ISM & AstrochemistryLecture 1

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.

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)

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

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

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

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

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

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

Dark Interstellar Clouds

Infrared and radio telescopes are best used to study star formation

Infrared image

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

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

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

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)

Protoplanetary DisksObserved directly around low-mass protostars

PPD Schematic

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