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 < \mathrm { [ Fe / H ] } < -0.9 and 4200 < T _ { \mathrm { eff } } \mathrm { ( K ) } < 4950 , using the HIRES spectrograph at the Keck Observatory ( R \approx 10 ^ { 5 } and 200 \leq \mathrm { S / N } \leq 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 } { M } g abundances start to became important , \mathrm { [ Fe / H ] } \sim - 1.4 \pm 0.1 .
The data with \mathrm { [ 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 .