He 2-113:: A multipolar planetary nebula with rings around a cool Wolf-Rayet star

We present Her imaging and molecular-line (CO) observations of He 2-113, an object which belongs to a rare class of planetary nebulae with Wolf-Rayet type central stars. The Her images, obtained with the Hubble Space Telescope Wide-Field Planetary Camera 2, show that He 2-113 has an overall bipolar shape and two bright, knotty, compact ringlike structures around the central star. The major bipolar axis and the axis of symmetry of the rings are not colinear. A third, smaller, lobe is found to extend along an axis different from both of the above. The central star of He 2-113 is conspicuously offset from the geometrical centers of the rings and the main bipolar lobes. A faint extended round halo surrounds the bipolar nebula, probably representing the spherical mass-loss phase of the progenitor AGB red giant. The CO data indicate that this halo is mostly neutral and expanding at 28 km s(-1); an extended weak blue wing indicates that some of the molecular gas has been accelerated to significantly larger velocities. Self-consistent modeling of the CO data and IRAS far-infrared flux, including heating-cooling to calculate the gas kinetic temperature, shows that the mass-loss rate of the progenitor AGE star is relatively high (similar to 10(-4) M. yr(-1)) and that the molecular gas is relatively cold; the gas-to-dust ratio (by mass) is 190. The very aspherical morphology of the He 2-113 nebula may allow ionizing radiation to escape preferentially from the nebula along directions with relatively low optical depth. The observed nebular structure is not consistent with the generalized interacting winds model of planetary nebula formation. The multilobed structure supports a new model by Sahai & Trauger in which collimated outflows with changing directionality are: the primary agent for the shaping of planetary nebulae. The origin of the rings and the offset of the central star remain puzzling.