Multiple Coaxial Rings in the Bipolar Nebula Hubble 12

A series of two-dimensional rings along a common axis aligned with the bipolar lobes is found in the planetary nebula Hb 12. The rings have separations of about 0.3'', which can be translated into physical separations of ~1000 AU, or time separations of the order of 50 yr. We suggest that the existence of the rings is a manifestation of the interaction between a time-variable, collimated fast outflow with the circumstellar envelope created by the stellar wind of an asymptotic giant branch star. Comparison of Hb 12 with other bipolar nebulae with hourglass structures such as Hen 2-104, MyCn 18, and SN 1987A suggests that the nebula of Hb 12 may represent the inner nebula of a much larger bipolar structures, and we propose to search for such outer structures with a wide-field infrared camera.