2002A&A...381..507K

In our investigation of the region of formation of the Fe II emission lines in RR Tel, we have applied the SAC method to the optical Fe II emission line fluxes measured by Crawford et al. (1999A&AS..139..135C). It is possible to determine physical parameters of the line emitting region, because of the presence both of lines with large and much smaller optical thicknesses. The values we obtained, given in Tables 3 and 4, are limits in most cases. The upper limit to the excitation temperature of the metastable levels in the permitted line region of 6600K, is close to the value of this temperature in the forbidden line region. Other excitation temperature limits corresponding to level population ratios were also found. The permitted lines are formed in a region with a radius between 1.8x10¹²cm and 1.6x10¹⁴cm, while the forbidden lines are formed in a region with a minimum radius of 5.3x10¹²cm. The populations of the lower levels of the permitted lines indicate a minimum column density of Fe+ of 3.0x10¹⁸atoms/cm² and, by assuming cosmic abundances, to a minimum column density of 7.5x10²²atoms/cm² for H. If we assume a minimum radius of 1.0x10¹⁴cm, which is supposed to be that of dust condensation of the cool Mira component of this symbiotic binary, a minimum mass loss rate of 3.5x10^–6M_☉/yr is found. Our results support a model according to which both the permitted and the forbidden lines are formed in the wind of the cool Mira component above the regions where absorption by dust is important. The forbidden and the permitted lines are not formed in quite the same region, the former being most probably formed further out in the part of an accelerated wind which is ionized by the hot component.