Variability of Emission Lines in the Planetary Nebulae IC 4997 and NGC 6572
Feibelman, Walter A.; Hyung, Siek; Aller, Lawrence H.
United States
Abstract
We compare the new IUE high dispersion data and recent ground-based spectra of the planetary nebulae IC 4997 and NGC 6572 with equivalent IUE observations taken in 1980 and optical data taken in 1974-1990. Significant changes are detected in many emission line fluxes in IC 4997, and primarily in He II and C IV for NGC 6572. We interpret these changes as due to recent temperature increases in the central stars of the planetaries which are known to belong to a small set which have shown variability in the past. (SECTION: Interstellar Medium and Nebulae)