Ultraviolet observations of planetary nebulae. I. Determination of extinction.

The problem of interstellar extinction in UV observations of planetary nebulae is discussed. Methods used to determine extinction are reviewed, and it is proposed that the excess interstellar extinction at 2200 A can be employed to determine the extinction along the line of sight to a planetary. Observational data are presented which confirm the two assumptions implicit in this technique, viz., that a common normalized extinction curve exists in all directions and that the intrinsic spectrum of a planetary is roughly continuous. (B-V) color excesses derived according to the proposed technique are given for 31 planetaries, and the values are compared with observed ratios of 6-cm radio flux to H-beta flux. The IR excess of planetary nebulae is examined, showing that the matter responsible for the measured excess is not the same as that responsible for the extinction. A value of approximately 3.2 is obtained for the ratio of total to selective extinction.