The origin of low [alpha/Fe] ratios in extremely metal-poor stars

Abstract

We show that the low ratios of $\alpha$ elements (Mg, Si, and Ca) to Fe recently found for a small fraction of extremely metal-poor stars can be naturally explained with the nucleosynthesis yields of core-collapse supernovae, i.e., $13-25M_{\odot }$ supernovae, or hypernovae. For the case without carbon enhancement, the ejected iron mass is normal, consistent with observed light curves and spectra of nearby supernovae. On the other hand, the carbon enhancement requires much smaller iron production, and the low [$\alpha$/Fe] of carbon enhanced metal-poor stars can also be reproduced with $13-25M_{\odot }$ faint supernovae or faint hypernovae. Iron-peak element abundances, in particular Zn abundances, are important to put further constraints on the enrichment sources from galactic archaeology surveys.

Accepted for publication in ApJ Letter, 7 pages, 2 figures

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• R-Process elements from magnetorotational hypernovae
• Disentangling the Galactic Halo with APOGEE: II. Chemical and Star Formation Histories for the Two Distinct Populations
• A Minimum Dilution Scenario for Supernovae and Consequences for Extremely Metal-Poor Stars
• Gemini/GRACES Spectroscopy of Stars in Triangulum II
• Evidence for the accretion origin of halo stars with an extreme r-process enhancement
• High resolution spectroscopic follow-up of the most metal-poor candidates from SkyMapper DR1.1
• Chemical trends in the Galactic Halo with APOGEE data
• The R-Process Alliance: Discovery of a low-alpha, r-Process-Enhanced Metal-Poor Star in the Galactic Halo
• The Pristine Inner Galaxy Survey (PIGS) X. Probing the early chemical evolution of the Sagittarius dwarf galaxy with carbon abundances
• The Influence of Environment on the Chemical Evolution in Low-mass Galaxies
• The COMBS Survey -- III. The Chemodynamical Origins of Metal-Poor Bulge Stars