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

TL;DR

This study measures magnesium isotopic ratios in metal-poor halo dwarf stars to determine the onset of AGB star nucleosynthesis and estimate the galactic halo formation timescale, finding it to be between 0.3 and 1.5 billion years.

Contribution

It provides new magnesium isotopic data for halo stars, nearly doubling existing measurements, and constrains the formation timescale of the galactic halo using these isotopic abundances.

Findings

Mg isotopic abundances start to increase at [Fe/H] ~ -1.4

Data suggest higher yields of neutron-rich isotopes than models predict

Halo formation timescale estimated between 0.3 and 1.5 Gyr

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.