Usage of Red supergiant spectral features as age indicators in starburst regions

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 (λλ 8498, 8542, 8662 Å) 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 7–14 Myr corresponding 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 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 usage of diagrams involving Ca ii triplet equivalent width, CO index and nebular recombination lines to infer the history as well as age of starburst regions. Subject headings: Galaxies: Magellanic Clouds — galaxies: starburst — stars: supergiants Fellow of the Programa Internacional de Astrofisica Avanzada “Guillermo Haro”