Some atomic physics u H I, O III, Fe X are spectra –Emitted by u H 0, O 2+, Fe 9+ –These are baryons u For absorption lines there is a mapping between these two –H I absorption is produced by H 0

Emission lines are ambiguous u H I emission is produced by –Recombination of H + –Impact excitation of H 0 u So H I lines trace H + or H 0, depending on circumstances u The H + region around a hot star produces H I emission u A hot star produces successive layers of H +, H 0, H 2

The primary mechanism 2012 Cloudy workshop

Luminosity of H II region u Set by total luminosity in ionizing photons ObjectL(Ha)Stars Orion M Dor

A non-equilibrium gas u Gas emitting spectrum has a very low density, exposed to a range of radiation fields and particles u Populations of levels, ionization, spectrum, determined by many micro-physical processes u Not characterized by a single temperature

Physical state of interstellar gas u Non-equilibrium microphysics –Te: heating & cooling –Ionization & chemistry u Grain physics –Heats gas, attenuates radiation field u Predict full spectrum u All done self-consistently –Few free parameters –Goal is no free parameters

groups.yahoo.com/neo/groups/cloudy_simulations/infogroups.yahoo.com/neo/groups/cloudy_simulations/info

Runaway O star H II regions u ~1/4 of O stars are runaways –Expelled from close binary by explosion or instability u Pass by diffuse ISM and photoionize it u H II region is –Very faint –Very low density –Small magnetic field, B ~ 6  G u The H II region is an innocent bystander

California Nebula APOD Xi Persei

Ferland+09 MNRAS, 392, 1475

Star forming H II regions u Hot young stars very close to the molecular cloud that formed it u Ionizing radiation and stellar winds strike nearby molecular cloud

ESO

NASA/CXC/PSU/L.Townsley et al.; Infrared: NASA/JPL

Idealized structure of an H II region Hot H + bubble Warm H + “H II region” Warm H 0, H 2 “PDR” Cool H 2 “molecular cloud”

Flow of evaporating material  H 2  H 0  H +

Will Henney, Morelia

Guedel Science 319, 309

OMC, 13 CO To Earth Figure 8.4, Osterbrock & Ferland AGN3

2012 Cloudy workshop

Extinction map u Combined VLA radio continuum, HST optical recombination lines u Lighter shade  greater extinction u O’Dell & Yusef-Zadeh 2000, AJ, 120, 382

The “real” Orion Nebula O’Dell and Yousef-Zadeh 2000

A Orion veil – B los from HI Zeeman effect Component A u Colors – B los

Grains in H II regions u Grains survive in the warm H + layer u Were they destroyed, very strong Ca, Fe, and Al lines would be present –Kingdon&Ferland 1995, ApJ, 439, 793 u Grains very likely destroyed, or never formed, in hot H + bubble u Commonly cited claim that grains do not exist in warm H+ layer is due to wrong geometry –See BFM, 1991, ApJ, 374, 580, Section 4.5

Idealized structure of an H II region Hot H + bubble Warm H + “H II region” Warm H 0, H 2 “PDR” Cool H 2 “molecular cloud”

The BFM Orion Model u The equation of state – how does the density vary with depth into the cloud? u Warm H + layer is hydrostatic u Starlight radiation pressure balancing gas pressure u Model stopped at H + - H 0 ionization front, where gas changes phase from warm H + to warm H 0

M 17

Orion and M 17 (not to scale) Pellegrini+ 2007, ApJ 658, 1119 Orion M17

Orion and M 17 (to scale) Pellegrini+ 2007, ApJ 658, 1119 Orion M17

Zeeman H I B field ~500 μG Brogan Troland 2001, AJ, 560, 821

Magnetostatic equilibrium u Starlight pushes back surrounding material u Field lines coupled to gas, so compressed u Establishes magnetic version of hydrostatic equilibrium –Outward momentum of starlight balanced by magnetic pressure in PDR u Establishes simple relationship between hot bubble, warm H +, and warm H 0 regions Pellegrini+ 2007, ApJ 658, 1119