Interstellar Medium 2ASTR 2110

Sarazin

Homework #11 Not due until Monday, December 4

Interstellar Medium 2ASTR 2110

Sarazin

Dust Temperatures

€

dEdt

"

# $

%

& ' absorbed

=dEdt

"

# $

%

& ' emitted

F ≡ flux of starlight (erg/cm2 /sec)dEdt

"

# $

%

& ' absorbed

= (πa2 )Fεabsorption

dEdt

"

# $

%

& ' emitted

= (4πa2 )σT 4εemission

Blackbody = perfect emitter and absorber εabsorption = εemission =1Real grains, ε ≈ a/λ

T ≈ 30 Ferg/cm2 /sec"

# $

%

& '

1/5

K

Far from star, F ≈ 0.02 erg/cm2/sec backgroundT ≥10 K

Dust Temperatures Dust not evaporate

T ≲ 1000 K

Heating from star light

T ≳ 10 K

Dust Infrared Emission λ = 0.3 cm / T Wien Law T ~ 10 – 1000 K

λ = 300 to 10 microns absorbed starlight re-radiated in IR

Orion – visible and IR

Galactic IR Emission

Origin of Dust Grains

nucleated in stellar outflows (winds, PNe, giants,

novae, SNe, . . .)

Ices freeze onto dust in cold clouds

destroyed in shocks, star formation, in hot gas

expect continuum, molecules → grains (PAHs)

Relation to Gas

strong correlation = mixed well together

NH ≡ ∫ nH dl = number density of H atoms x distance

NH ≈ 2 x 1021 cm-2 AV (mag)

~1% of mass of ISM in grains

Grains not mainly H, He

mass grains ~ mass of heavy elements (~2%)

much of heavy elements are in grains

Elemental Depletions in Gas in ISM

Gas element depletions

Volitiles (C, N, O, . . .) ~ 50%

Refractories (Fe, Al, . . .) > 90% depleted

Interstellar Medium Most of nearby material is in stars Interstellar space nearly empty (more than lab

vacuum) <n> ~ 1 atom/cm3

But, not empty 1.  Gas 2.  Dust = small solid particles 3.  Relativistic matter

1.  Light 2.  Cosmic rays = relativistic particles 3.  Magnetic fields

Interstellar Medium - GasASTR 2120

Sarazin

Interstellar Gas ISM consists of different phases of gas, different

temperatures and densities

Classify based on physical state of hydrogen

Molecular H2

Atomic H I = H0

Ionized H II = H+

<nH> ~ 1 atom/cm3

Very inhomogeneous

Almost no gas at <nH>

Neutral, Atomic Hydrogen (H I, Ho)

~50% of mass in local ISM

Interstellar Clouds

Warm Neutral Intercloud Medium

Most is fairly cold (~100 K), atoms in ground state, no normal atomic emission lines

How to see?

Interstellar gasAtomic Hydrogen HI

1904 – Hartmann – Interstellar absorption lines

Interstellar Absorption Lines •  Stationary lines from

binary stars

•  Narrower than stellar lines

•  Increase with distance

•  〈v〉 = ½ v(star)

→ Cold neutral gas

Most atomic lines in UV, can’t be seen except from space

Neutral Atomic Hydrogen Gas 21 cm Hyperfine Line of Hydrogen 1944 – van de Hulst predicts 21 cm line of atomic H 1951 – Ewen & Purcell detect

p e p e

21 cm Hyperfine Line of Hydrogen ΔE = 6 x 10-6 eV ν = ΔE/h = 1.42 x 109 Hz = 1420 MHz = 1.42 GHz λ = c/ν= 21.1 cm 3/4 of atoms in upper state, 1/4 in lower state 1 decay per 1.1 x 107 years

n=1 gs

n=2

n=3

3

1

p e

hydrogen

21 cm Hyperfine Line of Hydrogen

Great thing about 21 cm line: It simply counts hydrogen atoms!

Neutral Atomic Hydrogen Gas 21 cm Hyperfine Line of Hydrogen 1944 – van de Hulst predicts 21 cm line of atomic H 1951 – Ewen & Purcell detect

Atomic Hydrogen Standard Clouds

nH ~ 20 atoms/cm3

T ~ 100 K

D ~ 10 pc

Neutral Intercloud Gas

nH ~ 0.3 atoms/cm3

T ~ 2000 K

Molecular Gas Three kinds of energy levels and lines •  Electronic transitions

–  Just like individual atoms –  Mainly in UV and optical

•  Rotational transitions –  Mainly in mm and submm radio –  Except H, in IR

•  Vibrational Transitions –  Mainly in mid-IR –  Except H, in near IR

C O+ -

C O+ -

C O+ -

e

Molecular Gas 1969 – H2CO detected (Snyder, others) 1970 – CO (3 mm) detected in radio

Molecular Gas H2

~50% of mass

Often in dense clouds

n(H2) ~ 105 molecules/cm3

T ~ 10 K

AV > 10

> 200 molecules now known

Orion: visible and CO

Molecular Masers Tiny sources of VERY bright molecular radio

lines

If thermal, would require T > 1012 K

OH, H2O, SiO

Require very high densities and/or strong radiation fields

need protostars or old stars (giants, supergiants)

Photo-ionized Hydrogen

Always at T ~ 104 K

H II regions around OB stars

Planetary nebulae (?)

Diffuse ionized gas

Ionized Hydrogen (H II, H+)

Emission Nebulae 1930’s – Strömgren,

Menzel, Baker, Goldberg, . . . emission nebulae = photoionized hydrogen

gas

Why are H II regions sharply defined?

O, B stars make UV which ionizes H.

hν > 13.6 eV, λ < 912 Å = 91.2 nm

Optical Depth of ISM to these photons

If neutral, all UV absorbed in l << 0.05 pc, then stays neutral

If ionized, (nHI/nH) << 1, can stay ionized

€

σ ≈ 7×10−18 cm2

τ = nHoσ l = n

H

nHo

nH

σ l ≈ 20nH

1 cm-3

'

( )

*

+ , nHo

nH

'

( ) )

*

+ , , l

pc'

( )

*

+ ,

OB stars **

HII region, ionized

HI neutral

Narrow boundary

Size of H II regions

All ionizing UV photons absorbed in region

Radius = RS “Strömgren radius”

# absorptions = # ionizations = # recombinations

RS

*

€

e− + p+ ↔Ho + photon# recombinations

volume• time∝ nenp =α nenp

α ≈ 3×10−13 cm3 /sPure H, ionized, ne = np = nH# recombinations

sec=α nH

2 4π3RS

3

Q∗ ≡# ionizing photons

sec from star(s)

Q∗ =α nH2 4π

3RS

3

Emission nebulae: Emission lines from atomic hydrogen H I, helium He I

Why atomic H in ionized H region? Made by recombination, H+ + e- → Ho

Hγ Hα

Hβ

Emission nebulae: Emission lines from atomic hydrogen H I, helium He I

Why atomic H in ionized H region? Made by recombination, H+ + e- → Ho

n=2

n=3

n=1 gs

hydrogen continuum

e-

Hα

Lyα

Emission nebulae: Emission lines from atomic hydrogen H I, helium He I

Why atomic H in ionized H region? Made by recombination, H+ + e- → Ho

Forbidden lines of common elements (O, N, etc.) but only occur at very low densities, never in lab

O III = O+2

O II = O+

Collisionally Ionized Gas Generally at T ~ 106 to 108 K Due to shocks from SNe and stellar winds

Supernova remnants

Diffuse hot gas

Cas A – Chandra X-ray

Tycho – Chandra X-ray

Interstellar clouds ISM is inhomogeneous, gas and dust in clouds 1930’s – Beals and others

Pressure Equilibrium n increases as T decreases

n T ≈ constant

pressure equilibrium?

AB

PA PB

Supernova remnants

Tranquil or Violent ISM?

Tranquil (Swan Lake): Gas slowly cools, condenses into clouds,

clouds make stars, stars die, add to ISM. All near equilibrium.

Violent (We Will Rock You): Supernova blast through ISM, crunch gas

into clouds, clouds make stars, OB stars make SN.

Tranquil or Violent ISM?

Tranquil (Swan Lake): Gas slowly cools, condenses into clouds,

clouds make stars, stars die, add to ISM. All near equilibrium.

Violent (We Will Rock You): Supernova blast through ISM, crunch gas

into clouds, clouds make stars, OB stars make SN.