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

Dust Evolution and Growth in the Interstellar Medium Hiroyuki Hirashita (ASIAA, Taiwan)

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

1. Introduction Dust extinction in the optical ( λ ~ 0.5 µ m) Dark clouds/lanes in the Milky Way ← Dust extinction (= absorption + scattering) τ λ : optical depth for extinction (absorption + scattering) I λ ( τ λ ) = I λ (0) e - τ λ [ m λ (obs) = m λ (0) + A λ in mag.] Extinction Curve : τ λ = σ λ N dust (or A λ ) as a function of λ

Extinction Curves and Grain Properties Pei (1992): For nearby galaxies Fitting: Milky Way Grain size distribution n ( a ) ∝ a – 3.5 a min = 0.005 µ m Large Magellanic Cloud a max = 0.25 µ m τ λ / τ 0.44 µ m Extinction curves reflect the grain (abs + sca) Small Magellanic Cloud species and size IR Opt UV distribution . 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

Dense 
 AGB Accretion Dust Low V Stellar Coagulation Solve the dust enrichment for each gas particle Gas Dust Diffuse Shattering High V SN destruction SN Kuan-Chou Hou

Evolution of Grain Size Distribution One-zone galaxy evolution model • Stellar dust 10 Gyr production • Shattering 3 Gyr • Accretion • Coagulation 1 Gyr 0.3 Gyr 0.1 Gyr Hirashita & Aoyama (2018) Asano et al. (2013)

3. Grain Growth in the Dense ISM gas-phase metals grain grain X X grain grain accretion coagulation

Accretion important mechanism of dust mass increase  ∂ρ d ( m, t ) � m ρ d ( m, t )] + ˙ = − ∂ m ∂ m [ ˙ m ρ d ( m, t ) ∂ t ρ d ( m , t ): grain mass distribution n t : number density of dust-composing m = 4 π a 3 s /3 material [ n t ~ Z ( m H / m t )] ◆ 1 / 2 m t : atomic mass of ✓ k B T gas m = 4 π a 2 n t m t S acc dust-composing ˙ material 2 π m t S acc : sticking efficiency T gas : gas temperature ◆ ✓ Z ⌘ � 1 ✓ T gas ◆ � 1 / 2 ◆ � 1 ⇣ ✓ a n H m = 4 × 10 7 yr τ acc = m/ ˙ 10 2 cm � 3 0 . 1 µ m Z � 50 K Small grains grow more quickly.

Coagulation “Smoluchowski equation” Z ∞  ∂ρ d ( m, t ) � = − m ρ d ( m, t ) α ( m 1 , m ) ρ d ( m 1 , t )d m 1 ∂ t 0 Z ∞ Z ∞ + α ( m 1 , m 2 ) ρ d ( m 1 , t ) ρ d ( m 2 , t ) µ frag ( m ; m 1 , m 2 )d m 1 d m 2 0 0 Grain-grain collision rate 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 ~ v τ d = ( vas )/( c g ρ g ) v ∝ l 1/3 ⇒ v ∝ a 1/2 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 Hirashita & Voshchinnikov (2014) accretion + coagulation accretion coagulation Grain growth by accretion ⇒ Steepens Coagulation (grain-grain sticking) ⇒ Flattens the extinction curve.

Extinction Curve vs. Depletion Hirashita & Voshchinnikov (2014) Fraction of metals in dust R V = A V /( A B – A V ): Flatness of extinction curve in the optical Extinction curve becomes flatter as dust growth occurs.

Correlation between Extinction Features Hirashita & Voshchinnikov (2014)

Evolution of Polarization Curve Voshchinnikov & Hirashita (2014)

Effects of Growth on K – λ max Plane Voshchinnikov & Hirashita dust growth (2014)

µm-Sized Grains in Dense Cores Coreshine: Shining at ~ 3 µm in dense molecular cores 3.6 µm 4.5 µm 8 µm Interpreted as scattering by μ m grains Steinacker et al. (2010) a = 0.5 µm Production of large grains ( a > 0.5 µm)

µm-Sized Grains in Dense Cores Hirashita & Li (2013) The typical size of the interstellar dust grains is 0.1 µm. ⇒ Growth by coagulation up to 1 µm? How fast can coagulation occur? The timescale of grain growth puts an constraint to the lifetime of molecular cloud cores: • t coag > t ff ⇒ Favors those formation scenarios that involve persistent support against gravity. • t coag <~ t ff ⇒ Favors rapid star formation.

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 Hirashita & Li (2013) Evolution of grain size distribution for the three models Maximal coagulation Standard silicate model Sticky coagulation model model Coagulation stops Coagulation can proceed Even in this case, it because velocities beyond 0.1 µm. It takes takes several t ff to exceed the threshold. very long time to produce µm grains. produce µm-sized grains.

Constraint on the Lifetime Hirashita & Li (2013) Success diagram of the grain growth to 1 µm ○ : Success × : Failure At a typical density of molecular cloud cores ~ 10 5 cm -3 , it takes 5 t ff to produce 1-µm grains. Molecular clouds are long-lived objects with lifetimes > several free-fall times

Even mm-Sized Grains! Existence of mm-sized grains in the envelopes (~10 4–5 cm -3 ) of Class I protostars λ = 3 mm λ = 1.1 mm Miotello et al. (2014)

mm-Sized Grains in Protostellar Envelopes Wong, Hirashita, & Li (2016) Existence of mm-sized grains in the envelopes (~10 4–5 cm -3 ) of Class I protostars mm-sized grains are difficult to form in ~ 10 5 cm -3 ; require ~10 10 cm -3 .

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

Fraction of Destruction by Shattering Wong, Hirashita, & Li (2016) Shattered fraction mm-sized grains survive (destroyed fraction is <~ 0.1) after being injected into the envelope.

Collaboration with Astrochemistry Harada, …, Hirashita, et al. (2017) Grain growth is also checked with chemistry. Relevant lines for Band 1 SO: 36.2 GHz SO 2 : 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 .