2 X 10(32) erg. There is no evidence for the corotating cool prom inences that have been seen at some distance from the surfaces of othe r late-type rapid rotators. This may be interpreted as a geometric eff ect, whereby the low inclination deduced for BD + 22-degrees-4409, of 50-degrees +/- 10-degrees, combined with a centrifugal flattening of a ny cloud system towards the equatorial plane, renders clouds unobserva ble as Halpha absorption transients. Spot modelling of the photometric light curves yields an asymmetric spot component, covering at least 4 per cent of the total stellar surface. Variability of 10 +/- 3 per ce nt is seen in the strength of the Li I 6708-angstrom line over 7 h of a rotation peiod, in the sense that the peak Li I equivalent width cor relates with the time of maximum spot coverage deduced from the shapes of photospheric line profiles. This result is consistent with calcula tions of the effect of starspots on the Li I equivalent width, once pr ojection effects and limb-darkening are taken into account. Enhanced m olecular bands may also play a role, but very cool atmospheric models are required to investigate this question fully.