Rubidium In The Interstellar Medium

We present observations of interstellar rubidium toward o Per, zeta Per, AE Aur, HD 147889, chi Oph, zeta Oph, and 20 Aql. Theory suggests that stable (85)Rb and long-lived (87)Rb are produced predominantly by high-mass stars, through a combination of the weak s- and r-processes. The (85)Rb/(87)Rb ratio was determined from measurements of the Rb I line at 7800 angstrom and was compared to the solar system meteoritic ratio of 2.59. Within 1 sigma uncertainties, all directions except HD 147889 have Rb isotope ratios consistent with the solar system value. The ratio toward HD 147889 is much lower than themeteoritic value and similar to that toward rho Oph A; both lines of sight probe the Rho Ophiuchus Molecular Cloud. The earlier result was attributed to a deficit of r-processed (85)Rb. Our larger sample suggests instead that (87)Rb is enhanced in these two lines of sight. When the total elemental abundance of Rb is compared to the K elemental abundance, the interstellar Rb/K ratio is significantly lower than the meteoritic ratio for all the sight lines in this study. Available interstellar samples for other s- and r-process elements are used to help interpret these results.