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Interstellar Medium ASTR 2120 Sarazin

Interstellar Medium ASTR 2120 Sarazin - people.virginia.edupeople.virginia.edu/~cls7i/Classes/astr2120/Lecture22_ISM_Dust.pdf · Interstellar Medium Most of nearby material is in

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Interstellar MediumASTR 2120

Sarazin

Interstellar MediumPut you in contact with dead

relatives on other planetary systems?

NO

Interstellar MediumMost of nearby material is in starsInterstellar space nearly empty (more

than lab vacuum)<n> ~ 1 atom/cm3

But, not empty1. Gas2. Dust = small solid particles3. Relativistic matter

1. Light2. Cosmic rays = relativistic particles3. Magnetic fields

Interstellar MediumMost of nearby material is in starsInterstellar space nearly empty (more than lab

vacuum)<n> ~ 1 atom/cm3

But, not empty1. Gas2. Dust = small solid particles3. Relativistic matter

1. Light2. Cosmic rays = relativistic particles3. Magnetic fields

Dust1780 – W. Herschel – dark nebulae1847 – Struve – due to absorption, ~1 mag/kpc

1 kpc = 1000 pc1930 – Trumpler – uses

open star cluster sizes tomeasure distance, findsflux < L/(4pd2)

Dust ExtinctionExtinction = reduction of brightness due to

dust absorption and scattering

AV ≡ Vobs – Vemit in magnitudes

m = M + 5 log dpc – 5 + A

Reddening1940 – Stebbins & Whitford – photoelectric photometry,

reddening, extinction curveExtinction reddens starlight

distant red star

near blue star

redblue

flux

l

Extinction curveAl ~ 1/l → small, solid, dielectric particles = dust

grains

Al

l (µ)

Dust Extinction and Reddening

Dust Extinction and ReddeningAV ≡ Vobs – Vemit

m = M + 5 log dpc – 5 + A

EB-V ≡ (B – V)obs – (B – V)emit

AV ≈ 3 EB-V ≡ R EB-V

MV = −4.00, MB = −4.30 (Table A.5)EB−V = (B−V )obs − (B−V )em = (B−V )− (MB −MV )

= 0.90 − (−4.30 −−4.00) = 0.9 + 0.3 =1.20AV = R×EB−V = 3×1.2 = 3.6V = MV + 5 logdpc − 5 + AV

5 logdpc =V −MV + 5 − AV= 9.6 − (−4.0)− 3.6 + 5 =15

d =103 pc = 1 kpc

Example:B0 V (main sequence) star, observed with V =

9.60 and B-V = 0.90. Assume R = 3. What is distance (in pc)?

MV = −4.00, MB = −4.30 (Table A.5)EB−V = (B−V )obs − (B−V )em = (B−V )− (MB −MV )

= 0.90 − (−4.30 −−4.00) = 0.9 + 0.3 =1.20AV = R×EB−V = 3×1.2 = 3.6V = MV + 5 logdpc − 5 + AV

5 logdpc =V −MV + 5 − AV= 9.6 − (−4.0)− 3.6 + 5 =15

d =103 pc = 1 kpc

Example:B0 V (main sequence) star, observed with V =

9.60 and B-V = 0.90. Assume R = 3. What is distance (in pc)?

Dust CompostionUV 2200 Angstrom bump, graphite or other carbons

Infrared

Silicates 10, 18 µ

Ices 3.1 µ (H2O, NH3, etc.), dense cold clouds

Polycyclic Aromatic Hydrocarbons (PAHs)

Optical - UV Extinction Curves

Dust Scattering

Dust Scattering

Scattering = Blue Sky

Dust TemperaturesDust not evaporate

T ≲ 1000 K

Heating from star light

T ≳ 10 K

Dust Infrared Emissionl = 0.3 cm / T Wien LawT ~ 10 – 1000 K

l = 300 to 10 microns

absorbed starlight re-radiated in IR

Orion – visible and IR

Interstellar Medium - GasASTR 2120

Sarazin

Interstellar GasISM 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

Neutral, Atomic Hydrogen (H I, Ho)

~50% of mass in local ISM

Interstellar Clouds

Warm Neutral Intercloud Medium

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

How to see?

Interstellar Absorption Lines

Narrower than stellar lines

→ Cold neutral gas

Most atomic lines in UV, can�t be seen except from space

Neutral Atomic Hydrogen Gas21 cm Hyperfine Line of Hydrogen1944 – van de Hulst predicts 21 cm line of atomic H1951 – Ewen & Purcell detect

21 cm Hyperfine Line of HydrogenDE = 6 x 10-6 eVn = DE/h = 1.42 x 109 Hz = 1420 MHz

= 1.42 GHzl = c/n= 21.1 cm3/4 of atoms in upper state, 1/4 in lower state1 decay per 1.1 x 107 years

n=1 gs

n=2

n=3

3

1

p e

hydrogen

Neutral Atomic Hydrogen Gas21 cm Hyperfine Line of Hydrogen

Just Counts H atoms!!Image of Milky Way

Molecular Gas1969 – H2CO detected (Snyder, others)1970 – CO (3 mm) detected in radio

Molecular make emission lines due to rotation, in radio

Molecular Gas H2

~50% of mass

Often in dense clouds

n(H2) ~ 105 molecules/cm3

T ~ 10 K

AV > 10

> 100 molecules now known

Orion: visible and CO

Photo-ionized HydrogenAlways 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

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

HgHa

Hb

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

hydrogencontinuum

e-

Ha

Lya

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 GasGenerally at T ~ 106 to 108 K

Due to shocks from SNe and stellar winds

Supernova remnants

Diffuse hot gas

Cas A – Chandra X-ray