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Line emissivities and intensities inconsistent for optically thin gas?

Sylvia Ploeckinger
 

Dear Cloudy team,

  I have been calculating the intensity / emissivity of some emission lines with 
the following Cloudy commands (using c17.01):

intensity
save line list last absolute "lines_intensity.dat" "CloudyLineListMinimal.dat"

emissivity:
save lines, emissivity, last "lines_emissivity.dat"
C  5      40.2678A
C  6      33.7372A
N  7      24.7807A
O  7      21.6020A
O  8      18.9709A
end of lines

The file "CloudyLineListMinimal.dat" contains the same lines as listed for the emissivities. 
If I run a 1-zone model with Cloudy, I would expect that I can convert between the emissivity 
and the intensity outputs with:

intensity = emissivity * depth

This indeed works for high densities and high temperatures. For low densities (attached
is an example for nH = 10^-6 cm^-3), the intensity from the intensity command and the 
intensity calculated from the emissivity are the same for very high temperatures,
but seem to have a constant offset for lower temperatures. 
 
While I have not directly checked this, it seems to work okay for densities / temperatures
where collisional ionization dominates, but it has this offset where photo-ionization dominates. 

Does anyone know where this offset comes from? Is this something real?
 
Any help would be greatly appreciated!
 
Thanks a lot!
 
Regards,
Sylvia

Gary Ferland
 

Hi Sylvia,
The code does line optical depths (and self-shielding) self consistently and also includes continuum pumping (fluorescent excitation) for all transitions.  For one zone or thin clouds the line optical depths will be small, there is no self-shielding, and continuum pumping will be at a maximum.  As the column density increases, the lines become optically thick, and self-shielded, and continuum pumping goes away.  This is usually a source of complication when comparing small and large column density clouds.

To see if this is what is happening turn off continuum photoexcitation with the command
No induced processes
(described in hazy1) and see if that makes a better agreement.  it will change the one-zone model a lot but should have a modest effect on thick models.

hope that helps,
best,
Gary

On Thu, Jun 11, 2020 at 12:29 PM Sylvia Ploeckinger <ploeckinger@...> wrote:

Dear Cloudy team,

  I have been calculating the intensity / emissivity of some emission lines with 
the following Cloudy commands (using c17.01):

intensity
save line list last absolute "lines_intensity.dat" "CloudyLineListMinimal.dat"

emissivity:
save lines, emissivity, last "lines_emissivity.dat"
C  5      40.2678A
C  6      33.7372A
N  7      24.7807A
O  7      21.6020A
O  8      18.9709A
end of lines

The file "CloudyLineListMinimal.dat" contains the same lines as listed for the emissivities. 
If I run a 1-zone model with Cloudy, I would expect that I can convert between the emissivity 
and the intensity outputs with:

intensity = emissivity * depth

This indeed works for high densities and high temperatures. For low densities (attached
is an example for nH = 10^-6 cm^-3), the intensity from the intensity command and the 
intensity calculated from the emissivity are the same for very high temperatures,
but seem to have a constant offset for lower temperatures. 
 
While I have not directly checked this, it seems to work okay for densities / temperatures
where collisional ionization dominates, but it has this offset where photo-ionization dominates. 

Does anyone know where this offset comes from? Is this something real?
 
Any help would be greatly appreciated!
 
Thanks a lot!
 
Regards,
Sylvia



--
Gary J. Ferland
Physics, Univ of Kentucky
Lexington KY 40506 USA
Tel: 859 257-8795
https://pa.as.uky.edu/users/gary