Based on the near-IR spectra of 33 late-type stars in the wide metallicity range ( mainly dwarfs and partly giants ) obtained with IRCS+AO188 of the Subaru Telescope , we confirmed that He i 10830 \AA line is seen in absorption in almost all moderately to extremely metal-poor stars of thick disk and halo population ( from [ Fe/H ] \sim - 0.5 down to [ Fe/H ] \sim - 3.7 ) , the strength of which is almost constant irrespective of the metallicity .
This is an evidence that chromospheric activity at a basal level persists even for such old stars , despite that their rotations are considered to be slowed down and incapable of sustaining a dynamo , suggesting that some kind of chromospheric heating mechanism independent of rotation/magnetism ( e.g. , acoustic heating ) may take place .