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

VLA-A 3.6 cm continuum and H99 alpha observations and long-slit optica l 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 similar or equal to 2.7'' x 1.4'', elonga ted at position angle similar or equal to 54 degrees, and a bright, ci rcular inner shell of similar or equal to 0.28'' in diameter. The oute r shell presents an hourglass-like morphology and exhibits a striking mirror symmetry with respect to its minor axis. The deduced mean elect ron densities are similar or equal to 1.2 x 10(4) cm(-3) and similar o r equal to 1.4 x 10(5) cm(-3) in the outer and inner shells, respectiv ely. The knots in the outer shell present electron densities of simila r or equal to 3-4 x 10(4) cm(-3), somewhat higher than the mean. The a nalysis of the H alpha, [N II], and [S II] emission lines, spatially a nd spectrally resolved in the spectrum, allows us to deduce the basic kinematical properties of the two shells. The outer shell presents bip olar motions, expands at similar or equal to 12 km s(-1) in the equato rial plane, and was formed several hundred years ago. The inner shell expands at similar or equal to 25 km s(-1) and was ejected some decade s ago. The H alpha emission from the central star presents extremely w ide wings that can be traced up to similar or equal to 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 den se region may have been formed between the two shells. The physical co nditions in this dense region could vary as the inner shell expands, a nd, therefore, variations of the emitted spectrum from this region are expected. This interaction should be taken into account in order to e xplain the observed variability of the emission lines in the nebula.