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Session 72 - Open Clusters.
Display session, Wednesday, January 15
Spectra at \sim5 Å\ resolution in the 3600--4600 Å\ wavelength range have been obtained for 27 stars in the field of the old open cluster (M67) (NGC 2682) and for 14 stars of the younger Pleiades. Based on synthetic spectra (Briley 1996, private communication) of 23 M67 G-dwarf members and 14 Pleiads, the violet CN-band index is found to be adequate to detect a star-to-star spread in [N/H] for these G-dwarf spectra. No such [N/H] inhomogeneity was indicated. This is consistent with findings for unevolved stars in other open clusters, but is unlike the bimodality in CN-band strengths seen in globular cluster unevolved stars [Suntzeff 1989, in The Abundance Spread within Globular Clusters, (Paris, Obs. de Paris), 71: NGC 6752) and (Briley et al. 1994, AJ 108, 2183): 47 Tuc]. This homogeneity is also different than the significant spread in CN-band strengths of evolved stars in three other as old or older open clusters, Melotte 66, NGC 188, and NGC 6791 (Anthony-Twarog et al., 1994, AJ, 106, 486; McClure 1974, ApJ, 194, 355; and Hufnagel et al. 1995, AJ, 110, 693, respectively).
This result is consistent with an evolutionary process that creates a dispersion in the CN-band strength distribution of evolved stars, but is only effective for stars turning off the main-sequence with a mass lower than some threshold mass, as suggested by other workers (e.g., Gilroy 1989, ApJ, 347, 835).
The Pleiads were successfully modeled using enhanced [O/H] values (King 1994, PASP, 106, 423) and depleted [C/H] values, contrary to previous high-resolution results (e.g., Boesgaard amp; Friel 1990, ApJ, 351, 467).
Program listing for Wednesday