The peculiar chemistry of the inner ejecta of Eta Carina

We investigated continuum and molecular line emission of four species (CO, HCN, H¹³CN, and HCO⁺) at 0.8 mm in the inner region around η Car, using ALMA archival observations at a resolution better than 0.2 arcsec. We report the discovery of an asymmetric extended structure north-west of the star, independent from the continuum point source. The structure is only traced by continuum and HCO⁺, and not detected in the other lines. Kinematics of this structure reveal that the HCO⁺ gas likely arises from ejecta expelled in the 1890s eruption. The ejecta is propagating outwards within the cavity produced by the current wind-wind interaction of η Car A and its companion. Chemical analysis of the ejecta reveals an apparent lack of CO and nitrogen-bearing species. We explore possible explanations for this peculiar chemistry, that differentiates this structure from the ejecta of the Great Eruption, rich in HCN and H¹³CN. We also report an absorption component near the continuum point source, only traced by HCN and H¹³CN in their vibrational-ground and vibrationally excited states. This absorbing gas is attributed to a hot bullet of N-enriched material expelled at a projected velocity of 40 km s^-1.