Checking the validity of Busquet's ionization temperature with detailed collisional radiative models

Busquet's RADIOM model for effective ionization temperature T_z is an appealing and simple way to introduce non LTE effects in hydrocodes. We report here checking the validity of RADIOM in the optically thin case by comparison with two collisional radiative models, MICCRON (level-by-level) for C and Al and SCROLL (superconfiguration-by-superconfiguration) for Lu and Au. MICCRON is described in detail. The agreement between the average ion charge 〈 Z 〉 and the corresponding T_z obtained from RADIOM on the one hand, and from MICCRON on the other hand for C and Al is excellent. The absorption spectra at T_z agree very well with the one generated by SCROLL near LTE conditions (small β). Farther from LTE (large β) the agreement is still good, but another effective temperature gives an excellent agreement. It is concluded that the model of Busquet is very good in most cases. There is however room for improvement when the departure from LTE is more pronounced for heavy atoms and for emissivity. Improvement appears possible because the concept of ionization temperature seems to hold in a broader range of parameters.