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SPECTROSCOPY OF HOT HORIZONTAL-BRANCH STARS IN GLOBULAR CLUSTERS. I. NGC 6397.

Spectra have been obtained, with moderate signal-to-noise ratio, for four, blue horizontal-branch (BHB) stars in the metal-poor globular cluster NGC 6397 using a photon-counting detector. These photometrically selected stars have effective temperatures near 12,000 K. The observed H-delta profiles suggest values of log g near 2.8, putting these stars above the main sequence of population I. At an apparent signal-to-noise ratio of 25:1 between 3900 A and 4200 A, the only identifiable absorption lines other than the Balmer lines are weak He I 4026 A and Ca II 3933 A (presumably interstellar). The scatter in the observed strengths of He I 4026 A for several field stars makes it uncertain whether or not He I is underabundant in these cluster BHB stars. An examination of the cores of the H-delta profiles suggests that the cluster HB stars are probably slow rotators.