Excitation & Excavation of the Claws of the Southern Crab

TL;DR

This paper investigates the unique ionization pattern of the Southern Crab nebula, attributing it to shock ionization from a fast, latitude-dependent stellar wind, supported by hydrodynamic simulations.

Contribution

It introduces the first detailed analysis linking the nebula's reverse ionization pattern to shock ionization by a tapered stellar wind, supported by hydrodynamic modeling.

Findings

Ionization state increases with wall latitude, contrary to typical nebulae.

Shock ionization explains the reverse ionization pattern, not UV photoionization.

Hydrodynamic simulation supports the wind-driven excitation model.

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.