Low-Metallicity Star Formation:
From the First Stars to Dwarf Galaxies
Proceedings IAU Symposium No. 255, 2008
L.K. Hunt, S. Madden & R. Schneider, eds.

c 2008 International Astronomical Union
doi:10.1017/S1743921308024903

Evolution of newly formed dust in
Population III supernova remnants andits impact on the elemental composition ofPopulation II.5 stars
Takaya Nozawa1,2 , Takashi Kozasa1 , Asao Habe1 , Eli Dwek3 ,Hideyuki Umeda4 , Nozomu Tominaga5 , Keiichi Maeda2,6 , andKen’ichi Nomoto2,4,71

Department of Cosmosciences, Graduate School of Science, Hokkaido University,Sapporo 060-0810, Japanemail: tnozawa@mail.sci.hokudai.ac.jp
2Institute for the Physics and Mathematics of the Universe, University of Tokyo,
Kashiwa, Chiba 277-8568, Japan
3Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center,Greenbelt, MD 20771, USA
4
Department of Astronomy, School of Science, University of Tokyo,
Bunkyo-ku, Tokyo 113-0033, Japan
5
Division of Optical and Infrared Astronomy, National Astronomical Observatory of Japan,
Mitaka, Tokyo 181-8588, Japan
6
Max-Planck-Institut für Astrophysik, 85741 Garching, Germany
7Research Center for the Early Universe, School of Science, University of Tokyo,Bunkyo-ku, Tokyo 113-0033, JapanAbstract. We investigate the evolution of dust formed in Population III supernovae (SNe) byconsidering its transport and processing by sputtering within the SN remnants (SNRs). We findthat the fate of dust grains within SNRs heavily depends on their initial radii ain i . For Type II
SNRs expanding into the ambient medium with density of nH , 0 = 1 cm−3 , grains of ain i < 0.05 µm
are detained in the shocked hot gas and are completely destroyed, while grains of ain i > 0.2 µm
are injected into the surrounding medium without being significantly destroyed. Grains withain i = 0.05–0.2 µm are finally trapped in the dense shell behind the forward shock. We showthat the grains piled up in the dense shell enrich the gas up to 10−6 –10−4 Z⊙ , high enough toform low-mass stars with 0.1–1 M⊙ . In addition, [Fe/H] in the dense shell ranges from −6 to−4.5, which is in good agreement with the ultra-metal-poor stars with [Fe/H] < −4. We suggestthat newly formed dust in a Population III SN can have great impact on the stellar mass andelemental composition of Population II.5 stars formed in the shell of the SNR.
Keywords. dust, extinction, supernovae: general, hydrodynamics, shock waves, stars: abundances, stars: chemically peculiar, methods: numerical

1. Introduction
The first dust in the universe plays critical roles in the subsequent formation processesof stars and galaxies. Dust grains provide additional pathways for cooling of gas in metalpoor molecular clouds through their thermal emission and formation of H2 molecules on
the surface (e.g., Cazaux & Spaans 2004). In particular, the presence of dust decreases
the value of the critical metallicity to 10−6 –10−4 Z⊙ (Omukai et al. 2005; Schneideret al. 2006; Tsuribe & Omukai 2006), where the transition of star formation mode frommassive Population III stars to low-mass Population II stars occurs. Since absorption and254

Evolution of dust in Pop III SNRs and its impact on Pop II.5 stars

255

thermal emission by dust grains strongly depend on their composition, size distribution,and amount, it is essential to clarify the properties of dust in the early epoch of theuniverse, in order to elucidate the evolutionary history of stars and galaxies.
Dust grains at redshift z > 5 are considered to have been predominantly produced insupernovae (SNe). Theoretical studies have predicted that dust grains of 0.1–2 M⊙ and10–60 M⊙ are formed in the ejecta of primordial Type II SNe (SNe II, Todini & Ferrara
2001; Nozawa et al. 2003) and pair-instability SNe (PISNe, Nozawa et al. 2003; Schneider
et al. 2004), respectively. However, the newly formed dust is reprocessed via sputteringin the hot gas swept up by the reverse and forward shocks that are generated by theinteraction between the SN ejecta and the surrounding medium. Thus, the size and massof the dust can be greatly modified before being injected into the interstellar medium(ISM, Bianchi & Schneider 2007; Nozawa et al. 2007).
Here we present the results of the calculations for the evolution of newly formed
dust within Population III SN remnants (SNRs), based on the dust formation modelby Nozawa et al. (2003). We investigate the transport of dust and its processing by sputtering in the shocked hot gas, and report the size and amount of dust injected from SNe
into the ISM. It is also shown that a part of the surviving dust grains are piled up inthe dense SN shell formed behind the forward shock and can enrich the gas in the denseshell up to 10−6 –10−4 Z⊙ . We suppose that newly condensed dust in the SN ejecta hassignificant influence on the elemental abundances of Population II.5 stars, that is, thesecond-generation stars formed in the dense shell of Population III SNRs.

2. Evolution of Dust in Population III SNRs
We first briefly describe the models for the evolution of dust within SNRs. The time
evolution of a SNR is numerically solved by assuming spherical symmetry. We adopt
the hydrodynamic models of Population III SNe II with progenitor masses of Mpr = 13,20, 25, and 30 M⊙ and explosion energy of 1051 ergs by Umeda & Nomoto (2002) forthe initial condition of the gas in the ejecta. For the ambient medium, we consider auniform medium with a hydrogen number density of nH,0 = 0.1, 1, and 10 cm−3 . For themodel of dust in the He core, we adopt the dust grains formed in the unmixed ejecta byNozawa et al. (2003). Treating dust grains as test particles, we calculate the destruction
and dynamics of dust by taking into account the size distribution as well as the spatial
mass distribution of each grain species.
The results of the calculations are shown in Figure 1. Figure 1a shows the trajectoriesof C, Mg2 SiO4 , and Fe grains within the SNR for Mpr = 20 M⊙ and nH,0 = 1 cm−3 , andFigure 1b shows the time evolution of their radii relative to the initial ones. The positionsof the forward shock, the reverse shock, and the surface of the He core are depicted by
the thick solid lines in Figure 1a. Initially, newly formed dust grains are expanding withthe cool gas in the ejecta, and thus they undergo neither gas drag nor processing bysputtering. However, once they intrude into the hot gas swept up by the reverse shock
penetrating into the ejecta, they acquire the high velocities relative to the gas and areeroded by kinetic and/or thermal sputtering. The evolution of dust grains after colliding
with the reverse shock heavily depends on their initial radii aini and compositions.
For example, C grains formed in the outermost region of the He core encounter thereverse shock at 3650 yr. Since the deceleration by the gas drag is more efficient forsmaller grains, C grains with aini = 0.01 µm quickly slow down. These small grains are
eventually trapped in the hot gas (106 K) generated from the passage of the reverse andforward shocks and are completely destroyed by thermal sputtering. C grains of aini = 0.1µm reduce their sizes by sputtering but cannot be completely destroyed. These grains

256Nozawa et al.

Figure 1. (a) Spatial evolution of C, Mg2 SiO4 , and Fe grains within the SNR for Mp r = 20 M⊙
and nH , 0 = 1 cm−3 and (b) the temporal evolution of their radii relative to the initial values.
The evolution of dust with ain i = 0.01, 0.1, and 1 µm is shown by the dotted, dashed, and solidlines, respectively. The thick solid lines in (a) indicate the positions of the forward shock, thereverse shock, and the surface of the He core.

are finally captured in the dense SN shell formed behind the forward shock at ∼ 2 × 105
yr, where the gas temperature is too low (<105 K) to erode the dust grains by thermal
sputtering. C grains with aini = 1 µm, which are not efficiently decelerated by the gas
drag, can pass through the forward shock front and are injected into the ambient medium
without being significantly destroyed.
Mg2 SiO4 grains, which are formed in the O-rich layer, collide with the reverse shock at
about 6000 yr, but the dependence of their evolution on the initial radius is the same as
for C grains. On the other hand, Fe grains formed in the innermost region of the ejecta
hit the reverse shock after 13000 yr, and its 0.1 µm-sized grains are injected into theambient medium because of the high bulk density.
As shown above, the small dust grains formed in the SN ejecta are predominantly
destroyed by sputtering within the SNR. Thus, the size distribution of the surviving
dust is dominated by larger grains, compared to the sizes at formation. Note that theevolution of dust within SNRs and thus the resulting size distribution of dust does notdepend on the progenitor mass considered here, because their explosion energies are
the same and the time evolution of the gas temperature and density within SNRs aresimilar. On the other hand, the ambient gas density strongly affects the evolution ofdust in SNRs. The higher density in the ambient medium results in the higher densityof the shocked gas and causes the efficient erosion and deceleration of dust due to more
frequent collisions with the hot gas. Therefore, the initial radius below which dust iscompletely destroyed increases with increasing the ambient gas density and is 0.01, 0.05,
and 0.2 µm for nH,0 = 0.1, 1, and 10 cm−3 , respectively. As a result, the total mass of thesurviving dust is smaller for the higher ambient density and ranges from 0.01 to 0.8 M⊙
for nH,0 = 10 to 0.1 cm−3 , depending on the size distribution of dust formed in each SN.

Evolution of dust in Pop III SNRs and its impact on Pop II.5 stars

257

Table 1. Metallicities, [Fe/H], and abundances of C, O, Mg, and Si relative to Fe in the denseshell of primordial SN II remnants for various ambient gas densities.
Mp r (M⊙ )log(Z/Z⊙ )[Fe/H][C/Fe]

nH , 0 = 0.1 cm
13202530

−5.89
−5.44
−5.55
−5.33

−6.43
−5.20
−5.90
−5.56

[O/Fe][Mg/Fe][Si/Fe]−0.699−0.595−1.42−0.043−0.2300.034−0.500
0.739

1.92
0.410
−0.5520.866−0.5550.585−0.4120.242−0.459
1.16
0.407
1.09

1.01
1.87
0.989
1.26

−2.54
−2.15
0.122
0.375

−3.89
−1.80
0.232
−1.23

0.599
2.14
2.34
2.66

−3

−0.274
0.117
1.11
0.566

nH , 0 = 1 cm−31320
2530

−4.72
−4.68
−4.79
−4.60

−5.15−5.53
−5.23
−5.11

1.11
0.992
1.090.797

nH , 0 = 10 cm−313202530

−4.40
−4.09
−3.91
−3.84

−4.13
−4.92
−5.10
−5.11

0.284
0.946
1.60
−0.207

3. Metallicities and Elemental Abundances of Population II.5 Stars
In this section we discuss the influence of dust on the elemental composition of Population II.5 stars that are expected to form in the dense shell of Population III SNRs
(Mackey et al. 2003; Salvaterra et al. 2004; Machida et al. 2005). As shown in the lastsection, the dust grains surviving the destruction but not injected into the ISM are piled
up in the dense SN shell after 105 –106 yr. This implies that the elemental composition ofthese piled-up grains can play an important role in the elemental abundances of Population II.5 stars. Furthermore, the existence of dust in the shell may enable the formationof stars with solar mass scales through its thermal emission if the gas is enriched tothe critical metallicities (Omukai et al. 2005; Schneider et al. 2006; Tsuribe & Omukai2006). Thus, we calculate the metallicities and metal abundances in the dense shellbased on the elemental composition of piled-up grains, and compare with the observed
abundance patterns of low-mass hyper-metal-poor (HMP) and ultra-metal-poor (UMP)
stars.
Table 1 summarizes the calculated metallicities and elemental abundances in the denseshell of SN II remnants as a function of the ambient gas density. It should be noted thatthe metallicity in the shell ranges from 10−6 to 10−4 Z⊙ , which is considered to cause theformation of stars with 0.1–1 M⊙ . In addition, most of the calculated [Fe/H] abundances
are in the range of −6 to −4.5, which is in good agreement with those for HMP andUMP stars. We also plot in Figure 2a the abundances of C, O, Mg, and Si relative toFe in the shell for nH,0 = 0.1 and 1 cm−3 . We can see that the calculated abundances ofMg and Si showing 1–100 times overabundances are consistent with the observations ofHMP and UMP stars. Because the elemental composition of dust piled up in the shell canreproduce the abundance patterns of Fe, Mg, and Si in HMP and UMP stars, it is considered that the transport of dust separated from metal-rich gas within SNRs can be attributed to the elemental compositions of HMP and UMP stars, if they are Population II.5stars.
However, as can be seen from Figure 2a, no model considered here can reproduce
102 –104 times excesses of C and O observed in HMP stars. One of the reasons is that258

Nozawa et al.

Figure 2. Abundances of C, O, Mg and Si relative to Fe in the dense shell of SN II remnants fornH , 0 = 0.1 and 1 cm−3 (filled triangles); (a) derived from the elemental composition of the grains
piled up in the shell, and (b) derived from the elemental composition of the piled-up grains andthe gas outside the innermost Fe layer. For observational data of HMP and UMP stars, the 3-D
corrected abundances are adopted and are denoted by plus (HE0107-5240 with [Fe/H] = -5.62,
Collet et al. 2006), asterisk (HE1327-2326 with [Fe/H] = -5.96, Frebel et al. 2008), and cross(HE0557-4840 with [Fe/H] = -4.75, Norris et al. 2007).

in the calculation we assumed that the metal-rich gas in the SN ejecta does not mix
with the gas in the shell. Then we examine the abundance patterns in the shell by assuming that besides the piled-up grains, the gas outside the innermost Fe layer in theejecta is incorporated into the shell. The results are shown in Figure 2b. In this case
we can reproduce the very large overabundances of C and O, but the excesses of Mg
and Si are too large (100 times) to agree with the observations. However, it could bepossible to reproduce the abundance patterns of refractory elements observed in HMP
stars unless the Si-Mg-rich layer is mixed into the shell. Unfortunately, it is still being
debated to what extent the gas in the ejecta can mix into the SN shell when Population
II.5 stars form. Nevertheless, we can conclude that newly formed dust in a Population III SN can have great impact on the stellar mass and metal abundance of Population II.5 stars, if the metal-rich gas is not significantly incorporated into the dense gasshell.
Acknowledgements
This work has been supported in part by a Grant-in-Aid for Scientific Research fromthe Japan Society for the Promotion of Sciences (18104003, 19740094).
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Evolution of dust in Pop III SNRs and its impact on Pop II.5 stars
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