The spectroscopic challenges of sulfur

Alpha-elements can be used as cosmic clocks to constrain both stellar nucleosynthesis and galaxy's formation history.
Sulfur is the outcast alpha-element. Indeed, its analysis is often skipped in favor of the other alpha-elements, which are less complicated to measure. The challenges in question are the weakness of optical sulfur lines, the contamination due to telluric lines, and the deviation from local-thermodynamic equilibrium of near-infrared sulfur lines. Consequently, the behavior of sulfur has been mainly investigated in the solar neighborhood and a few stellar clusters.
Being volatile, sulfur is the proxy of alpha-elements for high-redshift systems. In fact, its abundances measured in local stars and gaseous phase in the far Universe (as Damped Ly-alpha systems, DLA) can be directly compared. These kinds of studies are relevant to understand the formation and evolution of galaxies, since DLAs have been proposed as progenitors of dwarf galaxies (i.e. the Milky Way building blocks). However, the Sculptor dwarf galaxy is the only extragalactic system for which sulfur abundances have been measured in stellar atmospheres.
This talk aims to show new outcomes on the sulfur behavior in the Milky Way major components. The results for 74 bulge stars, 21/30 thick/thin-disk stars and 24 halo stars observed with FLAMES/UVES@VLT will be presented. It will be shown that the chemical evolution of the Milky Way traced by sulfur provides clues on the formation of our Galaxy.