2013/10/29

Formation of Carbon Grains
in Red-Supergiant Winds ofVery Massive Population III stars
submitted to ApJ Letter

Takaya Nozawa (Kavli IPMU)
collaborators:
Yoon, S.-C. (SNU), Maeda, K. (Kyoto U.),
Kozasa, T. (Hokkaido U.), Nomoto, K. (K-IPMU),
Langer, N. (Bonn U.)

1-1. Sources of dust in the early unvierse
・ Origin of massive dust at high redshifts (z > 5)
- CCSNe may be promising sources ofdust grains (e.g., Dwek+2007)

Dwek & Cherchneff 2011

- the contribution from AGB stars is also
invoked to explain observed dust mass
(Valiante+2009; Dwek & Cherchneff 2011)

what stellar mass range can mainly
contribute dust budget in the early
universe depends on the stellar IMF
・ Typical mass scale of Pop III stars- ~40 Msun (Hosokawa+2011; Susa 2013)- >300 Msun (Omukai+2003; Ohkubo+2009)- 10-1000 Msun (Hirano+2013)

Hirano+2013

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 M ZAMS > 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

Newly formed grains are not likely
to be destroyed by the SN shock2-1. Model of 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:・ Free parameters- average mass-loss rate: Mdot = 0.003 Msun/yr➔ losing 90% (208 Msun) of envelope during 7x104 yr- wind velocity: vw = 20 km/s (also considering vw= 1-200 km/s)

2-2. Model of dust formation calculations
・ Formula of non-steady-state dust formation(Nozawa & Kozasa 2013, ApJ, 776, 24)
CMgSiO3

Λon = τsat/τcoll : ratio of supersaturation timescale to gas
collision timescale at the onset time (ton) of dust formation

Λon = τsat/τcoll ∝ τcool ngas

2-3. Chemical equilibrium calculations

・ Major carbon-bearing gas species other than CO- carbon atoms at T > ~1700K- C2H molecules at T = 1400-1700 K
chemical reactions considered in this study3-1. Results of dust formation calculations
behavior of dust formation

final size distribution of dust
thick line: Model A
thin line: Model B- carbon grains start to form at r = 7.2 Rstar (r = 11.5 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

3-2. Dependence on wind velocityΛon ∝ τcool ngas
- gas density: ngas ∝ 1/vw

- cooling time:Τcool = [ (1/T)(dT/dt) ]-1 ∝ 1/vw
, r = vw t

- The average grain radius is larger for a smaller vw, andscales as aave ∝ vw-1.75 for vw = 1-100 km/s
- The condensation efficiency is unity for vw = 1-100 km/s
 producing 1.7 Msun of C grains in total over thelifetime of the RSG

4-1. Discussion
・ Dust ejection efficiency by very massive Pop III RSGs
- XVMS = Mdust / MZAMS = 3.4x10-3 Msun
- Mdust / Mmetal = 0.24

・ Dust ejection efficiency by CCSNe (PISNe)- XCCSN = (0.1-30)x10-3 Msun (XPISN < 0.05)- Mdust / Mmetal = 0.01-0.25 (Mdust / Mmetal < 0.15)## depending on the destruction efficiency## of dust by the reverse shock

If NVMS ~ NCCSN in the Pop III IMF …
➔ The contribution of very massive RSGs
is comparable with, or even higher thanthat from CCSNe(XVMS NVMS) / (XCCSN NCCSN) > ~1Hirano+2013

4-2. Expected extinction curves
SMCMW

Gallerani+2010- Extinction curves derived in this study do not resemble any ofthe 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
➔ These extinction curves can be powerful tools to probe
the formation of C grains in very massive Pop III stars

4-3. Formation and composition of UMP stars- The ultra-metal-poor (UMP) stars with [Fe/H] < -4 show the large
excess of CNO- The formation of such low-mass metal-poor stars is believed tobe triggered through the cooling of gas by dust
(e.g., Schneider+2012; Chiaki+2013)

・ possible channel for 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 grainsenable the formation of low-mass stars whose chemicalcompositions are highly enriched with CNO- We do not predict the presence of any heavier elements➔ Further observations and more quantitative theoreticalstudies are needed to show whether any UMP stars haveformed through our scenario

5. Summary
We examine the formation of dust grains in a carbon-richmass-loss wind of a Pop III RSG with MZAMS = 500 Msun
・ For a steady stellar wind, carbon grains can form witha lognormal-like size distribution whose average radius
is sensitive to the wind velocity
・ As long as the mass-loss rate is as high as 10-3 Msun/yr,
the condensation efficiency is unity for vw = 1-100 km/s
・ The total mass of dust is 1.7 Msun, which would be highenough to have an impact on dust enrichment history
in the very early universe, especially if the IMF of Pop III
stars were top-heavy