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.Comment: Proceedings of the Monte Rosa Conference "GAIA spectroscopy, science
and technology" Gressoney Saint Jean (Valle d'Aosta, Italy), Sept 9-12, 2002,
ed. U. Munari, ASP Conf. Ser., in pres
