Accurate Abundances of Giant Stars in the Local disk: A manual analysis of IR APOGEE spectra

Abstract: Purpose: In order to study the Galactic Chemical Evolution, elemental abundances are needed. The purpose of this work is to investigate the chemical evolution of the Milky Way by determining several elemental abundances of giants in the local disk. Large surveys are up and coming, and several surveys are already working, and with these, the aim is high quality abundance measurements of several elements of several thousands of stars. Apache Point Observatory Galactic Evolution Experiment (APOGEE) analyses abundances using a fast pipeline, industrialized to encompass measurements of all stars with the wealth of data. This pilot-study aims to investigate how accurate and precise the abundances can be measured and gain as much scientific information from the spectra by reanalyzing a sub-sample of APOGEE abundance measurements manually. Method: The data used is a subset of IR spectra from the APOGEE survey, of giant stars that have also been analyzed accurately using high resolution optical spectra, which thus can be used as a benchmark. The stellar sample consists of 291 spectra in the local disk giants with a resolution of 22 500 and signal-to-noise of above 100. Giant stars enable us to probe deeper into the Milky Way. The stellar parameters of these stars have been determined in the optical in order to be as independent from APOGEE as possible. The abundances are determined by synthesizing a spectrum compared to the observed spectrum with focus on the line of interest using Spectroscopy Made Easy (SME), and the giants are assigned to the thin and thick disk using optical measurement. Results: Abundance elements of light elements (Na, Al, K), ↵-elements (Mg, Si, S, Ca, Ti), iron peak elements, (V, Cr, Co, Ni), and neutron-capture elements (Cu and Ce) are determined. The traditional method of spectral synthesis will allow for each line of interest of each element to be investigated and allow for manual inspection of each analysis, thus maximizing the accuracy and precision without calibration. Using available distance measurements of the sample, ages are determined, showing higher ↵-abundance in older stars. Conclusions: A manual analysis gives more accurate and precise abundance measurements, showing it is beneficial to measure APOGEE spectra with this method.