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

This is the final photometric study of TW Hya based on newMOSTsatellite observations.During 2014 and 2017, the light curves showed stable 3.75- and 3.69-d quasi-periodic os-cillations, 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 poledirected towards the observer. These major light variations were superimposed on a chaotic,flaring-type activity caused by hotspots resulting from unstable accretion – a situation remi-niscent 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 2008–2009data and typical for magnetized stars accreting in a strongly unstable regime. A rich set ofmulticolour data was obtained during 2013–2017 with the primary aim of characterizing thebasic spectral properties of the mysterious occultations in TW Hya. Although several possibleoccultation-like events were identified, they are not as well defined as in the 2011MOSTdata.The new ground-based andMOSTdata show a dozen previously unnoticed flares, as wellas small-amplitude 11 min–3 h brightness variations, associated with ‘accretion bursts’. Wecannot exclude the possibility that the shortest 11–15 min variations could also be caused bythermal instability oscillations in an accretion shock.Key words:stars: individual: TW Hya – stars: variables: T Tauri, Herbig Ae/Be.1 INTRODUCTIONTW Hya was shown to be a genuine classical T Tauri type star(CTTS: Rucinski & Krautter1983) in a young (about 7–10 Myr)association called TWA (Kastner et al.1997; Barrado y Navascues2006). It is one of the last two stars in the association that still showvigorous accretion (see the results of Tofflemire et al.2017for theyoung binary TWA 3A) and also the closest (59.5 pc, Gaia collabo-ration et al.2016) T Tauri type star to us. The pole-on geometry ofits transitional protoplanetary disc visibility (an inclination of 5–15◦between the stellar rotational axis and the observer was derivedfor the star – see Rucinski et al.2008for a review and also DonatiE-mail:siwak@oa.uj.edu.plet al.2011) favours TW Hya for detailed studies with modern imag-ing instrumentation: in addition to the previously known disc gapat 80 au found inHubble Space Telescope(HST) images (Debeset al.2013), Akiyama et al. (2015) found a gap localized in thedisc at a Uranus distance of 20 au using theSubaruHigh-ContrastCoronographic Imager for Adaptive Optics (Subaru–HiCIAO). Amost detailed image of TW Hya provided by the Atacama LargeMillimeter/submillimeter Array (ALMA: Andrews et al.2016)re-vealed numerous rings – the closest, localized at 1 au from the star,has popularly been interpreted as caused by an Earth-like planetsweeping disc matter in its orbit. Sophisticated, deep spectral dif-ferential imaging in the Paβline looking for accretion signaturesof possible planets was sensitive to 1.45–2.3 Jupiter-mass planetsbut did not reveal significant signals (Uyama et al.2017). SimilarC2018 The Author(s)Published by Oxford University Press on behalf of the Royal Astronomical SocietyDownloaded from https://academic.oup.com/mnras/article-abstract/478/1/758/5032347 by Cape Breton University user on 12 February 2019