Researchers in Statistics and Population Studieshttp://hdl.handle.net/10566/20242019-10-15T05:52:05Z2019-10-15T05:52:05ZModelling the rotation period distribution of M dwarfs in the Kepler fieldKoen, Chrishttp://hdl.handle.net/10566/33482018-01-05T00:01:09Z2018-01-01T00:00:00ZModelling the rotation period distribution of M dwarfs in the Kepler field
Koen, Chris
McQuillan et al. (Mon. Not. R. Astron. Soc.432:1203, 2013) presented 1570 periods P of M dwarf stars in the field of view of the Kepler telescope. It is expected that most of these reflect rotation periods, due to starspots. It is shown here that the data can be modelled as a mixture of four subpopulations, three of which are overlapping log-normal distributions. The fourth subpopulation has a power law distribution, with P −1/2. It is also demonstrated that the bulk of the longer periods, representing the two major sub-populations, could be drawn from a single subpopulation, but with a period-dependent probability of observing half the true period.
2018-01-01T00:00:00ZThe analysis of indexed astronomical time series – XII. The statistics of oversampled Fourier spectra of noise plus a single sinusoidKoen, Chrishttp://hdl.handle.net/10566/20232016-08-30T19:53:06Z2015-01-01T00:00:00ZThe analysis of indexed astronomical time series – XII. The statistics of oversampled Fourier spectra of noise plus a single sinusoid
Koen, Chris
With few exceptions, theoretical studies of periodogram properties focus on pure noise time
series. This paper considers the case in which the time series consists of noise together with a
single sinusoid, observed at regularly spaced time points. The distribution of the periodogram
ordinates in this case is shown to be of exponentially modified Gaussian form. Simulations
are used to demonstrate that if the periodogram is substantially oversampled (i.e. calculated
in a dense grid of frequencies), then the distribution of the periodogram maxima can be
accurately approximated by a simple form (at least at moderate signal-to-noise ratios). This
result can be used to derive a calculation formula for the probability of correct signal frequency
identification at given values of the time series length and (true) signal-to-noise ratio. A set of
curves is presented which can be used to apply the theory to, for example, asteroseismic data.
An illustrative application to Kepler data is given.
2015-01-01T00:00:00ZThe analysis of indexed astronomical time series – XI. The statistics of oversampled white noise periodogramsKoen, Chrishttp://hdl.handle.net/10566/20222016-08-30T19:52:56Z2015-01-01T00:00:00ZThe analysis of indexed astronomical time series – XI. The statistics of oversampled white noise periodograms
Koen, Chris
The distribution of the maxima of periodograms is considered in the case where the time
series is made up of regularly sampled, uncorrelated Gaussians. It is pointed out that if there
is no oversampling, then for large data sets, the known distribution of maxima tends to a oneparameter
Gumbel distribution. Simulations are used to demonstrate that for oversampling by
large factors, a two-parameter Gumbel distribution provides a highly accurate representation
of the simulation results. As the oversampling approaches the continuous limit, the twoparameter
Gumbel distribution takes on a simple form which depends only on the logarithm
of the number of data. Subsidiary results are the autocorrelation function of the oversampled
periodogram; expressions for the accuracy of simulated percentiles; and the relation between
percentiles of the periodogram and the amplitude spectrum.
2015-01-01T00:00:00ZA search for p-mode pulsations in white dwarf stars using the Berkeley Visible Imaging Tube detectorKilkenny, DavidWelsh, B.Y.Koen, ChrisGulbis, A.A.S.Kotze, M.M.http://hdl.handle.net/10566/20212016-08-30T19:52:24Z2014-01-01T00:00:00ZA search for p-mode pulsations in white dwarf stars using the Berkeley Visible Imaging Tube detector
Kilkenny, David; Welsh, B.Y.; Koen, Chris; Gulbis, A.A.S.; Kotze, M.M.
We present high-speed photometry (resolution 0.1 s) obtained during the commissioning of the
Berkely Visible Imaging Tube system on the Southern African Large Telescope (SALT). The
observations were an attempt to search for very rapid p-mode oscillations in white dwarf stars
and included three DA stars known to be g-mode pulsators (ZZ Cet, HK Cet and AF Pic), one
other DA star (WD 1056-384) not known to be variable and one AM Cvn star (HP Lib). No
evidence was found for any variations greater than about 1 mmag in amplitude (∼0.1 per cent)
at frequencies in excess of 60 mHz (periods <17 s) in any of the target stars, though several
previously known g-mode frequencies were recovered.
2014-01-01T00:00:00Z