First detection of silicon-bearing molecules in $\eta$ Car

Abstract

We present ALMA band 6 observations of the luminous blue variable Eta Car, obtained within the ALMAGAL program. We report SiO J=5-4, SiS J=12-11 and SiN N=5-4 emission in the equatorial region of the Homunculus nebula, constituting the first detection of silicon- and sulphur-bearing molecules in the outskirts of a highly evolved, early-type massive star. SiO, SiS and SiN trace a clumpy equatorial ring that surrounds the central binary at a projected distance of 2 arcsec, delineating the inner rims of the butterfly-shaped dusty region. The formation of silicon-bearing compounds is presumably related to the continuous recycling of dust due to the variable wind regime of Eta Car, that destroys grains and releases silicon back to gas phase. We discuss possible formation routes for the observed species, contextualizing them within the current molecular inventory of Eta Car. We find that the SiO and SiS fractional abundances in localised clumps of the ring, $6.7\times 10^{-9}$ and $1.2\times 10^{-8}$ respectively, are exceptionally lower than those measured in C- and O-rich AGB stars and cool supergiants; while the higher SiN abundance, $3.6\times 10^{-8}$, evidences the nitrogen-rich chemistry of the ejecta. These abundances must be regarded as strict upper limits, since the distribution of H2 in the Homunculus is unknown. In any case, these findings shed new light onto the peculiar molecular ecosystem of Eta Car, and establish its surroundings as a new laboratory to investigate the lifecycle of silicate dust in extreme astrophysical conditions.

17 pages, 7 figures. Accepted in ApJL

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