The EC 14026 stars -- X. A multi-site campaign on the sdBV star PG 1605+072     

D. Kilkenny  C. Koen  D. O'Donoghue  F. van Wyk  K. A. Larson  R. Shobbrook  D. J. Sullivan  M. R. Burleigh  P. D. Dobbie  S. D. Kawaler 《Monthly notices of the Royal Astronomical Society》1999,303(3):525-534

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The analysis of indexed astronomical time series V. Fitting sinusoids to high-speed photometry     

Chris Koen 《Monthly notices of the Royal Astronomical Society》1999,309(3):769-802

The estimation of the frequency, amplitude and phase of a sinusoid from observations contaminated by correlated noise is considered. It is assumed that the observations are regularly spaced, but may suffer missing values or long time stretches with no data. The typical astronomical source of such data is high-speed photoelectric photometry of pulsating stars. The study of the observational noise properties of nearly 200 real data sets is reported: noise can almost always be characterized as a random walk with superposed white noise. A scheme for obtaining weighted non-linear least-squares estimates of the parameters of interest, as well as standard errors of these estimates, is described. Simulation results are presented for both complete and incomplete data. It is shown that, in finite data sets, results are sensitive to the initial phase of the sinusoid.  相似文献   

Flow separation at the inner (convex) and outer (concave) banks of constant‐width and widening open‐channel bends     

Koen Blanckaert  Maarten G. Kleinhans  Stuart J. McLelland  Wim S. J. Uijttewaal  Brendan J. Murphy  Anja van de Kruijs  Daniel R. Parsons  Qiuwen Chen 《地球表面变化过程与地形》2013,38(7):696-716

There is a paucity of data and insight in the mechanisms of, and controls on flow separation and recirculation at natural sharply‐curved river bends. Herein we report on successful laboratory experiments that elucidate flow structure in one constant‐width bend and a second bend with an outer‐bank widening. The experiments were performed with both a flat immobile gravel bed and mobile sand bed with dominant bedload sediment transport. In the constant‐width bend with immobile bed, a zone of mainly horizontal flow separation (vertical rotational axis) formed at the inner bank that did not contain detectable flow recirculation, and an outer‐bank cell of secondary flow with streamwise oriented rotational axis. Surprisingly, the bend with widening at the outer bank and immobile bed did not lead to a transverse expansion of the flow. Rather, flow in the outer‐bank widening weakly recirculated around a vertical axis and hardly interacted with the inner part of the bend, which behaved as a constant‐width bend. In the mobile bed experiment, downstream of the bend apex a pronounced depositional bar developed at the inside of the bend and pronounced scour occurred at the outside. Moreover the deformed bed promoted flow separation over the bar, including return currents. In the constant‐width bend, the topographic steering impeded the generation of an outer‐bank cell of secondary flow. In the bend with outer‐bank widening, the topographic steering induced an outward expansion of the flow, whereby the major part of the discharge was conveyed in the central part of the widening section. Flow in the outer‐bank widening was highly three dimensional and included return currents near the bottom. In conclusion, the experiments elucidated three distinct processes of flow separation common in sharp bends: flow separation at the inner bank, an outer‐bank cell of secondary flow, and flow separation and recirculation in an outer‐bank widening. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

The analysis of indexed astronomical time series –VII. Simultaneous use of times of maxima and minima to test for period changes in long-period variables     

Chris Koen  Fred Lombard 《Monthly notices of the Royal Astronomical Society》2001,325(3):1124-1132

A statistical model is formulated that enables one to analyse jointly the times between maxima and minima in the light curves of monoperiodic pulsating stars. It is shown that the combination of both sets of data into one leads to analyses that are more sensitive. Illustrative applications to the American Association of Variable Star Observers data for a number of long-period variables demonstrate the usefulness of the approach.  相似文献   

Mode identification in the δ Scuti Star 1 Mon     

L. A. Balona  B. Bartlett  †  J. A. R. Caldwell  J. Gaobakwe  †  G. Handler  P. Kalebwe  †  K. Khoabane  †  C. Koen  D. Laney  T. Medupe  J. Menzies  M. Msikinya  †  M. Phillips †  T. Sono † 《Monthly notices of the Royal Astronomical Society》2001,321(2):239-246

We present new multicolour photometry and simultaneous high-dispersion spectroscopy for the δ Scuti star 1 Mon. The two main periodicities in the star are still present, but the third known period is not directly detected in the new radial velocities or light variations. However, it is detected in the periodogram of the second moment of the line profile variations. We use the cross-correlation function as an approximation for the line profile variations. By computing theoretical profiles for a given mode and comparing them with phased cross-correlation profiles, we are able to determine a goodness-of-fit criterion and estimate the most probable spherical harmonic degree,     the azimuthal order, m , of the pulsation and also the angle of inclination. We then compare the relative amplitudes and phases of the photometric variations in five wavebands and obtain the best estimates of     for the two visible periodicities. We confirm the earlier determinations that the main periodicity is a radial mode and that the other periodicity is probably         We show that the line profile variations and light variations give consistent results. We point out the importance of a long wavelength range when using the photometric mode identification technique. Finally, we attempt to match the two periods with unstable modes from linear, non-adiabatic calculations. We are able to show that the principal period is well matched by either the fundamental or first overtone radial mode, but could not find a satisfactory fit to the     mode. We discuss implications for mode identification of δ Scuti stars based on what we have learned from this star.  相似文献   

Testing the bivariate series of Mira light curve rise and fall times for changes     

Fred Lombard  Chris Koen  Diederick Potgieter 《Monthly notices of the Royal Astronomical Society》2001,325(3):1133-1141

The American Association of Variable Star Observers supplies the astronomical community with a large data base of times of light maxima and minima of Mira (long-period pulsating) stars. Period change studies using these data invariably use either times between maxima, or those between minima. A statistical analysis based on the two-component time series of light curve rise and fall times is developed. The results, which enable one to detect changes in the shapes of light curves, are applied to observations of seven long-period variables.  相似文献   

Apparent partial loss age spectra of Neoarchean hornblende (Murmansk Terrane,Kola Peninsula,Russia): the role of biotite inclusions revealed by 40Ar/39Ar laserprobe analysis     

Koen De Jong  Jan R. Wijbrans 《地学学报》2006,18(5):353-364

Metamorphic hornblende frequently yields spectra with progressively increasing ⁴⁰Ar/³⁹Ar age steps, often interpreted as caused by partial resetting due to thermally activated radiogenic argon loss by solid‐state diffusion. Yet, in many cases rising Ca/K ratio spectra for such samples imply the presence of minor inclusions of K‐contaminant minerals. To avoid parts of grains with mineral inclusions or compositional zoning we drilled tiny discs from thin sections under a petrographic microscope. Laser step‐heating of drilled biotite‐free hornblende discs yielded flat age and ratio spectra. In contrast, furnace step‐heated hornblende separates from the same samples produced apparent loss age spectra. Moreover, biotite‐free samples yielded flat spectra by laser and furnace dating. Consequently, apparent loss spectra result from degassing of included substantially younger biotite before its hornblende host during laboratory step‐heating; c. 2640 Ma hornblende ages constrain the Murmansk Terrane's cooling.  相似文献   

Near-infrared time-series photometry of six fields in the young open cluster IC 2391     

Chris Koen  Akika Ishihara 《Monthly notices of the Royal Astronomical Society》2006,369(2):846-852

Fields containing targets of spectral types later than M5 were monitored in   JHK_s   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.  相似文献   

δ Scuti pulsations in β Pictoris     

Chris Koen 《Monthly notices of the Royal Astronomical Society》2003,341(4):1385-1387

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KPD 0629–0016, a slowly pulsating hot subdwarf star     

C. Koen  E. M. Green 《Monthly notices of the Royal Astronomical Society》2007,377(4):1605-1609

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.  相似文献