A search for distant, pulsating red giants in the southern halo^⋆

¹ Université de Montpellier, Laboratoire Univers et Particules de Montpellier CNRS-IN2P3, Place Bataillon, 34095 Montpellier, France
e-mail: nicolas.mauron@umontpellier.fr
² NAS RA V.A. Ambartsumian Byurakan Astrophysical Observatory (BAO), Byurakan, Armenia
³ 18 Market Square, Northampton NN1 2DL, UK
⁴ Department of Astrophysics and Planetary Science, Villanova University, 800 Lancaster Avenue, Villanova, PA 19085, USA

Received: 28 October 2019
Accepted: 16 December 2020

Abstract

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 δ < −20°. We first focus on the 77 stars in the cap |b| > 30° 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_s0 = 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.