Flickering in FU Orionis

Abstract

We analyze new and published optical photometric data of FU Orionis,an eruptive pre-main-sequence star. The outburst consists of a 5.5mag rise at B with an e-folding timescale of ~50 days. The ratesof decline at B and V are identical, 0.015+/-0.001 mag yr$^{-1}$.Random fluctuations superimposed on this decline have an amplitudeof 0.035+/-0.005 mag at V and occur on timescales of 1 day or less.Correlations between V and the color indices U-B, B-V, and V-R indicatethat the variable source has the optical colors of a G0 supergiant.We associate this behavior with small-amplitude flickering of theinner accretion disk.