$\eta$ Carinae Baby Homunculus Uncovered by ALMA

We report observations of $\eta$ Carinae obtained with ALMA in the continuum of 100, 230, 280 and 660 GHz in 2012 November, with a resolution that varied from 2.88" to 0.45" for the lower and higher frequencies respectively. The source is not resolved, even at the highest frequency; its spectrum is characteristic of thermal bremsstrahlung of a compact source, but different from the spectrum of optically thin wind. The recombination lines H42$\alpha$, He42$\alpha$, H40$\alpha$, He40$\alpha$, H50$\beta$, H28$\alpha$, He28$\alpha$, H21$\alpha$ and He21$\alpha$ were also detected and their intensities reveal non local thermodynamic equilibrium (NLTE) effects. We found that the line profiles could only be fit by an expanding shell of dense and ionized gas, which produces a slow shock in the surroundings of $\eta$ Carinae. Combined with fittings to the continuum, we were able to constrain the shell size, radius, density, temperature and velocity. The detection of the He recombination lines is compatible with the high temperature gas and requires a high energy ionizing photon flux, which must be provided by the companion star. The mass loss rate and wind velocity, necessary to explain the formation of the shell, are compatible with a LBV eruption. The position, velocity and physical parameters of the shell coincide with those of the Weigelt blobs. The dynamics found for the expanding shell corresponds to matter ejected by $\eta$ Carinae in 1941, in an event similar to that which formed the Little Homunculus ; for that reason we called the new ejecta the "Baby Homunculus".