The Structure of the Homunculus: I. Shape and Latitude Dependence from H$_2$ and [Fe II] Velocity Maps of Eta Carinae

Nathan Smith

High resolution long-slit spectra obtained with the Phoenix
spectrograph on Gemini South provide our most accurate probe of the
three dimensional structure of the Homunculus Nebula around
$\eta$~Carinae. The new near-infrared spectra dramatically confirm
the double-shell structure inferred previously from thermal dust
emission, resolving the nebula into a very thin outer shell seen in
H$_2$ $\lambda$21218, and a warmer, thicker inner layer seen in
[Fe~{\sc ii}] $\lambda$16435. The remarkably thin and uniform H$_2$
skin has $\Delta$R/R of only a few per cent at the poles, hinting that
the most important mass loss during the 19th century eruption may have
had a very short duration of $\la$5 yr. H$_2$ emission traces the
majority of the more than 10 M$_{\odot}$ of material in the nebula,
and has an average density of order $n_H\ga$10$^{6.5}$ cm$^{-3}$.
This emission, in turn, yields our first definitive picture of the
exact shape of the nebula, plus a distance of 2350$\pm$50 pc and an
inclination angle of $\sim$41\arcdeg\ (the polar axis is tilted
49\arcdeg\ from the plane of the sky). The distribution of the H$_2$
emission provides the first measure of the latitude dependence of the
speed, mass loss, and kinetic energy associated with $\eta$~Car's 19th
century explosion. Almost 75\% of the total mass and more than 90\%
of the kinetic energy in the ejecta were released at high latitudes
between 45\arcdeg\ and the polar axis. This rules out a model for the
bipolar shape wherein an otherwise spherical explosion was pinched at
the waist by a circumstellar torus. Also, the ejecta could not have
been deflected toward polar trajectories by a companion star, since
the kinetic energy of the polar ejecta is greater than the binding
energy of the putative binary system. Instead, most of the mass
appears to have been directed poleward by the explosion itself --- or
the star failed to launch material from low latitudes, which would
have important consequences for the angular momentum evolution of the
star. In any case, comparing H$_2$ and [Fe~{\sc ii}] emission
resolves some puzzles about structure noted in previous studies.
H$_2$ emission also provides our first reliable picture of the
critical innermost waist of the Homunculus, yielding clues to the
observed morphology of the core and the more extended equatorial
debris.

Reference: 2006, ApJ, 644, 1151
Status: Manuscript has been accepted