Interstellar Medium ASTR 2120 Sarazin -...
Transcript of Interstellar Medium ASTR 2120 Sarazin -...
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
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
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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)?
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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)
Dust Infrared Emissionl = 0.3 cm / T Wien LawT ~ 10 – 1000 K
l = 300 to 10 microns
absorbed starlight re-radiated in IR
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
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
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