Prof. Chris Koen
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Position: | Senior Professor |
Department: | Statistics and Population Studies Programme |
Faculty: | Faculty of Natural Sciences |
Qualifications: | PhD (Astronomy), PhD (Mathematical Statistics) |
Research publications in this repository | |
ORCID iD | 0000 0003 2291 2660 |
Tel: | 021 959 3258 |
Fax: | 021 959 2909 |
Email: | ckoen@uwc.ac.za |
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Item JHKs time-series observations of a few ultracool dwarfs(Oxford University Press, 2005) Koen, Chris; Tanabe, T.; Tamura, M.; Kusakabe, N.The M8.5 object SSSPM J0109−5101 has recently been shown to be both a periodic and a flaring variable, based on optical observations in the extreme red. More than 16 h of monitoring in the near-infrared (NIR) reported here failed to show any variability. Similarly, no NIR variability could be detected in intensive monitoring of three other suspected optical variables. This paper also reports on photometry of half a dozen targets monitored over a few weeks, and on the comparison of intensive monitoring at different epochs. In only one case, that of the T dwarf binary Indi Bab, is there good evidence for variability. Our results allow stringent limits to be placed on the NIR variability levels in a large sample of ultracool dwarfs.Item IC and RC band time-series observations of some bright ultracool dwarfs(Oxford University Press, 2005) Koen, ChrisThe results of more than 100 h of CCD photometric monitoring of 10 L dwarfs and two T dwarfs are presented. Reasonable evidence is shown for IC band variability in DENIS 0255−4700, DENIS 1705−0516 and 2M 2104−1037. Substantial brightening of the T dwarf binary Indi Bab was observed over the course of 3.6 h; it is unlikely that this could have been caused by surface spots. No unequivocal variability could be detected in the RC band.Item The analysis of irregularly observed stochastic astronomical time-series – I. Basics of linear stochastic differential equations(Oxford University Press, 2005) Koen, ChrisThe theory of low-order linear stochastic differential equations is reviewed. Solutions to these equations give the continuous time analogues of discrete time autoregressive time-series. Explicit forms for the power spectra and covariance functions of first- and second-order forms are given. A conceptually simple method is described for fitting continuous time autoregressive models to data. Formulae giving the standard errors of the parameter estimates are derived. Simulated data are used to verify the performance of the methods. Irregularly spaced observations of the two hydrogen-deficient stars FQ Aqr and NO Ser are analysed. In the case of FQ Aqr the best-fitting model is of second order, and describes a quasi-periodicity of about 20 d with an e-folding time of 3.7 d. The NO Ser data are best fitted by a first-order model with an e-folding time of 7.2 d.Item A re-assessment of the nearest neighbour alignment of the X-ray isophotes of galaxy clusters(Oxford University Press, 2006) Koen, Chris; Lombard, FredAlignment is defined as the tendency of the distribution of pointing angles between the major axes of clusters and their nearest neighbours to be more concentrated towards small values for small nearest neighbour distances, whereas the distribution is expected to be uniform over all angles at larger distances. Conflicting pronouncements on the reality of this effect have been published in the astronomy literature. A re-assessment of the evidence for alignment is presented, based on three recently published X-ray data sets. We find that whereas there is evidence for alignment, it is not as convincing as previously claimed. In particular, the scale to which the effect has been claimed to extend seems to have been severely overstated.Item Near-infrared time-series photometry of six fields in the young open cluster IC 2391(Oxford University Press, 2006) Koen, Chris; Ishihara, AkikaFields containing targets of spectral types later than M5 were monitored in JHKs for about 7 h each. None of the targets showed variability at levels exceeding 0.02 (or smaller, in some cases). Seven new variable stars were discovered serendipitously. The large number of measurements (more than 50) of each field allows the construction of accurate colour–magnitude and colour– colour diagrams. These could be used to identify a small number of candidate very cool objects.Item On the upper limit on stellar masses in the Large Magellanic Cloud cluster R136(Oxford University Press, 2006) Koen, ChrisA truncated power-law distribution is fitted to the 29 largest stellar masses known in R136. Two different statistical techniques are used, with comparable results. An upper limit to the mass distribution of the order of 140–160 M is derived, while the power-law exponent is in the approximate range 0.9–1.7. A power-law distribution with no upper limit on the mass can be rejected with considerable confidence. It is recommended that the calculations be repeated when more reliable mass estimates are available.Item Evidence for rapid evolution of periodic variations in an ultracool dwarf(Oxford University Press, 2006) Koen, ChrisThe results of three short photometric monitoring runs on the L0 dwarf 2MASS J06050196−2342270, during three consecutive nights, are presented. The observations show the persistent presence of a 2.4-h period, with an IC band amplitude which decreased from 27 to 11 mmag over the three nights. The amplitude in the RC band appeared to be substantially smaller than in the IC band.Item The Nyquist frequency for irregularly spaced time-series: a calculation formula(Oxford University Press, 2006) Koen, ChrisEyer & Bartholdi showed that the Nyquist frequency of irregularly sampled time-series can be very high. In this paper, a calculation formula for the Nyquist frequency is presented. In practice there is an upper limit of 0.5/ on this frequency, where is the best accuracy with which time is recorded.Item Photometry of the magnetic white dwarf SDSS 121209.31+013627.7(Oxford University Press, 2006) Koen, Chris; Maxted, P.F.L.The results of 27 h of time series photometry of SDSS 121209.31+013627.7 are presented. The binary period established from spectroscopy is confirmed and refined to 0.061 412 d (88.43 min). The photometric variations are dominated by a brightening of about 16 mmag, lasting a little less than half a binary cycle. The amplitude is approximately the same in V, R and white light. A secondary small brightness increase during each cycle may also be present. We speculate that SDSS 121209.31+013627.7 may be a polar in a low state.Item The analysis of indexed astronomical time series – X. Significance testing of O − C data(Oxford University Press, 2006) Koen, ChrisIt is assumed that O − C (‘observed minus calculated’) values of periodic variable stars are determined by three processes, namely measurement errors, random cycle-to-cycle jitter in the period, and possibly long-term changes in the mean period. By modelling the latter as a random walk, the covariances of all O − C values can be calculated. The covariances can then be used to estimate unknown model parameters, and to choose between alternative models. Pseudo-residuals which could be used in model fit assessment are also defined. The theory is illustrated by four applications to spotted stars in eclipsing binaries.Item Further observations of Hipparcos red stars and standards for UBV(RI)C photometry(Oxford University Press, 2007) Kilkenny, David; Koen, Chris; van Wyk, F.; Marang, F.; Cooper, D.We present homogeneous and standardized UBV(RI)C JHK photometry for over 100 M stars selected from an earlier paper on the basis of apparent photometric constancy. L photometry has been obtained for stars brighter than about L = 6. Most of the stars have a substantial number of UBV(RI)C observations and, it is hoped, will prove useful as red supplementary standards. Additionally,we list JHK photometry for nearly 300 Hipparcos red stars not selected as standards, as well as L photometry for the brightest stars.Item HE0230–4323: an unusual pulsating hot subdwarf star(Oxford University Press, 2007) Koen, ChrisHE 0230−4323 is a known binary, consisting of a subdwarf star and a companion which is not observable in the optical. Photometric measurements reported in this paper have shown it to be both a reflection-effect and a pulsating variable. The dominant pulsation frequencies changed over the course of several nights of observing, from ∼32–39 d−1 to ∼8–16 d−1. Observations were obtained through B and V filters, and the variations in the two wavebands appear to be approximately 180◦ out of phase.Item Improved SAAO–2MASS photometry transformations(Oxford University Press, 2007) Koen, Chris; Marang, F.; Kilkenny, David; Jacobs, C.Near-infrared photometry of 599 stars is used to calculate transformations from the South African Astronomical Observatory (SAAO) JHK system to the Two-Micron All-Sky Survey (2MASS) JHKS system. Both several-term formal regression relations and simplified transformations are presented. Inverse transformations (i.e. 2MASS to SAAO) are also given. The presence of non-linearities in some colour terms is highlighted.Item Fitting sinusoids to observations of rotating spotted stars(Oxford University Press, 2007) Koen, ChrisIt is assumed that K blocks (e.g. seasons) of observations are available, and the parameters characterizing a fixed-frequency sinusoidal variability (mean light level, amplitude, phase) are constant within each of the blocks. The paper is concerned with estimation when any combination of these parameters varies between blocks of observations. This allows observations subject to changes in mean light level, spot sizes and/or spot locations to be modelled. Objective choices between competing models, and the calculation of the standard errors of model parameters, are also dealt with. Illustrative applications to simulated and real data are given.Item The detailed forms of the LMC Cepheid PL and PLC relations(Oxford University Press, 2007) Koen, Chris; Kanbur, S; Ngeow, CPossible deviations from linearity of the Large Magellanic Cloud Cepheid period–luminosity (PL) and period–luminosity–colour (PLC) relations are investigated. Two data sets are studied, respectively from the Optical Gravitational Lensing Experiment (OGLE) and MACHO projects. A non-parametric test, based on linear regression residuals, suggests that neither PL relation is linear. If colour dependence is allowed for, then the MACHO PL relation is found to deviate more significantly from the linear, while the OGLE PL relation is consistent with linearity. These findings are confirmed by fitting ‘Generalized Additive Models’ (non-parametric regression functions) to the two data sets. Colour dependence is shown to be non-linear in both data sets, distinctly so in the case of the MACHO Cepheids. It is also shown that there is interaction between the period and the colour functions in the MACHO data.Item Multivariate comparisons of the period–light-curve shape distributions of Cepheids in five galaxies(Oxford University Press, 2007) Koen, Chris; Siluyele, I.A number of published tests suitable for the comparison of multivariate distributions are described. The results of a small power study, based on realistic Cepheid log period – Fourier coefficient data, are presented. It is found that a statistic due to Henze has good general performance. The tests are applied to Cepheid observations in the Milky Way galaxy, Large Magellanic Cloud, Small Magellanic Cloud, IC 1613 and NGC 6822. The null hypothesis of equal populations is rejected for all pairs compared, except IC 1613 – NGC 6822.Item KPD 0629–0016, a slowly pulsating hot subdwarf star(Oxford University Press, 2007) Koen, Chris; Green, E.M.The results of nine CCD photometric observing runs on KPD 0629–0016 are presented. During six of the runs measurements were obtained alternately through B and V filters. Four periodicities, for which there is good agreement between the frequencies identified in the B and V data, were extracted: these lie in the range 46–81 min. A fifth lower frequency appears to be definitely present, but is very uncertain due to aliasing. The ratios of the mode amplitudes measured in B and V, and the phase differences between variations in the two colours, are compatible with pulsation theory.Item What can be deduced from open cluster metallicity measurements?(Oxford University Press, 2007) Koen, Chris; Lombard, FredThe dependence of [Fe/H] on galactocentric distance, distance from the galactic mid-plane and age is studied. Both ordinary least-squares and non-parametric regression in the form of a ‘generalized additive model’ are used. The radial metallicity slope is found to be shallower than previously claimed in the literature, and there is a significant abundance gradient perpendicular to the galactic plane. There may be a tendency for metallicity to increase with cluster age.Item Multicolour time series photometry of 2MASS 1207−3932(Oxford University Press, 2008) Koen, ChrisTime series photometry in VRI of the young brown dwarf Two-Micron All-Sky Survey (2MASS) J1207334−393254 is presented. Variability in I is insignificant, but amplitudes up to ∼0.2 mag in R, and ∼0.55 mag in V, are seen within a single night. There is also microvariability on time-scales of an hour or so, which appears to be present in data from all the three filters. There may be a non-sinusoidal periodicity of about half a day in the V and R data. A likely explanation for the variability is the presence of accretion hot spots.Item JL 82: a slowly pulsating hot subdwarf star in a close binary system(Oxford University Press, 2009) Koen, ChrisJL 82 is a known binary, consisting of an sdB star and a companion which is not directly observable in the optical. Photometric measurements reported in this paper show it to variable with both the binary period (∼0.75 d), as well as on much shorter time-scales. The shorter periods are ascribed to pulsation of the sdB star, making it a member of the PG 1716 class of pulsating stars.
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