"Heavy Metal from Ancient Superstars"

The compositions of progressively more and more metal-deficient stars allow us to reconstruct the synthesis of the elements in the Milky Way galaxy from the first stars to the present generation. The origin of heavy metals at the earliest times in the history of the Galaxy is uncertain, but the weak s-process operating in massive stars may have contributed the first neutron-capture elements, dominated by elements such as Sr, Y, and Zr. Heavier neutron capture elements, including the lanthanide elements, became more abundant only after the Galaxy reached a metallicity of 10-3 time solar, as the main r-process operating in Type II supernovae from 8-10 solar mass stars began to contribute. Until a metallicity of 10-2.4 solar, the abundance pattern of the heavy (Z>56) n-capture elements in most old stars is well-matched to a dominant r-process nucleosynthesis pattern, rather than the traditional mix of r- and s-process products that is observed in the Solar System. Contributions from the s-process can first be seen in some stars with metallicities as low as 10-2.75, and are present in most stars with metallicities above 10-2.3 solar. The appearance of s-process contributions as metallicity increases reflects the longer stellar evolutionary timescale of the low-mass stars which in which s-process nucleosynthesis operates. The abundances of the heavy metals in these metal poor stars show clear evidence for a large star-to-star dispersion in the heavy element-to-iron ratios. This condition must have arisen from individual nucleosynthetic events in rapidly evolving halo progenitors that injected newly manufactured n-capture elements into an inhomogeneous early Galactic halo interstellar medium.