The young galactic cluster Blanco 1 is a loose aggregate of stars lying
beyond the bright B-type star 
Sculptoris. It was first identified by
Blanco (1949) who noticed the area contained a
significantly higher
concentration of A0-type stars than the average value for similar galactic
latitudes.
Since its discovery the cluster has been subject to considerable photo-electric and photographic multi-colour photometry, e.g. Westerlund (1963), Epstein (1968), Eggen (1970, 1972), Perry, et al. (1978), Lyngå & Wramdemark (1984), Abraham de Epstein & Epstein (1985) and Westerlund et al. (1988).
Dolidze (1959) reported the presence of 
70
stars in the cluster region
(magnitude range: 
) that showed H
in emission.
These observations have since been questioned by Bond
(1972) who found no
stars exhibiting bright H
although he considered it possible that
very faint emission-line stars may exist between V = 14 and 16.
In a recent paper, Panagi et al. (1994)
(hereafter
Paper I), presented the
first detailed spectroscopic survey of the low-mass component of
the Blanco 1
cluster. The observations covered 115 stars from the survey of
Abraham de Epstein & Epstein (1985), taking in the
chromospheric lines of Ca II(H,K),
Ca II(I-R) triplet and H
. This preliminary survey
found that a very
low proportion of stars showed H
emission and all lie
approximately in the
magnitude range suggested by Bond.
Emission and absorption in chromospheric lines acts as an
important proxy for the dynamo that is thought to give rise to
heating in the chromosphere. This in turn depends on the
rotation rate and ultimately on the stellar age, so that
chromospheric studies can give important clues as to a cluster's
age. Stars that show H
emission are generally more
active than absorption-line stars. Studies of chromospheric
activity in young clusters such as the Pleiades (Prosser
et al. 1991), the Hyades (Stauffer et al. 1991) and
Persei (Prosser 1992) have shown that the mean
H
emission strength in the late K and M-type dwarfs
declines with age. An earlier study of the Pleiades by
Stauffer & Hartmann (1987) showed that a strong
correlation exists between H
emission and rotation, for stars earlier than about M0. This,
and other surveys, led Stauffer et al. (1991) to suggest that
the age-dependence of H
emission could be calibrated to
measure relative cluster ages. A paucity of H
emitters
in a cluster's late-type population is therefore an indication
of greater age. By comparing the ratio of the number of emitters
amongst the K dwarf population for a variety of clusters, we
attempt to calculate a new age for Blanco 1 (as first described
in O'Dell 1995).
Previous age estimates, derived by fitting isochrones to the
cluster's colour-magnitude diagram, suggested an age of
50 Myr (Perry et al. 1978;
Abraham de Epstein & Epstein 1985;
Westerlund et al. 1988) i.e. of
comparable age to the Pleiades and 
Persei clusters.
Following Paper I, we extended the lithium survey of the
cluster. Although recognised as an indicator of youth, an
understanding of the lithium depletion process is far from
established and ages for individual stars obtained from lithium
studies are subject to doubt. Nevertheless as a check we decided
to compare the spread of lithium abundances with colour, for
Blanco 1, with those of other young clusters to recover an
approximate age and compare to that obtained from the relative
number of H
emitters amongst the K dwarfs.
Finally we present accurate positions for all the stars surveyed by Abraham de Epstein & Epstein and revise cluster membership, incorporating all the proposed cluster members of Westerlund et al. (1988).