Jerusalem WS lecture notes: 04. Physics of Star-Forming Clouds

By Mark Krumholz, 'the only obstacle between you and the exciting opportunity to combine drinking with jet lag'. Here are the slides.
Observations:

• SF gas is cold: observations in radio, mm, far-IR
• Diffuse gas: emission lines, dust
• SF ISM is mostly molecular
• H_2: proof that nature has a cruel sense of humour:
  no electronic excitation in cold gas, vibration: mid-IR energy, too high. Rotations: H2 has no dipole mode, no J1 -> J0 transitions, the lowest transition is J2 --> J0. J2 state -- 511 K off ground: no H2 molecules emission.
• CO: proof that astronomers are stubborn bastards:
  
  • If density is high, radiation doesn't change energy distribution (Boltzmann, collisions). Else: way fewer excited molecules than excpected, because collisions don't happen often.
  • Brightness temperature
  • Integrated CO intensity is measure of velocity dispersion (=total gravitating mass), if T = const.
  • Intensity --> directly tells the column density (\Sigma) of CO and H2
  • Motion in gas: bulk, non-thermal, highly supersonic
• Gas properties:
  • cold (10K, 100K in starbursts): adiabatic compression, viscous dissipation, EUV ionisation/FUV photoelectric heating, CR/X ray heating, cooling processes: adiabatic expansion, lines. Dynamical timescales. CRs and X rays can penetrate high columns.
  • Isothermal gas -- efficient cooling if the gas is compressed. Equilibrium T ~ 10K, hard to change. CR -- main source of heating.
  • dense (n > 100cm^{-3})
  • very supersonic: magnetic forces are important, extremely turbulent (Re ~ 10^9).
  • linewidth-size relation (\sigma ~ size of the region), power spectrum
  • VT: thermal motion/thermal pressure prevent collapse, Bonnor-Ebert mass: for a given pressure, there is a maximum mass that can be stable against collapse.
  • for GMCs: M_{BE} ~= 10^7 M_{\odot}: that's why stars form in MCs.