  We present a pilot study on non-local thermodynamic equilibrium (NLTE)
line-formation computations for the isotopes 3He and 4He in the
mercury-manganese star kappa Cancri. The impact of NLTE effects on the
determination of isotopic abundances and the vertical stratification of helium
in the atmosphere is investigated. Modern NLTE line-formation computations were
employed to analyse a high-resolution and high signal-to-noise ratio
ESO-VLT/UVES spectrum of kap Cnc. The atmospheric parameters were determined
from fitting the hydrogen Balmer lines and the spectral energy distribution.
Multiple HeI lines were investigated, including HeI 4921A and 6678A, which show
the widest isotopic splits. Half of the observed HeI lines in the spectrum of
kap Cnc show significant NLTE strengthening, the effects are strongest in the
red lines HeI 5875A and HeI 6678A. NLTE abundances from individual HeI lines
are up to a factor of about 3 lower than LTE values. Helium is found to be
stratified in the atmosphere of kap Cnc. 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 3He/4He ratio of about 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. 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.
