The Interstellar Medium and Interstellar MoleculesRonald MaddalenaNational Radio Astronomy Observatory

**Interstellar Medium The Material Between the StarsConstituents:Gases:Hydrogen (92% by number)Helium (8%)Oxygen, Carbon, etc. (0.1%)Dust Particles1% of the mass of the ISMAverage Density: 1 H atom / cm3

**Interstellar Medium Properties

State of H & CTemperatureDensities (H/cm3)Percent VolumeHII Regions & Planetary NebulaeH, C Ionized5000 K0.5< 1%Diffuse ISMH, C Ionized1,000,000 K0.0150%Diffuse AtomicH2 < 0.1C Ionized30-100 K10-10030%Diffuse Molecular0.1 < H2 < 50%C+ > 50%30-100 K100-50010%Translucent MolecularH2 ~ 1C+ < 0.5, CO < 0.915-50 K500-5000?SmallDense MolecularH2 ~ 1CO > 0.910-50 K> 10410%

**Interstellar Medium Properties

Interstellar Medium Life Cycle

**HII Regions & Planetary NebulaeIsolated regions where H is ionized.UV from hot (20,000 50,000 K), blue stars produces ionization.HII Regions Young, massive, & short-lived (< few x 106 years) stars.HII Regions have short lives.Near regions where they formed.Planetary nebulaeEvolved (white dwarf) stars

Planetary Nebula and HII Regions

**Non-Thermal Continuum RadiationFree-Free EmissionIonized regions (HII regions and planetary nebulae)Free electrons accelerated by encounters with free protons

Thermal Continuum RadiationCharacteristics:Opaque Black BodyIsothermalIn EquilibriumPlancks Law:I = Intrinsic Intensity (ergs/cm2/sec/Hz). h = Plancks Constantk = Boltzmans ConstantT in K in HzRadio Approximation:

**Non-Thermal Continuum RadiationSynchrotron RadiationFree ElectronsMagnetic FieldsDiscrete SourcesSupernovae RemnantsGeneral Interstellar MediumI with between -0.2 and 1.2

**Spectral-Line RadiationRecombination LinesDiscovered in 1965 by Hogburn and MezgerIonized regions (HII regions and planetary nebulae)Free electrons temporarily recaptured by a protonAtomic transitions between outer orbital (e.g., N=177 to M = 176)

Spectral-Line RadiationHyperfine transition of HydrogenDiscovered by Ewen and Purcell in 1951.Found in regions where H is atomic.Spin-flip (hyperfine) transitionElectron & protons have spinIn a H atoms, spins of proton and electron may be aligned or anti-aligned. Aligned state has more energy.Difference in Energy = h vv = 1420 MHzAn aligned H atom will take 11 million years to flip the spin of the electron.But, 1067 atoms in Milky Way 1052 H atoms per second emit at 1420 MHz.

Atomic Hydrogen

**Spectral-Line RadiationWhat do they tell us?Number of emitting regions in that direction.Frequency of center of line Objects velocityDoppler EffectFrequency Observed = Frequency Emitted / (1 + V/c)Width of line Motion of gas within the regionHeight of the line Maybe temperature of the gasArea under the line Maybe number of atoms in that direction.

**Interstellar MoleculesHydroxyl (OH) first molecule found with radio telescopes (1964).Molecule Formation:Need high densitiesLots of dust needed to protect molecules for stellar UVBut, optically obscured need radio telescopesLow temperatures (< 100 K)Some molecules (e.g., H2) form on dust grainsMost form via ion-molecular gas-phase reactionsExothermicCharge transfer

**Interstellar MoleculesAbout 90% of the over 130 interstellar molecules discovered with radio telescopes.Rotational (electric dipole) TransitionsUp to thirteen atomsMany 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) Astronomers use CO as a tracer for H2

**Molecular CloudsDiscovered 1970 by Penzias, Jefferts, & Wilson and others.Coldest (5-30 K), densest (100 106 H atoms/cm3) parts of the ISM.Where stars are formed25-50% of the ISM massA few percent of the Galaxys volume.Concentrated in spiral armsDust Clouds = Molecular Clouds

Discovery of Ethanol

**Molecules Discovered by the GBT

Grain Chemistry

Ion-molecular gas-phase reactions

**Ion-molecular gas-phase reactionsExamples of types of reactionsC+ + H2 CH2+ + h (Radiative Association)H2+ + H2 H3+ + H (Dissociative Charge Transfer)H3+ + CO HCO+ + H2 (Proton Transfer)H3+ + Mg Mg+ + H2 + H (Charge Transfer)He+ + CO He + C+ + O (Dissociative Charge Transfer)HCO+ + e CO + H (Dissociative)C+ + e C + h (Radiative)Fe+ + grain Fe + h (Grain)

Importance of H3+

**Importance of H3+ -- Recent resultsFirst detected in 1994 in the infraredCreation: H2 + cr H2+ + eH2 + H2+ H3+ + HDestructionH3+ + e H + H2 or 3HNew laboratory measurements for reaction rates Dense Molecular clouds expected and measured H3+ agreeDiffuse Molecular clouds measured H3+ is 100x higher than expectedCosmic ray ionization rate has to be higher in diffuse clouds than in dark clouds. Why? Confinement of cr in the diffuse molecular cloudsHigher number of low energy cr than in current theory and which cant penetrate dark clouds

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**Maser Emission

Spectral-Line RadiationMilky Way Rotation and Mass?For any cloudObserved velocity = difference between projected Suns motion and projected cloud motion.For cloud BThe highest observed velocity along the line of siteVRotation = Vobserved + Vsun*sin(L)R = RSun * sin(L)Repeat for a different angle L and cloud BDetermine VRotation(R)From Newtons law, derive M(R) from V(R)

**Massive Supernovae

**Missing Mass

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Prebiotic Molecules

**The GBT and ALMA

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**Where to Get More InformationHarwitt: Astrophysical ConceptsVerschuur and Kellerman: Galactic and Extra-Galactic Radio AstronomyVerschuur: Invisible Universe RevealedKraus: Radio AstronomyBurke and Graham Smith: Radio Astronomy

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