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 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 angstroms 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 the meteoritic
value and similar to that toward rho Oph A (Federman et al. 2004); 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.
