The Impact of Starbursts on Element Abundance Ratios

Abstract

We investigate the impact of bursts in star formation on the predictions of one-zone chemical evolution models, adopting oxygen (O), iron (Fe), and strontium (Sr), as representative $\alpha$, iron-peak, and s-process elements, respectively. To this end, we develop the Versatile Integrator for Chemical Evolution (VICE), a python package. Starbursts driven by a temporary boost of gas accretion rate create loops in [O/Fe]-[Fe/H] evolutionary tracks and a peak in the stellar [O/Fe] distribution at intermediate values. Bursts driven by a temporary boost of star formation efficiency have a similar effect, and they also produce a population of $\alpha$-deficient stars during the depressed star formation phase that follows the burst. This $\alpha$-deficient population is more prominent if the outflow rate is tied to a time-averaged star formation rate (SFR) instead of the instantaneous SFR. Theoretical models of Sr production predict a strong metallicity dependence of supernova and asymptotic giant branch (AGB) star yields, though comparison to data suggests an additional source that is nearly metallicity-independent. Evolution of [Sr/Fe] and [Sr/O] during a starburst is complex because of the yield metallicity dependence and the multiple timescales in play. Moderate amplitude (10-20\%) sinusoidal oscillations in SFR produce loops in [O/Fe]-[Fe/H] tracks and multiple peaks in [O/Fe] distributions, which could be one source of intrinsic scatter in observed sequences. We investigate models that have a factor of ~2 enhancement of SFR at t = 12 Gyr, as suggest by some recent Milky Way observations. A late episode of enhanced star formation could help explain the existence of young stars with moderate $\alpha$-enhancements and the surprisingly young median age found for solar metallicity stars in the solar neighborhood, while also raising the possibility that this starburst has not fully decayed.