Researchers in Statistics and Population Studies
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Item An extensive search for rapid optical variability in ultracool dwarfs(Oxford University Press, 2013) Koen, ChrisIn this paper, a summary of optical time-series photometry of 125 ultracool dwarfs is given. The observing strategy was to monitor each object continuously for 2–3 h in order to ascertain whether itwas rapidly variable. Many of the targetswere observed atmultiple epochs, to follow up possible short time-scale variability, or to test for slow brightness changes on longer timescales. The 353 data sets obtained contain nearly 22 000 individual measurements. Optical (IC) magnitudes, accurate to roughly 0.1–0.2 mag, were derived for 21 objects for which there is no optical photometry in the literature. It is shown that photometry is affected by variable seeing in a large percentage of the time-series observations. Since this could give the appearance of variability intrinsic to the objects, magnitudes are modelled as functions of both time and seeing. Several ultracool dwarfs which had not been monitored before are variable, according to certain model-fitting criteria. A number of objects with multi-epoch observations appear to be variable on longer time-scales. Since testing for variability is far from being straightforward, the time-series data are made available so that interested readers can perform their own analyses.Item Time series photometry of the nearby brown dwarf DENIS-P J0041353-562112(Oxford University Press, 2011) Koen, ChrisDENIS-P J0041353−562112 is a brown dwarf, with a spectral type of M7.5, which may be younger than 20 Myr. On the basis of its Hα emission-line asymmetry, it has been suggested that the object is accreting material. If it is accreting, it is expected to rotate slowly. This paper reports on several photometric monitoring runs in the RC and IC bands, designed to study variability – such as that due to rotational modulation – of the object. Although there do not seem to be irregular brightness fluctuations typical of accretion, there is cyclical variability which may be due to a bright accretion-related spot. The rotation period is 2.8 h, although a double-wave variation with a period of 5.6 h cannot be ruled out completely. If the shorter period is correct, it is unlikely that the brown dwarf is disc-locked. Substantial flaring activity in the RC band suggests a strong magnetic field.