The C$_2$4668 Index

At a later stage in this project, we realized that carbon abundances affect the blue spectral region importantly enough that one would want to nail them down as tightly as possible. In order to improve our confidence in our carbon abundance determinations, we decided to add to our models the $C_2$4668 index, which is extremely sensitive to carbon abundances (Trippico & Bell 1995). This became possible when the Indo-US spectral library, by Valdes et al. (2004) became publicly available. This new spectral library covers the entire spectral range between $\sim$ 3500 and 9500 ${\rm\AA}$ (with a resolution of $\sim$ 1 ${\rm\AA}$, FWHM), without gaps such ast that in the Jones (1999) library. Most importantly, the set of 1273 stars in the Indo-US library contains almost all the Jones (1999) stars, for which we obtained accurate stellar parameters (see Section 2.3), so that incorporating the C$_2$4668 index in our models depended only on getting accurate measurements from Indo-US spectra. Such measurements were performed after convolving the Indo-US spectra into the Lick resolution (Table 1). Equivalent widths, fitting functions, and model predictions for other Lick indices not covered by the Jones (1999) library, such as Ca4455, Fe4531, and all indices redder than $\sim$ 5400 ${\rm\AA}$ will be presented elsewhere. Models for all the other indices presented in this paper are based on measurements taken on spectra from the Jones (1999) library.