2012/09/19

極めて金属量の低い星形成ガス雲中でのダスト成長と低質量星の形成
Nozawa et al. (2012, ApJ, 756, L35)

野沢 貴也（Takaya Nozawa）
東京大学 国際高等研究所
カブリ数物連携宇宙研究機構
共同研究者:
小笹 隆司（北海道大学）
野本 憲一（カブリ数物連携宇宙研究機構）

1. Introduction〇 HMP ([Fe/H] < -5) and UMP ([Fe/H] < -4) stars(Christlieb et al. 2002; Frebel et al. 2005; Norris et al. 2007)

Are these metal-poor stars the first generation or not?
How were these metal-poor low-mass stars formed?
➔ large excess of C, N, and O (Z > 1.0x10-3 Zsun)〇 Critical metallicity, Zcrit－ metal-line cooling (Bromm & Loeb 2003; Frebel et al. 2007)➔ Zcrit = log10(10[C/H] + 0.3x10[O/H])－ dust-emission cooling (Schneider et al. 2003; Omukai et al. 2005)➔ Zcrit = 10-6-10-4 Zsun (depends on fdep = Mdust/Mmetal)〇 SDSS J102915+172927 with [Fe/H] = -4.99(Caffau et al. 2011, see also Caffau et al. 2012)➔ no excess of CNO (most primitive: Z < 4.5x10-5 Zsun)

2. Critical dust-to-gas ratio
Schneider et al. (2011)・ minimum condition above which
the dust-induced fragmentationtakes place ➔ D = Z fdep

S : cross section per unit dust mass
D : dust-to-gas mass ratio
based on their dust model

Dcrit = Z fdep = 4.4x10-9
The cloud-fragmentation depends on depletion factor fdep, and it
is suppressed when the reverse shock destroys too much dust

3. Birth conditions of SDSS J102915+172927
dust mass and composition

thermal evolution of clouds

Schneider et al. (2012)－ fragmentation occurs at nH = 1012-1014 cm-3 if fdep > 0.01
➔ if dust formation in SNe is less efficient or strong dustdestruction occur, only M > 8 Msun fragments can form

4. Aim of this study
Type II SN

dust
destruction

metal-poor gas clouds

formation oflow-M star

dust

dust formation

metalgas

grain growth➔ increase of dustto-gas mass ratio

formation ofhigh-M starformation oflow-M star

5. Model of grain growth in collapsing clouds
・ Time evolution of gas density (collapsing with free-fall time)

initial single radius: ri,0
initial dust abundance: fi,0abundance of element i: Ai
・ equation of mass conservation

・ growth rate of grain radiusfi(t) = cidust(t) / ci(t)Yi(t) = cigas(t) / ci(t)Xi(t) = ri(t) / ri,0－ grain species: Fe and Si grains (not consider C grains)－ gas temperature: Tgas = 1000 K, sticking probability: s = 1

6. Grain growth in metal-poor gas clouds
growth of Fe grains－ grain growth activates in thegas clouds with [Fe/H] = -5
－ the gas density at which fFe
reaches > 0.5 is higher for alower Fe abundance

growth of Si grains

As fSi,0 decreases,－higher gas densities areneeded to achieve fSi > 0.5－ final grain radii are larger
when all Si atoms are used7. Critical metal abundances
fragmentation of cloud occurs
at cH = 1012-1014 cm-3
➔ before reaching this density,
growth must be significant

critical metal abundances

・ critical metal abundance:
above which the grain growth canaffect formation of low-mass stars

where－ Acrit is higher for a higher fi,* and/or for a lower fi,0－ for fi,* = 0.5 and 0.001 < fi,0 < 0.1,-4.12 < [Fe/H] < -3.2, -4.6 < [Si/H] < -3.3
ref. [Si/H] = -4.27 for SDSS J102915+2729

8. Dust-to-gas mass ratio
dust-to-gas mass ratio

・ dust-to-gas mass ratio

・ total metallicity

R : mass ratio of refractory elements
to all heavy elements

formation of low-mass stars withZ < 4.5x10-5 Zsun can be possible?
－ (SD)i,crit is well below the minimum value for the dustinduced fragmentation, whereas (SD)i,* exceeds this value➔ grain growth enhances SD in the clouds and enable thegas fragmentation into sub-solar mass clumps9. Fragmentation by metal-line cooling
MgSiO3 grain

Chiaki, TN, Yoshida (2012, in prep.)

f = 0.1 w/o growthf = 0.1  f =1.0 w/ growthf = 1.0

10. Summary
・ Growth of Fe and Si grains can operate efficiently
even in collapsing clouds with [Fe, Si/H] ~ -5
・ The critical abundances above which grain growth
could induce the gas fragmentation is [X/H] ~ -4.5
・ Even if the initial dust-to-gas mass ratio is well below
the minimum value for dust-induced fragmentation,grain growth increases the dust mass high enough tocause the gas fragmentation into low-mass clumps

・ As long as the critical abundance is satisfied, grain
growth could play an important role in the formationof low-mass stars with metallicity as low as 10-5 Zsun