New in C90


This page summarizes all major changes and improvements to Cloudy that will be included in the next release. You may also view the HotFixes and KnownProblems pages, or return to the main RevisionHistory page.


Commands

XE "Cloudy:90 vs 84"The abundances command now needs 29 numbers by default. A new command “init” allows a commonly used set of commands to be saved as a single file and used by a variety of scripts.

Continuum Transport

Versions 86 and before used a modified version of on-the-spot approximation (OTS) for the Lyman continua of hydrogen and helium. This method was numerically stable and gave results in excellent agreement with Van Blerkom and Hummer (1967). This has been changed to outward-only to obtain better agreement with predictions of Pat Harrington’s and Bob Rubin’s codes (Ferland et al. 1995). The OTS code is still in place and will be used if the diffuse OTS command is entered, but outward-only is the default. The two methods result in temperatures at the illuminated face which can differ by as much as several thousand degrees, but the resulting spectra are surprisingly similar.

Hydrogen

The model hydrogen atom has been generalized to an arbitrary multi-level atom (Ferguson and Ferland 1996). The hydrogen levels command is used to specify the number of levels to be used. The collision strengths have been changed to Vriens and Smeets (1980) for levels higher than 3, and Callaway (1994) for collisions with 1, 2 and 3.

Predicted infrared line intensities are now correct for all densities and temperatures greater than 103 K. Versions before 89 used a well l-mixed hydrogen atom, and its predictions were not correct for some infrared lines at low densities.

The routine that computes the free-free gaunt factors has been extended to include the full range the code can handle.

The helium ion

The helium ionization balance at low photon and particle densities, and at high particle densities, has always been exact, and this continues to be the case. There was a problem in the helium ion for high radiation densities, in versions 87 and before. The code used three pseudo levels to represent the levels between 7 and 1000, for H, He, and He+. This seemed to work well for the atoms for the cases of high densities, but testing has shown that it did not represent the physics of the high radiation density limit well. The problem is that the pseudo-levels had very large statistical weights, they represented line energies in the far infrared, and had A’s appropriate for lower levels. As a result they had very large induced rates when the photon occupation numbers were large, and this affected populations of lower levels. As a result the atom became too ionized - as much as a factor of two for He+. The following test illustrates this problem:

title helium ionization in high photon density limit

print departure coef

set dr 0

stop zone 1

constant temper 4

hden 11.000

phi(h) 20.750 range 1

stop thickness 11.7

table agn

In versions 88 and later no pseudo levels are used for any hydrogen or helium atom or ion.

Heavy elements

The atomic data base, the organization of aspects of the code dealing with storing heavy element ionization, and all the associated routines, have been totally rewritten. The lightest 30 elements are now included. Photoionization data are from Verner et al. (1996), recombination data partially from Verner and Ferland (1996), and roughly 104 lines of the heavy elements have been added (Verner, Verner, and Ferland 1996).

The number of resonance lines has increased by more than an order of magnitude. All resonance lines listed by Verner, Verner, and Ferland (1996) are included. As a result of these many additional lines the cooling function tends to be larger and smoother.

All lines are now fully transferred, and include pumping by the attenuated incident continuum as a general excitation mechanism. Pumping can be a significant contributor to the formation of weak high excitation lines.

The default solar mixture has been changed to Grevesse and Noel (1993). The biggest change is in the iron abundance. Previous versions had used a higher photospheric abundance. The current version is the 1993 suggested meteoritic abundance.

Free-free, line heating and cooling

These are counted in a different but equivalent manner. Now the difference between cooling and heating is used, since this is more numerically stable at high radiation densities. This difference has no physical affect on the predictions, but the printed contributors to the total heating and cooling do appear different.

Excited state photoionization cross sections

OP data are now used. For the excited state of Mg+ this is nearly ten times smaller than old screened hydrogenic values. This affects the intensity of Mg II l2798 in some BLR calculations.

The O+ photoionization cross section

Figure 17 This figure shows the O+ photoionization cross sections now used, compared to the Reilman and Manson values, and used in version 84 and before.

The Reilman and Manson photoionization cross sections, used before version 87, show a jump in the photoionization cross section at the 2s - 2p edge, and low values above that threshold extending up to the valence electron threshold. Opacity Project cross sections are used in the current version of the code, and these do not show the 2s edge (the OP calculations find that the 2s and 2p electrons are highly correlated). The cross section remains large up to the valence threshold. The difference approaches a factor of two, and this affects high ionization parameter clouds since O+ is the dominant opacity for some energies.

Verner, et al. (1996) comment on all other cases where the photoionization cross sections have changed. There are generally atoms and first ions where Opacity Project data are now available.


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