Origin of the white-light continuum in stellar flares

Determine whether the 1300–9000 Å continuum radiation observed during stellar flares is produced by photospheric heating to incandescence (i.e., optically thick blackbody-like emission), rather than alternative mechanisms such as optically thick hydrogen recombination or radiative backwarming, by establishing diagnostic spectroscopic criteria that can distinguish among these scenarios across flare phases and stellar types.

Background

Stellar flares exhibit prominent continuum radiation from the near-ultraviolet through the optical, often appearing hot and blackbody-like in the blue-optical while showing a Balmer jump at shorter wavelengths. Whether this emission is due to true photospheric incandescence or arises from chromospheric recombination processes has long been debated.

Clarifying the physical origin is essential for interpreting energy transport during flares, constraining atmospheric depths of emission, and connecting observations to radiative-hydrodynamic models. The review highlights that this foundational question remains unresolved and requires targeted spectroscopic diagnostics and modeling advances.