8 -The Interstellar Medium Emission-Line Nebulae H II Regions Planetary Nebulae Supernova Remnants Reflection Nebulae Dark Clouds Giant Molecular Clouds Bok Globules Diffuse Clouds ~1 cm -3 in spiral arms n He /n H ~0.1 (n O + n C + n N + n Ne )/n H ~ 3x10 -4 Face-on? (M 51) GAS CloudsGMCsDiffuse NebulaePlanetary Nebulae M M M M R20-80 pc~10 pc 0.1 pc n cm -3 ~10 2 cm cm -3 T~10 K~8000 K10,000-15,000 K HeatingCRs, XraysHot Young StarsHot Old Stars CoolingH 2, CO, Dust Mostly O +2, O +1, N +1 Intercloud Medium Warm Hot T~70 K5x10 5 K n~ cm -3 ~0.03 cm -3 Heating CRs, Xrays, UVshocks, Xrays, hard UV Cooling C +, Fe +, CO, DustIons, Bremsstrahlung Also Novae & Supernovae ejecta DUST Solid Grains C, Si, O, + ? T 1200 K Absorb & Scatter starlight Polarization Transmission & Scattering Thermal Emission Equilibrium States Thermal Equilibrium Detailed balancing for interacting systems Atomic States Atomic Energy Levels energy E 1 E 2 = energy E 2 E 1 Matter particles only mechanical equilibrium Matter + Radiation thermodynamic equilibrium TE In TE, all distributions are homogeneous and isotropic, and can be characterized by a single given temperature T. For atoms in ionization state j (j=0 for neutrals), having an ionization energy, and excitation state i, with an excitation energy relative to the ground level, and a statistical weight for occupation g i, the relative populations of i with respect to the ground i=0 is: Ionization and Excitation in TE And the relative populations of two adjacent ionization states is: Statistical Equilibrium Energy In = Energy Out of a particular state This is a less stringent condition than TE. The type of equilibrium that exists will depend on the way that the particles in the system interact. If the mean free path and mean free time between collisions are x and t, If the temperature is constant over:we have: a. times >> t, distances >> xthermal equilibrium b. times >> tstatistical equilibrium c. distances >> xno equilibrium d. none of the aboveno equilibrium If both matter and radiation are in thermal equilibrium (including with one another), we have TE. Sometimes, the conditions are not in perfect TE everywhere in the system. Nevertheless, if it is sufficiently close enough not to affect the processes sufficiently at a particular location, that location is said to be in Local Thermodynamic Equilibrium LTE. Interactions Particle-Particle Photon-Particle Example H II Region electron-ion collisions Typical n and T: n~10 and T~10 4 : x m =2x10 10 cmversus sizes ~ 3x10 18 cm t m =400 s versus ages ~ 3x10 13 s So: electron-ion (and electron-electron) interactions: mechanical equilibrium maxwellian velocity distribution Matter-Radiation Radiative lifetimes of atoms < 10 sec, and usually < sec, much shorter than matter-matter collisions usually do not have detailed balancing. Upward collisional transition is followed by downward radiative transition. Radiation Field: VERY ANISOTROPIC! Dilution Factor: T~T * inside T