Clean catalogues of blue horizontal-branch stars using Gaia EDR3
  Context: Blue horizontal-branch stars are very old objects that can be used
as markers in studies of the Galactic structure and formation history. To
create a clean sky catalogue of blue horizontal-branch stars, we cross-matched
the Gaia data release 2 (DR2) dataset with existing reference catalogues to
define selection criteria based on Gaia DR2 parameters. Following the
publication of Gaia early data release 3 (EDR3), these methods were verified
and subsequently applied to this latest release. Aims: The purpose of this
catalogue is to identify a set of blue horizontal-branch star candidates that
have been selected using photometric and astrometric observations and exhibits
a low contamination rate. Methods: We cross-matched reference blue
horizontal-branch datasets with the Gaia DR2 database and defined two sets of
selection criteria. Firstly, in Gaia DR2 - colour and absolute G magnitude
space, and secondly, in Gaia DR2 - colour and reduced proper motion space. The
main-sequence contamination in both subsets of the catalogue was reduced, at
the expense of completeness, by concentrating on the Milky Way's Galactic halo,
where relatively young main-sequence stars were not expected. Results: We
present a catalogue, based on Gaia EDR3, of 57,377 blue horizontal-branch
stars. The Gaia EDR3 parallax was used in selecting 16,794 candidates and the
proper motions were used to identify a further 40,583 candidates.
