Effect of interstellar objects on metallicity of low-mass first stars
  formed in a cosmological model
  We investigate metal pollution onto the surface of low-mass population III
stars (Pop. III survivors) via interstellar objects floating in the Galactic
interstellar medium. Only recently, Tanikawa et al. analytically estimated how
much metal should collide to an orbiting Pop. III survivor encouraged by the
recent discovery of 'Oumuamua and suggested that ISOs are the most dominant
contributor of metal enrichment of Pop. III survivors. When we consider a
distribution of interstellar objects in the Galactic disc, Pop. III survivors'
orbits are significant properties to estimate the accretion rate of them though
Tanikawa et al. assumed one modelled orbit. To take more realistic orbits into
calculating the accretion rate, we use a high-resolution cosmological $N$-body
simulation that resolves dark matter minihaloes. Pop. III survivors located at
solar neighbourhood have a number of chances of ISO($> 100$ m) collisions,
typically $5\times10^6$ times in the last $5$ Gyr, which is one order of
magnitude greater than estimated in the previous study. When we assume a
power-law parameter $\alpha$ of the ISO cumulative number density with size
greater than $D$ as $n \propto D^{-\alpha}$, $0.80 \, M_{\odot}$ stars should
be typically polluted [Fe/H]$\sim -2$ for the case of $\alpha=2.0$. Even in the
cases of $0.70$ and $0.75 \, M_{\odot}$ stars, the typical surface metallicity
are around [Fe/H]$=-6 \sim -5$. From the presence of stars with their [Fe/H],
we can constrain on the lower limit of the power $\alpha$, as $\alpha \gtrsim
2.0$, which is consistent with $\alpha$ of km-size asteroids and comets in the
solar system. Furthermore, we provide six candidates as the ISO-polluted Pop.
III stars in the case of $\alpha \sim 2.5$. Metal-poor stars so far discovered
are possible to be metal-free Pop. III stars on birth.
