High-spin early stars

High-spin early stars. Stellar nucleosynthesis by exothermic fusion ends with iron, the most tightly bound of all nuclei. Stars produce heavier species by two neutron-capture mechanisms—the rapid “r-process” and the slow “s-process”—and expel them in supernovae or stellar winds. The r-process avails itself of the enormous neutron flux during the supernova to convey nuclei up the atomic-number scale by way of short-lived stages faster than those stages can decay back down. The s-process, making do with weaker neutron fluxes inside quiescent stars, requires long-lived intermediate stages. It was long thought that the s-process occurs only in stars of modest mass. But now Cristina Chiappini (Leibniz Institute for Astrophysics, Potsdam, Germany) and coworkers have reported anomalously high abundances of two s-process products—yttrium and strontium—in spectra of a cluster of Milky Way stars so old that their Y and Sr must have been created in an early generation of very massive stars that exploded less than a billion years after the Big Bang. Yet model calculations of the s-process in early massive stars yield far too little Y and Sr to account for the overabundances—unless those stars were spinning very fast. Chiappini and company suggest that first-generation stars were not only very massive, as is generally thought, but also very rapidly spinning, with surface speeds as high as 800 km/s. They calculate that the resultant centrifugal mixing of interior layers boosted the s-process’s efficiency by the requisite four orders of magnitude. The spectra were taken with the European Southern Observatory’s Very Large Telescope, shown here. (C. Chiappini et al., Nature 472, 454, 2011 0028-0836.)