Topics

[O I] 6300 A transition

Ahmad Nemer
 

Hello all,

I am Ahmad Nemer, and I work with Jeremy Goodman on PPDs. I would like to ask a question about the code and I would really appreciate the help. I am trying to simulate a PPD environment using Cloudy, and the main purpose is to study the behavior of the [O I] 6300 A spectral line. I know the upper level of this transition can be excited in many ways; one of which is the dissociation of OH molecule into the upper level of this transition, so is this included in Cloudy. I am asking if Cloudy takes into consideration the reactions (e.g. OH > O + H) that would populate excited states of O, or does it assume all the dissociation products to be in their ground state. I would appreciate it if someone could point me to the part of the code where i can figure this out or tell me the answer if they know. Thank you!
--

Truly yours,
Dr. Ahmad Nemer
Research associate in astronomy
Princeton University
Department of astrophysical sciences
Tel: +13343330697
Email: anemer@...
aanimer@...
**Be the change you want to see in this world**

Gary Ferland
 

Hi there,

We did look into it some time ago. OH chemistry cannot produce observable [O I] emission for two reasons.  First, the OH transition occurs too deep in the cloud - Av is large.  See Figure 14 of Abel+ 2005ApJS..161...65A - in that calculation, OH starts at Av ~ 8.  Any 6300 produced by OH would be very heavily extinguished.  Second, the OH contribution to 6300 would be small compared to the direct impact excitation in the warm H+ layer.  OH-produced emission would be weak even without extinction just due to the reaction kinematics - the rate coefficient for most chemistry is about 1e-9 cm3 s-1 while the electron direct excitation rate is typically about 2 - 3 dex smaller.  But the electron density in the H+ layer is well more than 4 dex larger than the OH density in the PDR.  Nearly all PDRs are next to a bright H II region.

A PPD is not the same as a PDR but it surely has dust and likely has an ionized layer on its surface.  To get this process to produce observable emission there would have to be NO ionizing radiation producing O I in the warm ionized layer (homework problem PDR calculations are often set up with no ionizing radiation but real PDRs are next to H II regions).  Even then you would have the problem of the dust extinction.

I had a look and it does not appear that our trial code made it onto the mainline.  Likely it was done on a local copy, found to be insignificant, verified analytically, then forgotten.  

thanks,
Gary


On Mon, Jun 1, 2020 at 2:16 AM Ahmad Nemer <anemer@...> wrote:
Hello all,

I am Ahmad Nemer, and I work with Jeremy Goodman on PPDs. I would like to ask a question about the code and I would really appreciate the help. I am trying to simulate a PPD environment using Cloudy, and the main purpose is to study the behavior of the [O I] 6300 A spectral line. I know the upper level of this transition can be excited in many ways; one of which is the dissociation of OH molecule into the upper level of this transition, so is this included in Cloudy. I am asking if Cloudy takes into consideration the reactions (e.g. OH > O + H) that would populate excited states of O, or does it assume all the dissociation products to be in their ground state. I would appreciate it if someone could point me to the part of the code where i can figure this out or tell me the answer if they know. Thank you!
--

Truly yours,
Dr. Ahmad Nemer
Research associate in astronomy
Princeton University
Department of astrophysical sciences
Tel: +13343330697
Email: anemer@...
aanimer@...
**Be the change you want to see in this world**



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

Ahmad Nemer
 

Hi Gary,

Thank you so much for taking the time to answer my question, and I'm glad that you have thought about this before. 

Thank you for pointing out the dust extinction with this issue. I will have to study the effect of extinction more closely in my simulation, and I am not sure if OH is formed at those Av values. But it might be the reason why we do not see much contribution from OH to the O I 6300 line in our simulation. As for the direct electron impact excitation, that is definitely a competing mechanism. The thermal excitation, through electron collisions, are suspected to be the main source of the O I 6300. But some authors claim that at the surface of the disk, there is a significant emission of the O I 6300. In that region, the gas is mostly neutral (so there isn't much electrons nor dust), and the temperature is too low for collisional excitation. 

I tried to simulate that environment with Cloudy, and I saw no emission from that region. I did not see any emission from there of the O I 6300 line. It is one of 2 explanations. Either that the environment is not physically capable of producing this line (due to exctinction or some other reason), or that Cloudy does not include the population of the upper level of the transition through OH dissociation. So do you know if Cloudy includes that reaction, or does Cloudy put the resultant of O of that reaction in the ground state (as opposed to the upper level of the O I 6300)?

Thank you again and I hope to hear from you soon!

Gary Ferland
 

A warm layer emitting [O I] would result if FUV / XUV / X-ray light strikes the illuminated face of the cloud. I do not know your calculation setup, but it is important to use a realistic SED and not one of the "homework problem" PDR SEDs that are artificially constrained to have light only in a narrow energy range.  If the SED extends to high enough energies the warm layer and strong [O I] emission will result,
hope that helps,
Gary

On Sun, Jun 7, 2020 at 3:44 AM Ahmad Nemer <anemer@...> wrote:
Hi Gary,

Thank you so much for taking the time to answer my question, and I'm glad that you have thought about this before. 

Thank you for pointing out the dust extinction with this issue. I will have to study the effect of extinction more closely in my simulation, and I am not sure if OH is formed at those Av values. But it might be the reason why we do not see much contribution from OH to the O I 6300 line in our simulation. As for the direct electron impact excitation, that is definitely a competing mechanism. The thermal excitation, through electron collisions, are suspected to be the main source of the O I 6300. But some authors claim that at the surface of the disk, there is a significant emission of the O I 6300. In that region, the gas is mostly neutral (so there isn't much electrons nor dust), and the temperature is too low for collisional excitation. 

I tried to simulate that environment with Cloudy, and I saw no emission from that region. I did not see any emission from there of the O I 6300 line. It is one of 2 explanations. Either that the environment is not physically capable of producing this line (due to exctinction or some other reason), or that Cloudy does not include the population of the upper level of the transition through OH dissociation. So do you know if Cloudy includes that reaction, or does Cloudy put the resultant of O of that reaction in the ground state (as opposed to the upper level of the O I 6300)?

Thank you again and I hope to hear from you soon!



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

 

There seem to be two threads on the same topic.  I sent a pretty picture to the other one. 

Just to reiterate: yes, OH chemistry does produce observable [O I] emission - we see it!

This is particularly important for the [O I] 5577 Å auroral line.  This is very weak in ionization fronts, but is very bright from proplyd disks.   If it were to be thermally excited, then the implied T from 5577/6300 would be unrealistically large. 

A static model is bound to give you the wrong answer to this.  It would be necessary to include the advection as in https://ui.adsabs.harvard.edu/abs/2007ApJ...671L.137H/abstract (but with heavy element chemistry too, which was not included in that paper)

Cheers

Will


On Sat, Jul 25, 2020 at 6:05 PM Gary Ferland <gary@g.uky.edu> wrote:
A warm layer emitting [O I] would result if FUV / XUV / X-ray light strikes the illuminated face of the cloud. I do not know your calculation setup, but it is important to use a realistic SED and not one of the "homework problem" PDR SEDs that are artificially constrained to have light only in a narrow energy range.  If the SED extends to high enough energies the warm layer and strong [O I] emission will result,
hope that helps,
Gary

On Sun, Jun 7, 2020 at 3:44 AM Ahmad Nemer <anemer@...> wrote:
Hi Gary,

Thank you so much for taking the time to answer my question, and I'm glad that you have thought about this before. 

Thank you for pointing out the dust extinction with this issue. I will have to study the effect of extinction more closely in my simulation, and I am not sure if OH is formed at those Av values. But it might be the reason why we do not see much contribution from OH to the O I 6300 line in our simulation. As for the direct electron impact excitation, that is definitely a competing mechanism. The thermal excitation, through electron collisions, are suspected to be the main source of the O I 6300. But some authors claim that at the surface of the disk, there is a significant emission of the O I 6300. In that region, the gas is mostly neutral (so there isn't much electrons nor dust), and the temperature is too low for collisional excitation. 

I tried to simulate that environment with Cloudy, and I saw no emission from that region. I did not see any emission from there of the O I 6300 line. It is one of 2 explanations. Either that the environment is not physically capable of producing this line (due to exctinction or some other reason), or that Cloudy does not include the population of the upper level of the transition through OH dissociation. So do you know if Cloudy includes that reaction, or does Cloudy put the resultant of O of that reaction in the ground state (as opposed to the upper level of the O I 6300)?

Thank you again and I hope to hear from you soon!



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



--

  Dr William Henney, Instituto de Radioastronomía y Astrofísica,
  Universidad Nacional Autónoma de México, Campus Morelia

Gary Ferland
 

Hi Will,
Actually, I think the OH tests were done in Morelia when I was visiting with Robin, soon after the Hollenbach paper on the process.

The basic problem is the extinction to the OH dissociation layer in a static, time steady, PDR model.  The following is from  2005ApJS..161...65A and shows the problem:
image.png
The OH dissociation front is at about Av = 8.  It would be hard to get any 6300 out through that extinction.  

Advective flows can move OH into shallower layers which would increase the production rate and make the extinction to the layer smaller.  Will+ did such a model for H2 in the Helix in  2007ApJ...671L.137H.  This is an inherently dynamical rather than static effect.

thanks,
Gary


On Sat, Jul 25, 2020 at 8:23 PM William Henney <whenney@...> wrote:
There seem to be two threads on the same topic.  I sent a pretty picture to the other one. 

Just to reiterate: yes, OH chemistry does produce observable [O I] emission - we see it!

This is particularly important for the [O I] 5577 Å auroral line.  This is very weak in ionization fronts, but is very bright from proplyd disks.   If it were to be thermally excited, then the implied T from 5577/6300 would be unrealistically large. 

A static model is bound to give you the wrong answer to this.  It would be necessary to include the advection as in https://ui.adsabs.harvard.edu/abs/2007ApJ...671L.137H/abstract (but with heavy element chemistry too, which was not included in that paper)

Cheers

Will


On Sat, Jul 25, 2020 at 6:05 PM Gary Ferland <gary@g.uky.edu> wrote:
A warm layer emitting [O I] would result if FUV / XUV / X-ray light strikes the illuminated face of the cloud. I do not know your calculation setup, but it is important to use a realistic SED and not one of the "homework problem" PDR SEDs that are artificially constrained to have light only in a narrow energy range.  If the SED extends to high enough energies the warm layer and strong [O I] emission will result,
hope that helps,
Gary

On Sun, Jun 7, 2020 at 3:44 AM Ahmad Nemer <anemer@...> wrote:
Hi Gary,

Thank you so much for taking the time to answer my question, and I'm glad that you have thought about this before. 

Thank you for pointing out the dust extinction with this issue. I will have to study the effect of extinction more closely in my simulation, and I am not sure if OH is formed at those Av values. But it might be the reason why we do not see much contribution from OH to the O I 6300 line in our simulation. As for the direct electron impact excitation, that is definitely a competing mechanism. The thermal excitation, through electron collisions, are suspected to be the main source of the O I 6300. But some authors claim that at the surface of the disk, there is a significant emission of the O I 6300. In that region, the gas is mostly neutral (so there isn't much electrons nor dust), and the temperature is too low for collisional excitation. 

I tried to simulate that environment with Cloudy, and I saw no emission from that region. I did not see any emission from there of the O I 6300 line. It is one of 2 explanations. Either that the environment is not physically capable of producing this line (due to exctinction or some other reason), or that Cloudy does not include the population of the upper level of the transition through OH dissociation. So do you know if Cloudy includes that reaction, or does Cloudy put the resultant of O of that reaction in the ground state (as opposed to the upper level of the O I 6300)?

Thank you again and I hope to hear from you soon!



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



--

  Dr William Henney, Instituto de Radioastronomía y Astrofísica,
  Universidad Nacional Autónoma de México, Campus Morelia



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

 

Hi Gary

Yes, but my point was that this is only a problem in that model, not in real photoevaporating disks.  In the proplyds, the AV to the OH dissociation front is clearly only 1 or 2 max. This difference may be partly due to advection, but there is also evidence that AV/NH is significantly smaller than the standard ISM value, due to grain evolution in the disks. 

Anyway, the take-away for Ahmad should be that yes it is an important process for exciting [O I] 6300 (and even more-so [O I] 5577) in photoevaporating disks, but no it is not currently possible to use Cloudy to predict it without a fair amount of work. 

Cheers

Will

On Sat, Jul 25, 2020 at 7:50 PM Gary Ferland <gary@g.uky.edu> wrote:
Hi Will,
Actually, I think the OH tests were done in Morelia when I was visiting with Robin, soon after the Hollenbach paper on the process.

The basic problem is the extinction to the OH dissociation layer in a static, time steady, PDR model.  The following is from  2005ApJS..161...65A and shows the problem:
image.png
The OH dissociation front is at about Av = 8.  It would be hard to get any 6300 out through that extinction.  

Advective flows can move OH into shallower layers which would increase the production rate and make the extinction to the layer smaller.  Will+ did such a model for H2 in the Helix in  2007ApJ...671L.137H.  This is an inherently dynamical rather than static effect.

thanks,
Gary


On Sat, Jul 25, 2020 at 8:23 PM William Henney <whenney@...> wrote:
There seem to be two threads on the same topic.  I sent a pretty picture to the other one. 

Just to reiterate: yes, OH chemistry does produce observable [O I] emission - we see it!

This is particularly important for the [O I] 5577 Å auroral line.  This is very weak in ionization fronts, but is very bright from proplyd disks.   If it were to be thermally excited, then the implied T from 5577/6300 would be unrealistically large. 

A static model is bound to give you the wrong answer to this.  It would be necessary to include the advection as in https://ui.adsabs.harvard.edu/abs/2007ApJ...671L.137H/abstract (but with heavy element chemistry too, which was not included in that paper)

Cheers

Will


On Sat, Jul 25, 2020 at 6:05 PM Gary Ferland <gary@g.uky.edu> wrote:
A warm layer emitting [O I] would result if FUV / XUV / X-ray light strikes the illuminated face of the cloud. I do not know your calculation setup, but it is important to use a realistic SED and not one of the "homework problem" PDR SEDs that are artificially constrained to have light only in a narrow energy range.  If the SED extends to high enough energies the warm layer and strong [O I] emission will result,
hope that helps,
Gary

On Sun, Jun 7, 2020 at 3:44 AM Ahmad Nemer <anemer@...> wrote:
Hi Gary,

Thank you so much for taking the time to answer my question, and I'm glad that you have thought about this before. 

Thank you for pointing out the dust extinction with this issue. I will have to study the effect of extinction more closely in my simulation, and I am not sure if OH is formed at those Av values. But it might be the reason why we do not see much contribution from OH to the O I 6300 line in our simulation. As for the direct electron impact excitation, that is definitely a competing mechanism. The thermal excitation, through electron collisions, are suspected to be the main source of the O I 6300. But some authors claim that at the surface of the disk, there is a significant emission of the O I 6300. In that region, the gas is mostly neutral (so there isn't much electrons nor dust), and the temperature is too low for collisional excitation. 

I tried to simulate that environment with Cloudy, and I saw no emission from that region. I did not see any emission from there of the O I 6300 line. It is one of 2 explanations. Either that the environment is not physically capable of producing this line (due to exctinction or some other reason), or that Cloudy does not include the population of the upper level of the transition through OH dissociation. So do you know if Cloudy includes that reaction, or does Cloudy put the resultant of O of that reaction in the ground state (as opposed to the upper level of the O I 6300)?

Thank you again and I hope to hear from you soon!



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



--

  Dr William Henney, Instituto de Radioastronomía y Astrofísica,
  Universidad Nacional Autónoma de México, Campus Morelia



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



--

  Dr William Henney, Instituto de Radioastronomía y Astrofísica,
  Universidad Nacional Autónoma de México, Campus Morelia