Discovery of CH and OH in the -513 km s^-1 Ejecta of η Carinae

Abstract

The very massive star η Carinae (η 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_H~10⁷ 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) on board the Hubble Space Telescope (HST) confirms the model's predictions. The molecules H₂, 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 an 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 offer us a unique environment for studying the chemistry, dust formation processes, and nucleosynthesis in the ejected layers of a highly evolved massive star.