We present measurements of the neutron-capture elements Rb and Pb for bright giants in the globular clusters M4 and M5 .
The clusters are of similar metallicity ( [ Fe/H ] \simeq - 1.2 ) but M4 is decidedly s -process enriched relative to M5 : [ Ba/Fe ] = +0.6 for M4 but 0.0 for M5 .
The Rb and Pb abundances were derived by comparing synthetic spectra with high-resolution , high signal-to-noise ratio spectra obtained with MIKE on the Magellan telescope .
Abundances of Y , Zr , La , and Eu were also obtained .
In M4 , the mean abundances from 12 giants are [ Rb/Fe ] = 0.39 \pm 0.02 ( \sigma = 0.07 ) , [ Rb/Zr ] = 0.17 \pm 0.03 ( \sigma = 0.08 ) , and [ Pb/Fe ] = 0.30 \pm 0.02 ( \sigma = 0.07 ) .
In M5 , the mean abundances from two giants are [ Rb/Fe ] = 0.00 \pm 0.05 ( \sigma = 0.06 ) , [ Rb/Zr ] = 0.08 \pm 0.08 ( \sigma = 0.11 ) , and [ Pb/Fe ] = - 0.35 \pm 0.02 ( \sigma = 0.04 ) .
Within the measurement uncertainties , the abundance ratios [ Rb/Fe ] , [ Pb/Fe ] and [ Rb/X ] for X = Y , Zr , La are constant from star-to-star in each cluster and none of these ratios are correlated with O or Na abundances .
While M4 has a higher Rb abundance than M5 , the ratios [ Rb/X ] are similar in both clusters indicating that the nature of the s -products are very similar for each cluster but the gas from which M4 ’ s stars formed had a higher concentration of these products .