Simultaneous Production of Disk and Lobes: A Single‐Wind MHD Model for the η Carinae Nebula

Abstract

The luminous blue variable {η} Carinae is surrounded by a complex andhighly structured nebula of ejected material. The best-studied andaxisymmetric components of this outflow consist of bipolar lobes (the``homunculus'') and an equatorial ``skirt.'' Recent proper-motionmeasurements suggest that the skirt was ejected at the same time as thelobes, contrary to the assumptions of all current theoretical models forthe formation of the nebula (which use the skirt to collimate stellarwinds into lobes). We present a magnetohydrodynamic (MHD) stellar windmodel that produces an outflowing disk and bipolar lobes in a single,steady state wind. The basic model consists of a wind from a rotatingstar with a rotation axis-aligned dipole magnetic field. The azimuthalcomponent of the magnetic field, generated by stellar rotation,compresses the wind toward the equator and also toward the rotationaxis, simultaneously producing an outflowing disk and jet. We usenumerical MHD simulations to study the wind for various amounts ofstellar rotation and to show a range of wind morphologies. In order toproduce wide-angle lobes similar to the homunculus (which have roughly a30{\deg} opening angle), a high-speed polar wind (with enhanced energydensity) from the star is also required. In that case, the structure ofthe wind bears a remarkable resemblance to the skirt plus homunculusmorphology of the {η} Car nebulae, and a significant fraction of thestellar angular momentum is carried away by the wind. Although the modelassumes a steady state wind (rather than an eruption) and thermal winddriving (rather than radiation pressure), the structure of the wind isencouraging.