AGB Stars as Tracers of Star Formation Histories: Implications for GAIA Photometry and Spectroscopy

We argue that tracing star formation histories with ESA's space mission GAIA using main sequence turn-off (MSTO) point dating will mainly be effective in cases of mild interstellar extinction (E(B-V)<0.5). For higher reddenings the MSTO approach will be severely limited both in terms of traceable ages (t<0.5 Gyr at 8 kpc; E(B-V)=1.0) and/or distances (d<2 kpc for t=15 Gyr; E(B-V)=1.0), since the MSTO will be located at magnitudes too faint for GAIA. AGB stars may alternatively provide precise population ages with GAIA for a wide range of ages and metallicities, with traceable distances of up to 250 kpc at E(B-V)=0 (d=15 kpc if E(B-V)=2.0). It is essential however that effective temperatures precise to 0.01 dex, metallicities to 0.2 dex, and E(B-V) to 0.03 are derived for individual stars, in order to obtain their ages precise to 0.2 dex. This task is quite challenging for GAIA photometry and spectroscopy, though preliminary tests show that comparable precisions may be achieved with GAIA medium band photometry.