Dust Evolution and Growth in the Interstellar Medium Hiroyuki - - PowerPoint PPT Presentation

Dust Evolution and Growth in the Interstellar Medium

Hiroyuki Hirashita

(ASIAA, Taiwan)

• 1. Importance of Grain Size Distribution
• 2. Grain Processing in the ISM
• 3. Grain Growth in the dense ISM
• 4. Summary

Topics

• 1. Introduction

Dust extinction in the optical (λ ~ 0.5 µm) Dark clouds/lanes in the Milky Way ← Dust extinction (= absorption + scattering) τλ: optical depth for extinction (absorption + scattering) Iλ(τλ) = Iλ(0) e-τλ [mλ(obs) = mλ(0) + Aλ in mag.] Extinction Curve: τλ = σλNdust (or Aλ ) as a function of λ

Extinction Curves and Grain Properties

Pei (1992): For nearby galaxies

Fitting: Grain size distribution n(a) ∝ a – 3.5

amin = 0.005 µm amax = 0.25 µm

τλ/τ0.44µm

(abs + sca)

Extinction curves reflect the grain species and size distribution.

UV Opt IR Milky Way Large Magellanic Cloud Small Magellanic Cloud See also Mathis et al. (1977; MRN) Weingartner & Draine (2001), etc.

What Determines the Grain Size Distribution?

Answer: Lifecycle of Dust in the ISM

grain-grain collision by turbulence

The Purpose of This Talk

Evolution of the evolution of grain size distribution in the ISM. Particular focus: Grain growth in the dense ISM

• 2. Dust Processing in the

ISM

AGB SN Dust Gas

SN destruction

Dust Low V

Accretion Dense


High V

Shattering Diffuse

Solve the dust enrichment for each gas particle

Stellar Coagulation

Kuan-Chou Hou

Evolution of Grain Size Distribution

0.1 Gyr 0.3 Gyr 1 Gyr 3 Gyr 10 Gyr

• Stellar dust

production

• Shattering
• Accretion
• Coagulation

One-zone galaxy evolution model

Hirashita & Aoyama (2018) Asano et al. (2013)

• 3. Grain Growth in the Dense ISM

X X

grain accretion

gas-phase metals

grain grain grain coagulation

Accretion

important mechanism of dust mass increase

˙ m = 4πa2ntmtSacc ✓kBTgas 2πmt ◆1/2

nt: number density of dust-composing material [nt ~ Z(mH/mt)] mt: atomic mass of dust-composing material Sacc: sticking efficiency Tgas: gas temperature

τacc = m/ ˙ m = 4 × 107 yr ✓ a 0.1 µm ◆ ✓ Z Z ◆1 ⇣ nH 102 cm3 ⌘1 ✓ Tgas 50 K ◆1/2

Small grains grow more quickly. ρd(m, t): grain mass distribution m = 4πa3s/3

∂ρd(m, t) ∂t

• = − ∂

∂m[ ˙ mρd(m, t)] + ˙ m mρd(m, t)

Coagulation

Grain-grain collision rate “Smoluchowski equation”

∂ρd(m, t) ∂t

• = −mρd(m, t)

Z ∞ α(m1, m)ρd(m1, t)dm1 + Z ∞ Z ∞ α(m1, m2)ρd(m1, t)ρd(m2, t)µfrag(m; m1, m2)dm1dm2

Grain velocity (induced by (MHD) turbulence)

Yan et al. (2004); Ormel et al. (2009); Hoang et al. (2011); etc.

Grain motion is coupled on a scale l: l ~ vτd = (vas)/(cgρg) v ∝ l1/3 ⇒ v ∝ a1/2 Larger grains are coupled with larger-scale gas motions, which have larger velocities.

Grain Growth on Grain Size Distribution

Hirashita & Voshchinnikov (2014) accretion coagulation

Grain Growth vs. Extinction Curves

Grain growth by accretion ⇒ Steepens Coagulation (grain-grain sticking) ⇒ Flattens the extinction curve.

accretion + coagulation accretion coagulation Hirashita & Voshchinnikov (2014)

RV = AV /(AB – AV): Flatness of extinction curve in the optical Extinction curve becomes flatter as dust growth occurs. Fraction of metals in dust

Extinction Curve vs. Depletion

Hirashita & Voshchinnikov (2014)

Correlation between Extinction Features

Hirashita & Voshchinnikov (2014)

Evolution of Polarization Curve

Voshchinnikov & Hirashita (2014)

Voshchinnikov & Hirashita (2014) dust growth

Effects of Growth on K–λmax Plane

µm-Sized Grains in Dense Cores

Coreshine: Shining at ~ 3 µm in dense molecular cores Interpreted as scattering by μm grains

Steinacker et al. (2010)

Production of large grains (a > 0.5 µm)

a = 0.5 µm

3.6 µm 4.5 µm 8 µm

How fast can coagulation occur?

µm-Sized Grains in Dense Cores

The typical size of the interstellar dust grains is 0.1 µm. ⇒ Growth by coagulation up to 1 µm? The timescale of grain growth puts an constraint to the lifetime of molecular cloud cores:

• tcoag > tff ⇒ Favors those formation scenarios that

involve persistent support against gravity.

• tcoag <~ tff ⇒ Favors rapid star formation.

Hirashita & Li (2013)

Three Models

(1) Standard silicate model: Apply coagulation threshold velocity of silicate. (2) Sticky coagulation model: Grains always stick without any velocity threshold. (3) Maximal coagulation model: Apply 5 × cross section for coagulation to consider non-compact aggregates (Ormel et al. 2009). (3) gives the lower limit for the coagulation timescale.

Results

Evolution of grain size distribution for the three models

Hirashita & Li (2013) Standard silicate model Coagulation stops because velocities exceed the threshold. Maximal coagulation model Even in this case, it takes several tff to produce µm grains. Sticky coagulation model Coagulation can proceed beyond 0.1 µm. It takes very long time to produce µm-sized grains.

Constraint on the Lifetime

Success diagram of the grain growth to 1 µm ○: Success ×: Failure At a typical density of molecular cloud cores ~ 105 cm-3, it takes 5 tff to produce 1-µm grains. Molecular clouds are long-lived objects with lifetimes > several free-fall times

Hirashita & Li (2013)

Even mm-Sized Grains!

Existence of mm-sized grains in the envelopes (~104–5 cm-3)

• f Class I protostars

Miotello et al. (2014)

λ = 1.1 mm λ = 3 mm

mm-Sized Grains in Protostellar Envelopes

Existence of mm-sized grains in the envelopes (~104–5 cm-3) of Class I protostars mm-sized grains are difficult to form in ~ 105 cm-3; require ~1010 cm-3.

Wong, Hirashita, & Li (2016)

Possibility of Transport by Outflow

Simulation: outflow with ~ 1 km/s Machida & Hosokawa (2013) 1D calculation with a constant wind speed ~ 1 km/s (drag vs gravity): Wong, Hirashita, & Li (2016)

Fraction of Destruction by Shattering

mm-sized grains survive (destroyed fraction is <~ 0.1) after being injected into the envelope.

Wong, Hirashita, & Li (2016) Shattered fraction

Collaboration with Astrochemistry

Harada, …, Hirashita, et al. (2017)

Grain growth is also checked with chemistry. Relevant lines for Band 1 SO: 36.2 GHz SO2: 44.1 GHz

Grain growth

• 4. Summary

Grain Growth and Extinction/Polarization Curves

• Accretion and coagulation cause different effects on the

extinction curve.

• Model explains the observed variations in extinction

curves and depletion. µm−mm sized grains

• 1. Existence of µm-sized grains in dense molecular cores

means that they are sustained against free fall.

• 2. mm sized grains are difficult to form in situ. They may

be transported from stellar vicinity (proto-planetary disk?) to the envelope.

Thank you.