Distance Estimations of Blue Horizontal-branch Stars from the LAMOST and Sloan Digital Sky Survey Spectroscopic Surveys

Abstract

Blue horizontal-branch (BHB) stars are standard candles for tracing the structure and assembly history of the Galactic halo. We present color–absolute-magnitude relations for BHB stars in multiple optical and near-infrared bands. Using a calibration sample of 164 BHB stars with precise Gaia Early Data Release 3 (EDR3) parallaxes, we derive linear relations in the observed Gaia (BP, G, RP) and Two Micron All Sky Survey (J, H, Ks) systems. We also provide supplementary calibrations from Gaia DR3 synthetic photometry for commonly used optical bands, including UBVI, griz, y, and Hubble Space Telescope F606W and F814W. The MG–(G − RP)0 relation, with an intrinsic scatter of 0.143 mag, is applied to 8767 BHB stars from LAMOST and Sloan Digital Sky Survey (SDSS). The distances are up to ∼101 kpc, with a median uncertainty of 6.7%. We validate our distance scale through comparison with Gaia EDR3 geometric distances for 1143 high-quality stars, finding a median difference of 6.4%. Additional tests using globular clusters and two high-precision reference catalogs get median offsets of −1.1% and −5.8%, respectively. These results show that our calibrations provide reliable BHB distances and offer a useful complement to Gaia parallaxes in the distant Galactic halo.