Use of Red Supergiant Spectral Features as Age Indicators in Starburst Regions

Abstract

We investigate techniques that can be used to determine ages of starburst regions containing populations beyond their early nebular phase. In particular, we study the strength of the Ca II triplet (λλ8498, 8542, 8662) and the CO index (2.31-2.40 μm band) using synthetic models as the starburst evolves. For an instantaneous burst of star formation, both of these absorption features remain strongest between 7 and 14 Myr, which corresponds to the red supergiant population. The detailed evolutionary behavior of the starburst is strongly metallicity dependent. Low-metallicity starburst models successfully reproduce the distribution of equivalent widths of the Ca II triplet with age in Large Magellanic Cloud clusters. The clusters in the red supergiant phase strongly favor the stellar evolutionary models incorporating mass-loss rates higher than the standard values. We suggest using of diagrams involving Ca II triplet equivalent width, CO index, and nebular recombination lines to infer the history as well as the age of starburst regions.