The APM survey for cool carbon stars in the Galactic halo - I

A by-product of the APM high-redshift quasar survey was the discovery of several distant (20-100 kpc) N-type carbon stars at high galactic latitude. Following on from this, we have started a systematic all-sky survey at galactic latitudes --b-->30 deg to find further examples of these rare objects, and we report here on the results from the first season of follow-up spectroscopy. Faint, high-latitude carbon (FHLC) giants make excellent probes of the kinematic structure of the outer Galactic halo. Therefore, in addition to detailed spectrophotometry covering a wide wavelength range, we have obtained high-resolution (~1A) spectra centred on the CN bands at ~8000A, and have derived accurate (<~10 km s^-1) radial velocities for the known FHLC stars. From the initial phase of our survey covering ~6500 deg^2, we find a surface density of faint N-type carbon stars in the halo of ~1 per 200 deg^2, roughly a factor of 4 less than the surface density of CH-type carbon stars in the halo. Intermediate-age, N-type carbon stars seem unlikely to have formed in the halo in isolation from other star-forming regions, and one possibility that we are investigating is that they either arise from the disruption of tidally captured dwarf satellite galaxies or are a manifestation of the long-sought optical component of the Magellanic Stream.