2014/03/20

種族III巨大質量星の赤色超巨星星風中における炭素質ダスト形成
Formation of Carbon Dust in Red-supergiantWinds of Very Massive Population III Stars

野沢 貴也（Takaya Nozawa）
Kavli IPMU, University of Tokyo
Collaborators:
Yoon, S.-C. (SNU), Maeda, K. (Kyoto University.),
Kozasa, T. (Hokkaido University), Nomoto, K. (K-IPMU),
Langer, N. (Bonn University)

1-1. Sources of dust in the early unvierse
・ Origin of massive dust at high redshifts (z > 5)
- core-collapse supernovae (CCSNe) may be promising sourcesof dust grains (e.g., Todini & Ferrara 2001; TN+2003; Dwek+2007)- the contribution from AGB stars is also invoked to explain theobserved dust mass (e.g., Valiante+2009; Dwek & Cherchneff 2011)➔ what stellar mass range can mainly contribute dust
budget in the early universe depends on the stellar IMFHirano+2014

・ Typical mass of Pop III stars➔ Pop III stars may be much more
massive than Pop I/II stars- ~40 Msun (Hosokawa+2011; Susa 2013)- >300 Msun (Omukai+2003; Ohkubo+2009)- 10-1000 Msun (Hirano+2014)

1-2. Very massive Population III stars
・ Role of very massive stars (MZAMS > ~250 Msun)- emitting numerous ionizing photons➔ reionization of the universe- finally collapsing into black holes➔ serving as seeds of SMBHs

・ Evolution of massive Pop III stars- non-rotating stars with MZAMS > 250Msun
undergo convective dredge-up of C andO during the RSG phase (Yoon+2012)

Yoon+2012- enriching the surrounding medium
with CNO through the RSG winds➔ serving as formation sites of dust
m500vk00

Dust grains formed in the winds are not
likely to be destroyed by the SN shocks

3-1. Model of Pop III red-supergiant winds・ RSG model: m500vk00 (Yoon+2012)- MZAMS = 500 Msun (no rotation)- L = 107.2 Lsun, Tstar = 4440 K, Rstar = 6750 Rsun
- AC = 3.11x10-3, AO = 1.75x10-3 ➔ C/O = 1.78, Z = 0.034

・ Model of circumstellar envelope- spherically symmetry, constant wind velocity- density profile:- temperature profile:・ Fiducial values of Mdot and Vw- wind velocity: vw = 20 km/s
- mass-loss rate: Mdot = 0.003 Msun/yr➔ losing 90% (208 Msun) of envelope during 7x104 yr

3-2. Chemical equilibrium calculations
major carbon-bearing gas
species other than CO:- atomic carbon
at T > ~1800K- C2H molecules
at T = 1400-1700 K## Formation of PAHs would not## be expectedchemical reactions considered in this study- Dust formation calculations (TN & Kozasa 2013)
formulation of non-steady-state dust formation

4-1. Results of dust formation calculations
behavior of dust formation

final size distribution of dust

thick: Model A
thin: Model B

thick: Model A
thin: Model B- carbon grains form around r = 7.5 Rstar (r = 12 Rstar) for
Model A (Model B)- final condensation efficiency is unity for both of the models- final average radius is similar in both Model A and Model B the results are almost independent of chemical reactions

4-2. Dependence on Mdot and vw∝ Mdot0.88

solid: Model A
dashed Model B∝ vw-1.75- The condensation efficiency of dust is unity for the condition;- for the fiducial case (Mdot = 3x10-3 Msun/yr, vw=20 km/s, fc=1)
 1.7 Msun of C grains is produced over the lifetime of the RSG5-1. How efficient is dust formation?
・ Dust ejection efficiency by very massive Pop III RSGs
- XVMS = Mdust / MZAMS < 3.4x10-3 = ~0.3 %- Mdust / Mmetal < 0.24

・ Dust ejection efficiency by CCSNe- XCCSN = (0.1-30)x10-3 = 0.1-3.0 %- Mdust / Mmetal = 0.01-0.25 (Mdust / Mmetal < 0.15)

Hirano+2014## The ranges above reflects the destruction## efficiency of dust by the reverse shock
CCSNe

If NVMS ~ NCCSN in the Pop III IMF …
➔ The contribution of dust from verymassive RSGs is comparable with,or even higher than that from CCSNe(XVMS NVMS) / (XCCSN NCCSN) > ~1VMS5-2. Expected extinction curves
SMCextinction curves for high-z QSOs

MWGallerani+2010- Extinction curves derived in this study do not resemble any of
the known extinction law such as those in the MW and SMC- The extinction curves observed for high-z quasars do not show
a bump structure, being inconsistent with those given here➔ The derived extinction curves can be powerful tools to probe
the formation of C grains in very massive Pop III stars

5-3. Composition of low-mass UMP stars- The ultra-metal-poor (UMP) stars with [Fe/H] < -4 would record
chemical imprints of Population III stars- The formation of such low-mass metal-poor stars is triggered
through the cooling of gas by dust produced by Pop III SNe(e.g., Schneider+2012a, 2012b; Chiaki+2014)

・ Possible channel for C-rich UMP star formation- Very massive Pop III RSGs are sources of carbon grains aswell as CNO elements➔ In the gas clouds enriched by Pop III RSGs, carbon grains
enable the formation of CNO-rich low-mass stars- We do not predict the presence of heavier elements (Mg, Si, Fe)➔ Further observations and more quantitative theoreticalstudies are needed to show whether UMP stars formed
through our scenario## SMSS J0313-6708: [C/H] = -2.6, [Fe/H] < -7 (Keller+2014)

6. Summary
We have examined the possibility of dust formation in a carbon-richmass-loss wind of a Pop III RSG with M ZAMS = 500 Msun

- For a steady stellar wind, C grains can form with a lognormal-likesize distribution whose average radius is sensitive to wind velocity- The condensation efficiency is unity for➔ the first dust grains in the universe ??
- The mass of C grains is <1.7 Msun （Mdust/MZAMS < 3.4x10-3), which
would be high enough to have impacts on dust enrichment history
in the early universe, if the IMF of Pop III stars were top-heavy# The extinction curves expected from ejected C grains are different fromany known ones
# The chemical feedback by PopIII VMSs predicts a new type of UMP stars