The Double-Shell Structure of the Variable Young Planetary Nebula IC 4997

Abstract

VLA-A 3.6 cm continuum and H92␣ 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 32"7 ϫ 1"4, elongated at position angle 354Њ, and a bright, circular inner shell of 30"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 31.2 ϫ 10 4 cm Ϫ3 and 31.4 ϫ 10 5 cm Ϫ3 in the outer and inner shells, respectively. The knots in the outer shell present electron densities of 33-4 ϫ 10 4 cm Ϫ3 , somewhat higher than the mean. The analysis of the H␣, [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 312 km s Ϫ1 in the equatorial plane, and was formed several hundred years ago. The inner shell expands at 325 km s Ϫ1 and was ejected some decades ago. The H␣ emission from the central star presents extremely wide wings that can be traced up to 35375 km s Ϫ1 . 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. Subject headings: ISM: kinematics and dynamics -planetary nebulae: individual (IC 4997)

VLA observations at 7 mm of the planetary nebula IC 4997

“…This optical depth implies an electron density of 2.6 ± 0.3 × 10 6 cm −3 for the core. The parameters that we derive for the core are consistent with the values estimated from previous optical observations (Hyung et al 1994; Miranda et al 1996). In Fig.…”

“…An electron temperature of 14 000 K is taken from Kostyakova (1999). Finally, adopting an electron density of 1.2 × 10 4 cm −3 for the halo (Miranda et al 1996), we derive an optical depth of 0.78 at a frequency of 1 GHz. We have two free parameters.…”

“…All these data indicate not only that IC 4997 is still in the process of its formation as a PN, but also that considerable activity exists in the inner region of the PN. From optical spectroscopy and VLA 3.6‐cm imaging, Miranda et al (1996) showed that IC 4997 consists of two shells. The outer shell shows bipolar motions, expands at ≃ 12 km s −1 in the equatorial plane, and has a dynamical age of several hundred years.…”

High-Resolution Spectroscopy of the Planetary Nebulae PM 1–242, PM 1–318 and PM 1–322

“…These PNe are characterized by differences between the forbidden lines and Hα profiles, a double-peaked Hα profile, with very similar properties to these observed in PM 1-322, and extended Hα wings (e.g. Balick 1989;Miranda, Torrelles & Eiroa 1996;Guerrero et al 2001;Arrieta & Torres-Peimbert 2003). These results reinforce the idea that the central star of PM 1-322 is a symbiotic star.…”

Polarization of the broad Hα wing in symbiotic stars

“…Similar broad wings also appear in young planetary nebulae, including M2‐9, Mz3 and IC 4997 (e.g. Miranda, Torrelles & Eiroa 1996 ; Arrieta & Torres‐Peimbert 2002 ). In particular, for M2‐9, Balick (1989) reported the existence of very broad Hα wings with a width ∼10 4 km s −1 .…”