Origins of the Thick Disk as Traced by the Alpha-Elements of Metal-Poor Giant Stars Selected from RAVE

TL;DR

This study investigates the elemental abundances of metal-poor giant stars from the RAVE survey to understand the origins of the thick disk, revealing rapid star formation and similarities with the halo, thus challenging dwarf galaxy accretion models.

Contribution

It provides detailed alpha-element abundance measurements for metal-poor thick disk stars, offering new insights into their formation history and the processes involved.

Findings

Alpha-elements are enhanced, indicating rapid enrichment by core-collapse supernovae.

Low scatter in [alpha/Fe] ratios suggests well-mixed interstellar medium prior to star formation.

Similarities with halo star ratios imply a common massive star IMF, challenging dwarf galaxy accretion scenarios.

Abstract

Theories of thick disk formation can be differentiated by measurements of stellar elemental abundances. We have undertaken a study of metal-poor stars selected from the RAVE spectroscopic survey of bright stars to establish whether or not there is a significant population of metal-poor thick-disk stars ([Fe/H] <~ -1.0) and to measure their elemental abundances. In this paper, we present abundances of four alpha-elements (Mg, Si, Ca, Ti) and iron for a subsample of 212 RGB and 31 RC/HB stars from this study. We find that the [alpha/Fe] ratios are enhanced implying that enrichment proceeded by purely core-collapse supernovae. This requires that star formation in each star forming region had a short duration. The relative lack of scatter in the [alpha/Fe] ratios implies good mixing in the ISM prior to star formation. In addition, the ratios resemble that of the halo, indicating that the halo and thick disk share a similar massive star IMF. We conclude that the alpha enhancement of the metal-poor thick disk implies that direct accretion of stars from dwarf galaxies similar to surviving dwarf galaxies today did not play a major role in the formation of the thick disk.