First Detection of Silicon-bearing Molecules in η Car

• Published in: Astrophysical journal. Letters Vol. 939; no. 2; p. L30
• Main Authors: Bordiu, C., Rizzo, J. R., Bufano, F., Quintana-Lacaci, G., Buemi, C., Leto, P., Cavallaro, F., Cerrigone, L., Ingallinera, A., Loru, S., Riggi, S., Trigilio, C., Umana, G., Sciacca, E.
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01.11.2022; IOP Publishing
• Subjects: Abundance; Astrochemistry; Asymptotic giant branch stars; Circumstellar matter; Clumps; Dust; Dust destruction; Ejecta; Equatorial regions; Life cycles; Luminous blue variable stars; Massive stars; Molecular gas; Nebulae; Silicon; Silicon compounds; Stellar mass loss; Sulfur; Supergiant stars; Vapor phases; Variable stars; Wind regime
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ac9b10

Abstract We present ALMA band 6 observations of the luminous blue variable η 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 sulfur-bearing molecules in the outskirts of a highly evolved, early-type massive star. The SiO, SiS, and SiN trace a clumpy equatorial ring that surrounds the central binary at a projected distance of ∼2″, 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 η Car, which destroys grains and releases silicon back to the gas phase. We discuss possible formation routes for the observed species, contextualizing them within the current molecular inventory of η Car. We find that the SiO and SiS fractional abundances in localized clumps of the ring, 6.7 × 10 −9 and 1.2 × 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 × 10 −8 , evidences the nitrogen-rich chemistry of the ejecta. These abundances must be regarded as strict upper limits, since the distribution of H 2 in the Homunculus is unknown. In any case, these findings shed new light on the peculiar molecular ecosystem of η Car and establish its surroundings as a new laboratory to investigate the life cycle of silicate dust in extreme astrophysical conditions.