Multiwavelength observations of a giant flare on CN Leonis I. The chromosphere as seen in the optical spectra

TL;DR

This study presents multiwavelength optical and X-ray observations of a major flare on the M5.5 dwarf star CN Leo, revealing extensive chromospheric emission lines, line asymmetries indicating mass motions, and continuum enhancements during the flare.

Contribution

First detailed optical spectral atlas of a stellar flare with 1143 emission lines, including the first observation of asymmetries in Ca II lines caused by a flare.

Findings

1143 emission lines identified, expanding red spectrum observations tenfold

Detected line asymmetries in H I, He I, Ca II lines indicating mass motions

Observed continuum enhancement and slope inversion during the flare

Abstract

Flares on dM stars contain plasmas at very different temperatures and thus affect a wide wavelength range in the electromagnetic spectrum. While the coronal properties of flares are studied best in X-rays, the chromosphere of the star is observed best in the optical and ultraviolet ranges. Therefore, multiwavelength observations are essential to study flare properties throughout the atmosphere of a star. We analysed simultaneous observations with UVES/VLT and XMM-Newton of the active M5.5 dwarf CN Leo (Gl 406) exhibiting a major flare. The optical data cover the wavelength range from 3000 to 10000 Angstrom. From our optical data, we find an enormous wealth of chromospheric emission lines occurring throughout the spectrum. We identify a total of 1143 emission lines, out of which 154 are located in the red arm, increasing the number of observed emission lines in this red wavelength range by about a factor of 10. Here we present an emission line list and a spectral atlas. We also find line asymmetries for H I, He I, and Ca II lines. For the last, this is the first observation of asymmetries due to a stellar flare. During the flare onset, there is additional flux found in the blue wing, while in the decay phase, additional flux is found in the red wing. We interpret both features as caused by mass motions. In addition to the lines, the flare manifests itself in the enhancement of the continuum throughout the whole spectrum, inverting the normal slope for the net flare spectrum.