Strontium and Barium In Early-Type Galaxies
  The detailed abundance patterns of the stars within galaxies provide a unique
window into the history of star formation (SF) at early times. Two widely used
`chronometers' include the alpha and iron-peak elements, which are created on
short and long timescales, respectively. These two clocks have been widely used
to estimate SF timescales from moderate-resolution spectra of early-type
galaxies. Elements formed via s-process neutron captures (e.g., Sr and Ba)
comprise a third type of chronometer, as the site of the main s-process is
believed to be intermediate and low-mass asymptotic giant branch stars. The
[alpha/Ba] ratio in particular should provide a powerful new constraint on the
SF histories of galaxies, in part because it is insensitive to the uncertain
distribution of type Ia SNe detonation times and the overall Ia rate. Here we
present new measurements of the abundance of Sr and Ba in nearby early-type
galaxies by applying stellar population synthesis tools to high S/N optical
spectra. We find a strong anti-correlation between [Mg/Fe] and [Ba/Fe], and a
strong positive correlation between [Mg/Ba] and galaxy velocity dispersion.
These trends are consistent with the idea that more massive galaxies formed
their stars on shorter timescales compared to less massive galaxies, and rule
out several other proposed explanations for the observed super-solar [Mg/Fe]
values in massive galaxies. In contrast, [Sr/Fe]~0, with no strong variation
across the sample. It is difficult to interpret the Sr trends without detailed
chemical evolution models owing to the multiplicity of proposed nucleosynthetic
sites for Sr.
