formation condition and classification of extremely metal
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Formation Condition and Classification of
Extremely Metal-Poor Stars:
Absent Region in the A(C)-[Fe/H] Plane
Gen Chiaki (Georgia Tech)Nozomu Tominaga (Konan)
Takaya Nozawa (NAOJ)
Chiaki, Tominaga, & Nozawa (2018, MNRAS, 472, L115) arXiv: 1710.04365
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Sub-groups of CEMP starsYoon et al. (2016)
From SAGA database(Suda et al. 2008)
Group I
Group III
Group II
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Sub-groups of CEMP starsYoon et al. (2016)
From SAGA database(Suda et al. 2008)
Group I ([Fe/H]>−3)Relatively high Fe & CEnhancement of s-, r/s-elementsMost of stars are binary
Binary transfer of C and s-, r/s-elements
Group I
Group III
Group II
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Sub-groups of CEMP stars
From SAGA database(Suda et al. 2008)
Group I ([Fe/H]>−3)Relatively high Fe & CEnhancement of s-, r/s-elementsMost of stars are binary
Binary transfer of C and s-, r/s-elements
Group I
Group III
Group II
Group II, III ([Fe/H]<−3)Extremely Metal-PoorNo enhancement of s-, r/s-elementsFew of them are binaries
They would inherit one or several progenitors.
★SN
Yoon et al. (2016)
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Sub-groups of CEMP starsYoon et al. (2016)
From SAGA database(Suda et al. 2008)
Group II
Group III
CEMP-no starsmainly in [Fe/H]<-3consist of Groups II and IIIAnd what is the physical origin of the discrimination of Groups II and III?
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From SAGA database(Suda et al. 2008)
Our findings (1/3)Group III stars show large C-enhancement ([C/Fe] > 2)Group II stars show moderate C-enhancement ([C/Fe] < 2)
Group III
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Our findings (2/3)The distribution of Group II appears to continuously connectwith that of C-normal stars.
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Our findings (3/3)Stars with large C-enhancement show lower limit of Acr(C)~6
Stars with moderate C-enhancement show lower limit of [Fe/H]cr ~ −5
No stars have been found
Group III
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In this work, we reconsider the classification of EMP stars from a
theoretical point of view
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First, we focus on the lower-limits of A(C) & [Fe/H]
No stars have been found
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The lower-limits reflect dust amounts required for cloud fragmentation
Sufficient amount of dust
Unstable to fragment
★
low-massstellar cluster
★★
Multiple low-mass objectsStrong gas cooling bydust thermal emission
protostars
accretion disk
Insufficient amount of dust
Single massive object
★
Weak gas cooling byhydrogen molecules
Stable to fragment
(Omukai 2000; Schneider et al. 2003; Dopcke et al. 2011, 2013; Bromm et al. 2014; Safranek-Shrader 2014, 2016)
long-livedWe can observe
SN
short-livedcan not observe
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For the formation of C-enhanced/normal EMP stars,
carbon/silicate grains are importantCarbonaceous grainsComposition: C
Lodders & Amari (2005)
Silicate grainsComposition: O, Si, Mg, …
Moyce et al. (2015)
Carbon grain cooling
Low-mass fragmentation
Critical C abundance is defined
Silicate grain cooling
Low-mass fragmentation
Critical Mg and Si abundance is defined
Marassi et al. (2014)
Chiaki et al. (2015)
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★ ★★
Carbon grain rich
Cloud fragmentation to form low-mass stars by carbon grains
★ ★★
Silicate grain rich
Cloud fragmentation to form low-mass stars by silicate grains
★lack of both
single massive star
Only two classes where low-mass star formation is triggered by
carbon/silicate grain cooling
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Group III
Interestingly,For Group III star formation, carbon grains are dominant.For Group II and C-normal star formation, silicate are dominant.
Carbon grains dominant
Silicate grains dominantIneffective grain cooling
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★ ★★
Carbon grain rich
Cloud fragmentation to form low-mass stars by carbon grains
★ ★★
Silicate grain rich
Cloud fragmentation to form low-mass stars by silicate grains
★lack of both
single massive star
So, let’s estimate the boundary dividing EMP stars into two classes
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The condition established on this boundary is
★ ★★
Carbon grain rich
Cloud fragmentation to form low-mass stars by carbon grains
★ ★★
Silicate grain rich
Cloud fragmentation to form low-mass stars by silicate grains
★lack of both
single massive star
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We need estimate the carbon and silicate grain cooling efficiency
Carbon grain cooling efficiency = Silicate grain cooling efficiency
? ?
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To estimate the cooling efficiencies, we need dust properties:
condensation efficiency of metal into dustdust size
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To estimate the cooling efficiencies, we need dust properties:
condensation efficiency of metal into dustdust size
Metal
dust
Larger condensation efficiency → Larger cooling efficiency
Gas
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To estimate the cooling efficiencies, we need dust properties:
condensation efficiency of metal into dustdust size
Metal
Smaller dust size→ Larger cooling efficiency
Gas
Total grain cross-section increases.
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How can we determine these dust properties in the early Universe?
Direct measurement.→Too hard to directly measure them by observations.
Earlier studies estimate them by theoretical models.(Schneider et al. 2006; Chiaki et al. 2013, 2014, 2015; Marassi et al. 2014, 2015)
→Model dependent…
So, we here present a new way to estimate the dust properties.
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We pay attention to Acr(C) & [Fe/H]cr
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Λcarbon > Γcomp
Λcarbon = Γcomp
Λcarbon < Γcomp
Λsilicate < ΓcompΛsilicate > ΓcompΛsilicate = Γcomp
On the critical lines, energy balance equations are established
dust cooling rate Λdust = gas compressional heating Γcomp
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Energy balance equation on the critical lines
3XH μdustmHnH2vgas
rdust
Λdust = (2kTgas−2kTdust) (σdustvgas)ngasndust
cool4ςdust
fdust←metal≈
Dust cooling rate:
Gas compressional heating:
dust bulk density ~g cm−3
Hydrogen mass fraction molecular weight of dust
gas thermal velocitygrain cross section
condensation efficiency
average grain radius
ymetal
Γcomp =ngaskTgas
tcollapse
collapse timescale = free-fall time
metal number fraction
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Energy balance equationon the critical lines
3μdust XH
cool4ςdust rdust / fdust←metal
ymetal = 1.4×10−2 g cm−1
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Energy balance equationon the critical lines
cool4ςdust rdust / fdust←metal
ymetal = 1.4×10−2 g cm−1
Acr(C) ~ 6
[Fe/H]cr ~ −5
We can estimate grain size / condensation efficiency as
rcarbon / fcarbon←C ~ 10 μm
rsilicate / fsilicate←Mg ~ 0.1 μm
3μdust XH
We consider that they are valid for all EMP stars.
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We can reconstruct the critical lines with these dust properties.
3μcarbon XH
cool4ςcarbon rcarbon / fcarbon←C
yC +3μsilicate XH
cool4ςsilicate rsilicate / fsilicate←Mg
yMg = 1.4×10−2 g cm−1
In general, both carbon and silicate grains contribute to gas cooling. Therefore,
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3μcarbon XH
cool4ςcarbon rcarbon / fcarbon←C
yC +
rcarbon / fcarbon←C ~ 10 μm
rsilicate / fsilicate←Mg ~ 0.1 μm
3μsilicate XH
cool4ςsilicate rsilicate / fsilicate←Mg
yMg = 1.4×10−2 g cm−1
using
10[C/H]−1.93+10[Mg/H]= −4.70
We can reconstruct the critical lines with these dust properties.
In general, both carbon and silicate grains contribute to gas cooling. Therefore,
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3μcarbon XH
cool4ςcarbon rcarbon / fcarbon←C
yC +
rcarbon / fcarbon←C ~ 10 μm
rsilicate / fsilicate←Mg ~ 0.1 μm
3μsilicate XH
cool4ςsilicate rsilicate / fsilicate←Mg
yMg = 1.4×10−2 g cm−1
using
10[C/H]−1.93+10[Mg/H]= −4.70
from average [Mg/Fe] = 0.368 of EMP stars,
10[C/H]−2.30+10[Fe/H]= −5.07
We can reconstruct the critical lines with these dust properties.
In general, both carbon and silicate grains contribute to gas cooling. Therefore,
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10[C/H]−2.30+10[Fe/H]= −5.07
rcarbon / fcarbon←C ~ 10 μm
rsilicate / fsilicate←Mg ~ 0.1 μm
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Finally, the boundary of two regions with carbon and silicate is
3μcarbon XH
cool4ςcarbon rcarbon / fcarbon←C
yC =3μsilicate XH
cool4ςsilicate rsilicate / fsilicate←Mg
yMg
[C/Fe]= 2.30
Carbon grain cooling efficiency = Silicate grain cooling efficiency
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[C/Fe]= 2.30
10[C/H]−2.30+10[Fe/H]= −5.07
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Acr(C) & [Fe/H]cr suggest the larger carbon grains than silicate
rcarbon / fcarbon←C ~ 10 μm
rsilicate / fsilicate←Mg ~ 0.1 μm
We confirm it from a model calculation.
Pop II star-form. cloud
SN
Metal & Dust
Gas-phase metal condenses into dust in an expanding ejecta.
•Dust grains in the early universe are mainly supplied by supernovae (SNe) of first (Pop III) stars (Todini & Ferrara 2001).
•We calculate condensation of metals in an expanding ejecta.
•1D hydrodynamics calculationRadiative transferenergy deposition of 56Ni decay(Iwamoto et al. 2000)
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SN modelsCEMP stars - faint (SN) models (Umeda & Nomoto 2003)
COSiFe
ejected
fall-back
C-normal stars - normal type II
COSiFe
ejected
dust formation
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The model predicts larger carbon grains than silicate!
favored region for Keller star
favored region for Caffau star
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Summery•We reconsider the classification of EMP stars from a theoretical point of viewStars formed through cloud fragmentation induced by carbon grains ([C/Fe]>2)Stars formed through cloud fragmentation induced by silicate grains ([C/Fe]<2)
•From the critical abundances Acr(C) and [Fe/H]cr, we derive the dust properties in EMP star-forming clouds asrcarbon / fcarbon←C ~ 10 μm
rsilicate / fsilicate←Mg ~ 0.1 μm
•Using the dust properties, we reconstruct the critical condition for low-mass EMP star formation as10[C/H]−2.30+10[Fe/H]= −5.07
~~
•The boundary of two classes is estimated as[C/Fe]= 2.30
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