Photometric variability of TW Hya from seconds to years as seen from space and the ground during 2013-2017

Abstract

This is the final photometric study of TW Hya based on new&nbsp;<em>MOST</em>&nbsp;satellite observations. During 2014 and 2017, the light curves showed stable 3.75- and &thinsp;3.69-d quasi-periodic oscillations, respectively. Both values appear to be closely related to the stellar rotation period, as they might be created by changing visibility of a hotspot formed near the magnetic pole directed towards the observer. These major light variations were superimposed on a chaotic, flaring-type activity caused by hotspots resulting from unstable accretion &ndash; a situation reminiscent of that in 2011, when TW Hya showed signs of a moderately stable accretion state. In 2015, only drifting quasi-periods were observed, similar to those present in 2008&ndash;2009 data and typical for magnetized stars accreting in a strongly unstable regime. A rich set of multicolour data was obtained during 2013&ndash;2017 with the primary aim of characterizing the basic spectral properties of the mysterious occultations in TW Hya. Although several possible occultation-like events were identified, they are not as well defined as in the 2011&nbsp;<em>MOST</em>&nbsp;data. The new ground-based and&nbsp;<em>MOST</em>&nbsp;data show a dozen previously unnoticed flares, as well as small-amplitude 11&nbsp;min&ndash;3 h brightness variations, associated with &lsquo;accretion bursts&rsquo;. We cannot exclude the possibility that the shortest 11&ndash;15&nbsp;min variations could also be caused by thermal instability oscillations in an accretion shock.