Molecules Between the Stars: The Astrochemistry of the ISM

Paul RuffleNational Radio Astronomy Observatory

Our Galactic Home

• We are in a galaxy whose projected image on the night sky is the Milky Way

• Dust in the Interstellar Medium (ISM) obstructs our optical view

• Our perspective is from a star in the outer regions of the Milky Way

Our Galactic Home

The Interstellar Medium (ISM)

• Average Density 1 particle cm–3

• Accounts for ~60% of Galactic mid-plane mass

• Gases:

‣ Hydrogen (92% by number)

‣ Helium (8%)

‣ Oxygen, Carbon, Nitrogen, etc (0.1%)

• Dust Particles:

‣ 1% of the mass of the ISM

‣ Silicates, silicon carbide, graphite, metal oxides

Properties of the ISM

State of Hydrogen Temperature

Densities (H cm–3)

Percent Volume

HII Regions & Planetary

NebulaeIonised 5,000 K 0.5 < 1%

Hot ISM Ionised 1,000,000 K 0.01 50%

Warm ISM Atomic 3,000 K 0.3 30%

Cold ISM Atomic 300 K 30 10%

Molecular Clouds Molecular < 30 K > 300 10%

Thermal Continuum RadiationCharacteristics

• Opaque “Black” Body

• Isothermal

• In Equilibrium (or LTE)

Planck’s Law

• I = Intrinsic Intensity (W m–2 Hz–1 sr–1)

• h = Planck’s Constant

• k = Boltzman’s Constant

• T in K

• ν in Hz

Rayleigh-Jeans Approximation

• Where hν << kT

Thermal Continuum Radiation

Relationship between mean photon energy and temperature given by

Eph,mean = 2.7kT where k = 8.6 × 10−5 eVK−1

inten

sity

X-ray ultraviolet infrared microwave radio wave!-ray2"

108 K

5000

K

2"

105 K

3 K200 K

wavelength10#15 m 10#12 m 1 nm 1$m 1 mm 1 m 1 km

visible

NGC 604 Giant HII Region

Emission nebula in the Triangulum Galaxy (M33). HST wide field and planetary camera 2

HII Regions & Planetary Nebulae

• Isolated regions where H is ionised (H+)

• HII Regions:

‣ UV from hot, blue stars (20,000 – 50,000 K)

‣ Young, massive and short-lived (< 106 years)

‣ HII regions have short lives

‣ Near regions where stars formed

• Planetary Nebulae (PNe):

‣ Evolved stars (white dwarf)

‣ Ejected stellar atmosphere

Omega Nebula HII Region

GBT Radio Image + DSS Optical Image + 2MASS IR Image

Planetary Nebulae

Bruce Balick, Howard Bond, R. Sahai, their collaborators, and NASA

Non-Thermal Continuum Radiation

Free-Free Emission

• Ionised regions: HII regions and Planetary Nebulae

• Free electrons accelerated by encounters with free protons

Non-Thermal Continuum Radiation

Synchrotron Radiation

• Free Electrons

• Magnetic Fields

• Discrete Sources:

Supernovae Remnants (SNR)

• General Interstellar Medium

• I ∝ να with α between –0.2 and –1.2

Spectral-Line Radiation

Recombination Lines

• Discovered in 1965 by Hogburn and Mezger

• Ionised regions (HII regions and Planetary Nebulae)

• Free electrons temporarily recaptured by a proton

• Atomic transitions between outer orbital (e.g. n = 177 to m = 176)

Spectral-Line Radiation

Hyperfine Transition of Hydrogen

• Discovered by Ewen and Purcell in 1951

• Found in regions where H is atomic

• Spin-flip (hyperfine) transition

‣ Electron and protons have “spin”

‣ Spins of proton and electron may be aligned or anti-aligned

‣ Aligned state has more energy

‣ Difference in Energy = hν (ν = 1420 MHz λ ≈ 21 cm)

‣ Aligned H atom will take 11 million years to flip the spin of the electron

‣ 1067 atoms in Milky Way – 1052 H atoms emit every second

Spectral-Line Radiation

What does it tell us?

• Number of emitting regions in that direction

• Frequency of centre of line → object’s velocity

• Doppler effect: νobs = νem / (1+ V/c)

• Width of line → motion of gas within the region

• Height of the line ∝ temperature of the gas

• Area under the line ∝ column density (cm–2)

Spectral-Line Radiation

Milky Way rotation and mass

For any cloud• Observed cloud velocity = difference

between projected Sun’s motion and projected cloud motion

For cloud B• Highest observed velocity along

line of site

• Vrot = Vobs + Vsun sin(L)

• R = Rsun sin(L)

Repeat for cloud B with new angle L• Determine Vrot (R)

• From Newton’s 2nd Law derive m(R) from V(R)

Interstellar MoleculesMolecule Formation

• Need high densities (~104 cm–3)

• Dust needed to protect molecules for UV

• Extinction – optically obscured – radio telescopes

• Low temperatures < 100 K

• Some molecules (e.g. H2) form on dust grains

• Most form via ion-molecular gas-phase reactions

• Exothermic

• Charge transfer, e.g. H3+ and C+

Interstellar Molecules

• About 90% of the over 130 interstellar molecules discovered with radio telescopes.

• Rotational (electric dipole) transitions

• Up to thirteen Atoms

• Most carbon-based (organic)

• Many cannot exist in normal laboratories (e.g. OH)

• H2 most common molecule:‣ No dipole moment so no radio transition

‣ Only observable in UV (rotational) or Infrared (vibrational) transitions

‣ Astronomers use CO as a tracer for H2

• A few molecules (OH, H2O, CH3OH) maser

Interstellar MoleculesDiatomic Triatomic 4 Atoms 5 Atoms 6 Atoms 7 Atoms 8 Atoms 9 Atoms 10 Atoms 11 Atoms 12 Atoms 13 Atoms

CH H2O NH3 HC3N CH3OH CH3NH2 CH3C3N CH3OCH3 (CH3)2CO HC9N C6H6 HC11N

CN HCO+ H2CO HCOOH CH3CN CH2CHCN C7H C2H5OH NH2CH2COOH HOCHC2CH2OH CO(CH2OH)2

CH+ HCN HNCO CH2NH NH2HCO CH3CHO CH3COOH CH3CH2CN CH3CH2CHO CH3C6H

OH OCS H2CS NH2CN CH3CHO CH3C2H CH2OHCHO HC7N CH3C5N

CO H2S C3N H2C2O C2H4 HC5N C6H2 CH3C4H

H2 HNC HNCS C4H C5H C6H CH2CHCHO C8H

SiO N2H+ HOCO+ SiH4 CH2CHO c-C2H4O CH2CCHCN CH3CONH2

CS C2H C3H c-C3H2 HC3NH+ CH2CHOH

SO SO2 C3O CH2CN C5N C6H-

SiS HDO HCNH+ C5 H2COH+

NS HCO H3O+ SiC4 C4H-

C2 HNO C3S l-C3H2 C4H2

NO OCN- c-C3H CH4 HC4N

HCl HCS+ C2H2 HC2NC c-C2H3O

NaCl HOC+ HC2N HNC3 CH2CNH

AlCl c-SiC2 H2CN H2COH+

KCl MgNC SiC3 C4H-

AlF C2S CH3

PN C3

SiC CO2

CP CH2

NH C2O

SiN NH2

SO+ NaCN

CO+ N2O

HF MgCN

LiF H3+

SH SiCN

FeO AlNC

N2 SiNC

CF+

O2

Interstellar Molecules

CO used to trace distribution of mass

H3+ plays role in gas-phase reactions

H2CO organic molecule widely distributed

NH3 used to determine temperature

CH4 and CH3OH

HCN

CO as a tracer for H2

Optical and CO images of the Milky Way in the northern sky

12CO Maps and Beam Filling

Telescope beam sizes (hpbw = 1.22λ/D) centred on: (a) EC2 position A (α1950 = 02:44:52.6, δ = 58:16:00.2)superimposed on JCMT EC2 map 1 of observed CO 2–1 spectra. (b) EC2 position K (α1950 = 02:44:41.0, δ = 58:11:00.0) superimposed on JCMT EC2 map 2 of observed CO 2–1 spectra (axis offsets in arcsec). Colour key: Yellow = JCMT 15m; Blue = ARO 12m; Red = Effelsberg 100m.

NH3

CO2-1

CO 1-0

H2CO 1-1

HCO+ 1-0

H2CO 2-2

NH3

CO2-1

CO 1-0

H2CO 1-1

HCO+ 1-0

H2CO 2-2

Molecular Line Spectra

Observations of Edge Cloud 2 (EC2) taken at the ARO 12m Telescope February to June 2002

Variation in Noise (rms)

372 minutes of observing HCO+ in Edge Cloud 1 at the Onsala 20m telescope in May 2005

Variation in Noise (rms)

372 minutes of observing HCO+ in Edge Cloud 1 at the Onsala 20m telescope in May 2005

Addition of Data to Improve S/N

Signal to noise ratio improves (2t → √2), and the noise (rms) is reduced from around 2 K to 28 mK

Addition of Data to Improve S/N

Signal to noise ratio improves (2t → √2), and the noise (rms) is reduced from around 2 K to 28 mK

Ammonia as a Thermometer

Trot = 20 ± 3 K. Assuming LTE and a Boltzmann distribution. Combined statistical weight factors = 5/3

N(J, K) = 1.55 ! 1014J(J + 1)!K2

TmbdVN(2,2)N(1,1)

=gop(2)gop(1)

g2g1exp(!!E/kT )

Molecular Clouds

• Discovered in 1970(Penzias, Jefferts & Wilson and others)

• Cold (5-30 K) and dense (102 -106 cm–3)

• Where stars are formed

• A few percent of the Galaxy’s volume

• But 50% of the ISM mass

• Concentrated in spiral arms

• Dust Clouds = Molecular Clouds

The Eagle Nebula (M16), which comprises cold gas and dustNASA, ESA, and The Hubble Heritage Team (STScI/AURA)

Cloud Chemical Evolution

Fractional abundances varying over time for molecular cloud Model 8

1e-20

1e-18

1e-16

1e-14

1e-12

1e-10

1e-08

1e-06

1e-04

1 10 100 1,000 10,000 1e+05 1e+06 1e+07

Frac

tiona

l Abu

ndan

ce

Time (yr)

CCOCNHCNNH3CSSOH2COHCO+

C+

OC2HHNCNSS+

Model 8

C CO

CN

NH3

CS

SO

H2COHCO+ HNC

HCN

C+ON

S

S+

C2H

Dust Clouds = Molecular Clouds

The open star cluster NGC 6520 and the dark nebula Barnard 86 (B86).Credits: Fred Calvert, Adam Block, NOAO/AURA/KPNO/NSF.

Dust Scatters Starlight

Barnard’s Merope Reflection Nebula (IC 349) illuminated by nearby hot bright star Merope. Credits: NASA and The Hubble Heritage Team (STScI/AURA).

Shells of Ejected Dust

Expanding halo of light around V838 Monocerotis.Credits: NASA, the Hubble Heritage Team (AURA/STScI) and ESA.

Dust Continuum Emission

CO 2–1 intensity contours overlaid on a 1.2mm dust map of Edge Cloud 2 (axis offsets in arcsec)

More Information• Tielens: “The Physics and Chemistry of the Interstellar Medium”

• Dyson and Williams: “The Physics of the Interstellar Medium”

• Evans: “The Dusty Universe”

• Millar and Williams: “Dust and Chemistry in Astronomy”

• Whittet: “Dust in the Galactic Environment”

• Harwitt: “Astrophysical Concepts”

• Verschuur and Kellerman: “Galactic and Extra-Galactic Radio Astronomy”

• Verschuur: “Invisible Universe Revealed”

• Kraus: “Radio Astronomy”

• Burke and Smith: “Radio Astronomy”

Paul Rufflewww.paulruffle.com paulruffle@paulruffle.com