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Astron. Astrophys. 357, 61-65 (2000)

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1. Introduction

An ideal instantaneous burst of star formation generates a so-called Simple Stellar Population (SSP), that is a stellar system which is coeval and initially chemically homogeneous (see Renzini & Buzzoni 1986). The integrated near IR luminosity of a SSP is dominated by red stars since its very early stage of evolution ([FORMULA]10 Myr), when massive stars ([FORMULA]40 [FORMULA]) evolve as red supergiants. When the stellar system gets older ([FORMULA]100 Myr) intermediate mass giants evolving along the AGB and, after a few Gyr, low mass giants near the tip of the Red Giant Branch (RGB) dominate the integrated IR and bolometric luminosities (e.g. Renzini & Buzzoni 1986, Chiosi et al. 1986). The time evolution of the observable parameters related to a SSP, such as e.g. photometric colours and spectral indices, provides the basic ingredient for constructing evolutionary models of star forming galaxies.

Among the photometric and spectroscopic indices used to study the red stars of a SSP, the CO index has attracted quite some attention as a potential tool to trace red supergiants, i.e. young stellar systems. This idea primarily derives from the fact that field stars of similar spectral types show different CO indices depending on their spectral class, the strongest features being found in supergiants (see e.g. Fig. 4 of Kleinmann & Hall 1986, hereafter KH86). Several attempts of predicting the evolution of the CO index of a SSP appeared in the literature and were applied to the interpretation of IR spectral observations of starburst galaxies (e.g. Doyon et al. 1994, Shier et al. 1996, Goldader et al. 1997, Mayya 1997, Leitherer et al. 1999). Most of the models are restricted to solar metallicities and predict a pronounced maximum at [FORMULA]10 Myr followed by a quite rapid and steady decline. The CO index drops by almost a factor of 3 at [FORMULA]100 Myr and, noticeably, reaches values much lower than those observed in old ([FORMULA]10 Gyr) Galactic globular clusters and spheroidal galaxies of quasi-solar metallicities. The few models at sub-solar metallicities predict a similar time evolution with shallower CO features at all epochs.

Taken at face value, these models would imply that star forming galaxies with prominent CO absorption features must be dominated by a young ([FORMULA]100 Myr) star formation event, while more mature, but still relatively young systems of a few [FORMULA]100 Myr should be characterized by quite weak CO absorption features. In other words, the CO index could provide a powerful tool to constrain the age of the major star formation event of galaxies.

This paper is a critical re-analysis of the time evolution of the CO index in simple SSPs and star forming galaxies. In Sect. 2 we describe the, sometimes confusing, definition of CO index and discuss its relationship with stellar parameters. In Sect. 3 we present theoretical curves based on different stellar evolutionary models and briefly discuss the possible reasons for their very different behaviours. In Sect. 4 we compare the predicted evolution of [CO] with measured parameters of template stellar clusters in the Magellanic Clouds, old globular clusters in the Galaxy, normal and starburst galaxies. In Sect. 5 we draw our conclusions.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000