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Session 54 - Star Clusters, Kinematics and the Galactic Center.
Display session, Wednesday, June 10
The old open cluster NGC 6791 is metal-rich, yet harbors a variety of blue horizontal branch (BHB) stars. Analysis of an echelle spectrum of the coolest BHB star confirms that the cluster metallicity is high, [Fe/H] = +0.4 \pm 0.1, more than twice solar (Peterson amp; Green, ApJL, submitted). To explore its HB population further, we obtained moderate S/N echelle spectra at Lick Observatory of over two dozen RHB candidates for radial-velocity confirmation of membership. These were combined to form a single composite spectrum representative of the typical cluster RHB star. A direct comparison of this composite spectrum with an existing echelle spectrum of the prototypical super-metal-rich field red giant Mu Leo shows that the two spectra are nearly indistinguishable. This is surprising, since analyses of Mu Leo have yielded for decades a temperature cooler by 300K or more and a surface gravity 0.5 lower for the star, resulting in a somewhat lower metallicity. A check by Grenon of recent HIPPARCOS data for Mu Leo confirms that its absolute magnitude is Mv = +0.84 \pm 0.08 and its color is B-V = 1.222; both are consistent with the NGC 6791 RHB upon adoption of a reddening E(B-V) = 0.14 \pm 0.03, at the low end of the range of current determinations. We will discuss in detail the nature and limitations of this similarity, and its consequences for cluster reddening and distance. We will also comment on the ramification that our own and others' analyses of cool giants as strong-lined as these are much less reliable than previously believed.
Program listing for Wednesday