High-mass Star Formation
across the Universe
Takashi Hosokawa / 細川 隆史
hosokawa@tap.scphys.kyoto-u.ac.jp(Department of Physics, Kyoto Univ.)
22/08/2022 @ 夏の学校2022 online

High-mass stars@present-day

30doradus@LMC

Looks very energetic… How do they form?

The First (Pop III) Stars

Our GalaxyBigBang

観測目標

They should have formed at some point… How did they form?

High-mass Stars @ early universe
Ø more popular than the present-day (suggested by theorists…)
Ø heavy-elements factory, cosmic reionization, SMBH seedsØHigh-z observations see their emission… How did they form?
First Stars(PopIII)
Cosmic reionization

Seed BHs forSMBHs

High-mass Star Formation
across the Universe
Many generations of high-mass ( >10M8) stars have formed
across cosmic history, including the present-day and early universe.
How do they form in different epochs of the cosmic history?
なぜこれを知りたいのか? 何が面白いのか?
+ feedback, big influences on the surrounding environments+ factories of heavy elements (our origin)+ origins of SMBHs+ High-z observations see their emission+ First (Pop III) stars … why not?

A Big Picture
Typical caseRare case

Z = Z8(Pop I)

low-mass SF〜1M8

high-mas SF
Up to 〜100 M8Z=0(Pop III)The First Stars〜10 – 100 M8(?)

Supermassive Stars
Up to 〜105 M8(?)”Direct Collapse”

A Big Picture
Rare case

Typical case
Z = Z8(Pop I)em
sa

Z=0(Pop III)high-mas SF
Up to 〜100 M8

low-mass SF〜1M8

The First Stars〜10 – 100 M8(?)

ssamgenra

Supermassive Stars
Up to 〜105 M8(?)”Direct Collapse”

星形成の基本シナリオ
前期段階
ガス雲の重力崩壊

後期段階
原始星へのガス降着“run-away”
collapse

何らかの冷却過程で
γ_eff < 4/3 である限り続く

γ_eff > 4/3になったら原始星誕生(原始星質量) << (ガス雲質量)↓
ガス降着で原始星質量増

初期宇宙/現在の宇宙の星形成、ともに共通の基本描像
大質量星形成の研究 ＝ 後期段階の研究

The First (Pop III) Stars

Our GalaxyBigBang

観測目標

How, When, and Where did they form?
What kind of stars were they? Can we observe them?

コンピューターで宇宙最初の星を作ろう
NAOJ super-computer: ATERUIMatter distribution just after the Big Bang＋
Basic physical laws
(gravity, gas dynamics, radiation)

With the well-defined initial condition, it is straightforward tofollow the evolution from the Big Bang to the birth of the first stars.

Collapse stage in 3D
Yoshida et al. 03

Yoshida et al. 08

600h-1kpc

Zoom-up

Primordial protostar：～0.01M8

How massive were they?
early collapse stage ⇒ late accretion stage
Yoshida, Omukai & Hernquist (2008)

Zoom-up“seed” protostar

10-2 M8 protostar
surrounded by >103 M8 gas envelope

for stellar lifetime (〜Myr) → 〜1000M8 star

本当は何が起こるか?

2D numerical simulations
HII region

Tρ+v

Acc. rate (M8/yr )TH+11, 12; Hirano, TH et al. 14

No Feedback

With Feedback

Stellar mass (M8)

Stellar UV feedback halts the accretion at M*～43 M8“Ordinary massive” Pop III starForming >100 Pop III Stars

Pick up > 100 of the star-forming clouds found in cosmologicalsimulations. The later evolution until the stellar mass is fixed isfollowed by 2D RHD simulations
Hirano, TH, Yoshida et al. (2014)
bipolarHII regions

The UV feedback finally shuts off themass accretion in all the cases

The “Mass Spectrum”

With more than 1000 (!) star-forming clouds taken from cosmological simulations
Hirano, TH, Yoshida+ (2015)

Diversity is originally created through the cosmological structure formation.

Era of Gravitational Wave

Pop III stars are possible progenitors of merging BH-BH binaries!

First ”binary” stars© 杉村和幸(京大)High-massstar formation@ z 〜 25

Sugimura, Matsumoto, TH+(2020)

Birth of a massive binary systems with 〜50M8 stars

Hot plasma heated bystellar irradiation (HII)

Advent of JWST

https://webb.nasa.gov/content/science/firstLight.html

BUT expected to be impossible to detect individual 〜1000M8 stars at z〜10
(common knowledge in the community)
Realistic targets are larger high-z galaxies / star clustersthat may contain some stellar component of Pop III stars.
⇒ 極めて金属量の少ない大質量星団の形成過程が面白いだろう(like young version of globular clusters)

JWST + grav. lensing

z〜20 candidates!?
(Yan+22, arXiv:2207.11558 )©NASA, ESA, STScl

Young Massive Clusters (YMCs)
Young analoguesof globular clusters?

Antenna galaxy (interacting)

Whitmore+ (99)

Such massive clusters rarely form in nearby starburst galaxies.
What physical conditions are necessary for their formation?
Cloud-Cloud collisions prepare such conditions... (e.g., Tsuge+21; Enokiya+21)

Fukushima+Yajima+TH+20Fukushima & Yajima (2021)
RHD simulations following the star cluster formation, including YMCs (w/ SFUMATO).
YMC formation occurs if the surface density of a cloud Σ exceeds a threshold value.
Cloud mass:
106M8,+
Cloud radius:
20pc
Cloud mass:
106M8,+
Cloud radius:
40pc
電離バブル膨張より十分早く、電離ガスをtrapできるだけ星団がmassiveになるか否かObservational Signatures?
Particularly, YMCs in the formation stage (when the stellar mass increases)+ Photodissociation region (PDR)
clouds irradiated by stellar FUV photons
tracer: CII (158μm), OI (63μm) …
line emission, warm dust etc.

Sollins+ (2005)©JWST+ gas infall motion toward the forming cluster← motion detected by the molecular (NH3) absorption line
against the background free-free emission
Molecular gas infalling toward the central HII region

molecular line absorption

Aim
What are observationalsignatures of forming
YMCs?

Method: Post-process calculation (synthetic observation)Line emission

level population calculations solving the detailed balance (local)↓radiation transfer (integration along lines of sight)

molecular absorption: 2D velocity distributions weighted byforeground CO column densities

Motion of Molecular Gas
Case of YMC formation

V_{line of sight}[ km/s ]V_{line of sight}[ km/s ]averaged over 6 lines of sight

20-2

Distance from cluster center: r[pc]Inner infall motion (infalling molecular gas trapped by the cluster’s gravity)+Outer outflow motion (molecular gas blown away by escaping ionizing photons)

Comparing time evolution
YMC formation

NO YMC formation

Inner infall motion continues long(normalized by t_ff)

Inner infall motion only in an early stage→ outflow motion dominates everywhere

Comparing time evolution
YMC formation

NO YMC formation

How to distinguish these cases?

CII emission map
NO YMC formation

I [CII] (erg cm-2 s-1 str-1)

I [CII] (erg cm-2 s-1 str-1)

YMC formation

Very strong emission at the center
Centrally-condensed distribution

Weaker emission at the center
Less centrally-condensed distribution

Obs. Signatures of YMC formation
As far as we have explored…
Molecular gas infalling
Centrally-condensed, very strong
toward the central HII region
CII emission (+ OI, CO, H2, and dust)+These features co-exist only during the YMC formation stage
Currently only at Z〜Z8 and to be extended for low-metallicity cases

Summary+ Pop III star formation as a challenge of the computational astrophysics
3D simulations reveal great details of their formation process,e.g., mass distributions, binality, etc.
技術的には詳細観測と比較している銀河系の星形成研究と遜色なし+ Observational signatures of their on-going formation process?
Low-metallicity massive clusters may provide some chance