The Integrated Spectrum of M 67

In Paper III an integrated spectrum of M 67 was obtained from coaddition of individual spectra of cluster members, weighted according to their luminosities and relative numbers, assuming a Salpeter IMF. Since publication of that work a few revisions have been made to the cluster integrated spectrum and Lick indices measured in it, so that an update on these observables is made necessary here. The first important change relative to the values published in Paper III refers to the EW measurements, which had to be retaken, for the reasons exposed in Section 2.2. The second important revision relates to star #6472 (ID from Montgomery, Marschall & Janes 1993), which has been erroneously included in the coaddition as a first-ascent giant star. Inspection of Figure 1 in Paper III suggests that this star is too blue (by $\mathrel{\copy\simgreatbox}$ 0.2 mag in B-V) to be on the red giant branch of M 67, which is confirmed by its warm spectrum. It is also too bright and too blue to be an early-AGB star. Because of its uncertain evolutionary stage, we decided to remove this star from the coaddition to produce the cluster integrated spectrum. In the third relevant change, we performed a test which showed that main sequence M 67 stars fainter than V $\sim$ 15 (the magnitude limit in Paper III sample) contribute significantly to the integrated Lick indices of the cluster. In order to correct for this effect, we used the Padova isochrones that best matched the cluster color-magnitude diagram (Girardi et al. 2000, solar-scaled, solar metallicity, 3.5 Gyr-old) to compute model predictions including and excluding stars less massive than $\sim$ 0.85 $M_\odot$. The difference between these two model predictions was used to correct the Lick indices measured in the cluster integrated spectrum for the contribution of low-mass stars. Both corrected and uncorrected Lick indices for M 67 are listed in Table 26. Comparison between the two sets of indices shows the corrections are small, but not negligible for some indices. These numbers supersede the values provided in Paper III and will be used throughout this paper in our comparison with model predictions.