Jan 17, 2016
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
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vard
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/mm
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O_o
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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