Distant field BHB stars and the mass of the Galaxy I: Classification of halo A-type stars

Abstract

This is the first in a series of three papers presenting a new calculation of the mass of the Galaxy based on radial velocities and distances measured for a sample of some 100 faint 16 < B < 20 field blue horizontal-branch (BHB) stars. This study aims to reduce the uncertainty in the measured mass of the Galaxy by increasing the number of halo objects at Galactocentric distances r>30 kpc with measured radial velocities by a factor five. Faint A-type stars in the Galactic halo have been identified from UBR photometry in six UK Schmidt fields. We obtain accurate CCD photometry and spectra to classify these stars. This paper describes our methods for separating out clean samples of BHB stars in a way that is efficient in terms of telescope time required. We use the high signal-to-noise ratio (S/N) spectra of A-type stars of Kinman, Suntzeff & Kraft (published in 1994) to assess the reliability of two methods, and to quantify the S/N requirements. First we revisit, refine and extend the hydrogen line width versus colour relation as a classifier. The second method is new and compares the shapes of the Balmer lines. Using the equivalent width of the Ca II K line as an additional filter we find we can reproduce Kinman, Suntzeff & Kraft's classifications with both methods.