Nonthermal Motions and Atmospheric Heating of Cool Stars

Abstract

The magnetic processes associated with the nonthermal broadening of optically thin emission lines appear to carry enough energy to heat the corona and accelerate the solar wind. We investigate whether nonthermal motions in cool stars exhibit the same behavior as on the Sun by analyzing archival stellar spectra taken by the Hubble Space Telescope, and full-disk Solar spectra taken by the Interface Region Imaging Spectrograph. We determined the nonthermal velocities by measuring the excess broadening in optically thin emission lines formed in the stellar atmosphere; the chromosphere, the transition region, and the corona. Assuming the nonthermal broadening is caused by the presence of Alfvén waves, we also determined the associated wave energy densities. Our results show that with a nonthermal velocity of ∼23 km s ^−1 the Sun-as-a-star results are in very good agreement with values obtained from spatially resolved solar observations. The nonthermal broadening in our sample shows a correlation to stellar rotation, with the strength of the nonthermal velocity decreasing with decreasing rotation rate. Finally, the nonthermal velocity in cool Sun-like stars varies with atmospheric height or temperature of the emission lines, and peaks at transition region temperatures. This points toward a solar-like Alfvén wave-driven heating in stellar atmospheres. However, the peak is at a lower temperature in some cool stars suggesting that other magnetic processes such as flaring events could also dominate.

Keywords

• Stellar atmospheres
• Stellar winds