初代星形成
an overview
Takashi Hosokawa / 細川 隆史(Kyoto Univ. / 京都大学)
12.Nov.2019 初代星・初代銀河研究会 2019@名古屋

Review of Reviews…
Most of the previous review slides are still available on the web.
First visit a summary site at Konan, http://tpweb2.phys.konan-u.ac.jp/~shodai/
2018.11 @ 水戸 “初期宇宙構造形成シミュレーション”by 吉田さん“連星形成” by 町田さん
2018.2 @ 呉 “初代星形成理論の現状” by 須佐さん“連星の形成について” by 釣部さん
2016.10 @ 金沢 “初代星形成: an overview” by 細川2015.12 @ 草津温泉 “初代星の形成” by 大向さん
2015.1 @ 仙台 “初代星形成過程の理論的研究” by 平野くん
…and more!

Binary formation is a recent ”hot topic”!

Advent of GW Era

What are the origins of such massive BH-BH binaries?
massive stellar binaries ⇒ binary evolution (e.g., common envelope)⇒ BH-BH binary (w/ 〜0.1AU separation)
Consider the first stage of the formation of massive stellar binaries

Pop III Origin?

(Kinugawa+14,16; Nakamura+16; Inayoshi+17 etc)

(Monte-Carlo simulations)
Kinugawa+16

Peak around 30M8
only for Pop III case

Pop III
GW150914

Number of events: dN/dM/Ntotal

Assume millions of stellar binaries, and then derive the chirp-mass
distribution of merger events that occur within the cosmic age…

Pop II1

10Chirp mass [ solar mass ]It was predicted in 2014,
before the GW detection

100And then GW150914 came
just on the peak…

Key Questions
Stars in the earliest universe: What is their
typical mass? / mass distribution? /binarity and multiplicity?
To answer these, we understand the key processes:(I) stellar radiative feedback + (II) fragmentation

Chon, TH+18

Pop III UV feedback(TH+11, 16; Stacy+16, 12; Susa+14, 13 etc)

caused by the stellar ionizing radiation, which heats up the accreting gas

HII region

protostar

TH+16

The mass accretion onto the star is shut off by the UV feedback→ it determines how massive star is finally formed.

※Gas pressure effect (UV radiation enhances the gas pressure)Fragmentation
The grav. fragmentation yield multiple star-disk systems ⇒ binary formation?
Chon, TH & Yoshida (2018)

Desperately complex evolution?
The fragmentation is often followed by merger events.
But not for all. Some fragments evolve into binary systems, and survive.

Binary v.s. Merger
What determines such different fates? Chon & TH (2019)
numerical experiments
Artificially put a clump in a rapidly accreting disk,
and then follow the orbital evolution(initial positions and clump masses are free parameters)
binarymerger

Evolution is diverse: it sensitively depends on the parameter choiceExtract Key Physics
Planet formation theories help us (though the situation is so different!)
Ansatz
1. orbital ang. momentum is lost by (I) type-I migration, (II) tidal disruption
2. the binary formation occurs when gap/cavity is cleared in the disk
Analytic evaluations↓We can explain thedivergent evolution withthe above ansatz.

The binaries generallyexpand
while accreting the gas.

UV feedback + Fragmentation
Sugimura et al (incl. TH) in prep.

UV feedback + Fragmentation

The disk fragmentation ⇒ binary w/ single star + triple system⇒ stellar mass growth under the UV feedback w/ bipolar HII regions

Binary Separationt 〜 1.5 x 104 years

t 〜 3.5 x 104 years

The massive binary expands while accreting the gas! (c.f. Chon & TH 19)

Pop I Massive Star Formationt 〜 4.2 x 104 yearsKrumholz+09

t 〜 5.6 x 104 years

The similar binary expansion has been also reported
in the context of the present-day high-mass star formation

Massive Binaries Expand?
※ BH-BH binary separation must be <〜 0.1AU
for the merger to occur within the Hubble time

Why?
”Inside-Out Accretion” onto stars
The gas near the star accretes first↓
As time goes by, the gas with the larger
specific angular momentum starts to accrete↓
The binary mass increases, butthe binary separation also increases
Magnetic fields may change the story…
⇒ see Eric-san + Harada-san’s talks

Density evolution
in the accretion stage

Pop III v.s. Pop I
High-mass Star Formation〜100M8 star formation is considered for the both cases…
Typical Pop III caseRare Pop I case

Sugimura+19

Krumholz+09

Radiation force feedback
Photoionization feedback(rad. pressure effect w/ dust)(gas pressure effect)“Bridge the gap” btw Pop I and III high-mass star formation

-2
10 Z

8 Massive Star Formation
Fukushima + TH in prep. 19

Stellar mass: M [M8]〜Myr evolution of the protostellar acc.,
with a cosmological halo (H2)studied in Chiaki+

The net feedback is dominated by the photo-ionization, as in Pop III cases.

Accretion rate: M [M8/yr]

Feedback and Metallicity.

Stellar mass: M [M8]

In comparison to 〜1Z8 case, the net feedback is actually stronger
Weaker Rad.-force feedback ⇔ Stronger Photoionization feedback
The latter effect dominates the metallicity-dependence.

Summary+ PopIII binaries: possible origin of BH-BH merger?
A large number of Pop III stars may be in binaries.
BUT no one answers how massive and tight binaries could form
(actually it is also the same for Pop I and II cases)+ We shall bridge the gap btw Pop I and III massive star formation