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 MOST satellite\nobservations. During 2014 and 2017 the light curves showed stable 3.75 and 3.69\nd quasi-periodic oscillations, respectively. Both values appear to be closely\nrelated with the stellar rotation period, as they might be created by changing\nvisibility of a hot-spot formed near the magnetic pole directed towards the\nobserver. These major light variations were superimposed on a chaotic,\nflaring-type activity caused by hot-spots resulting from unstable accretion - a\nsituation reminiscent of that in 2011, when TW Hya showed signs of a moderately\nstable accretion state. In 2015 only drifting quasi-periods were observed,\nsimilar to those present in 2008-2009 data and typical for magnetised stars\naccreting in a strongly unstable regime. A rich set of multi-colour data was\nobtained during 2013-2017 with the primary aim to characterize the basic\nspectral properties of the mysterious occultations in TW Hya. Although several\npossible occultation-like events were identified, they are not as well defined\nas in the 2011 MOST data. The new ground-based and MOST data show a dozen\npreviously unnoticed flares, as well as small-amplitude, 11 min - 3 hr\nbrightness variations, associated with 'accretion bursts'. It is not excluded\nthat the shortest 11-15 min variations could also be caused by thermal\ninstability oscillations in an accretion shock.\n