Metal–poor turnoff and subgiant field stars in the Galaxy

I. RGU photometry in a high–latitude field near M101

¹ Department of Astronomy and Space Sciences, Istanbul University Observatory, Istanbul, Turkey
² Astronomisches Institut der Universität Basel, Venusstr. 7, CH–4102 Binningen, Switzerland

Send offprint request to: R. Buser

Received: 29 October 1998
Accepted: 2 January 1998

Abstract

Photographic RGU photometry for about 1800 stars down to a limiting G-magnitude of 19.5 in a high-latitude field (() = (101°, +60) 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 & Rong (1995) and Buser et al. (1998a).