Modeling of C IV pumped fluorescence of Fe II in symbiotic stars

Aims. We describe how the C IV lambda 1548.18 line pumps the 1548.20 and 1548.41 angstrom channels of Fe II in symbiotic stars through the process known as photo-ionization by accidental resonance (PAR). We describe where and why Fe II fluorescence arises in symbiotic stars and whether the Fe II lambda 1548.41 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. Methods. We calculate intensities of the C IV-pumped Fe II fluorescence lines in symbiotic stars, corresponding to the y(4)H(11/)2 and w(2)D(3/2) levels, based on a simple geometrical model of the emitting regions. We apply the model to seven symbiotic stars, known to have Fe II fluorescence lines pumped by C IV in their spectra. We compare the predicted intensities to the observed intensities of the selected symbiotic stars. Results. 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 Fe II lines corresponding to the w2D3/2 level in certain symbiotic stars provide evidence of hot shells expanding with velocities of at least hundreds km s(-1) in those systems.