Metal-poor turnoff and subgiant field stars in the Galaxy - I. RGU photometry in a high-latitude field near M101

Photographic RGU photometry for about 1800 stars down to a limiting G-magnitude of 19.5 in a high-latitude field ((l, b) = (101 degrees, +60 degrees)) near the galaxy M101 is investigated. We use an improved variant of the classical three-color method which is based on recent UBV-RGU-photometric transforms and calibrations (Buser 1988; Buser & Fenkart 1990) and on models of the stellar density distributions of the Galactic population components. In particular, predictions from the Galactic models of Gilmore & Wyse (1985) are employed as guides in determining the density functions from the present data, which are then used in turn to derive consistent local luminosity functions. We find that a significant fraction of the intermediate and extreme Population II stars must be (mildly) evolved (rather than main-sequence) stars whose higher-luminosity nature cannot be inferred from their two-color positions only. This conclusion is also consistent with results obtained from combined RGU-photometric and proper motion data in two other Basel fields by Buser & Chiu (1981a,b), as well as from detailed model-calculations for the larger survey of RGU-data in seven high-latitude fields by Buser St Pong (1995) and Buser et al. (1998a).