Download - ISM & Astrochemistry Lecture 1
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ISM & AstrochemistryLecture 1
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Interstellar Matter
• Comprises Gas and Dust
• Dust absorbs and scatters (extinguishes) starlight
Top row – optical images of B68
Bottom row – IR images of B68
Dust extinction is less efficient at longer wavelengths
– Astrochemistry is the study of the synthesis of molecules in space and their use in determining the properties of Interstellar Matter, the material between the stars.
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Interstellar Gas
• HII (H+) Region – T ~ 104 K, n ~ 10-2 cm-3
Surround hot starsPhotoionised by stellar UV photonsMajor ion is H+. Other ions:He+, C+, N+, O+, etcNo photons with energy > IP(H) =
13.6 eV permeate the ISM.
Triffid Nebula (M20)
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Interstellar Gas• Coronal Gas - T ~ 106 K, n ~ 10-2 cm-3
Detected in UV observations of highly ionised atoms
such as OVI (O5+) which has IP = 114 eV.Fills about 20% of volume of the Galaxy.Thought to be produced by interacting supernova
remnants.
SN Remnant in the Large Magellanic Cloud
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Diffuse Interstellar CloudsTemperature: 80-100K
Density: 102 cm-3
Slab-like, thickness ~ 1019 cm
Clouds permeated by UV radiation
- with photon energies less than IP(H)
Carbon is photoionised
f(e-) ~ 10-4
Cloud mostly atomic
f(H2) < 0.3
Few simple diatomics – CO, OH, CH, CN, CH+
f(M) ~ 10-6-10-8 The Pleiades
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Interstellar Gas
• Giant Molecular Clouds (GMCs)T ~ 10-50 K, n ~ 105 - 107 cm-3, <n> ~ 6 102 cm-3
Material is mostly molecular. About 100 molecules detected. Most massive objects in the Galaxy.
Masses ~ 1 million solar masses, size ~ 50 pc
Typically can form thousands of low-mass stars and several high-mass stars.
Example – Orion Molecular Cloud, Sagittarius,
Eagle Nebula
1 pc = 3.1 1018 cm = 3.26 light years
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Interstellar GasThe Orion Nebula
Optical picture of Orion Nebula (M42)
Image of the Orion Molecular Cloud in Carbon Monoxide – size 30 light years (2 times larger in the sky than the full moon
The Constellation of Orion
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Star-Forming Hot CoresDensity: 106 - 108 cm-3
Temperature: 100-300 K
Very small UV field
Small saturated molecules: NH3, H2O, H2S, CH4
Large saturated molecules: CH3OH, C2H5OH, CH3OCH3
Large deuterium fractionation
Few molecular ions - low ionisation ?
f(CH3OH) ~ 10-6
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Dark Interstellar Clouds
• Dark Clouds - T ~ 10 K, n ~ 1010 - 1012 m-3
Not penetrated by optical and UV photons. Little ionisation. Material is mostly molecular, dominant species is H2. Over 60 molecules detected, mostly via radio astronomy.
Masses 1 – 500 solar masses, size ~ 1-5 pcTypically can form 1 or a couple of low-mass
(solar mass) stars.
B68: A dark cloud imaged in the IR by the VLT
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Dark Interstellar Clouds
Infrared and radio telescopes are best used to study star formation
Infrared image
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Interstellar Dust
• Interstellar extinction
- absorption plus scattering
- UV extinction implies small (100 nm) grains
- Vis. Extinction implies normal (1000 nm) grains
- n(a)da ~ a-3.5da
- Silicates plus carbonaceous grains
- Mass dust/Mass gas ~ 0.01
- Dense gas – larger grains with icy mantles
- Normal – nd/n ~ 10-12
- Within interstellar clouds, characterise extinction of UV photons by the visual extinction, AV, measured in magnitudes
- Iλ = I0λexp(-Aλ)
The interstellar extinction curve
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Interstellar Ices
Mostly water ice
Substantial components:
- CO, CO2, CH3OH
Minor components:
- HCOOH, CH4, H2CO
Ices are layered
- CO in polar and non-polar
ices
Sensitive to f > 10-6
Solid H2O, CO ~ gaseous H2O, CO
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Interstellar Gas Phase Abundances
H 1.0(D 1.6e-5)He 0.1C 0.000073N 0.00002O 0.00018S <1e-6Mg, Si, Fe, < 1e-9
IS Gas is oxygen-rich – O/C > 1
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Evolved StarsIRC+10216 (CW Leo)• Nearby (~130 pc) high mass-
loss carbon star (AGB)• Brightest object in the sky at 2
microns – optically invisible• Carbon dust envelope detected
out to 200’’ = 25,000 AU ( ~ 1 lt yr)
• Molecular shells at ~ 1000 - 4000 AU
• >60 molecules detected: CO, C2H2, HC9N ...
• Newly discovered anions C8H-,
C6H-, C4H- , C3N- , C5N-, CN-
• Recent detections of H2O, OH and H2CO
Figures from Leao et al. (2006) Lucas and Guelin et al. (1999)
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Protoplanetary DisksObserved directly around low-mass protostars
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PPD Schematic
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Protoplanetary Disks
Thin accretion disks from which protostar forms
Inflow from large radii (300 AU) onto central protostar
Temperature of outer disk is cold (10 K)
n(H2) ~ 105 – 1014 cm-3
Molecular gas is frozen on to dust grains in outer disk
Temperature of inner disk is ~ 100 K at 10 AU, ~1000 K at 1 AU
Ices evaporate in inner disk