ゼロ・低金属量星におけるコロナ加熱とX線放射
Coronal Heating and X-ray Emission in Zero/Low-metal Stars

The University of Tokyo

Haruka Washinoue
Collaborator : Takeru Suzuki
2019.11.11

Population III Stars
Clark et al. 2011

• Metal free• (typically) Massive stars• Fragment of protostellar disk

Machida & Doi 2013

→ merge or survive
Low-mass ﬁrst stars M ≤ 0.8M⊙
Lifetime ≥ 10Gyr
No discovery
Greif et al. 2011!2

Stellar Coronae
・Low-mass main sequence stars
photosphere : T ≈ 4000 − 7000K
corona : T ≥ 106K

・Magnetic structure
closed loop : UV/X-ray emission
open region : Stellar wind
Fraknoi 2016!3

Stellar Coronae
・Low-mass main sequence stars
photosphere : T ≈ 5000 − 7000Kcorona : T ≥ 106K

・Magnetic structure
closed loop : UV/X-ray emission
open region : Stellar wind

HINODE / XRT!4

Heating Mechanism
Surface convection

Coronal heating

Energy transfer
waves, magnetic
reconnection …

✔ Low-mass, low-metal stars have surface convection：zero-metal stars … M ≤ 0.9M⊙ ( Richard et al. 2002 )→ Low-mass Pop.II/III stars should have coronae!5

Motivation? UV/X-ray radiation from metal-free coronaeContribution to cosmic reionization
Observability of low-mass Pop.III stars

・ We investigate
the heating of coronal loops on zero/low-metal stars
̶ Method : 1D MHD simulation+ thermal conduction, radiative cooling!6

1D MHD Simulation
・Method・1D calculation along✳

s coordinate

B ,⃗ v ⃗ have 3 components

・Heating by dissipation of Alfven waves

f(s) = 1
Moriyasu et al. 2004

f(s) = 200

・MHD waves are excited from footpoints by injection of δv!7

1D MHD Simulation
・Settings
Stellar mass : M = 0.8M⊙
Loop length :l = 0.8 × 105km

Magnetic ﬁeld : Bph = 2kG Bs = Bph /f(s)Velocity perturbation : δv = 1.0km/s✳

f(s) = 1Moriyasu et al. 2004

f(s) = 200f(s) : expansion factor−1−2−3Z=
1,10,10,10,0Z⊙
Metallicity!8MHD Equations・∂ρ+ ∇ ⋅ (ρ v ⃗) = 0∂tρGM ̂∂v⃗
B21⃗⃗・ ρ ∂t = − ∇(P + 8π ) + 4π ( B ⋅ ∇) B − ρ( v ⃗ ⋅ ∇) v ⃗ − R2 R sin θ・・∂B ⃗= ∇ × ( v ⃗ × B )⃗∂tdv2B2GMhB2B⃗ ⃗⃗ + ∇ ⋅ Fc + q R = 0ρ {e +++ 2 cosθ} + ∇ ⋅ [(P +) v ⃗ − ( B ⋅ v )]dt28πρ
R8π
4π
1 ∂⃗∇⋅ X =f Xsf ∂s1 ∂⃗∇× X =f ∂sf Xs

R : stellar radiush : loop height e : internal energy
Fc : thermal conduction qR : cooling rate f(s) : expansion factor!9Radiative Cooling・Optically thin region( corona )Sutherland & Dopita 1993

qR = ΛnneΛ : cooling functionn, ne : ion, electron number density
・Optically thick region( chromosphere )Z
qR = 4.5 × 10 ρ(0.2 + 0.8 )Z⊙
9Suzuki 2018!10

Coronal Temperature / Density! = !⨀! = 10'(!⨀! = 10')!⨀! = 10%& !⨀!=0c are higher in lower-Z
・Temperature and density← ineﬃcient radiative cooling + chromospheric evaporation!11

Heating Rate! = !⨀! = 10'(!⨀! = 10%& !⨀! = 10')!⨀!=02B
11Q(s) = ∇ ⋅ {− Bs( B ⊥⃗ ⋅ v ⊥⃗ ) + vs( ρv⊥2 + ⊥ )}4π
24π

↑Poynting ﬂux of MHD waves

・eﬃcient heating in lower-Z!12

UV / X-ray Fluxes✳ We suppose the stellar surface is covered by simulated loops

LX =LUV =

1ftop
1ftop

× 4πR 2∫sΛnne f(s)ds

at T > 5 × 105K

× 4πR 2

sat T ≤ 5 × 105K∫sΛnne f(s)ds

at T ≥ 2 × 104 K・Lower-Z gives larger L・not depend on Z monotonically!13

Summary
1D MHD simulations for heating of coronal loops
to study the dependence on metallicity• In lower-Z, ・Hotter and denser corona is formed
・UV/X-ray luminosities tend to be larger→ Coronae on low-mass Pop.III stars emits strong radiation

・The stellar/loop models need to be improved for low-metal cases→ ? Impact of metal-free coronae on cosmic history!14