color indices, ($b-y$) and ($u-b$), and the $\beta$-index,

$$\beta = \mathrm{H}\beta_\text{narrow} - \mathrm{H}\beta_\text{wide},$$

are all sensitive to temperature and weakly dependent on surface gravity for late A- and F-type stars. The Balmer discontinuity index,

$$c_1 = (u-v) - (v-b),$$

is sensitive to temperature for early type (OB) stars and surface gravity for intermediate (AF) spectral types. Finally, the metal line index,

$$m_1 = (v-b) - (b-y),$$

is sensitive to the metallicity $[M/H]$.

For each index, there is a corresponding intrinsic, dereddened index denoted by a naught subscript with e.g $c_0, (b-y)_0,$ and $(u-b)_0$, referring to the intrinsic, dereddened equivalents of the indices $c_1, (b-y),$ and $(u-b)$, respectively. Furthermore, although reddening is expected to be negligible for the nearby sources of primary interest to us, automated classification schemes that divide a large sample of stars for analysis into groups corresponding to earlier than, around, and later than the Balmer maximum will sometimes rely on the reddening-independent indices defined by [@crawfordmandwewala1976] for A-type dwarfs:

$$\begin{aligned}
    [c_1] &= c_1 - 0.19 (b-y) \\
    [m_1] &= m_1 + 0.34 (b-y) \\
    [u-b] &= [c_1] + 2 [m_1].
    \end{aligned}$$

Finally, two additional indices useful for early A-type stars, $a_0$ and $r^*$, are defined as follows:

