[Ti ii] and [Ni ii] emission from the strontium filament of η Carinae |
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| Authors: | M A Bautista H Hartman T R Gull N Smith K Lodders |
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| Institution: | Centro de Física, IVIC, PO Box 21827, Caracas 1020A, Venezuela;Lund Observatory, Lund University, Box 43, SE-22100 Lund, Sweden;Code 667, NASA Goddard Space Flight Center, Greenbelt, MD, USA;Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, CO 80309, USA;Planetary Chemistry Laboratory, Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA |
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| Abstract: | We study the nature of the Ti ii ] and Ni ii ] emission from the so-called strontium filament found in the ejecta of η Carinae. To this purpose, we employ multilevel models of the Ti ii and Ni ii systems, which are used to investigate the physical condition of the filament and the excitation mechanisms of the observed lines. For the Ti ii ion, for which no atomic data were previously available, we carry out ab initio calculations of radiative transition rates and electron impact excitation rate coefficients. It is found that the observed spectrum is consistent with the lines being excited in a mostly neutral region with an electron density of the order of 107 cm?3 and a temperature around 6000 K. In analysing three observations with different slit orientations recorded between 2000 March and 2001 November, we find line ratios that change among various observations, in a way consistent with changes of up to an order of magnitude in the strength of the continuum radiation field. These changes result from different samplings of the extended filament due to the different slit orientations used for each observation, and yield clues on the spatial extent and optical depth of the filament. The observed emission indicates a large Ti/Ni abundance ratio relative to solar abundances. It is suggested that the observed high Ti/Ni ratio in gas is caused by dust–gas fractionation processes and does not reflect the absolute Ti/Ni ratio in the ejecta of η Carinae. We study the condensation chemistry of Ti, Ni and Fe within the filament and suggest that the observed gas phase overabundance of Ti is likely the result of selective photoevaporation of Ti-bearing grains. Some mechanisms for such a scenario are proposed. |
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| Keywords: | atomic data atomic processes line: formation stars: abundances |
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