An A star on an M star during a flare within a flare

TL;DR

This study presents a near-UV/optical spectrum of a secondary flare on an M dwarf, revealing a hot spot with significantly higher temperature than current models predict, challenging existing flare mechanisms.

Contribution

The paper provides the first spectrum of a secondary flare during an M dwarf flare, showing a hot spot with a temperature increase much greater than current models suggest.

Findings

The spectrum resembles an early-type star with absorption lines.

The hot spot temperature increase (~16,000K) exceeds model predictions (~1,200K).

Secondary flares can produce intense localized heating on M dwarfs.

Abstract

M dwarfs produce explosive flare emission in the near-UV and optical continuum, and the mechanism responsible for this phenomenon is not well-understood. We present a near-UV/optical flare spectrum from the rise phase of a secondary flare, which occurred during the decay of a much larger flare. The newly formed flare emission resembles the spectrum of an early-type star, with the Balmer lines and continuum in absorption. We model this observation phenomonologically as a temperature bump (hot spot) near the photosphere of the M dwarf. The amount of heating implied by our model (\Delta T_phot ~ 16,000K) is far more than predicted by chromospheric backwarming in current 1D RHD flare models (\Delta T_phot ~ 1200K).