Spectra were obtained using Hydra, the multiple-fiber optic fed
spectrograph on the 4-m telescope at Kitt Peak National
Observatory, on the nights 1-4 January 1993 and
21-24 Decmber 1993. We used the red fiber cable in
conjunction with the BL181 grating (316 l/mm) set at a central
wavelength of 7600Å, and a Tektronix 2048 × 2048 CCD
to obtain spectra in the range 
5500 - 9500 Å. The
spectral dispersion was 1.9 Å per pixel; the
effective resolution was 3 pixels or 6Å. The spectra were
reduced using the DOHYDRA package within IRAF ²,
and further analysed using the Hydra and ONEDSPEC packages.
The spectra were flat-fielded and wavelength calibrated, but no attempt
was made to flux calibrate them.
Our sample is drawn primarily from the open clusters M67 and Praesepe. We chose cluster stars because it is an efficient use of the multi-object spectrometer used in our pre-main sequence star program; we could obtain spectra for a large number of stars having a substantial range of spectral types simultaneously. While cluster members' common age, metallicity, and distance offer some advantages in forming a spectral sequence, they also present some limitations in its use; for example, the variation in line strength with metallicity cannot be investigated. Likewise, since Praesepe is believed to be similar to the Hyades in composition and M67 is thought to have solar metallicity, this spectral sequence may not be appropriate for the classification of very metal-poor stars.
We observed only those stars which have a high probability (based on proper
motion studies) of cluster membership.
In Praesepe, 74% of our sample Table 1
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have a 90% or higher probability of membership (Jones and
Stauffer 1991; hereafter JS),
and in M67, 71% of our sample Table 2
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are members at or above the 90% probability level (Girard et al. 1989).
Color-magnitude diagrams for the two cluster samples are
presented in Figure 1.
In order to extend our sample coverage to late-type luminous stars,
we also obtained spectra of several isolated giants (types K7-M7 III).
These are listed in Table 3
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Figure 1. (a) V vs. (B - V) for the stars observed in Praesepe and in M67, and listed in tables 1 and 2. (b) V vs. (V - I) for the stars observed in Praesepe and in M67. In both figures, Praesepe's distance modulus has been shifted by 3.4 magnitudes to match that of M67.
² IRAF is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.
