The isotope ratio, 85Rb/87Rb, places constraints on models of thenucleosynthesis of heavy elements, but there is no precise determination of theratio for material beyond the Solar System. We report the first measurement ofthe interstellar Rb isotope ratio. Our measurement of the Rb I line at 7800 Afor the diffuse gas toward rho Oph A yields a value of 1.21 +/- 0.30 (1-sigma)that differs significantly from the meteoritic value of 2.59. The Rb/Kelemental abundance ratio for the cloud also is lower than that seen inmeteorites. Comparison of the 85Rb/K and 87Rb/K ratios with meteoritic valuesindicates that the interstellar 85Rb abundance in this direction is lower thanthe Solar System abundance. We attribute the lower abundance to a reducedcontribution from the r-process. Interstellar abundances for Kr, Cd, and Sn areconsistent with much less r-process synthesis for the solar neighborhoodcompared to the amount inferred for the Solar System.
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