Excitation and Excavation of the Claws of the Southern Crab

Abstract

We show that the Southern Crab (aka Hen2–104) presents an auspicious opportunity to study the form and speed of the invisible winds that excavate and shock the lobes of various types of bipolar nebulae associated with close and highly evolved binary stars. A deep three-color image overlay of Hen2–104 reveals that the ionization state of its lobe edges, or “claws,” increases steadily from singly to doubly ionized values with increasing wall latitude. This “reverse” ionization pattern is unique among planetary nebulae (and similar objects) and incompatible with UV photoionization from a central source. We show that the most self-consistent explanation for the ionization pattern is shock ionization by a fast (∼600 km s−1) “tapered” stellar wind in which the speed and momentum flux of the wind increase with equatorial latitude. We present a hydrodynamic simulation that places the latitude-dependent form, the knotty walls, and the reverse ionization of the outer lobes of Hen2–104 into a unified context.