The Interstellar Rubidium Isotope Ratio toward Ophiuchi A

The isotope ratio 85 Rb/ 87 Rb places constraints on models of the nucleosynthesis of heavy elements, but there is no precise determination of the ratio for material beyond the solar system. We report the first measurement of the interstellar Rb isotope ratio. Our measurement of the Rb I line at 7800 Å for the diffuse gas toward ρ Oph A yields a value of 1.21 ± 0.30 (1 σ) that differs significantly from the meteoritic value of 2.59. The Rb/K elemental abundance ratio for the cloud also is lower than that seen in meteorites. Comparison of the 85 Rb/K and 87 Rb/K ratios with meteoritic values indicates that the interstellar 85 Rb abundance in this direction is lower than the solar system abundance. We attribute the lower abundance to a reduced contribution from the r -process. Interstellar abundances for Kr, Cd, and Sn are consistent with much less r -process synthesis for the solar neighborhood compared to the amount inferred for the solar system.