Insights into the inner regions of the FU Orionis disc

Abstract

<p><span><em>Context</em>. We investigate small-amplitude light variations in FU Ori occurring in timescales of days and weeks. Aims. We seek to determine the mechanisms that lead to these light changes. </span></p> <p><span><em>Methods</em>. The visual light curve of FU Ori gathered by the MOST satellite continuously for 55 d in the 2013&ndash;2014 winter season and simultaneously obtained ground-based multi-colour data were compared with the results from a disc and star light synthesis model. </span></p> <p><span><em>Results</em>. Hotspots on the star are not responsible for the majority of observed light variations. Instead, we found that the long periodic family of 10.5&ndash;11.4 d (presumably) quasi-periods showing light variations up to 0.07 mag may arise owing to the rotational revolution of disc inhomogeneities located between 16 and 20 R_⊙. The same distance is obtained by assuming that these light variations arise because of a purely Keplerian revolution of these inhomogeneities for a stellar mass of 0.7 M_⊙. The short-periodic (&sim;3 &ndash; 1.38 d) small amplitude (&sim;0.01 mag) light variations show a clear sign of period shortening, similar to what was discovered in the first MOST observations of FU Ori. Our data indicate that these short-periodic oscillations may arise because of changing visibility of plasma tongues (not included in our model), revolving in the magnetospheric gap and/or likely related hotspots as well. </span></p> <p><span><em>Conclusions</em>. Results obtained for the long-periodic 10&ndash;11 d family of light variations appear to be roughly in line with the colour-period relation, which assumes that longer periods are produced by more external and cooler parts of the disc. Coordinated observations in a broad spectral range are still necessary to fully understand the nature of the short-periodic 1&ndash;3 d family of light variations and their period changes.</span></p>