A low-density cloud with solar (or higher) abundances seemed to crash or stop unexpectedly. What happened?
The first thing to do in any calculation is understand why the code stopped. In most cases it stops because the electron temperature falls below 4000K, unless this is reset with the stop temperature command. For classical nebulae the gas grows this cool only in neutral regions beyond the H+ – Ho ionization front, so the effect is to stop the calculation near this ionization front. If the gas has solar or higher abundances, and the density is 1,000 cm-3 or lower, the ionized part of the cloud can equilibrate at temperatures well below 1000K despite the high ionization and photoelectric heating. This is discussed, among other places, in Ferland et al. (1984; ApJ 281, 194, the test case dqher.in illustrates the phenomenon) and for HII regions by Shields and Kennicutt (1995, ApJ 454, 807). This is not a problem, and should occur in real nebulae. (It is not understood why pure recombination line HII regions are not observed.) Cloudy stopped because the gas was too cool. To compute the full
The first thing to do in any calculation is understand why the code stopped. In most cases it stops because the electron temperature falls below 4000K, unless this is reset with the stop temperature command. For classical nebulae the gas grows this cool only in neutral regions beyond the H+ – Ho ionization front, so the effect is to stop the calculation near this ionization front. If the gas has solar or higher abundances, and the density is 1,000 cm-3 or lower, the ionized part of the cloud can equilibrate at temperatures well below 1000K despite the high ionization and photoelectric heating. This is discussed, among other places, in Ferland et al. (1984; ApJ 281, 194, the test case dqher.in illustrates the phenomenon) and for HII regions by Shields and Kennicutt (1995, ApJ 454, 807). This is not a problem, and should occur in real nebulae. (It is not understood why pure recombination line HII regions are not observed.) Cloudy stopped because the gas was too cool. To compute the full
