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° 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 (~1 Å) spectra centred on the CN bands at ~8000 Å, and have derived accurate (≲10 km s−1) radial velocities for the known FHLC stars. From the initial phase of our survey covering ≈6500 deg2, we find a surface density of faint N-type carbon stars in the halo of ≈1 per 200 deg2, 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.