Time-dependent Stellar Flare Models of Deep Atmospheric Heating

• Published in: The Astrophysical journal 2024-07, Vol.969 (2), p.121
• Main Authors: Kowalski, Adam F., Allred, Joel C., Carlsson, Mats
• Format: Article
• Language: English
• Subjects: Astronomical models; Atmospheric heating; Atmospheric models; Blackbody; Electron beams; Flare stars; High energy electrons; Line spectra; Optical flares; Optical properties; Pressure broadening; Red dwarf flare stars; Short pulses; Spectra; Stellar flares; Stellar models; Time dependence; White light
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad4148

Optical flares have been observed from magnetically active stars for many decades; unsurprisingly, the spectra and temporal evolution are complicated. For example, the shortcomings of optically thin, static slab models have long been recognized when confronted with the observations. A less incorrect—but equally simple—phenomenological T ≈ 9000 K blackbody model has instead been widely adopted in the absence of realistic (i.e., observationally tested) time-dependent, atmospheric models that are readily available. We use the RADYN code to calculate a grid of 1D radiative-hydrodynamic stellar flare models that are driven by short pulses of electron-beam heating. The flare heating rates in the low atmosphere vary over many orders of magnitude in the grid, and we show that the models with high-energy electron beams compare well to the global trends in flux ratios from impulsive-phase stellar flare, optical spectra. The models also match detailed spectral line-shape properties. We find that the pressure broadening and optical depths account for the broad components of the hydrogen Balmer γ lines in a powerful flare with echelle spectra. The self-consistent formation of the wings and nearby continuum level provides insight into how high-energy electron-beam heating evolves from the impulsive to the gradual decay phase in white-light stellar flares. The grid is publicly available, and we discuss possible applications.