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Spatial distribution and kinematics of the molecular material associated with $η$ Carinae

Single-dish sub-millimeter observations have recently revealed the existence of a substantial, chemically peculiar, molecular gas component located in the innermost circumstellar environment of the very massive luminous blue variable star $η$ Carinae. Here, we present 5$"$-resolution interferometric observations of the 1$\rightarrow$0 rotational transition of hydrogen cyanide (HCN) obtained with the Australia Telescope Compact Array (ATCA) toward this star. The emission is concentrated in the central few arcseconds around $η$ Carinae and shows a clear 150 km s$^{-1}$ velocity gradient running from west-north-west (blue) to east-south-east (red). Given the extent, location, and kinematics of this molecular material, we associate it with the complex of dusty arcs and knots seen in mid-infrared emission near the center of the Homunculus nebula. Indeed, the shielding provided by this dust could help explain how molecules survive in the presence of the intense UV radiation field produced by $η$ Carinae. The dust located in the central few arcseconds around $η$ Carinae and the molecular component described here have most likely formed in situ, out of material expelled by the massive interacting binary system. Thus, $η$ Carinae offers us a rare glimpse on the processes leading to the formation of dust and molecules around massive stars that are so relevant to the interpretation of dust and molecule detections at high redshifts.