A Minimum Dilution Scenario for Supernovae and Consequences for Extremely Metal-Poor Stars

TL;DR

This paper establishes a lower limit on supernova mixing with the interstellar medium, revealing that previous assumptions may overestimate the dilution and thus affect interpretations of metal-poor star abundances.

Contribution

It introduces a minimum dilution scenario for supernova ejecta, challenging previous abundance fitting methods and highlighting the importance of aspherical supernova mixing in early chemical enrichment.

Findings

The minimum dilution limit aligns with existing simulations.

Faint supernova models cannot explain certain metal-poor star abundances.

Dilution assumptions significantly impact supernova progenitor identification.

Abstract

To date no metal-free stars have been identified by direct observations. The most common method of constraining their properties is searching the spectra of the most metal-poor stars for the chemical elements created in the first stars and their supernova. In this approach, modelled supernova yields are compared to the observed abundance patterns in extremely metal-poor stars. The method typically only uses the abundance ratios, i.e., the yields are diluted to the observed level. Following the usual assumption of spherical symmetry we compute a simple lower limit of the mass a supernova can mix with and find that it is consistent with all published simulations of early chemical enrichment in the interstellar medium. For three different cases, we demonstrate that this dilution limit can change the conclusions from the abundance fitting. There is a large discrepancy between the dilution found in simulations of SN explosions in minihaloes and the dilution assumed in many abundance fits. Limiting the dilution can significantly alter the likelihood of which supernovae are possible progenitors of observed CEMP-no stars. In particular, some of the faint, very low-yield SNe, which have been suggested as models for the abundance pattern of SMSS0313-6708, cannot explain the measured metal abundances, as their predicted metal yields are too small by two orders of magnitude. Altogether, the new dilution model presented here emphasizes the need to better understand the mixing and dilution behaviour of aspherical SNe.