default zone thickness gives incorrect line emissivity
I was contacted by a participant at the last Belfast workshop who was comparing the workshop version of Cloudy with c13. His group was generating large grids of line emissivities for various densities and kinetic temperatures, for gas exposed to the H&M background. He found very large differences in the emissivities of strong lines such as Lya. The problem can be distilled to the attached sims.
The sim is a single zone, constant temperature, exposed to the H&M background. I compared the trunk with c13_branch. The attached tests report the emissivity of Lya in the fiile *.em.
The file sdr.in behaves correctly and gets the same emissivity with c13 and trunk. It sets the zone thickness to 1 cm.
c13dr.in uses c13 and lets the thickness of the zone be set by the code. tdr.in does the same thing for the trunk. Here are the results for the zone thickness and the emissivity, erg/cm3/s in Lya
|| sim || thickness || 4pi J (Lya) ||
|| sdr || 1 || -28.7937 ||
|| c13dr || 4.647E+20 || -29.0139 ||
|| tdr || 4.613E+20 || -35.9298 ||
The emissivity is per unit volume so should be the same for all three, and equal to the first. Both c13 and the trunk get the same answer for sdr.