  We determine magnesium isotopic abundances of metal poor dwarf stars from the
galactic halo, to shed light on the onset of asymptotic giant branch (AGB) star
nucleossynthesis in the galactic halo and constrain the timescale of its
formation. We observed a sample of eight new halo K dwarfs in a metallicity
range of -1.9 < [Fe/H] < -0.9 and 4200 < $T_{eff}$(K) < 4950, using the HIRES
spectrograph at the Keck Observatory (R~10^5 and 200 < S/N < 300). We obtain
magnesium isotopic abundances by spectral synthesis on three MgH features and
compare our results with galactic chemical evolution models. With the current
sample, we almost double the number of metal poor stars with Mg isotopes
determined from the literature. The new data allow us to determine the
metallicity when the $^{26}$Mg abundances start to became important, $[Fe/H]
\sim -1.4 \pm 0.1$. The data with $[Fe/H] > -1.4$ are somewhat higher (1-3
$\sigma$) than previous chemical evolution model predictions, indicating
perhaps higher yields of the neutron-rich isotopes. Our results using only AGB
star enrichment suggest a timescale for formation for the galactic halo of
about 0.3 Gyr, but considering also supernova enrichment, the upper limit for
the timescale formation is about 1.5 Gyr.
