Scandium and chromium in the strontium filament in the Homunculus of η Carinae

Abstract

We continue a systematic study of chemical abundances of the strontium filament found in the ejecta of η Carinae. To this end we interpret the emission spectrum of Sc ii and Cr ii 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 107 cm−3 and a temperature between 6000 and 7000 K, conditions previously determined from [Ni ii], [Ti ii] and [Sr ii] 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.

Electron-impact excitation of Cr ii

“…Collision and effective collision strengths are presented for all transitions among the lowest 74 Jπ states of Cr ii and comparisons made with the work of Bautista et al (2009). While the effective collision strengths agree well for some transitions, significant discrepancies exist for others.…”

“…3 we discuss the collisional calculation itself. Section 4 details the results obtained with a graphical and detailed comparison made with the previous data of Bautista et al (2009) for a selection of important low-lying transitions. This qualitative comparison involves both the collision strengths and the astrophysically desirable Maxwellian averaged effective collision strengths for a range of temperature values.…”

“…To date the largest calculation performed for this ion was reported by Bautista et al (2009) primarily as part of an ongoing study into the chemical abundances of the strontium filament found in the ejecta of η Carinae. In this calculation electron-impact collision strengths for transitions among the lowest 162 levels of Cr ii were evaluated.…”

“…In the present model we extend the calculation of Bautista et al (2009) by computing the fine-structure collisional data for…”

Unsporting scandium

“…This stellar anomaly is still being studied, as is a curious scandium-rich nebula that surrounds the Eta Carinae star system 8,000 light years away. It has been observed over the centuries to brighten and dim quite inexplicably 1 , and the role of scandium in this observation is still under investigation.…”