Physical structure of planetary nebulae. III. The large and evolved NGC 1360

NGC 1360 is a large planetary nebula (PN) without an obvious shell morphology. We have analyzed Halpha images and high-dispersion echelle spectra of NGC 1360 in order to construct spatio-kinematic models and to determine its density distribution. The best-fit model indicates that NGC 1360 is a prolate ellipsoidal shell whose major axis is twice as long as its minor axis and is tilted by 60degrees with respect to the line of sight. The large kinematic age of the shell, similar to10,000 yr, and the low density of the nebula, less than or equal to130 H atom cm(-3), imply that NGC 1360 is an evolved PN and has begun to merge with the interstellar medium. The observed morphology and surface brightness profiles of NGC 1360 can be described well as a thick shell with a Gaussian radial density profile without a sharp inner edge, indicating a lack of ongoing compression by a fast stellar wind. The fast, low-ionization emission regions observed in NGC 1360 near the end of its major axis expand faster than the shell and are younger than the nebular shell.