Active Stars and He I 10830 Å: The EUV Connection

Abstract

The metastable lower level of the He I 10830 line in active cool stars is believed to be populated by recombination following photoionization of neutral Helium present in the upper chromosphere. This transition is a powerful diagnostic of atmospheric dynamics of cool stars [1,4]. The EUV flux radiated in the corona (the ionization edge of He I is at 24.58 eV, or 504.27 Å) is responsible for the photoionization, and so it is decoupled from local thermodynamic conditions. Absorption of photospheric radiation can trace the atmospheric motions through the line profiles. Some of the studies that can be carried out from high-resolution spectra of the He I 10830 line include: (i) stellar activity in late type stars; (ii) measurements of winds and mass loss in late type giants and young stars; (iii) search for Helium present in the atmospheres of planets in close orbit around late type stars.