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

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.

Origin of metals in old Milky Way halo stars based on GALAH and Gaia

“…In contrast, the hydrogen dilution mass required to explain both [X/Fe] ratios and [Fe/H] is determined to be 100-1000 𝑀 , which is much smaller than the analytic prescription. This value is also not compatible with a new estimate of the minimum dilution mass by Magg et al (2020). In those respect, it is unlikely that only a single Pop III supernova dominate in producing metals in the OHS.…”

“…Taking into account the contamination from normal CCSNe, the majority of the OHS are best fitted by the 25𝑀 Pop III CCSN yield models that produce ∼ 10 −1 𝑀 of 56 Fe (Table A2). Similar to Model A, however, the Fe yields from Pop III CCSN that best explain observed abundances violate the constraint on the amount of diluting hydrogen gas to be compatible with the observed [Fe/H] ( Magg et al 2020). We discuss the validity of the Models A and B in Section 5.2.…”

“…To explain observed [Fe/H] values of the OHS by a single Pop III yield that fits the abundances, the ejected Fe should be diluted with only 10 2 − 10 3 𝑀 of hydrogen in most cases (Table A1). These values are extremely small compared to simulations of metal mixing and analytical models (Magg et al 2020). We discuss the validity of this scenario in Section 5.2.…”

When did Population III star formation end?

“…Metal mixing is also crucial for inferring the properties of Pop III stars from observations of extremely metal-poor stars in the local Universe, i.e. 'stellar archaeology' (e.g Frebel & Norris 2015;Ji et al 2015;Hartwig et al 2015;Ishigaki et al 2018;Magg et al 2019;Magg et al 2020…”