We present spectroscopic and photometric observations of BD + 22-degrees-4409, a nearby, low-mass star which, on the basis of its Galactic space motions and high EUV-to-bolometric luminosity ratio, has been previously 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 velocity 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 photometry and a trigonometric parallax, leads to the conclusion that the star is young, although probably no younger than 20-30 Myr, and is a prime Local Association candidate in all respects. Considerable variability is seen in the rotationally broadened Halpha emission line, and can be interpreted in terms of transient flare activity at very high latitudes or the combination of a rotationally modulated, high-latitude active region and a rather smaller flare at an unconstrained latitude. In either case, the flare radiative losses in the Halpha line are at least 2 X 10(32) erg. There is no evidence for the corotating cool prominences that have been seen at some distance from the surfaces of other late-type rapid rotators. This may be interpreted as a geometric effect, whereby the low inclination deduced for BD + 22-degrees-4409, of 50-degrees +/- 10-degrees, combined with a centrifugal flattening of any cloud system towards the equatorial plane, renders clouds unobservable 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 cent 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 correlates with the time of maximum spot coverage deduced from the shapes of photospheric line profiles. This result is consistent with calculations of the effect of starspots on the Li I equivalent width, once projection effects and limb-darkening are taken into account. Enhanced molecular bands may also play a role, but very cool atmospheric models are required to investigate this question fully.
BD+22-DEGREES-4409 - A RAPIDLY ROTATING, LOW-MASS MEMBER OF THE LOCAL ASSOCIATION
LANZAFAME, Alessandro Carmelo
1994-01-01
Abstract
We present spectroscopic and photometric observations of BD + 22-degrees-4409, a nearby, low-mass star which, on the basis of its Galactic space motions and high EUV-to-bolometric luminosity ratio, has been previously 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 velocity 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 photometry and a trigonometric parallax, leads to the conclusion that the star is young, although probably no younger than 20-30 Myr, and is a prime Local Association candidate in all respects. Considerable variability is seen in the rotationally broadened Halpha emission line, and can be interpreted in terms of transient flare activity at very high latitudes or the combination of a rotationally modulated, high-latitude active region and a rather smaller flare at an unconstrained latitude. In either case, the flare radiative losses in the Halpha line are at least 2 X 10(32) erg. There is no evidence for the corotating cool prominences that have been seen at some distance from the surfaces of other late-type rapid rotators. This may be interpreted as a geometric effect, whereby the low inclination deduced for BD + 22-degrees-4409, of 50-degrees +/- 10-degrees, combined with a centrifugal flattening of any cloud system towards the equatorial plane, renders clouds unobservable 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 cent 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 correlates with the time of maximum spot coverage deduced from the shapes of photospheric line profiles. This result is consistent with calculations of the effect of starspots on the Li I equivalent width, once projection effects and limb-darkening are taken into account. Enhanced molecular bands may also play a role, but very cool atmospheric models are required to investigate this question fully.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
