科研之友
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摘要
Aims. We present a pilot study on non-local thermodynamic equilibrium (NLTE) line-formation computations for the isotopes He-3 and He-4 in the mercury-manganese star. Cancri. The impact of NLTE effects on the determination of isotopic abundances and the vertical stratification of helium in the atmosphere is investigated. %26lt;br%26gt;Methods. Modern NLTE line-formation computations were employed to analyse a high-resolution and high signal-to-noise ratio ESO-VLT/UVES spectrum of kCnc. The atmospheric parameters were determined from fitting the hydrogen Balmer lines and the spectral energy distribution. Multiple He I lines were investigated, including He I lambda 4921 angstrom and lambda 6678 angstrom, which show the widest isotopic splits. %26lt;br%26gt;Results. Half of the observed He I lines in the spectrum of kappa Cnc show significant NLTE strengthening, the effects are strongest in the red lines He I lambda 5875 angstrom and He I lambda 6678 angstrom. NLTE abundances from individual He I lines are up to a factor of similar to 3 lower than LTE values. Helium is found to be stratified in the atmosphere of kCnc. While the LTE analysis indicates a step-like profile of the helium abundance, a gradual decrease with height is indicated by the NLTE analysis. A He-3/He-4 ratio of similar to 0.25-0.30 is found. With the available data it cannot be decided whether the two isotopes follow the same stratification profile, or not. %26lt;br%26gt;Conclusions. This work implies that NLTE effects may be ubiquitous in the atmospheres of HgMn stars and may have a significant impact on abundance determinations and the interpretation of the vertical abundance stratification of elements.
- 出版日期2014-12
