B-type main-sequence stars, although fainter than O-types, have the additional advantage that their atmospheres can be modelled with sufficient reliability to yield accurate atmospheric parameters and chemical compositions (see, for example, [8, Gies & Lambert 1992;] [13, Kilian 1992).] Estimates are also less subject to the uncertainties implicit in other methods, such as dust affecting abundances from emission line plasmas (e.g. HII regions), or contamination from mixing with nuclear material processed from the stellar interior for highly evolved objects (e.g. Planetary Nebulae).
Our investigations of the chemical composition of the galactic disk have concentrated mainly on the anticentre direction, with evidence for small scale spatial structure [30, (Smartt et al. 1996a, 1996b),] the first reliable stellar estimate of large scale abundance gradients [29, (Smartt & Rolleston 1997)] and the use of element abundance ratios to identify the predominant nucleosynthetic environment, e.g. supernovae, low or high mass stars [11, (Hibbins et al. 1998).] By contrast, studies towards the galactic centre have been hampered by the large extinction at optical wavelengths. However, preliminary investigations of stars with galactic latitudes have shown that it is possible to map both the inner region of the galaxy [33, (Smartt et al. 1999)] and even beyond the galactic centre [25, (Ryans et al. 1997).]
In these last two papers (and also in [39, Venn et al. 1998),] we have used the catalogues of southern luminous stars by [21, Reed (1993)] and [22, Reed & Beatty (1995)] to search for early-type candidates. Using a combination of colour-colour and colour-magnitude criteria, we identified a number of apparently normal B-type main-sequence stars within 4kpc of the galactic centre [33, (Smartt et al. 1999),] together with A-type supergiants (their evolutionary descendents), which are slightly closer to the Sun. However this procedure was not straightforward as the catalogues have a faint visual magnitude cut-off of 12.5 and may not be complete. Additionally we now believe that we have exhausted the available published photometry for these galactic directions (at least for the colour and magnitude criteria appropriate for blue stars within the inner few kiloparsecs of the disk). Hence in order to further theses studies, we need to identify more potential targets which necessarily requires a (very) wide-field survey technique employing both multi-colour photometry and preliminary low-resolution spectroscopy for spectral-type classification.
The UK Schmidt telescope provides appropriate instrumentation to fulfill our needs. In this paper, we present the first results of a systematic survey to identify further early-type stars towards the galactic centre using its imaging and spectroscopic capabilities. Suitable photographic material for a mosaic of 19 Schmidt fields (yielding a total survey area of 475 square degrees) has already been obtained, and we have measured a subset of 14 of these fields, to produce lists of candidate blue stellar objects. For three fields, low dispersion multi-object spectroscopy of the blue targets has been gathered and we present the photometric and spectroscopic details associated with these three fields. It is intended that spectroscopy will be obtained for the other fields and these will be presented in future papers. The data reported here forms the basic selection criteria for a high resolution spectroscopic campaign to map the chemical composition of the inner galaxy.
Copyright The European Southern Observatory (ESO)