Non-thermal motions and atmospheric heating of cool stars

Abstract

The magnetic processes associated with the non-thermal broadening of optically thin emission lines appear to carry enough energy to heat the corona and accelerate the solar wind. We investigate whether non-thermal motions in cool stars exhibit the same behaviour as on the Sun by analysing archival stellar spectra taken by the Hubble Space Telescope, and full-disc Solar spectra taken by the Interface Region Imaging Spectrograph. We determined the non-thermal 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 non-thermal broadening is caused by the presence of Alfvén waves, we also determined the associated wave energy densities. Our results show that, with a non-thermal velocity of $\sim$23 kms${}^{-1}$ the Sun-as-a-star results are in very good agreement with values obtained from spatially-resolved solar observations. The non-thermal broadening in our sample show correlation to stellar rotation, with the strength of the non-thermal velocity decreasing with decreasing rotation rate. Finally, the non-thermal velocity in cool Sun-like stars varies with atmospheric height or temperature of the emission lines, and peaks at transition region temperatures. This points towards 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 process such as flaring events could also dominate.

13 pages, 8 Figures, accepted for publication in The Astrophysical Journal

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