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 va riant of the classical three-color method which is based on recent UBV -RGU-photometric transforms and calibrations (Buser 1988; Buser & Fenk art 1990) and on models of the stellar density distributions of the Ga lactic population components. In particular, predictions from the Gala ctic models of Gilmore & Wyse (1985) are employed as guides in determi ning 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 s tars must be (mildly) evolved (rather than main-sequence) stars whose higher-luminosity nature cannot be inferred from their two-color posit ions only. This conclusion is also consistent with results obtained fr om combined RGU-photometric and proper motion data in two other Basel fields by Buser & Chiu (1981a,b), as well as from detailed model-calcu lations for the larger survey of RGU-data in seven high-latitude field s by Buser St Pong (1995) and Buser et al. (1998a).