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Variable Spectra of IC 4997 and NGC 6572

Published online by Cambridge University Press:  07 August 2017

Siek Hyung ,
Lawrence H. Aller  and
Walter A. Feibelman
Siek Hyung
Affiliation:
University of California, 8979 Math Science bldg., 405 Hilgard Ave. Los Angeles, California 90024-1562
Lawrence H. Aller
Affiliation:
University of California, 8979 Math Science bldg., 405 Hilgard Ave. Los Angeles, California 90024-1562
Walter A. Feibelman
Affiliation:
NASA/Goddard Space Flight Center, Code 684, Greenbelt, Maryland 2077

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Variability of the [OIII]4363/4340 H γ ratio in IC 4007 was established in 1956 by William Liller and L.H. Aller who attributed the changes to a gradual decrease of electron density with time. Subsequent 4363/4340 ratio fluctuations negated this explanation. Ferland pointed out that small changes in the radiative flux of the Planetary nebula nucleus (PNN) could explains the variations. Our pervious study emphasized IUE observations, here we compare high dispersion spectra obtained with the Hamilton Echelle Spectrograph with previous measurements to asses line intensity variations. Emission line variability in PNN spectra as noted by Mendez et al. (1988) and by other for HeII 4686 in NGC 6572 may offer significant clues. PNN 4686 appeared by 1990 in IC 4997. Possibly both of these PNN may be evolving into Wolf-Rayet objects, but this development does not necessarily imply that the nebular excitation will increase with time.

Type
V. Planetary Nebulae Connection: Evolution to White Dwarfs
Information
Symposium - International Astronomical Union , Volume 155: Planetary Nebulae , 1993 , pp. 488
Copyright
Copyright © Kluwer 1993