Rubidium in metal-deficient disk and halo stars

We report the first extensive study of stellar Rb abundances. High-resoluti on spectra have been used to determine, or set upper limits on, the abundan ces of this heavy element and the associated elements Y, Zr, and Pa in 44 d warfs and giants with metallicities spanning the range -2.0 < [Fe/H] < 0.0. In metal-deficient stars Rb is systematically overabundant relative to Fe; we find an average [Rb/Fe] of +0.21 for the 32 stars with [Fe/H] < -0.5 an d measured Rb. This behavior contrasts with that: of Y, Zr,and Ba, which, w ith the exception of three new CH stars (HD 23439A and B and ED + 5 degrees 3640), are consistently slightly deficient relative to Fe in the same star s; excluding the three CH stars, we find that the stars with [Fe/H] < -0.5 have average [Y/Fe], [Zr/Fe], and [Ba/Fe] of -0.19 (24 stars), -0.12 (28 st ars), and -0.06 (29 stars), respectively. The different behavior of Rb on t he one hand and Y, Zr, and Ba on the other can be attributed in part to the fact that in the Sun and in these stars Rb has a large r-process component while Y, Zr, and Ba are mostly s-process elements with only small. r-proce ss components. In addition, the Rb s-process abundance is dependent on the neutron density at the s-processing site. Published observations of Rb in s -process enriched red giants indicate a higher neutron density in the metal -poor giants. These observations imply a higher s-process abundance for Rb in metal-poor stars. The calculated combination off he Rb r-process abundan ce, as estimated for the stellar Eu abundances, and the s-process abundance , as estimated for red giants, accounts satisfactorily for the observed run of [Rb/Fe] with [Fe/H].