Some atomic physics

H I, O III, Fe X are spectra– Emitted by

H0, O2+, Fe9+

– These are baryons For absorption lines there is a mapping

between these two– H I absorption is produced by H0

Emission lines are ambiguous

H I emission is produced by– Recombination of H+

– Impact excitation of H0

So H I lines trace H+ or H0, depending on circumstances

The H+ region around a hot star produces H I emission

A hot star produces successive layers of H+, H0, H2

2012 Cloudy workshop

The primary mechanism

Luminosity of H II region

Set by total luminosity in ionizing photons

Object L(Ha) • Stars

Orion 5.0 1036 1

M17 50

30 Dor 2.7 1040 5000

A non-equilibrium gas Gas emitting spectrum has a very low density,

exposed to a range of radiation fields and particles Populations of levels, ionization, spectrum,

determined by many micro-physical processes Not characterized by a single temperature

Physical state of interstellar gas

Non-equilibrium microphysics– Te: heating & cooling– Ionization & chemistry

Grain physics– Heats gas, attenuates

radiation field Predict full spectrum All done self-consistently

– Few free parameters– Goal is no free parameters

www.nublado.org groups.yahoo.com/neo/groups/cloudy_simulations/info

Runaway O star H II regions

~1/4 of O stars are runaways– Expelled from close binary by explosion or

instability Pass by diffuse ISM and photoionize it H II region is

– Very faint– Very low density– Small magnetic field, B ~ 6 μG

The H II region is an innocent bystander

California Nebula APODXi Persei

Ferland+09 MNRAS, 392, 1475

Star forming H II regions

Hot young stars very close to the molecular cloud that formed it

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 H0, H2

“PDR”

Cool H2

“molecular cloud”

Flow of evaporating material

H2 H0 H+

http

://cf

a-w

ww

.har

vard

.edu

/mm

w/C

O_o

ptic

al_o

rion

.htm

l

Will Henney, Morelia

Will Henney, Morelia

Guedel+ 2008 Science 319, 309

1 Oriexciting

stars

Ionized gas

Atomic gas

Veil

Molecular gas100”

BN0.5 pc

A

A

B

B OMC, 13CO

To EarthFigure 8.4, Osterbrock & Ferland AGN3

Extinction map

Combined VLA radio continuum, HST optical recombination lines

Lighter shade greater extinction

O’Dell & Yusef-Zadeh 2000, AJ, 120, 382

The “real” Orion Nebula

O’Dell and Yousef-Zadeh 2000

A

Orion veil – Blos from HI Zeeman effect

Component A

Colors – Blos

Grains in H II regions

Grains survive in the warm H+ layer Were they destroyed, very strong Ca, Fe,

and Al lines would be present– Kingdon&Ferland 1995, ApJ, 439, 793

Grains very likely destroyed, or never formed, in hot H+ bubble

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 H0, H2

“PDR”

Cool H2

“molecular cloud”

The BFM Orion Model

The equation of state – how does the density vary with depth into the cloud?

Warm H+ layer is hydrostatic Starlight radiation pressure balancing gas

pressure Model stopped at H+ - H0 ionization front,

where gas changes phase from warm H+ to warm H0

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

Starlight pushes back surrounding material Field lines coupled to gas, so compressed Establishes magnetic version of hydrostatic

equilibrium– Outward momentum of starlight balanced by

magnetic pressure in PDR

Establishes simple relationship between hot bubble, warm H+, and warm H0 regions

2

24 8

L B

r c

Pellegrini+ 2007, ApJ 658, 1119