The EUV spectra of young planetary nebulae.

Abstract

High- and low-resolution EUV observations of the possible young planetary nebulae V1016 Cyg, HM Sge, and IC 4997 are reported which were made with the IUE satellite over the wavelength ranges from 1150 to 1950 A and from 1900 to 3200 A. The strongest emission lines observed in V1016 Cyg are identified, the spectra of the three objects in the region of the C III doublet at 1907 and 1909 A are illustrated, and electron densities are estimated. All three EUV spectra are shown to be those expected for planetary nebulae. Line widths and expansion velocities are determined for the C III line at 1909 A and the C IV line at 1549 A. The results indicate electron densities in excess of 1 million per cu cm for V1016 Cyg and HM Sge, an electron density of approximately 500,000 per cu cm for IC 4997, and nebular expansion velocities of 50 to 120 km/s. These features suggest that all three objects are recently formed planetary nebulae.