Hen 2–104: a close-up look at the Southern Crab

Abstract

Aims. The kinematics, shaping, density distribution, expansion distance, and ionized mass of the nebula Hen 2–104, and the nature of its symbiotic Mira are investigated. Methods. A combination of multi-epoch HST images and VLT integral field high-resolution spectroscopy is used to study the nebular dynamics both along the line of sight and in the plane of the sky. These observations enable a 3-D spatio-kinematical model of the nebula to be constructed, which, with the measurement of its apparent expansion in the plane of the sky over a period of 4 years, provides the expansion parallax for the nebula. The integral-field data featuring the [Sii] λλ671.7, 673.1 emission line doublet provide a density map of the inner lobes of the nebula, which with the distance estimation provide a measurement of its ionized mass. Results. We measure densities ranging from ne = 500 to 1000 cm −3 in the inner lobes, and from 300 to 500 cm −3 in the outer lobes. We determine an expansion-parallax distance of 3.3 ± 0.9 kpc to Hen 2–104, which implies an unexpectedly large ionized mass for the nebula of approximately one tenth of a solar mass.