Metal Pollution of Low-Mass Population III Stars through Accretion of Interstellar Objects like `Oumuamua

TL;DR

This paper investigates how interstellar objects like 'Oumuamua can pollute low-mass Population III stars with metals through accretion, potentially explaining their observed surface metallicities.

Contribution

It provides a quantitative estimate of surface pollution of Pop. III stars by ISOs, highlighting the significance of ISO size distribution and galactic location in metal enrichment.

Findings

Pop. III stars can accrete significant ISO mass if size distribution slope is shallow.

Surface metallicity of Pop. III stars can reach [Fe/H] ~ -6 to -8 due to ISO accretion.

ISO accretion could explain the observed metallicities in some metal-poor stars.

Abstract

We calculate accretion mass of interstellar objects (ISOs) like `Oumuamua onto low-mass population III stars (Pop.~III survivors), and estimate surface pollution of Pop.~III survivors. An ISO number density estimated from the discovery of `Oumuamua is so high ($\sim 0.2$~au$^{-3}$) that Pop.~III survivors have chances at colliding with ISOs $\gtrsim 10^5$ times per $1$~Gyr. `Oumuamua itself would be sublimated near Pop.~III survivors, since it has small size, $\sim 100$~m. However, ISOs with size $\gtrsim 3$~km would reach the Pop.~III survivor surfaces. Supposing an ISO cumulative number density with size larger than $D$ is $n \propto D^{-\alpha}$, Pop.~III survivors can accrete ISO mass $\gtrsim 10^{-16}M_\odot$, or ISO iron mass $\gtrsim 10^{-17}M_\odot$, if $\alpha < 4$. This iron mass is larger than the accretion mass of interstellar medium (ISM) by several orders of magnitude. Taking into account material mixing in a convection zone of Pop.~III survivors, we obtain their surface pollution is typically [Fe/H] $\lesssim -8$ in most cases, however the surface pollution of Pop.~III survivors with $0.8M_\odot$ can be [Fe/H] $\gtrsim -6$ because of the very shallow convective layer. If we apply to Pop.III survivors located at the Galactocentric distance of 8 kpc, the dependence of the metal pollustion is as follows. If $\alpha > 4$, Pop.~III survivors have no chance at colliding with ISOs with $D \gtrsim 3$~km, and keep metal-free. If $3 < \alpha < 4$, Pop.~III survivors would be most polluted by ISOs up to [Fe/H] $\sim -7$. If $\alpha < 3$ up to $D \sim 10$~km, Pop.~III survivors could hide in metal-poor stars so far discovered. Pop.~III survivors would be more polluted with decreasing the Galactocentric distance. Although the metal pollution depends on $\alpha$ and the Galactocentric distance, we first show the importance of ISOs for the metal pollution of Pop.~III survivors.