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

摘要

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 phenomenologically as a temperature bump (hot spot) near the photosphere of the M dwarf. The amount of heating implied by our model (ΔT_(phot) ~ 16,000 K) is far more than predicted by chromospheric backwarming in current 1D RHD flare models (ΔT_(phot) ~ 1200 K).