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

TL;DR

This study demonstrates that metal-poor stars exhibit persistent chromospheric activity, evidenced by the He I 10830A absorption line, indicating a basal heating mechanism independent of stellar rotation or magnetic dynamo effects.

Contribution

It provides observational evidence of chromospheric activity in extremely metal-poor stars, suggesting a non-magnetic heating process like acoustic heating is at work.

Findings

He I 10830A line seen in almost all metal-poor stars studied

Chromospheric activity persists regardless of metallicity

Indicates a rotation/magnetism-independent heating mechanism

Abstract

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 10830A line is seen in absorption in almost all moderately to extremely metal-poor stars of thick disk and halo population (from [Fe/H]~ -0.5 down to [Fe/H]~ -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.