Probing stellar evolution through spectroscopy of horizontal branch stars

This dissertation describes a new detailed abundance study of field red horizontal branch stars, RR Lyrae stars and blue horizontal branch stars. To carry out this study, we obtained high-resolution and high signal-to-noise ratio echelle spectra at the McDonald observatory and Las Campanas Observatory. In addition, new pulsational emphemerides were derived to analyze the spectra of RR Lyrae stars throughout the pulsational cycles. We find that the abundance ratios are generally consistent with those of field stars of similar metallicity in different evolutionary stages and throughout the pulsational cycles for RR Lyrae stars. We also estimated the red and blue edges of the RR Lyrae instability strip using the derived effective temperatures of RHB and BHB stars. New variations between microturbulence and effective temperature are found among the HB population. For the first time the variation of microturbulence as a function of phase is empirically shown to be similar to the theoretical calculations. Finally, through the study of a rare eclipsing sdB and M dwarf binary, we discovered an unusually low mass for this type of HB star, which observationally proved the existence of a new group of low-mass sdB stars that was theoretically predicted in the past.