A non-LTE spectral analysis of the 3He and 4He isotopes in the HgMn star kappa Cancri

TL;DR

This study demonstrates that non-LTE spectral analysis significantly alters helium abundance and stratification measurements in the HgMn star kappa Cancri, highlighting the importance of NLTE effects in stellar atmosphere studies.

Contribution

The paper introduces the first detailed NLTE analysis of helium isotopes in a HgMn star, revealing the impact of NLTE effects on abundance and stratification determinations.

Findings

NLTE effects cause up to a threefold decrease in helium abundance estimates.

Helium in kappa Cancri is stratified with a gradual decrease in abundance with height.

NLTE analysis suggests different stratification profiles for 3He and 4He, but data is inconclusive.

Abstract

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.