Evidence for an additional source of opacity during the impulsive phase of stellar flares

Abstract

Broad-band photometric observations of YZ CMi show a 1.2 magnitude U-band flare at 19:55 UT on 4 March 1985. During the flare the Ca I feature at 4227A changed dramatically. In the spectrum taken during the rise phase, Ca I was both narrower and weaker than that of either the preflare spectrum or spectra taken later in the flare. We interpret this as evidence that the photospheric flare continuum observed here is a combination of a normal dM4.5e continuum plus an overlying continuum with a temperature of less than 4000 – 5000 K and a pressure of at least an order of magnitude less than the photosphere of a dM4.5e star.

A similar result was inferred from Walraven photometry of G1 867A, where the Walraven B-band (centered at ∼4300Å showed evidence for an additional source of opacity overlying the continuum source during the early stages of a flare.