Studies of RR Lyrae variable stars in the unusual globular cluster Omega Centauri. I - Spectroscopic observations

Abstract

Spectroscopic measurements of a large number of w Cen RR Lyrae stars are reported; [Fe/H] values have been determined for nearly half of the RR Lyrae stars. The observed range in [Fe/H] is real and is very close to the range needed to explain the great width of the giant branch in the color-magnitude diagram. Both Oosterhoff groups appear to be present: the "metal-rich" ([Fe/H] > -1.0) RR Lyrae stars have P5ab = 0.57, and the "metal-poor" ([Fe/H] < -1.0) ones have = 0.66. A superposition of evolutionary tracks with the color-magnitude diagram for w Cen RR Lyrae stars provides evidence which supports composition-dependent mass loss in earlier evolutionary stages. If A log Z A[Fe/H], then the metal-rich RR Lyrae stars in w Cen are 0.1 M0 less massive than the metal-poor ones. Subject headings: clusters: globular - stars: abundances - stars: mass loss - stars: RR Lyrae