Measuring copper abundances in the Galactic Bulge

Abstract: Context: The chemical evolution of Cu in the Galactic bulge have been studied in a few studies using red giant branch stars, but the ages of individual red giant stars are difficult to determine. In addition, the bulge has been a challenging region of the Milky Way galaxy to obtain high-resolution spectra of stars, except when observing through certain few small fields into the bulge that are known to have low extinction. Fortunately, a sample of dwarf and subgiant stars were observed during gravitational microlensing events, whose ages were easily determined for each individual star using isochrone fitting and are widely spread across the longitude of the bottom-half of the Galactic bulge. Previous studies that have analysed the sample of microlensed bulge dwarf stars were successful in measuring the abundances for a variety of elements, but have not made measurements for Cu. Therefore, this work has focused on measuring the abundance of Cu in the Galactic bulge using the sample of microlensed bulge dwarf stars. Method: The Cu abundances for a sample of 71 microlensed dwarf and subgiant stars that are located in the Galactic bulge were analysed by generating a synthetic spectra with known stellar parameters and fitting it to the observed spectra with the best chosen Cu abundance value for the absorption line at 5782 Å. Results: The Cu abundances for 31 microlensed bulge stars were reliably measured. It was found that Cu/Fe remained flat across metallicities Fe/H between 0.7 ≤ Fe/H ≤ 0.55. Comparing the Cu/Fe-Fe/H trend of the microlensed bulge stars with that of the Galactic chemical evolution model for the bulge did not agree well with each other, where the bulge model has an relative overproduction of Cu/Fe for higher metallicities Fe/H > 0.5. The results fit well with the observed Cu/Fe-Fe/H trend seen in the sample of thick disk dwarf stars for 0.7 ≤ Fe/H ≤ 0.0 and the ages for the microlensed bulge dwarfs agreed well with the ages of the thick disk dwarf stars. Finally, comparing the Cu abundances with the C and O abundances measured in the microlensed bulge dwarf star sample showed that Cu and C were in lockstep when plotting Cu/C vs. C/H and it was found that Cu/O rises for increasing O/H abundances. Conclusion: From the results, the linear Cu/Fe-Fe/H trend showed a lockstep behaviour between Cu and Fe, which suggests a dependency between the production of Cu and metallicity. A few microlensed bulge dwarf stars show signs that the formation of the bulge and thick disk are similar. Finally, the rising trend for Cu/O for larger O/H implies that massive stars, which are believed to be the major producers of Cu, are not the only contributor of Cu and contribution from other sources need to be considered.