We report the first extensive study of stellar Rb abundances .
High-resolution spectra have been used to determine , or set upper limits on , the abundances of this heavy element and the associated elements Y , Zr , and Ba in 44 dwarfs 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 and measured Rb .
This behavior contrasts with that of Y , Zr , and Ba , which , with the exception of three new CH stars ( HD 23439A and B and BD +5 ^ { o } 3640 ) , are consistently slightly deficient relative to Fe in the same stars ; excluding the three CH stars , we find the stars with [ Fe/H ] < -0.5 have average [ Y/Fe ] , [ Zr/Fe ] , and [ Ba/Fe ] of –0.19 ( 24 stars ) , –0.12 ( 28 stars ) , and –0.06 ( 29 stars ) , respectively .
The different behavior of Rb on the 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 -process 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 of the Rb r -process abundance , 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 ] .