New light on faint stars - VII. Luminosity and mass distributions in two high galactic latitude fields

The stellar distribution in apparent V magnitude and B–V colour has been determined for complete samples of ∼ 10 000 stars in 11.5 square degrees towards (l, b) = (0°, − 90°), and of ∼ 28 000 stars in 17 square degrees towards (l, b) = (37°, − 51°). V–I colour data for the second sample are also discussed here, while those for the first field have been published elsewhere. From these data we derive the structural parameters of the Galactic spheroid, and the form of the stellar luminosity function in the solar neighbourhood. The low-mass stellar luminosity function shows a broad maximum near MV ∼ + 13, and a decline for less luminous stars, in agreement with our earlier studies. Any missing mass in the solar neighbourhood cannot be explained by normal low-mass luminous stars. The old, metal-poor stars in the Galactic spheroid are distributed in an oblate spheroid, with the flattening becoming less pronounced with increasing Galactocentric distance. In addition to this population, a large proportion of the stars in the spheroid belong to a population with a red main-sequence turn-off, consistent with an old, relatively metal-rich population. The spatial distribution of this population is not well defined, but is consistent with the ‘thick disc’ which is suggested by other evidence to be a component of the Galaxy. No published model of the stellar distribution in the Galaxy is in acceptable agreement with these data.