Insights into the inner regions of the FU Orionis disc^⋆

¹ Mount Suhora Observatory, Krakow Pedagogical University, ul. Podchorazych 2, 30-084 Krakow, Poland
e-mail: siwak@oa.uj.edu.pl
² Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Krakow, Poland
³ Département de Physique, Université de Montréal, CP 6128, Succursale: Centre-Ville, Montréal, QC, H3C 3J7, Canada
⁴ Department of Astronomy and Astrophysics, University of Toronto, 50 George St., Toronto, Ontario, M5S 3H4, Canada
⁵ Department of Mathematics, Physics & Geology, Cape Breton University, 1250 Grand Lake Road, Sydney, NS, B1P 6L2, Canada
⁶ Canadian Coast Guard College, Dept. of Arts, Sciences, and Languages, Sydney, Nova Scotia, B1R 2J6, Canada
⁷ Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Rd, Vancouver, BC, V6T 1Z1, Canada
⁸ Universität Wien, Institut für Astrophysik, Türkenschanzstrasse 17, 1180 Wien, Austria
⁹ Institute for Computational Astrophysics, Department of Astronomy and Physics, Saint Marys University, Halifax, NS, B3H 3C3, Canada
¹⁰ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138, USA

Received: 9 May 2018
Accepted: 4 July 2018

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 d 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 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 (∼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.