Eta Carinae & the Homunculus: Far Infrared, Sub-millimeter Spectral
  Lines
  The evolved massive binary star Eta Carinae underwent eruptive mass loss
events that formed the complex bi-polar Homunculus nebula harboring tens of
solar masses of unusually nitrogen-rich gas and dust. Despite expectations for
the presence of a significant molecular component to the gas, detections have
been observationally challenged by limited access to the far-infrared and the
intense thermal continuum. A spectral survey of the atomic and rotational
molecular transitions was carried out with the Herschel Space Observatory,
revealing a rich spectrum of broad emission lines originating in the ejecta.
Velocity profiles of selected PACS lines correlate well with known
substructures: H I in the central core; NH and weak [C II] within the
Homunculus; and [N II] emissions in fast-moving structures external to the
Homunculus. We have identified transitions from [O I], H I, and 18 separate
light C- and O-bearing molecules including CO, CH, CH+, and OH, and a wide set
of N-bearing molecules, NH, NH+, N2H+, NH2, NH3, HCN, HNC, CN, and N2H+. Half
of these are new detections unprecedented for any early-type massive star
environment. A very low ratio [12C/13C] LE 4 is estimated from five molecules
and their isotopologues. We demonstrate that non-LTE effects due to the strong
continuum are significant. Abundance patterns are consistent with line
formation in regions of carbon and oxygen depletions with nitrogen
enhancements, reflecting an evolved state of the erupting star with efficient
transport of CNO-processed material to the outer layers. The results offer many
opportunities for further observational and theoretical investigations of the
molecular chemistry under extreme physical and chemical conditions around
massive stars in their final stages of evolution.
