Mapping the present-day chemical abundance structure of the Solar neighbourhood: O & Si

Sammanfattning: Context. Large scale chemical abundance gradients in the Galactic disks, small-scale abundance structures, and the mean chemical abundance values in the Solar vicinity, are important constraints to Galactic chemo-dynamical formation and evolution models. The formation and evolution of stars, and interstellar gas and dust depends on the distribution and evolution of matter in the Galaxy. Therefore, metallicity structures within the Milky Way can be mapped from the abundance analysis of its gas and stars. Data. A sample of 379 mostly spectral type B main sequence stars within 1.5 kpc radius from the Sun, was observed with the MIKE high-resolution spectrograph on the Magellan 6.5-m telescope on Las Campanas in Chile in 2007. Projected rotational velocities and photometric effective temperatures were determined for these stars by Bragança et al. (2012). B-stars are good indicators for present-day cosmic abundances due to their short lifetimes. They preserve the interstellar medium abundances, which they were born from, in the photosphere, and they do not migrate far from their birth environment. Aims. The purpose of this Master’s project was to start mapping the present-day silicon and oxygen abundance structure of the Solar neighbourhood, by determining the stellar atmospheric parameters, and Si and O abundances, for the low-rotating (< 22 km/s ) subsample. The thesis at hand is part of an international study aimed to determine the elemental abundances of B-type main sequence stars near the Solar vicinity and in the outer Galactic disk (Bragança et al. 2012, 2015; Garmany et al. 2015, Bragança et al. in prep.). Methods. Line-profile fitting of Si, O and Balmer lines, with full NLTE synthetic spectra, and an iterative analysis methodology, was used to constrain stellar atmospheric parameters and elemental abundances at high accuracy and precision. Results & Conclusions. With this Master’s thesis, an extensive mapping the Solar neighbourhood has begun: stellar parameters, and Si and O abundances were established for 17 stars. The mean elemental abundance of the Solar neighbourhood of a 1.25 kpc radius, was found to be 7.42±0.13 dex for silicon and 8.73±0.06 dex for oxygen. The results are consistent with the cosmic abundance standard from Nieva & Przybilla (2012) within error-bars, and provide reliable present-day anchor points for Galactic chemical evolution models. However, the stars in our sample have on average smaller abundance values and the Solar neighbourhood seems to be more heterogeneous, than determined by Nieva & Przybilla (2012) study. Small scale abundance structures were not discovered, due to yet small sample size. In the future, other low-rotating stars in the observed stellar sample will be analysed, up to a v sin i = 80 km/s limit. This would amount to about 100 stars. Combined with Gaia DR2 positions, and chemical abundances for the fast rotating B-type stars in the Solar vicinity (Cazorla et al. 2017), would provide an unprecedented view of the present-day Solar neighbourhood.