Disc light variability in the FUor star V646 Puppis as observed by TESS and from the ground

Abstract

Context. We investigate small-scale light variations in V646 Pup occurring on timescales of days, weeks, and years. Aims. We aim to investigate whether this variability is similar to that observed in FU Ori. Methods. We observed V646 Pup on six occasions at the SAAO and CTIO between 2013 and 2018 with Johnson and Sloan filters, typically using a one-day cadence maintained for two to four weeks. We also utilised the public-domain 1512-day-long ASAS-SN light curve and TESS photometry obtained in 2019 over 24.1 days with a 30 min cadence. New SAAO low-resolution spectra assist in updating major disc parameters, while the archival high-resolution Keck spectra are used to search for temporal changes in the disc rotational profiles. Results. The ground-based observations confirm the constantly decreasing brightness of V646 Pup at the rate of 0.018 mag yr−1. Precise i-band sensitive TESS data show that the slight, 0.005−0.01 mag, light variations imposed on this general trend do consist of a few independent wave trains of an apparently time-coherent nature. Assuming that this is typical situation, based on an analysis of colour–magnitude diagrams obtained for earlier epochs, we were able to make a preliminarily inference that the bulk of the light changes observed could be due to the rotation of disc photosphere inhomogeneities, arising between 10–12 R⊙ from the star. We do not exclude the possibility that these inhomogeneities could also manifest themselves in the rotational profiles of the disc, as obtained from the high-resolution spectra. Assuming Keplerian rotation of these inhomogeneities, we give a preliminary determination of the stellar mass at 0.7–0.9 M⊙. Conclusions. Over certain weeks, at least, V646 Pup has shown time-coherent light variability pattern(s) that could be explained by the rotation of an inhomogeneous disc photosphere. These preliminary results are similar to those better established for FU Ori, which suggests a common driving mechanism(s).