Scandium and chromium in the strontium filament in the Homunculus of ηCarinae

Abstract

We continue a systematic study of chemical abundances of the strontium filament found in the ejecta of ηCarinae. To this end we interpret the emission spectrum of ScII and CrII using multilevel non-local thermodynamic equilibrium models. Since the atomic data for these ions were previously unavailable, we carry out ab initio calculations of radiative transition rates and electron impact excitation rate coefficients. The observed spectrum is consistent with an electron density of the order of 10⁷cm^-3 and a temperature between 6000 and 7000K, conditions previously determined from [NiII], [TiII] and [SrII] diagnostics. The observed spectrum indicates an abundance of Sc relative to Ni more than 40 times the solar value, while the Cr/Ni abundance ratio is roughly solar. Various scenarios of depletion and dust destruction are suggested to explain such abnormal abundances.