  The mechanism of formation of the He I 10830 A triplet in cool stars has been
subject of debate for the last 30 years. A relation between the X-ray
luminosity and the He I 10830 A flux was found in cool stars, but the dominant
mechanism of formation in these stars (photoionization by coronal radiation
followed by recombination and cascade, or collisional excitation in the
chromosphere), has not yet been established. We use modern instrumentation
(NOT/SOFIN) and a direct measurement of the EUV flux, which photoionizes He I,
to investigate the formation mechanism of the line for the most active stars
which are frequently excluded from analysis. We have observed with an
unprecedented resolution (R~170,000) the He I 10830 A triplet in a set of 15
stars that were also observed with the Extreme Ultraviolet Explorer (EUVE) in
order to compare the line strengths with their EUV and X-ray fluxes. Active
dwarf and subgiant stars do not exhibit a relation between the EUV flux and the
equivalent width of the He I 10830 A line. Giant stars however, show a positive
correlation between the strength of the He I 10830 A absorption and the EUV and
X-ray fluxes. The strength of the C IV 1550 A emission does not correlate with
coronal fluxes in this sample of 15 stars. Active dwarf stars may have high
chromospheric densities thus allowing collisional excitation to dominate
photoionization/recombination processes in forming the He I 10830 A line.
Active giant stars possess lower gravities, and lower chromospheric densities
than dwarfs, allowing for photoexcitation processes to become important.
Moreover, their extended chromospheres allowfor scattering of infrared
continuum radiation, producing strong absorption in He I and tracing wind
dynamics.
