2009ApJ...706..614W

We present observations of interstellar rubidium toward o Per, ζ Per, AE Aur, HD 147889, χ Oph, ζ Oph, and 20 Aql. Theory suggests that stable ⁸⁵Rb and long-lived ⁸⁷Rb are produced predominantly by high-mass stars, through a combination of the weak s- and r-processes. The ⁸⁵Rb/⁸⁷Rb ratio was determined from measurements of the Rb I line at 7800 Å and was compared to the solar system meteoritic ratio of 2.59. Within 1σ 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 the meteoritic value and similar to that toward ρ Oph A; both lines of sight probe the Rho Ophiuchus Molecular Cloud. The earlier result was attributed to a deficit of r-processed ⁸⁵Rb. Our larger sample suggests instead that ⁸⁷Rb 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.