Clean catalogues of blue horizontal-branch stars using Gaia EDR3

Abstract

Context. Blue horizontal-branch stars evolve from low-mass stars that have completed their main-sequence lifetimes and undergone a helium flash at the end of their red-giant phase. As such, 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. Previous catalogues of blue horizontal-branch stars were developed using spectral analyses or were restricted to individual globular clusters. 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. This has been deemed important as the success of the Gaia mission has changed the way that targets are selected for large-scale spectroscopic surveys, meaning that far fewer spectra will be acquired for blue horizontal-branch stars in the future unless they are specifically targeted. 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. The entire catalogue is limited to those stars with no apparent neighbours within 5 arcsec. These methods were verified and subsequently applied to the Gaia Early Data Release 3 (EDR3). 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.

Probing the inner Galactic halo with blue horizontal-branch stars

“…Here, we present a new release of the Gaia DR3-based catalogue of halo BHB stars that is more complete than the original version of the catalogue (Culpan et al 2021) based on Gaia Early Data Release 3 (EDR3) in crowded regions, particularly close to the Galactic Plane, and at the bluer end of the HB where late B-type BHB stars are expected. The catalogue of Galactic halo BHB candidate stars with stellar parameters derived from their spectral energy distributions (SEDs) is a subset of the newly released Gaia DR3 based catalogue.…”

“…In Sect. 2, we explain the generation of stellar parameters using SEDs and apply this method to the 16 794 BHB candidates in the parallax selection of Culpan et al (2021) and use the results to revise and extend the Gaia DR3 CMD selection criteria. Section 3 describes how we selected and verified the applicability of various astrometric, photometric, and blending quality criteria when applied to our particular Gaia DR3 data set.…”

Identification of Blue Horizontal-branch Stars from LAMOST DR5

“…Montenegro et al (2019) discovered 12,554 BHB stars using the globular cluster M22 as a reference standard and constructing color-magnitude and colorcolor diagrams with precise cuts in the ZYJHK from the Vía Láctea ESO Public Survey data. Culpan et al (2021) selected 57,377 BHB stars using the color and absolute magnitude (G, G BP − G RP ) from Gaia early data release 3 (EDR3; Gaia Collaboration et al 2021), and the purity was about 70%. But the effect of extinction is inevitable, which can usually lead to serious contamination.…”

Identification of Blue Horizontal Branch Stars with Multimodal Fusion

“…Their use as tracers is widely sought in the studies concerning the kinematics and structural composition of our Galaxy (Vickers et al 2021(Vickers et al , 2012. Due to the considerable age of BHBs (Dotter et al 2010), BHBs have become ideal for studying the structure of the older parts of the Galaxy (Culpan et al 2021). Many works focus on using a growing sample of BHB halo tracers to probe the Milky Way's enclosed mass: Xue et al (2008) used BHBs to derive precise constraints on the masses of Galactic dark matter halos; Gnedin et al (2010) utilized 910 hypervelocity halo BHBs and blue straggler stars to map 80 kpc of the mass profile; Utkin & Dambis (2020) employed BHBs to simultaneously determine the distance from the Sun to the center of motion of the halo velocity field and a distance scale correction factor.…”

“…Therefore, it is necessary to identify an increasing number of pure BHBs and thus obtain a more extensive sample of BHBs, which helps make further, more comprehensive studies of the Galaxy. The discrimination of BHBs from their main-sequence counterparts poses a critical challenge in pursuing BHB identification (Culpan et al 2021). The presence of a polluting population of high surface-gravity A-type stars and the blue stragglers makes identifying BHBs in distant halos unfeasible (Clewley et al 2005).…”