To date, the most extensive survey of the cluster is that
published by Abraham de Epstein & Epstein (1985) who were the
first to examine stars fainter than 
. Their
photometric survey encompasses a considerable proportion of the
late-type population of the cluster, notably the G and K-dwarfs.
Westerlund et al. (1988) studied 130 stars in a much larger
region surrounding the known centre of the cluster. This survey
was restricted to stars brighter than 
and
therefore Abraham de Epstein & Epstein (1985) remains the
primary data source for the late-type stars. Neither of these
papers presents accurate astrometry, although Abraham de Epstein
& Epstein (1985) give X and
Y plate co-ordinates for 262 stars taken from the photographic
plate used in their survey.
As all the observed stars were from the Abraham de Epstein &
Epstein survey, we transformed all the X and Y co-ordinates to
(
, 
) (B1950.0) using the APM facility at the
Royal Greenwich Observatory (Irwin & McMahon 1992) and
cross-checked the target stars with the appropriate plate of the
Schmidt southern sky survey.
The epoch B1950.0 positions of all the objects that we consider
to be members are given in Table A1 (available electronically) (the
criteria for membership is discussed in
Sect. 3.1 (click here)). Some stars were not recovered by the
APM. These invariably lay in regions close to bright stars
(e.g. within the diffraction ring of 
Sculptoris).
Others were not detected as stellar objects due to the shape
of their intensity profile and were classified non-stellar.
Although a few of these were extragalactic objects the
majority, on visual inspection, were found to be close stellar
pairs that were unresolved by the scan. Those stars whose
positional accuracy is in doubt are annotated accordingly in
Table A1.
This detailed study of the Abraham de Epstein & Epstein sample revealed some interesting anomalies. The `stars' ZS 51, 59, 86 and 87 are in fact elliptical galaxies. Close examination of the Schmidt plate reveals that stars of comparable brightness exhibit diffraction spikes whereas these particular objects exhibit characteristic galactic haloes. The finding charts show ZS 250 and 255 to be the same object. This is confirmed by examining the photometry of Abraham de Epstein & Epstein (1985) which is virtually identical for these stars. Table A2 in the appendix lists the positions of all the non-members, derived either from previous publications, inspection of the Schmidt plate, photometry or spectroscopy.
The observations were taken between the 24
and
27
October 1993, using the AUTOFIB (Sadler et al.
1991) fibre optic
feed from the prime focus of the AAT to the RGO spectrograph,
with 25cm camera, Tektronix 1024
1024 pixel CCD. The
1200R grating used gave a resolving power
14000. In all, the 64 available
fibres present a 38
field, with the added
constraint that stars in a given exposure must be within 2
magnitudes of each other. As the cluster is spread out over some
1.5 degrees, only 16 stars at most were used for any one
exposure. The remaining fibres were used to obtain an accurate
measure of the sky background. Stellar positions need to be
accurate to 
, because of the small
entrance apertures to the fibres. Exposure times were 1500 s and
6000 s (2
3000 s) at H
and Ca II, for stars
with magnitude 
, and 600 s and 1200 s respectively for
stars with 
.
The standard CCD reduction methods of bias subtraction, flat-fielding, scattered light and sky subtraction, fibre transmission, and wavelength calibration were performed with the STARLINK software collection (Lawden 1995).
The spectral range centred on H
covered approximately
the region 6300-6800 Å, encompassing also the Li I
(6708 Å) line. For our subsequent analysis, we only required
H
equivalent widths, so that flux calibration was not
necessary. The blue region covered the range 3800-4100 Å,
and flux calibration of the Ca II data followed the
empirical transformations of Linsky et al. (1979), as described
in equations (1-5) of Paper I. In particular, the flux (
) in
a 50 Å region centred on 3950 Å is given by
The mean colour excess E(B-V) = 0.02 mag for the Blanco 1 region (Westerlund et al. 1988).
We have obtained H
region spectra for 114 cluster members in
this survey, of which 13 had too weak an equivalent width to measure (see
Table 1). Of the remaining 101 spectra, 23 were in common with Paper I, so
that, in total, we have accumulated H
spectra of 125 stars of the
230 in the cluster survey of Abraham de Epstein & Epstein (1985).
Figure 1 (click here) shows the variation of
H
with (B-V), where we have included the solar neighbourhood
compilation of Panagi & Mathioudakis (1993) on the
diagram as we make comparisons later.
Figure 1: Blanco 1 and the solar neighbourhood
variation with (B-V). The lack of Blanco 1 observations beyond
(B-V) 
1.4 is due to target selection and not real
Of the 23 stars in common between this paper and
Paper I, we find agreement to within 10% for the majority of
H
absorption equivalent widths. The exceptions are
ZS 108, 109, which are about 50% and 100% greater here than
in Paper I. We cannot comment precisely as to the discrepancy,
except to say that we believe that there was no errors in the
stellar positions during either observation. The sole
H
emission star, ZS 61, shows somewhat different
behaviour. From Paper I, ZS 61 is the earliest spectral type
to show emission (K2V-K3V), yet there was no discernible
emission (or indeed absorption) in the present observations.
Including the results of Paper I, we have covered some 60% of
the cluster's K stars, of which we find 9 with H
in
emission. As the Abraham de Epstein & Epstein
study was extensive, it is therefore unlikely that many more
H
emitters remain to be discovered amongst the K star
population.
Following Paper I we searched for the presence of lithium in those stars for
which we had good quality medium resolution spectra around H
.
By comparing our lithium measurements with those of other young clusters we
hoped to find clues to the relative age of Blanco 1. Our
previous study found
15 stars exhibited the Li I feature at 6707.8 Å. The present
observations extend this list to 50 stars.
In addition to the H
region, we have taken spectra of
some 58 stars in Ca II(K). The 30 F and G star spectra
showed no core emission at our S/N, but of the remaining 28 K
stars, we were able to identify six stars with measurable Ca
II(K). Four of the six Ca II(K) emission stars,
ZS 35, 37, 144 and 172 are also strong H
emitters,
whereas ZS 46 had too weak an H
profile to measure. Not
withstanding the poor seeing, the fact that the remaining K
dwarfs, numbering 22 stars, did not show signs of Ca II
emission cores is rather puzzling. This is not a problem of
dispersion. Indeed, we have been able to measure H
for
nearly all the K dwarfs. Closer scrutiny reveals that 8 stars
have possible errors in their positions, however, for all these
stars we have been able to measure H
. If these stars
possess chromospheres, then we must expect to see Ca II(K)
in emission at these effective temperatures. Alternatively,
these stars may be field giants, in which the Ca II
emission will be considerably weaker. We have decided to
classify them as non-members, and they have been annotated
accordingly in Table A2 (available electronically).
Based on a visual inspection and comparison with single star
spectra of the same spectral type, we have found four new
binaries in this survey, ZS 5, 14, 65 and 176, in addition to
confirming that ZS 30a is a binary, based on its H
profile. Including ZS 76, 80 and 120 found in Paper I,
the total number of binaries is eight. We should note that
auto-correlation of the spectra proved ambiguous.
The measured H
and Li I(6708) equivalent widths and
Ca II(K) surface fluxes are presented in Table 1.
Table 1: Star No. assigned by Abraham de Epstein &
Epstein (1985), apparent V magnitude, (B-V) colour,
equivalent width of H
line (+ indicates emission),
and equivalent width of Li I(6708) line.
= Poor s/n, or too weak be measured.
= no discernible Ca II(K) emission
Newly evolved clusters such as the Pleiades and 
Persei
are known to possess broad spreads in rotation rate,
chromospheric activity and lithium abundance (Soderblom et al.
1993; Balachandran et al. 1988). It was therefore
expected that Blanco 1 would exhibit similar patterns of lithium
abundance. As part of our spectroscopic survey we searched for
the Li I feature at 6707.8 Å and were successful in
measuring lithium abundances for 34 possible cluster members (4
of these were also measured in Paper I).
Figure 2 (click here) plots lithium equivalent width
(
[Li]) as a function of un-reddened colour
for Blanco 1 and compares it to the Hyades, Pleiades
and 
Persei clusters. This figure is deliberately
constructed to resemble the plots shown in Soderblom et al.
(1993). The data for the 
Persei cluster were taken from
Balachandran et al. (1988), the Pleiades from
Soderblom et al. (1993) and the Hyades from
Soderblom et al. (1990). A large spread of
[Li] is noticeable at any
given spectral type for the
Pleiades, 
Persei and
Blanco 1 clusters. The spread appears greatest for the lower
mass stars where lithium depletion occurs more rapidly, such
that by the age of the Hyades lithium has become no longer
detectable. It is interesting to note that, except for a few
instances (e.g. 
) the Blanco 1
stars follow the same pattern as the Pleiades and 
Persei stars, which would imply a similar age.
Lithium is a coarse estimator of relative age, the process by which it is
depleted is far from understood and it is questionable how much
weight should
be attributed to our results. It can separate the Hyades from
the Pleiades, 
Persei and Blanco 1, yet it cannot resolve the latter
with certainty.