2008A&A...477..255E

We describe how the CIV λ1548.18 line pumps the 1548.20 and 1548.41Å channels of FeII in symbiotic stars through the process known as photo-ionization by accidental resonance (PAR). We describe where and why FeII fluorescence arises in symbiotic stars and whether the FeII λ1548.41 channel can only be activated when there is a white-dwarf wind present in the system. Further, we aim to show how an analysis of the PAR-pumped lines helps to understand the phyisical conditions that they manifest.We calculate intensities of the CIV-pumped FeII fluorescence lines in symbiotic stars, corresponding to the y⁴H_11/2 and w²D_3/2 levels, based on a simple geometrical model of the emitting regions. We apply the model to seven symbiotic stars, known to have FeII fluorescence lines pumped by CIV in their spectra. We compare the predicted intensities to the observed intensities of the selected symbiotic stars.We find that we can reproduce the observed fluorescence intensities of the seven symbiotic stars in our sample, using parameters that are consistent with their known properties. We show that PAR-produced lines can be used as a diagnostic tool to derive important physical parameters of a stellar system. We suggest that the detection of FeII lines corresponding to the w²D_3/2 level in certain symbiotic stars provide evidence of hot shells expanding with velocities of at least hundreds km/s in those systems.