Transcript Low-Mass Star Formation Triggered by Supernova in Primordial

Low-Mass Star Formation, Triggered
by Supernova in Primordial Clouds
Masahiro N. Machida
Kohji Tomisaka
Fumitaka Nakamura
Masayuki Y. Fujimoto
(Chiba University)
(NAOJ)
(Niigata University)
(Hokkaido University)
Introduction
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Is the formation of the low mass, metal deficient star
possible in early universe ?
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Why do we discuss the low mass star?
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They have a long life time and survive up to now
They can be observed (Beers et al. 1992, Norris et al. 1999)
Why do we discuss the metal deficient star?
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They formed in early universe that has not been polluted
They have an information on the early universe
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Recently many metal deficient star ([Fe/H]<-3) has been
observed
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However, not yet understood when and how such stars form
In this study, we examine the low mass star
formation processes, triggered by the first
generation SNe
How to form the low mass star
in early Universe
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In a present star formation, the gas cools to ～10K for dust and metal
cooling
low temperature gas ⇒ small Jeans mass ⇒ low mass star formation
In early Universe (no dust, no metal), the gas cools to about 300-500K
(Nakamura & Umemura 1998)
⇒ It is difficult to form the low mass star in a primordial composition
How to form the low mass star in early Universe
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The H2 and HD are effective coolant at low temperature
The H2 and HD increase if the shock heating ( or re-ionization) occurs
Shock heating is caused by the Supernova Remnant (SNR)
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First star is massive ⇒ short life time ⇒ Supernova explosion occurs
in short period
Gas can cool to low temperature by first generation SNR (shock heating)
⇒ low mass star formation
DM+baryon
First Collapsed Objects
1.
2.
3.
4.
5.
6.
7.
106~107Msun
Scenario
Fragmentation in the SNR shell
cylindrical fragmentation
First collapsed object is formed
(M= 106~107Msun , z=10~50)
Baryon component cools for radiative cooling
First star is formed (massive
star)
SNR shell
Supernova
Next generation low mass stars
radiative cooling explosion occurs
SNR evolves in the host cloud
Fragmentation occurs in the shell of Star
theformation
SNR in the fragments
Low mass star forms in the fragments
Our Study
SNR
first star formation
Supernova explosion
Recent Studies
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Bromm, Yoshida & Hernquist (2003)
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SPH simulation of an SNR by a pair-instability supernova (PISN)
High energy SN (1053 erg)
Low-mass star formation does not occurs
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host cloud (~106Msun) is completely disrupted
gas with Z > 10-2 Zsun is ejected into IGM
No including HD, only two models
Salvaterra, Ferrara & Schneider (2004)
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Calculation of the SNR triggered by the PISN: analytical SNR evolution, many models
Fragmentation (low mass star formation) occurs only when the following condition is fulfilled:
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Bromm, Yoshida & Hernquist 2004
high explosion energy (e0 > 3x1052 erg)
high ambient density (14< n0< 790 cm-3)
They does not calculate the chemical reaction (approximation form of chemical composition)
HD cooling effect is not considered, only H2
They assume the high constant ambient gas pressure (~104 K)
⇒ suppress the SNR evolution, promote fragmentation
⇒ They does not calculate the ambient gas evolution
Our study (Machida, Tomisaka, Nakamura & Fujimoto ApJ 2005 in press)
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SNR evolution (analytical), chemical reaction (1-zone approximation) for the SNR shell and the
ambient gas
Including HD, effect of ambient gas pressure; we calculate 20 models
Low mass star formation occurs, trigged by low energetic SN (1051 erg) in minihalo (106-7 Msun)
Numerical Model(1)
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Initial Conditions
and Calculation
results
We Parameters,
solve the followings
at the same
time
1.
SNR
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2.
Shocked Shell
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3.
radius (RSNR), velocity
outer pressure (Pout)
(VSNR),
density (nSH), temperature
(Xi), shell pressure (Psh)
inner pressure
(Tsh),
chemical composition
Ambient gas
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temperature (Thc), ambient pressure
composition (Yi), (constant density)
Psh
ambient gas
nSH
Shell
ambient gas Thc Phc Yi
Pout
Shell
e0
RSNR
VSNR
Pin
(Phc),
chemical
Parameters
Inside of SNR
SNR
r 0 ∝ (1+z)3
(Pin),
・e0: explosion energy
=1, 3, 5, 10 x 1051 erg
・z: redshift (or ambient density)
=10, 20, 30, 40, 50
◆20 models
◆Chemical composition:
primordial composition
(Galli & Palla 1998)
Numerical Model (2)
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Evolution of the SNR
 Sedov-Taylor phase (tcool>texp)
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Evolution of gas in the shell and
ambient gas
 Temperature
where
Pressure-driven phase (tcool<texp)
(
,
 Fragmentation
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)
(tff = tdyn)
(Λ：He、H、H2、HD、IC)
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Post fragmentation phase
Density
Before fragmentation
(tff<tdyn)
(
,
)
After fragmentation
・Four typical timescales
(Ostreiker 1964)
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Chemical Composition
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H、H+ 、H-、H2、He、He+、He++、D、D+、
HD、HD+、e
Results
★Evolution of the SNR, shell temperature and ambient gas
temperature with different initial density (redshift: z)
Evolution of SNR
Evolution of Shell
r (pc)
shell temperature
SNR radius
time (yr)
Evolution of ambient gas
v (km/s)
ambient gas temperature
ambient gas temperature
SNR velocity
time (yr)
time (yr)
・SNR radius evolve to about 100 pc in 106～107 yr
・The SNR radius increases with lower ambient density
・The temperature of gas shell and ambient temperature falls more quickly in the model with
high-z because radiative cooling is more effective
・Ambient gas temperature cools enough ⇒ ambient gas hardly affects the SNR evolution
★Four typical timescales
Fragmentation (tdyn=tff)
(a) e0=1x1051, z=20
tff
tcool
Sedov-Taylor (texp<tcool)
Pressure-Driven (texp>tcool)
texp
tdyn
t (yr)
・tff: free fall timescale
・texp: expansion timescale
Post-Fragmentation (tdyn>tff)
・tdyn: dynamical timescale
・tcool: cooling timescale
◆After fragmentation, the time scale shortens very much
because the density may increase in the fragments
◆The feature of the timescale does not depend on the parameter so much
◆Fractional abundances
Model: (z, e0)=(20, 1051 erg)
Before Fragmentation
After Fragmentation
temperature
Number density
The time variation of the fractional abundances for 10 species
initial
Fragmentation
epoch
optically thick
◆H2 :1.1×10-6 ⇒ 1.7×10-3 ⇒ 0.85
◆HD:1.2×10-11 ⇒ 7.5×10-6 ⇒ 1.7×10-4
◆Cooling rates due to contribution from various molecules
Model: (z, e0)=(20,1051 erg)
Before Fragmentation
After Fragmentation
Total
H
HD
H2
He
H2
HD
HD
H2
◆Because H2 and HD is increased for shock heating, H2 and HD become effective coolant
at low temperature (H2: 5000<T<200k HD: T<200K) in the ante-fragmentation phase
◆In the post-fragmentation phase, H2 and HD are effective coolant
◆n>108 cm-3, H2 increase for 3-body reaction
◆Fragmentation condition
VSNR = Cs,hc (T)
Fragmentation region is plotted on the e0 -z parameter space
◆Solid lines: the relation that swept mass is equal to the baryonic mass in the host cloud
◆Dotted line: the relation that the shell expansion speed at the fragmentation is equal to
the sound speed in the host cloud of cs,0=2,3,5 km/s
Post Fragmentation Phase
Temperature and Jeans mass
when the gas becomes optically thick,
Jeans mass becomes
・MJ = 0.16 Msun for cylindrical geometry
・MJ = 0.89 Msun for spherical geometry
Low mass star formation is possible !
Metallicity of the low mass star
・[Fe/H] is estimated by the ratio of the
swept mass and the ejected iron mass
・Ejected iron mass is assumed to be
MFe,ej = 0.1 , 0.05 and 0.5 Msun
[Fe/H]=log (x[Fe] / x[H]) – log (x[Fe] / x[H])sun
≈ -3.5 +log [ (Mej,Fe / 0.1Mo)/ (Msw/5x104Mo)]
-4.5< [Fe/H] < -2.5 metal deficient !
Summary
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Before Fragmentation Phase
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H2 and HD increase by about 103 times compared with primordial
composition because of re-ionization caused by SNR shock
Fragmentation (or low mass star formation) condition:
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Mhc = 1x106 Msun : fragmentation impossible
Mhc = 3x106 Msun : z>20 e0>3x1051
Fragments (formed low mass star) have the metal abundance of
-2.5~-4.5 ([Fe/H])
After Fragmentation Phase
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In both of cylindrical and spherical collapse, Only H2 and HD are
effective coolant
In both of cylindrical and spherical geometry, the low mass star
that survives up to now forms