2014/11/18

星間ダストの形成・破壊過程とサイズ分布の進化(Formation and destruction processes of interstellar dust
grains and evolution of their size distribution)

野沢 貴也 （Takaya Nozawa）
National Astronomical Observatory of Japan

Collaborators:
Asano, S. Ryosuke (Nagoya University)
Hirashita, Hiroyuki (ASIAA)
Takeuchi, T. Tsutomu (Nagoya University)

References
・ Asano, Takeuchi, Hirashita, Nozawa (2013, MNRAS, 432, 637)
・ Asano, Takeuchi, Hirashita, Nozawa (2014, MNRAS, 440, 134)
・ Nozawa, Asano, Hirashita, Takeuchi (2014, in press, arXiv:1410.7861)

Outline
1. Introduction- Aim of this study and extinction curves

2. Physical processes of dust
3. Model and Results- Evolution of extinction curves in galaxies- Extinction curve in the Milky Way

4. Extinction curves at high-z quasars

1. Introduction

1-1. Dust alters the SEDs of galaxies

Noll+2009

Takeuchi+2005

Dust grains absorb UV/opticalphotons and re-radiate thermal
emission at infrared wavelengths
70% of the star formation activity
at 0.5 <z< 1.2 is obscured by dust

1-2. Life-cycle of interstellar matter (dust)

植田稔也さんのスライドより

1-3. Aim of our study
to correct the obscuration by dust, many (observational) studies
have assumed the MW, SMC, LMC extinction curves or the Calzettiextinction (attenuation) law➔ however, the size distribution of interstellar dust must change
as the galaxy evolves

we construct the evolution model of grain size distribution, withthe aim at understanding how the extinction curve is modified in
the course of galaxy evolution
・ dust processes considered in our works- production of dust in SNe II and AGB stars- destruction of dust by interstellar shocks- grain growth due to metal accretion in molecular clouds- shattering and coagulation due to grain-grain collisions

1-4. Extinction curves in the Milky Way〇 MRN dust model(Mathis, Rumpl, & Nordsieck 1977)

・ composition :silicate and graphite
・ size distribution :n(a) ∝ a^{-q} with q=3.5,0.005 μm ≤ a ≤ 0.25 μm○ CCM relation(Cardelli, Clayton, & Mathis 1989)・ small grains (small Rv)➜ steep extinction curve
・ large grains (large Rv)➜ flat extinction curve## RV = AV / (AB – AV)

2. Physical processes of dust

2-1. Properties of dust ejected from SNe II-P
Nozawa+07, ApJ, 666, 955

SNe II

total mass of dust surviving thedestruction in Type II SNRs;
0.07-0.8 Msun (nH,0 = 0.1-1 cm-3)
size distribution of dust after theshock-destruction is domimatedby large grains (> 0.1 μm)

2-2. Properties of dust ejected from AGB stars
solid: carbon
dashed: silicate
dotted: the others

Zhukovska & Gail 2008; Valiante+2009
SiC grainmass of dust grains injectedfrom a AGB star into the ISM:
0.001-0.01 Msun (Mstar > 1 Msun)
average radius of dust ejectedfrom AGB stars is likely to belarge (a = 0.1-1.0 μm)
Yasuda & Kozasa 2012

2-3. Destruction of dust in SN shocks
sputtering

projectiles: H and He

sputtering: ejection of atoms
from dust surface

・ erosion rate by sputtering: da/dt ~ 10-6 (nH / 1.0 cm-3) μm yr-1
(e.g., Nozawa+2006, ApJ, 648, 435)

・ timescale of dust destruction by SN shocks, τdest
τdest = [ (1/Mdust)(dMdust/dt) ]-1 = [ ε Mswept RSN / Mgas ]-1~ 5x108 yr (ε / 0.3)-1 (Mswept / 3000 Msun)-1 (RSN / 0.02 yr-1)-1x (Mgas / 1010 Msun)

2-4. Growth of dust in molecular clouds
grain growth

grain growth:
accretion of gas-phase heavy
elements onto pre-existing dust

Sirono+2013

・ timescale of grain growth, τgrow
τgrow = [ (1/mdust)(dmdust/dt) ]-1 = [ (1/3a) αs nmetal V0 <v> ]-1~ 2x107 yr (αs / 0.2)-1 (a / 0.01 μm) (Z / 0.02)-1 (ngas / 30 cm-3)-1➔ grain growth is more efficient for a higher gas density, a higher
metallicity (higher abundance of metals), and a smaller grain## grain growth is more efficient for a large surface-to-volume
ratio of dust grains

2-5. Examples of change of size distribution (1)Modification of grain size distribution
due to sputtering in SN shocks

Nozawa+2006

Modification of grain size distribution
due to grain growth

Chiaki. Marrasi, TN+2014

2-6. Shattering and coagulation of dust
shattering・ shattering at vrel > vshat
where vshat = 1-3 km/s

・ coagulation at vrel < vcoag
where vcoag = 0.001-0.1 km/s
in the interstellar turbulence, vrel
is higher for a lower gas density
and a larger grain radius (Yan+2004)
Hirashita & Yan+2009

These processes do not reduce dust
mass but change size distribution

coagulation

・ timescale of shattering, τshat
τshat ~ 1 (τSF / Gyr)1/2 GyrSirono+2013## grain-grain collision processes## becomes efficient once dust## grains are enriched sufficiently

2-7. Examples of change of size distribution (2)Modification of grain size distribution
due to shattering

Modification of grain size distribution
due to coagulation

Hirashita, TN+2010

Hirashita, 2012

3. Model and results

3-1. Dust evolution model in a galaxy (1)‐one-zone closed-box model (no inflow and no outflow)‐star formation rate (SFR)
Schmidt law with n = 1: SFR(t) = Mgas(t)/τSF with τSF = 5 Gyr‐initial mass function (IMF)
Salpeter IMF: φ(m) = m-q with q=2.35 for Mstar = 0.1-100 Msun
‐two dust species
graphite (carbonaceous grains)astronomical silicate (silicate and the other grains species)‐two-phase ISM
WNM (warm neutral medium): T = 6000 K, n = 0.3 cm-3
CNM (cold neutral medium): T = 100 K, n = 30 cm-3
➜ ηWNM = ηCNM = 0.5

3-2. Dust evolution model in a galaxy (2)‐mass evolution of dust ΔMd(a,t) with radii between a and a+da
xSFR(t), astration
dust production by SNe II and AGB stars

shock destruction
grain growth

shattering

coagulation

3-3. Evolution of extinction curves in galaxies
dust amount

grain size distribution

extinction curve

MRN

Asano, Takeuchi, Hirashita, TN+2013, 2014

‐early phase : formation of dust in SNe II and AGB stars➔ large grains (>0.1 μm） are dominant ➔ flat extinction curve

Asano+12‐middle phase : shattering, grain growth due to accretion of gas metal➔ small grains (< 0.03 μm） are produced ➔ steep extinction curve‐late phase : coagulation of small grains➔ shift of peak of size distribution ➔ making extinction curve flatter3-4. Reproducing the MW extinction curve
steep extinction curve is due to the presence of too much small grains‐no grain growth in CNM➜ producing the MW-likeextinction curve➜ not explaining the total
mass of dust‐efficient coagulation➜ producing flatter extinction curve than original➜ still too steep to be consistent with MW curve
Asano, Takeuchi, Hirashita, TN+14, MNRAS, 440, 134

3-5. Reproducing the MW extinction curve‐two-phase ISM
・ WNM (T = 6000 K, n = 0.3 cm-3)

WNM=0.5
CNM=0.3
MC=0.2

・ CNM (T = 100 K, n = 30 cm-3)‐three-phase ISM

・ WNM (T = 6000 K, n = 0.3 cm-3)
・ CNM (T = 100 K, n = 30 cm-3)・ MC (molecular clouds)➜ T = 25 K, n = 300 cm-3

Nozawa+2014- three-phase ISM model including the MC phase can reproduce
the average extinction curve in the MW- ISM phase is one of the important quantities in constructing theevolution model of interstellar dust

4. Extinction curve at high-z QSOs

4-1. Extinction curves in high-z quasars
SDSS J1048+4637 at z=6.2 :broad absorption line (BAL) quasars

Hirashita+2008

Maiolino+2004, Nature, 431, 533➔ interstellar dust in the early
universe is SN origin?

Gallerani+2010

4-2. A large amount of dust in high-z quasarsCalura+2014‐Huge amounts of dust grains are observed for the host galaxies ofquasars at z < 5➜ it is suggested that the grain growth is needed to account forsuch massive dust contents## it seems only the contribution of dust from SNe II cannot explain## the observed amount of dust grains in high-z quasars

How can we explain the dust mass and unusual extinction
curves observed for high-z quasars in a consistent way?

4-3. Explaining massive dust in high-z quasars
high-z quasar host: starburst galaxies➜ indicating a high fraction of MC
MH2/MH,total ~ 0.7-0.97 (Calura+2014)‐two-phase ISM:
WNM=0.3 and MC=0.7‐τSF = 0.5 Gyr

Grain growth is necessary toachieve the observed high D/GNozawa+2014

4-4. Explaining the high-z extinction curves
The presence/absence of 2175 A
bump may be related to the dust
composition of dust rather thanthe dust evolution model- graphite and silicate- amorphous carbon & silicate➜ the derived extinction curve
well match the observed highz extinction curve
Nozawa+2014

The origin of the 2175 A bump is still unclear➜ small size (<0.02 µm) of graphite? (e.g., Draine & Lee 1984)➜ PAHs (polycyclic aromatic hydrocarbon?) (e.g., Joblin+1992)
・ formation site of PAHs
- AGB stars? (bottom-up scenario) (e.g., Cherchneff+1993)- shattering of C grains? (up-down scenario) (e.g., Seok+2014)

5. Summary of this talkWe investigate the evolutions of grain size distribution
and the extinction curves in galaxies‐ early phase : large grains (>~0.1 μm) from SNe II and AGB stars➔ flat extinction curve‐ mid phase : small grains (<0.03 μm) via shattering/grain growth➔ steep extinction curve‐ late phase : shift of peak of size distribution due to coagulation➔ making extinction curve flatter・ the average extinction curve in the MW can be reproduced byour three-phase (WNM, CNM, MC phases) ISM model・ our model can explain the unusual extinction curves and large
amounts of dust grains observed for high-z quasars➜ a large fraction of MCs (>0.5), silicate & amorphous carbon