Chapter 14 The Interstellar Medium

All of the material other than stars, planets, and degenerate objects Composed of gas and dust ~1% of the mass of the galaxy Site of star formation ISM

Evidence for Interstellar Dust: Interstellar extinction: scattering, absorption, reflection Dust grains scatter and absorb background starlight

Evidence for dust: Interstellar reddening Longer wavelengths pass through Shorter (bluer) light is more easily scattered

Interstellar extinction curve 2200 A bump o

The densest ISM: Molecular clouds, birthplace of stars

Gas and dust collect into clouds: Dust radiates in the thermal infrared

Phases of the ISM Compare the pressures (P=nkT) of each of these phases!

Comparing the pressures of different phases of ISM

The Milky Way at 100 microns Warm dust traces out the ISM

Observing cold neutral hydrogen, H I (“H-one”) Interaction between electron spin and nuclear spin

21-cm emission from the entire Milky Way

Spectroscopy of molecular clouds First detected in 1937 !!! 146 compounds have been identified in the ISM as of June 2006 Most are organic (contain C and at least one atom other than O) List does not include deuterated species or ions Good reference: More than half originally detected in Sgr B2 (massive star forming region near galactic center) My personal faves: glycine, ethanol, acetic acid !!

Compounds are identified by their rotational spectrum Sgr B2

The rich molecular spectrum of Sgr B2

Beam-averaged column densities 〈 NT 〉 determined from Sgr B2(N-LMH) interferometric measurements: acetic acid (CH3COOH); formic acid (HCOOH); acetone ((CH3)2CO); ethyl cyanide (CH3CH2CN); and methyl formate (HCOOCH3). [Reproduced with permission from Snyder et al Snyder L E PNAS 2006;103: ©2006 by National Acadey of Sciences

Molecular spectra Three main types of transitions emit photons (corresponding to specific spectral lines): 1.Electronic Hot gases highest E photons: ~ few eV ~ visible, UV 2.Vibrational For gas phase molecules, always comes with rotation Solids have pure vibrational spectra ~ IR 3.Rotational Lowest E photons ~ radio, microwave (mm to m) Cold gas-phase molecules

Rotational spectra

A diatomic molecule modeled as a “rigid rotor” Energy is rotational KE Rotational energy levels of a diatomic molecule: Diatomic molecules l = (rotational) angular momentum quantum #. Our book uses J

How to find momentum of inertia, I

Rotational energy levels for a diatomic molecule **Our book uses J instead of l

Vibrational energy levels n = vibrational quantum number

Vibrational levels for a diatomic molecule (harmonic oscillator): Equally spaced

Formation of interstellar molecules Hydrogenated (H 2 O, CH 4, NH 3 ) CO, CO 2, N 2, etc

Vibrational and rotational energy levels Selection rules: If vibrational level changes, n must increase by 1 if a photon is absorbed n must decrease by 1 if a photon is emitted

Rotation-vibration spectrum of HCl

Radio Spectrum of a molecular cloud