The Morphology of the Planetary Nebula NGC 3242

The study of planetary nebulae is useful in determining a methodology by which stars can return material to the interstellar medium. Double-shell planetary nebulae present a unique problem in which material appears to have been ejected at two different epochs in the stars' history. Through the study of these shells, insights can be gained into the energetics and structure of the nebulae. This in turn leads to a better understanding of not only how material is returned to the interstellar medium, but also to the formation processes of the planetary nebulae themselves. In this investigation, observations of the Balmer decrement were used to obtain information about the internal structure on the double-shell planetary nebula NGC 3242. Data were collected using the CCD direct imaging system at Kitt Peak National Observatory with interference filters for the principal three Balmer lines. This information was processed to obtain high resolution images of the Balmer -line ratios and subsequently calibrated using scanning spectrophotometer data obtained at Blue Mesa Observatory. It is known that variations in the Balmer decrement are associated with reddening, self-absorption, and changes in excitation mechanisms. In this work, variations in the intensity line ratios allow the definition and position of a shock wave to be determined. By using high resolution plots of the Balmer-line ratios, a simple straightforward method for finding and locating weak shocks in H II regions is here proposed. Evidence presented demonstrates that a shock front exists at the boundary between the inner and outer shells of NGC 3242. The discontinuity is moving into the outer shell at a speed of 1.3 times the speed of sound in that material. Temperature, density, and velocity variations in this nebula, as found by other investigators, support the model developed in this investigation for the existence of a shock front in NGC 3242.