add PRINT CRITICAL DENSITY command (trac #404)

[cloudy-bot]

reported by: peter

A frequently asked question is: what is the critical density for collisional de-excitation of level xxx in some atom or ion. The answer changes every time the atomic data is updated. So it would be useful to have a print command that gives this information based on the atomic data that is in Cloudy. The syntax could be something like

PRINT CRITICAL DENSITY [ levels = n ] [ Te = T ]

which would print the critical density for the lowest n levels (excluding the ground level of course). The default for n could be 5 and the default for Te could be 10,000 K.

Migrated from https://www.nublado.org/ticket/404

{
    "status": "new",
    "changetime": "2019-02-04T12:10:21Z",
    "_ts": "1549282221449479",
    "description": "A frequently asked question is: what is the critical density for collisional de-excitation of level xxx in some atom or ion. The answer changes every time the atomic data is updated. So it would be useful to have a print command that gives this information based on the atomic data that is in Cloudy. The syntax could be something like\n\nPRINT CRITICAL DENSITY <species> [ levels = n ] [ Te = T ]\n\nwhich would print the critical density for the lowest n levels (excluding the ground level of course). The default for n could be 5 and the default for Te could be 10,000 K.",
    "reporter": "peter",
    "cc": "",
    "resolution": "",
    "time": "2017-11-20T12:25:52Z",
    "component": "atomic/molecular data base",
    "summary": "add PRINT CRITICAL DENSITY command",
    "priority": "good to do",
    "keywords": "",
    "version": "",
    "milestone": "c19 branch",
    "owner": "nobody",
    "type": "enhancement"
}

[cloudy-bot]

@peter-van-hoof-noaccount commented:

Gary replied:

We already have that in the SAVE LINE DATA ouput. Here is a sample of the output: Level 2 transferred lines

Ion WL gl gu gf A CS n(crt) damp

the critical density is the n(crt) number. It is evaluated at save_linedata.cpp:335 using the stored collision strength.

but this was not consistently supported as the lines were developed. we need to check that Coll().col_str() is really kept up to date for electrons, And make it clear that this is an electron collision strength. there are species in PDRs where atomic collisions may be more important.

[cloudy-bot]

@peter-van-hoof-noaccount commented:

Peter replied:

I searched Hazy 1 for the string "critical density" before opening this ticket. It popped up nothing relevant. This feature in the SAVE LINE DATA output is not documented.

I also see other problems. We always say that the critical density is a feature of a level and not a line, so it is confusing to have it as part of the SAVE LINE DATA output.

A quick look suggests to me that the formula used to calculate the critical density does not agree with Eq. 3.30 in AGN3.

[cloudy-bot]

@peter-van-hoof-noaccount commented:

Gary replied:

OK - I agree. it should be defined in the level solver, just after the matrix is set up. the info is all there.

[cloudy-bot]

• @peter-van-hoof-noaccount commented:

Milestone renamed

• @peter-van-hoof-noaccount changed milestone from "C19_branch" to "c19 branch"