The Structure of the Homunculus. I. Shape and Latitude Dependence from H₂and [Feii] Velocity Maps of η Carinae

Abstract

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 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 and a warmer, thicker inner layer seen in [Fe ii]. The remarkably thin and uniform H 2 skin hints that the most important mass loss during the 19th century eruption had a very short duration of P5 yr. H 2 emission traces the majority of the more than 10 M of material in the nebula and has an average density of order n H k 10 6:5 cm À3. This emission, in turn, yields our first definitive picture of the exact shape of the nebula, plus a distance of 2350 AE 50 pc and an inclination angle of $41 . 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 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 and the polar axis. This rules out a model for the bipolar shape in which an otherwise spherical explosion was pinched at the waist by a circumstellar torus. Instead, most of the mass appears to have been directed poleward by the explosion itself. 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.

A disrupted molecular torus around Eta Carinae as seen in 12CO with ALMA

“…The spatial offset between the locations of the warmest dust and the CO is only about 0. ′′ 5−1 ′′ , comparable to the thickness of the walls of the polar lobes themselves (Smith 2006). The hotter dust seen in the 8.8 µm emission traces a thin skin on the inside walls of the hollow polar lobes exposed to direct starlight, whereas the cooler dust and CO is shielded by the dense walls of the polar lobes and resides at slightly larger distances from the star.…”

“…Bottom: Same contours from the top panel, but superposed on the near-IR H 2 emission from the Homunculus, taken with the Phoenix spectrograph on Gemini South. This is the average of several slits oriented along the same position angle, from Smith (2006). torus traces the IR torus (see above), the kinematic structure of the CO emission overlaps perfectly with the near-IR H2 emission.…”

“…While we of course cannot see the far side of the equatorial region at visible wavelengths because of obscuration, it is clear that on the near side, the faint CO features are pushed to larger radii than the rest of the CO/IR torus, concident with the region of the peculiar equatorial ejecta. In fact, the CO emission a few arcseconds to the N and E of star corresponds to dark patches in the equatorial ejecta, which have been inferred to be regions of higher visual-wavelength (Smith 2006). This is an approximation of a "pole-on" view of the CO as seen by an observer in the SE polar lobe.…”

“…Figure 2 (bottom) compares the kinematics of CO seen by ALMA to the kinematics of molecular hydrogen, as seen in high-resolution spectra of the Homunculus in the 2.122 µm H2 line (Smith 2004(Smith , 2006. On the redshifted side of the CO torus (and toward negative positional offsets in Figure 1), where the CO emission is brightest and where the CO Figure 2.…”

“…The first reported detection of molecules in the Homunculus arose via its near-IR emission from H2 in several lines around 2 µm Smith 2002). This H2 emission traced very dense and very thin walls of the polar lobes of the Homunculus (Smith 2002(Smith , 2004(Smith , 2006Smith & Ferland 2007), but was not seen from the equatorial ejecta except at the pinched waist of the polar lobes. Absorption from H2 and several diatomic molecules like CH and OH was detected in UV spectroscopy of η Car obtained with HST, where the radial velocity and narrow line width indicated that this molecular gas was located in the thin walls of the south-east polar lobe Verner et al 2005).…”