He I emission lines in symbiotic stars

We present an analysis of the He i|λλ 5876, 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 |λλ 5876, 6678| lines distinguish between S- and D-type symbiotics in that |I (λ 6678)/I ( 5876) 0.50| for S-types and |I(λ 6678)/I(λ 5876) ~ 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 He i intensity ratios require large optical depths in the He i|λ 3889| 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.