EDP Sciences
Free access
Volume 477, Number 1, January I 2008
Page(s) 255 - 265
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20078420

A&A 477, 255-265 (2008)
DOI: 10.1051/0004-6361:20078420

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

M. Eriksson1, 2, H. Nilsson 3, H. Veenhuizen 1, and K. S. Long2

1  University of Kalmar, 391 82 Kalmar, Sweden
    e-mail: mattias@astro.lu.se
2  Space Telescope Science Institute, 3700 San Martin drive, Baltimore, MD 21218, USA
3  Atomic Astrophysics, Lund Observatory, Box 43, 221 00 Lund, Sweden

(Received 4 August 2007 / Accepted 12 September 2007)

Aims.We describe how the $\ion{C}{iv}$ $\lambda$1548.18 line pumps the 1548.20 and 1548.41 Å channels of $\ion{Fe}{ii}$ in symbiotic stars through the process known as photo-ionization by accidental resonance (PAR). We describe where and why $\ion{Fe}{ii}$ fluorescence arises in symbiotic stars and whether the $\ion{Fe}{ii}$ $\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 $\ion{C}{iv}$-pumped $\ion{Fe}{ii}$ fluorescence lines in symbiotic stars, corresponding to the y4H11/2 and w2D3/2 levels, based on a simple geometrical model of the emitting regions. We apply the model to seven symbiotic stars, known to have $\ion{Fe}{ii}$ fluorescence lines pumped by $\ion{C}{iv}$ 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 $\ion{Fe}{ii}$ 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.

Key words: binaries: symbiotic -- radiation mechanisms: non-thermal -- stars: atmospheres

© ESO 2007