Effect of interstellar objects on metallicity
of low-mass first stars formed
in a cosmological model

Kirihara et al. MNRAS, in press
arXiv: 1905.02974v1

Takanobu Kirihara (Chiba University)
Collaborators

Ataru Tanikawa (The University of Tokyo)
Tomoaki Ishiyama (Chiba University)

Population III survivors

Susa+14z=14Pop. III stars
 formed in pristine gas typically very massive SN explosions at high-z(e.g., Tegmark +97; Omukai & Nishi +98; Nakamura & Umemura +01;
Abel+02; Bromm+02; Hosokawa +11; Stacy+12)

Low-mass Population III stars (Pop. III survivors)

 Born in the disks via fragmentation(e.g., Greif et al. 2012) 𝑀𝑀∗(Lifetime ≥ Cosmic age) They should be observed in the MW

No Metal-free stars have been discovered so far.

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Metal pollution of Pop.III survivors

Can they get metals from ISM while wandering in the MW?
• ISM (interstellar medium) accretion→ [Fe/H]～－５

assuming Bondi-Hoyle accretion（Yoshii 1981, Komiya+15, Shen+17）• Steller wind block the accretion→ [Fe/H]<-14(Tanaka et al. 2017, Suzuki 2018)

Tanaka+17 3Interstellar objects (ISOs)’Oumuamua

Wikipedia- The first ISO observed passing throughthe Solar System (Meech et al. 2017).
- Size: ~100 m

Estimated cumulative number densityof such ISOs (>100 m) is(Do +18)

Meech+17, Nature

Pop. III survivors polluted by ISOs

By Tanikawa et al. 2018

ISOs (>100 m) accretion rate
Pop. III survivor

ISO size distribution−αRadiativezone

MW diskISOs

Pop. III survivors polluted by ISOs
ISOs (>100 m) accretion rate

Surface metallicity of a survivor

ISO size distribution−αRadiativezone=5 GyrTanikawa+18

ISOs are the most dominant contributor of metal enrichment.
They assumed one modelled orbit for the analytical estimation.

Cosmological N-body simulation
We consider more realistic orbits of Pop. III survivors.
N-body simulation+ Pop. III formation model N=20483(Ishiyama +16)
Boxsize: 8 ℎ−1 cMpc

Minimum halo mass: 2.4× 105 M⦿
We calculate(We set the number density of ISOs

≡5Gyr)of the Galactic thin disk.

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Accretion ratesolar neighborhood

Pop. III survivors have experienced typically 5M times of ISO(> 100 m) collisions in the last 5 Gyr.
The value is one order of magnitude greater than estimated in Tanikawa+18.

Surface metallicity of Pop.III survivors

solar neighborhood(α=2.0)0.80: typically polluted to [Fe/H]～ −2.
0.70 and 0.75
stars: the typical surface metallicity are around [Fe/H]= −6 ～ −5.

Surface metallicity of Pop.III survivors

solar neighborhood(α=2.0)0.80: typically polluted to [Fe/H]～ −2.
0.70 and 0.75
stars: the typical surface metallicity are around [Fe/H]= −6 ～ −5.

Metal-poor stars as candidates of Pop.III
We choose non- C-enhanced metal poor (nonCEMP) and main-sequence stars in the MW.
Most metal-poor stars with 0.8
6 stars with −3.8 < [Fe/H] < −3.6
SAGA database (Suda +08)

α=3.0: there is no Pop. III candidate.
α=2.5: 6 stars with 0.80 are the most promising
candidates of Pop. III survivors.
α～2.0: non-CEMP and main-sequence starswith Teff ∼ 6500 K (0.80 ) and [Fe/H]≲−3 arecandidates of Pop. III survivors.
1 star with 0.75
SDSS J164234+443004 (Teff = 6280 K and [Fe I/H]= −4.05), which as small perigalacticon (～2 kpc) by Gaia
(Sestito et al. 2019)

Summary• We investigated metal pollution onto the surface of Pop. IIIsurvivors by ISOs floating in the Galactic disk.
• ISOs are the most dominant contributor of metal pollution.
• Metal-poor stars so far discovered at solar neighborhood
are possible to be metal-free Pop. III stars on birth.
• Pop. III survivors could hide in extremely metal-poor stars
so far discovered.