O, B, and A Stars

As in the case of F and G field dwarfs, stellar parameters of hotter stars were inferred from Strömgren photometry. We used a combination of the $\beta$, $a_0$, and $c_1$ indices. The $\beta$ index (Crawford 1958; Crawford & Perry 1966) provides a photometric measurement of the strength of $H\beta$ and therefore it is a good $T_{\rm eff}$ indicator for A and F stars. The calibration adopted in Paper I for F and G stars cannot be used here, though, as Alonso et al. did not extend to stars as hot as spectral type A. Therefore, for stars with $\beta > 2.7$ we adopted a relation obtained from a polynomial fit to the data of Smalley & Dworetsky (1993). Metallicities for these stars were computed from the $m_1$ index (Strömgren 1966), adopting the relation by Smalley (1993) and the standard calibration from Perry, Olsen & Crawford (1987).

The $\beta$ index cannot be used along the entire BA sequence to determine uniquely $T_{\rm eff}$, because Balmer lines get weaker for stars hotter than A0. Therefore, the $T_{\rm eff}$s of stars with $-0.034 \leq (b-y) \leq 0.066$ were determined from the $a_0$ parameter (Strömgren 1966), and for bluer stars, from the $c_1$ index (Strömgren 1966), in both cases adopting the calibrations by Ribas et al. (1997).