He I emission lines in symbiotic stars

Abstract

We present an analysis of the HeI λλ5876, 6678, 7065 emission lines for a large sample of symbiotic binary stars. As in earlier studies, we find significant deviations from case B predictions for every system considered. In most cases, the λλ5876, 6678 lines distinguish between S-and D-type symbiotics in that I(λ6678)/I(λ5876) ≳ 0.50 for S-types and I(λ6678)/I(λ5876) ∼ 0.25 for D-types. We follow Almog & Netzer and derive predicted intensity ratios for comparison with the observations. Our models indicate densities of ne∼1010 cm-3 for most S-type systems and ne ≲ 1010 cm-3 for all D-type systems. The HeI intensity ratios require large optical depths in the HeI λ3889 line, τ3889 ∼ 100-1000, for most sources; the nebular sizes are then roughly 1 au for S-types and 10 au for D-types. These results agree with previous density and size estimates for symbiotics; thus our He I model provides a useful diagnostic for the physical conditions within dense photoionized nebulae.