2002MNRAS.337...87C

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 of 5. Faint A-type stars in the Galactic halo have been identified from ub_jr photometry in six UK Schmidt fields. These samples include field BHB stars as well as less luminous stars of main-sequence surface gravity, which are predominantly field blue stragglers. We obtain accurate charge-coupled device 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 (S/N) ratio spectra of A-type stars of Kinman, Suntzeff & Kraft (published in 1994), and their definitive spectrophotometric Λ classifications, to assess the reliability of two methods, and to quantify the S/N ratio requirements. First we revisit, refine and extend the hydrogen linewidth versus colour relation as a classifier (here called the D_0.15-colour method). The second method is new and compares the shapes of the Balmer lines. With this method (here called the scale-width-shape method) there is no need for colours or spectrophotometry. Using the equivalent width of the Caii K line as an additional filter we find we can reproduce Kinman, Suntzeff & Kraft's Λ classifications with both methods. In a sample of stars with strong Balmer lines, equivalent width Hγ > 13 Å [equivalent to the colour range 0 ≤ (B - V)₀≤ 0.2 ], halo BHB stars can be separated from halo blue stragglers reliably. For the spectroscopy (i.e. both classification methods) the minimum required continuum S/N ratio is 15 Å^–1. For the D_0.15-colour method (B - V)₀colours accurate to 0.03 mag are needed.