A formation timescale of the Galactic halo from Mg isotopes in dwarf stars

Abstract

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.

8 pages, 5 figures. Accepted for publication on ApJ

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Cited by in corpus (5)

• The Cosmic Evolution of Magnesium Isotopes
• Nucleosynthetic yields of Z=$10^{-5}$ intermediate-mass stars
• Isotopic ratios for C, N, Si, Al, and Ti in C-rich presolar grains from massive stars
• Magnesium isotope ratios in Milky Way and dwarf galaxy stars
• The complex stellar system M 22: constraining the chemical enrichment from AGB stars using magnesium isotope ratios