Insights into the inner regions of the FU Orionis disc

TL;DR

This study analyzes small-amplitude light variations in FU Orionis using continuous satellite and ground data, revealing that long-period variations are due to disc inhomogeneities and short-period variations may involve plasma tongues and hotspots.

Contribution

It provides new insights into the mechanisms behind short- and long-term light variations in FU Ori, linking them to disc inhomogeneities and magnetospheric phenomena.

Findings

Long-period variations (10.5-11.4 days) likely caused by disc inhomogeneities at 16-20 solar radii.

Short-period variations (1.38-3 days) show period shortening, possibly due to plasma tongues or hotspots.

Long-period variations align with the colour-period relation indicating cooler, outer disc regions.

Abstract

Context. 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. Methods. The visual light curve of FU Ori gathered by the MOST satellite continuously for 55 days in the 2013-2014 winter season and simultaneously obtained ground-based multi-colour data were compared with the results from a disc and star light synthesis model. Results. Hotspots on the star are not responsible for the majority of observed light variations. Instead, we found that the long periodic family of 10.5-11.4 d (presumably) quasi-periods showing light variations up to 0.07 mag may arise owing to the rotational revolution of disc inhomogeneities located between 16-20 solar radii. 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 solar mass. The short-periodic (3-1.38 d) small amplitude (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. Conclusions. Results obtained for the long-periodic 10-11 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 1-3 d family of light variations and their period changes.