The SDSS-Gaia View of the Color-Magnitude Relation for Blue Horizontal-branch Stars

TL;DR

This paper refines the selection and calibration of blue horizontal-branch stars using SDSS and SEGUE data, employing a probabilistic approach to distinguish BHB stars and updating their color-magnitude relations for better distance estimates.

Contribution

It introduces a probabilistic method for identifying BHB stars and provides new calibrations for their absolute magnitudes based on updated globular cluster distances.

Findings

Probabilistic classification improves BHB star identification.

New color-magnitude calibrations for BHB stars.

Enhanced distance estimation accuracy for BHB stars.

Abstract

We present an updated sample of blue horizontal-branch (BHB) stars selected from the photometric and spectroscopic data from Sloan Digital Sky Survey and its associated project Sloan Extension for Galactic Understanding and Exploration (SEGUE). With this data, we selected candidates for A-type stars in the color-color space and then a mixture modeling technique was implemented in order to distinguish between BHB and main-sequence/blue-straggler stars based on their surface gravity values ($\log \rm{g}$) estimated by the SEGUE Stellar Parameter Pipeline. Our robust approach allows us to attribute individual probabilities of each star truly being in the BHB stage. Hence, our method is advantageous in comparison to previous SEGUE BHB selections that adopted simple $\log \rm{g}$ cuts. We also revisit the color-magnitude relation for these stars and propose two calibrations, based on updated distances for Galactic globular clusters, to estimate absolute magnitudes with $(g-r)_0$ and $(u-r)_0$ colors.