  MOST (Microvariability & Oscillations of STars) and ASAS (All Sky Automated
Survey) observations have been used to characterize photometric variability of
TW Hya on time scales from a fraction of a day to 7.5 weeks and from a few days
to 8 years, respectively. The two data sets have very different uncertainties
and temporal coverage properties and cannot be directly combined, nevertheless,
they suggests a global variability spectrum with "flicker noise" properties,
i.e. with amplitudes a ~ 1/sqrt(f), over >4 decades in frequency, in the range
f = 0.0003 to 10 cycles per day (c/d). A 3.7 d period is clearly present in the
continuous 11 day, 0.07 d time resolution, observations by MOST in 2007.
Brightness extrema coincide with zero-velocity crossings in periodic (3.56 d)
radial velocity variability detected in contemporaneous spectroscopic
observations of Setiawan et al. (2008) and interpreted as caused by a planet.
The 3.56/3.7 d periodicity was entirely absent in the second, four times longer
MOST run in 2008, casting doubt on the planetary explanation. Instead, a
spectrum of unstable single periods within the range of 2 - 9 days was
observed; the tendency of the periods to progressively shorten was well traced
using the wavelet analysis. The evolving periodicities and the overall
flicker-noise characteristics of the TW Hya variability suggest a combination
of several mechanisms, with the dominant ones probably related to the accretion
processes from the disk around the star.
