The nature of the metal-rich stellar population in the galactic halo

The spectrophotometry of metal-rich A stars (V in the range 12 to 15) in 100 square degrees centered on the South Galactic Pole is presented. It is found that the solar abundance stars have surface gravities that are near main sequence values. Values of Z(max) between 1200 and 8000 pc are obtained; the stellar ages involved are 1-2.5 Gyr so that the stars are dynamically well-mixed with a velocity dispersion at the galactic plane of 110 km/s in the Z direction. It is suggested that the A (and K) giant stars represent a sample of a second stellar population in the galactic halo. Several hypotheses for the origin of this population are discussed; it is suggested that the most plausible hypothesis involves an encounter between the Galaxy and a third Magellanic-type object that took place about two-billion years ago.