Chromospheres in Metal-Poor Stars Evidenced from the He I 10830 Å Line

Based on the near-IR spectra of 33 late-type stars in a wide metallicity range (mainly dwarfs and partly giants) obtained with IRCS+AO188 of the Subaru Telescope, we confirmed that the He I 10830 angstrom line is seen in absorption in almost all moderately to extremely metal-poor stars of thick disk and halo population (from [Fe/H] similar to -0.5 down to [Fe/H] similar to -3.7), the strength of which is almost constant irrespective of the metallicity. This is 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.