Is the [CO] index an age indicator for star forming galaxies?

The classical [CO] index, i.e. the strength of the CO first overtone absorption bands starting at 2.29 micron, is sometimes used to constrain the maximum age of star formation events in galaxies. In this paper we critically analyze

• theoretical models which could predict either a factor of >2 drop or a pronounced increase of [CO] at ages older than 100 Myr, depending on the evolutionary tracks one adopts
• observational data for young clusters in the LMC which do not show any strong relationship between the CO index and cluster age. The above scenario indicates that the value of [CO] does not provide a reliable tool for estimating the age of stellar populations older than about 10 Myr, i.e. after the first red supergiants have been formed. The contradictory results of theoretical models reflect problems in treating the evolution along the Asymptotic Giant Branch (AGB). In particular, those evolutionary synthesis models using stellar tracks which do not include the thermal pulsing AGB phase produce too weak CO features at 100-1000 Myr, i.e. in the range of ages when the near infrared emission is dominated by thermal pulsing AGB stars.