Ultraviolet spectra of planetary nebulae - X. Physical conditions in the compact planetary nebula SW ST 1.

Abstract

The authors analyse photoelectric visual and ultraviolet observations of the compact planetary nebula Sw St 1. The electron density, determined from the C III λλ1907/1909 line ratio, is N_e = (1.1±0.1)×10⁵cm^-3, consistent with the high emission measure and high critical frequency determined from observations of the thermal radio emission. The C/O abundance ratio in the nebula is found to be N(C)/N(O) = 0.72±0.1, i.e. the envelope is oxygen rich, as suggested by the identification of the silicate feature in the 8 - 13 μm infrared spectrum. Difficulties remain in accurately determining the reddening constant to the nebula and its electron temperature.