Stellar flares

Abstract

Magnetic storms on stars manifest as remarkable, randomly occurring changes of the luminosity over durations that are tiny in comparison to the normal evolution of stars. These stellar flares are bursts of electromagnetic radiation from X-ray to radio wavelengths, and they occur on most stars with outer convection zones. They are analogous to the events on the Sun known as solar flares, which impact our everyday life and modern technological society. Stellar flares, however, can attain much greater energies than those on the Sun. Despite this, we think that these phenomena are rather similar in origin to solar flares, which result from a catastrophic conversion of latent magnetic field energy into atmospheric heating within a region that is relatively small in comparison to normal stellar sizes. We review the last several decades of stellar flare research. We summarize multi-wavelength observational results and the associated thermal and nonthermal processes in flaring stellar atmospheres. Static and hydrodynamic models are reviewed with an emphasis on recent progress in radiation-hydrodynamics and the physical diagnostics in flare spectra. Thanks to their effects on the space weather of exoplanetary systems (and thus in our search for life elsewhere in the universe) and their preponderance in Kepler mission data, white-light stellar flares have re-emerged in the last decade as a widely-impactful area of study within astrophysics. Yet, there is still much we do not understand, both empirically and theoretically, about the spectrum of flare radiation, its origin, and its time evolution. We conclude with several big-picture questions that are fundamental in our pursuit toward a greater understanding of these enigmatic stellar phenomena and, by extension, those on the Sun.

Time-dependent Stellar Flare Models of Deep Atmospheric Heating

“…The optical color temperature attains the hottest values in the rise and peak phases (Mochnacki & Zirin 1980) when the Balmer jumps are smallest (Kowalski et al 2013). In the fast decay phase, the optical color temperatures rapidly decrease and settle to a cooler value around 5500 K through the gradual decay phase (see reviews in Kowalski 2024). The evolution of blackbody temperatures in stellar flares has been investigated with broadband photometry (Hawley et al 2003;Robinson et al 2005;Zhilyaev et al 2007;Howard et al 2020), but the interpretation of the continuum radiation can be difficult without spectra (Allred et al 2006;Kowalski et al 2019b).…”

“…The semiempirical model atmospheres of Cram & Woods (1982) explored six variations of temperature and electron density adjustments within the deep chromosphere and photosphere, one of which produced very broad Balmer line wings and a hot optical blackbody color temperature. For a detailed review of stellar flares and stellar flare modeling, see Kowalski (2024).…”