Identification of BHB stars using Synthetic SkyMapper colors from Gaia XP spectra

TL;DR

This study constructs a large, high-purity catalog of BHB stars using synthetic SkyMapper colors derived from Gaia XP spectra, enabling detailed mapping of the Galactic stellar halo up to 20 kpc and exploring future prospects with CSST data.

Contribution

It introduces a novel method to identify BHB stars from Gaia XP spectra using synthetic SkyMapper colors, achieving high completeness and purity, and calibrates their distances for Galactic halo mapping.

Findings

Catalog of 49,733 BHB stars with >90% purity and completeness.

Achieved 5% distance uncertainty using calibrated $g$-band magnitude relation.

Demonstrated potential of CSST photometry to extend BHB detection to the Galaxy's outer halo.

Abstract

Blue horizontal-branch (BHB) stars are ideal tracers for mapping the structure of Galactic stellar halo. Traditionally, BHB sample stars are built from large-scale spectroscopic surveys utilizing their spectral features, however, the resulting sample sizes have been quite limited. In this paper, we construct a catalog of BHB stars based on synthetic colors $(u-v)_{0}$ and $(g-i)_{0}$ in SkyMapper photometric systems, which are convolved from Gaia XP spectra. A total of 49,733 BHB stars are selected from nearly the entire sky (excluding regions of low Galactic latitudes $|b| \le 8^{\circ}$ with heavy reddening), with a completeness and purity exceeding 90\%. Using member stars of globular clusters with precise distance determinations, we carefully calibrate the relationship between the $g$-band absolute magnitude and $(g-i)_{0}$, achieving a precision of 0.11\,mag, which corresponds to a 5\% uncertainty in distance. This relation is applied to derive distances for all BHB stars in the constructed sample. Given current capabilities of Gaia XP observations, the constructed BHB sample is primarily located within 20 kpc, enabling detailed mapping of the inner stellar halo. To extend this depth to the outer halo or even the edge of our Galaxy, we explore the potential of the Chinese Space Station Telescope (CSST) and its broad-band photometry for detecting BHB stars. Using mock data from synthetic spectra, we find that it is feasible to distinguish BHB stars from blue stragglers (BS) stars using CSST near-ultraviolet bands ($NUV, u$) photometry. Thanks to the deep limiting magnitude of CSST, its data will provide a groundbreaking perspective on our Galaxy, particularly regarding the outer halo, in an unprecedented volume.