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The mechanism of formation of the He I 10830 A triplet in cool stars has beensubject of debate for the last 30 years. A relation between the X-rayluminosity and the He I 10830 A flux was found in cool stars, but the dominantmechanism of formation in these stars (photoionization by coronal radiationfollowed by recombination and cascade, or collisional excitation in thechromosphere), 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 starswhich are frequently excluded from analysis. We have observed with anunprecedented resolution (R~170,000) the He I 10830 A triplet in a set of 15stars that were also observed with the Extreme Ultraviolet Explorer (EUVE) inorder to compare the line strengths with their EUV and X-ray fluxes. Activedwarf and subgiant stars do not exhibit a relation between the EUV flux and theequivalent width of the He I 10830 A line. Giant stars however, show a positivecorrelation between the strength of the He I 10830 A absorption and the EUV andX-ray fluxes. The strength of the C IV 1550 A emission does not correlate withcoronal fluxes in this sample of 15 stars. Active dwarf stars may have highchromospheric densities thus allowing collisional excitation to dominatephotoionization/recombination processes in forming the He I 10830 A line.Active giant stars possess lower gravities, and lower chromospheric densitiesthan dwarfs, allowing for photoexcitation processes to become important.Moreover, their extended chromospheres allowfor scattering of infraredcontinuum radiation, producing strong absorption in He I and tracing winddynamics.

Active cool stars and He I 10830 \AA: the coronal connection