Elemental Composition of Hyper-Metal-Poor Stars Formed in Dust-Enriched
Dense Shells of Population III Supernova Remnants
Takaya Nozawa1 (takaya.nozawa@ipmu.jp)
T. Kozasa2, K. Nomoto1, K. Maeda1, N. Tominaga3, H. Umeda4
1Kavli IPMU, University of Tokyo, 2Hokkaido University, 3Konan University, 4University of Tokyo〇 Abstract
Dust grains in the early universe are considered to be important agents to trigger the formation of low-mass stars in metal-poor environments(e.g., Omukai 2000; Schneider et al. 2003). The condition that the dust-induced fragmentation occurs depends on dust-to-gas mass ratio as wellas size distribution of dust (Omukai et al. 2005; Schneider et al. 2006; 2012a; 2012b). We investigate the origin of dust gra ins in metal-poor starforming clouds and the feasibility of the grain growth through accretion of refractory elements in the collapsing dense cores .
In Nozawa et al. (2007), we found that a fraction of dust grains formed in the ejecta of Pop III supernovae (SNe) are trapped in the dense shells
behind the forward shocks. Here we present the abundances of refractory elements in the dust-enriched SN shells, finding that the abundance
patterns are in good agreement with that observed for a Galactic halo star, SDSS J102915+172927 (Caffau et al. 2011). On the other hand, oursimple analysis of grain growth shows that the growth of dust can activate even for the metallicity as low as Z = 10 -5 Zsun before dust emission
cooling causes cloud-fragmentation. This implies that the grain growth could have potentials to affect the formation of the first low-mass stars.

〇 Dust Evolution in Pop III SNe (Nozawa et al. 2007)－ Dust model: dust formed in the unmixed ejecta (Nozawa et al. 2003)－ The fates of dust grains depend on their initial radii, aini (Figure 1)
・ aini < 0.05 μm: completely destroyed in the shocked hot gas
・ 0.05 μm < aini < 0.2 μm: trapped in the shells behind the forward shocks
・ aini > 0.2 μm: injected into the ISM without being eroded significantly➔ A part of newly formed grains is piled up in the dense SN shells !

〇 Elemental abundances in the dust-enriched dense SN shells
Abundance patterns of C, O, Mg, and Si relative to Fe in the SN shells
are in good agreement with that in SDSS J102915+17927 (Figure 2)

Fig. 1 – Time evolutions of (a) positions and (b) ratios of grain
radii to the initial ones for a SNR model of MZAMS = 20 Msun andnH,0 = 1.0 cm-3. The trajectories of the forward and the reverse
shocks are depicted by the thick red curves in (a). Dust species
are C, Mg2SiO4, and Fe with the initial radii of 0.01 μm (dotted),
0.1 µm (solid) and 1.0 µm (dashed).

Fig. 2 – Abundances of C, O, Mg, and Si relative to Fe in thedense shells of Pop III SNRs with MZAMS = 13, 20, 25, and 30
Msun for nH,0 = 0.1 cm-3 (triangles) and nH,0 = 1.0 cm-3 (circles).
The abundance pattern of a halo star SDSS J102915+172927(Z < 4.5x10-5 Zsun) is plotted by the red crosses (Caffau et al.
2011). The open triangles and circles are, respectively, theresults for MZAMS = 30 Msun and nH,0 = 0.1 cm-3, and MZAMS =25 Msun and nH,0 = 1.0 cm-3, which show good agreements withthe observed composition of SDSS J102915+172927.

〇 Is grain growth possible in metal-poor star-forming clouds?
－ Grain growth in dense clouds collapsing with the free-fall timescale
・ single-component dust, Fe and Si, with a single initial radius, r i,0・ initial dust abundance fi,0 (number fraction of element i originally contained in dust)－ Basic equations
・ mass conservation
・ growth rate of grain radius－ Results of calculations of grain growth (Figures 3 and 4)
・ Grain growth is efficient even in the gas clouds with [Fe, Si/H] ~ -5
・ For lower [X/H] and lower fi,0, grain growth proceeds at higher densities

Fig. 3 – Time evolution of the condensation efficiency f(t)(red) and grain radius r(t) (blue) through the growth of Fe
grains with the initial dust abundance f Fe,0 = 0.1 and theinitial grain radius r Fe,0 = 0.01 µm as a function of number
density of hydrogen c H(t). Dot-dashed, solid, and dashedcurves give the results for the collapsing clouds with theFe abundance of [Fe/H] = -3, -4, and -5, respectively.

Fig. 4 – Time evolution of the condensation efficiency f(t)(red) and grain radius r(t) (blue) through the growth of Si
grains of the initial radius r Si,0 = 0.01 µm in the collapsingcores with the Si abundance of [Si/H] = -4. Solid, dashed,
and dot-dashed lines correspond to the results for theinitial dust abundance of f Si,0 = 0.1, 0.01, and 0.001,respectively.

Fig. 5 – Critical abundances of Fe (upper) and Si (lower) above
which the grain growth becomes effective before dust emission
cooling causes the gas fragmentation, as a function of the initialdust abundance fi,0. The initial grain radius is set as r i,0 = 0.01
μm. Dot-dashed, solid, and dashed curves give the abundancesat which the condensation efficiency reaches f i,* = 0.8, 0.5, and0.25, respectively, at the density c H,* = 1012 cm-3.

Fig. 6 – Products (SD) of dust-to-gas mass ratio D andcross section per unit dust mass S for Fe (blue) and Si
grains (red) as a function of the initial dust abundance f i,0.
Dashed lines depict (SD)i,crit corresponding to the criticalabundances given in Figure 5, whereas solid lines display(SD)i,* when the condensation efficiency reaches f i,* = 0.5
through the grain growth in dense clouds. The horizontal
green dot-dashed line is the minimal criterion above which
the dust cooling causes the fragmentation into low-mass
clumps (Schneider et al. 2012a).

〇 Critical abundance and Dust-to-gas mass ratio－ Critical abundances, Ai,crit, above which grain growth becomes effective(Figure 5)
where Ki is a function of fi,0 and fi,* (initial and final condensation efficiencies)－ Dust-to-gas mass ratio corresponding to the critical abundance is well
below the criterion for the dust-induced fragmentation (Figure 6)－ While, dust-to-gas mass ratio after grain growth can exceed this criteria➔ The increase of dust mass due to grain growth can cause the cloudfragmentation as long as the above critical abundance is fulfilled〇 Discussion－ Abundance patterns of the SN shells enriched with SN dust match with that in SDSS J102915+17927➔ SDSS J102915+17927 might be the second-generation star formed in the dense shell of Pop III SN－ Even if the initial dust-to gas mass ratio is below the criterion for the dust-driven fragmentation, thegrain growth increases the dust mass in dense clouds and can drive the formation of low-mass stars➔ It is not always required that a significant fraction of dust grains formed in SNe should survive

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