A White Light Megaflare on the dM4.5e Star YZ CMi

TL;DR

This study documents a rare, extremely energetic white light megaflare on the star YZ CMi, analyzing its spectral components and temporal evolution to better understand stellar flare physics.

Contribution

It provides the first spectral evidence that the blue continuum in stellar flares is a combination of Balmer continuum and blackbody emission, with detailed areal coverage analysis.

Findings

The flare lasted over 7 hours with peak brightness nearly 6 magnitudes above quiescence.

The blue continuum can be modeled as a sum of Balmer continuum and ~10,000K blackbody emission.

The Balmer continuum region is significantly larger than the blackbody emitting region during decay.

Abstract

On UT 2009 January 16, we observed a white light megaflare on the dM4.5e star YZ CMi as part of a long-term spectroscopic flare-monitoring campaign to constrain the spectral shape of optical flare continuum emission. Simultaneous U-band photometric and 3350A-9260A spectroscopic observations were obtained during 1.3 hours of the flare decay. The event persisted for more than 7 hours and at flare peak, the U-band flux was almost 6 magnitudes brighter than in the quiescent state. The properties of this flare mark it as one of the most energetic and longest-lasting white light flares ever to be observed on an isolated low-mass star. We present the U-band flare energetics and a flare continuum analysis. For the first time, we show convincingly with spectra that the shape of the blue continuum from 3350A to 4800A can be represented as a sum of two components: a Balmer continuum as predicted by the Allred et al radiative hydrodynamic flare models and a T ~ 10,000K blackbody emission component as suggested by many previous studies of the broadband colors and spectral distributions of flares. The areal coverage of the Balmer continuum and blackbody emission regions vary during the flare decay, with the Balmer continuum emitting region always being significantly (~3-16 times) larger. These data will provide critical constraints for understanding the physics underlying the mysterious blue continuum radiation in stellar flares.