The very massive star , Eta Carinae ( \eta Car ) , is enshrouded in an unusual complex of stellar ejecta , which is highly depleted in C and O , and enriched in He and N. This circumstellar gas gives rise to distinct absorption components corresponding to at least 20 different velocities along the line-of-sight .
The velocity component at - 513 km s ^ { -1 } exhibits very low ionization with predominantly neutral species of iron-peak elements .
Our statistical equilibrium/photoionization modeling indicates that the low temperature ( T = 760 K ) and high density ( n _ { \mathrm { H } } \sim 10 ^ { 7 } cm ^ { -3 } ) of the - 513 km s ^ { -1 } component is conducive to molecule formation including those with the elements C and O .
Examination of echelle spectra obtained with the Space Telescope Imaging Spectrograph ( STIS ) aboard the Hubble Space Telescope ( HST ) confirms the model ’ s predictions .
The molecules , H _ { 2 } , CH , and most likely OH , have been identified in the - 513 km s ^ { -1 } absorption spectrum .
This paper presents the analysis of the HST /STIS spectra with the deduced column densities for CH , OH and C I , and upper limit for CO .
It is quite extraordinary to see molecular species in a cool environment at such a high velocity .
The sharp molecular and ionic absorptions in this extensively CNO- processed material offers us a unique environment for studying the chemistry , dust formation processes , and nucleosynthesis in the ejected layers of a highly evolved massive star .