USE OF RED SUPERGIANT SPECTRAL FEATURES AS AGE INDICATORS IN STARBURST REGIONS

We investigate techniques that can be used to determine ages of starbu rst regions containing populations beyond their early nebular phase. I n particular, we study the strength of the Ca II triplet (lambda lambd a 8498, 8542, 8662) and the CO index (2.31-2.40 mu m band) using synth etic models as the starburst evolves. For an instantaneous burst of st ar formation, both of these absorption features remain strongest betwe en 7 and 14 Myr, which corresponds to the red supergiant population. T he detailed evolutionary behavior of the starburst is strongly metalli city dependent. Low-metallicity starburst models successfully reproduc e 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 ma ss-loss rates higher than the standard values. We suggest using of dia grams involving Ca II triplet equivalent width, CO index, and nebular recombination lines to infer the history as well as the age of starbur st regions.