Where are the Low-Mass Population III Stars ?

TL;DR

This study models the distribution and observability of low-mass Population III stars in the Milky Way, constraining their formation models and IMF based on current and future observational prospects.

Contribution

It introduces a high-resolution cosmological simulation of Pop III star formation, analyzing their distribution and providing observational constraints on their numbers and IMF.

Findings

Survivors concentrate in the halo center and subhalos.

Current observations exclude models with more than ten stars per minihalo.

Less than ten dwarf galaxies within 100 kpc are needed to find one Pop III survivor.

Abstract

We study the number and the distribution of low mass Pop III stars in the Milky Way. In our numerical model, hierarchical formation of dark matter minihalos and Milky Way sized halos are followed by a high resolution cosmological simulation. We model the Pop III formation in H2 cooling minihalos without metal under UV radiation of the Lyman-Werner bands. Assuming a Kroupa IMF from 0.15 to 1.0 Msun for low mass Pop III stars, as a working hypothesis, we try to constrain the theoretical models in reverse by current and future observations. We find that the survivors tend to concentrate on the center of halo and subhalos. We also evaluate the observability of Pop III survivors in the Milky Way and dwarf galaxies, and constraints on the number of Pop III survivors per minihalo. The higher latitude fields require lower sample sizes because of the high number density of stars in the galactic disk, the required sample sizes are comparable in the high and middle latitude fields by photometrically selecting low metallicity stars with optimized narrow band filters, and the required number of dwarf galaxies to find one Pop III survivor is less than ten at <100 kpc for the tip of redgiant stars. Provided that available observations have not detected any survivors, the formation models of low mass Pop III stars with more than ten stars per minihalo are already excluded. Furthermore, we discuss the way to constrain the IMF of Pop III star at a high mass range of > 10 Msun.