High-resolution spectroscopy and NLTE calculations of the HeI 10830A line in magnetic chemically peculiar stars. A search for indirect evidences of hot stellar envelopes.

Observations of CIV and SIIV lines in the ultraviolet spectra, X-ray and radio emission for some of the hottest chemically peculiar stars have led to the suggestion that high temperature plasmas are present around them. A possible indirect way to prove the existence of a corona around the CP stars is to observe the HeI 10830A line since, in presence of a X-ray emitting corona, this line should disappear in the spectrum of B-type stars (Mitskevich & Tsymbal 1992). Searching for possible differences, we have obtained high resolution HeI 10830A spectra for 7 early type magnetic CP and 8 normal stars. The equivalent width of the HeI 10830A line, always in absorption, increases from A1 to B3 and then decreases. The HeI 10830A line is present in the spectrum of CP stars with the same strength as in normal stars. Comparison with Mitskevich & Tsymbal (1992) calculations suggests that no X-ray emitting plasma is present around the observed CP stars. We also present new NLTE calculations for HeI 10830A, analysing the sensitivity of this line to abundance variations. For normal main sequence stars of spectral type A1-B3 our computations are in closer agreement with observations than in previous work. In this spectral range, the line is found to be weakly dependent on the chemical composition.