A Search For Distant, Pulsating Red Giants In The Southern Halo

To investigate the asymptotic giant branch (AGB) population in the Galactic halo, we search for pulsating AGB stars at a heliocentric distance D>50 kpc. Our research is based on the Catalina Southern Survey (CSS) catalogue of variables, comprising 1286 long-period variables (LPVs) with delta<-20. We first focus on the 77 stars in the cap |b| > 30 degrees for which spectral M-type or C-type classification can be derived from Hamburg-ESO objective prism spectra. Most of these are oxygen-rich (M-type) and very few are carbon rich. The periods are in the range 100-500 days, and CSS amplitudes are up to 3 mag. In this small sample, no halo AGB star is fainter than K-s(0)=12.5. This may be due to the scarcity of AGBs in the outer halo, or insufficient instrumental depth. Leaving aside spectral information, we then searched for even fainter pulsators (K-s>12.5) in the entire CSS catalogue. Gaia astrometry makes it possible to identify some contaminants. Our final result is the identification of ten candidate distant LPVs. If these ten stars obey the fundamental mode K-band period-luminosity relation used for Miras and small-amplitude Miras, their distances are between 50 and 120 kpc from the Sun. In a diagram showing distance versus Gaia tangential velocity, these ten stars have positions consistent with that of other objects in the halo, such as globular clusters and dwarf galaxies. We detect some underluminous AGBs that deserve further study. Finally, the halo LPVs ressemble the slow redder variable of globular clusters when colour and periods are compared.