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

We argue that tracing star formation histories with GAIA using main sequence turn-off (MSTO) point dating will mainly be effective in cases of very mild interstellar extinction (EB−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; EB−V = 1.0) and/or distances (d = 2 kpc if t ≤ 15 Gyr; EB−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 EB−V = 0 (15 kpc if EB−V = 2.0). It is essential however that effective temperatures, metallicities, and reddenings of individual stars are derived with the precision of σ(log Teff) ∼ 0.01, σ([M/H]) ∼ 0.2, and σ(EB−V ) ∼ 0.03, to obtain σ(log t) ∼ 0.15. 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.