THE DOUBLE-SHELL STRUCTURE OF THE VARIABLE YOUNG PLANETARY NEBULA IC 4997

摘要

VLA-A 3.6 cm continuum and H92α observations and long-slit optical spectroscopy of IC 4997 are presented. The radio continuum map shows that IC 4997 is a double-shell planetary nebula consisting of a faint, knotty outer shell of size approx=2.7″x 1.4″, elongated at position angle approx=54°, and a bright, circular inner shell of approx=0.28″ in diameter. The outer shell presents an hourglass-like morphology and exhibits a striking mirror symmetry with respect to its minor axis. The deduced mean electron densities are approx=1.2 x 10~4 cm~(-3) and approx=1.4 x 10~5 cm~(-3) in the outer and inner shells, respectively. The knots in the outer shell present electron densities of approx=3-4 x 10~4 cm~(-3), somewhat higher than the mean. The analysis of the Hα, [N Ⅱ], and [S Ⅱ] emission lines, spatially and spectrally resolved in the spectrum, allows us to deduce the basic kinematical properties of the two shells. The outer shell presents bipolar motions, expands at approx=12 km s~(-1) in the equatorial plane, and was formed several hundred years ago. The inner shell expands at approx=25 km s~(-1) and was ejected some decades ago. The Hα emission from the central star presents extremely wide wings that can be traced up to approx=5375 km s~(-1). This value is much higher than those previously reported for IC 4997. Our results suggest that the two shells are interacting and that a dense region may have been formed between the two shells. The physical conditions in this dense region could vary as the inner shell expands, and, therefore, variations of the emitted spectrum from this region are expected. This interaction should be taken into account in order to explain the observed variability of the emission lines in the nebula.