UBVRIJH photometry of two new luminous δ Scuti stars and the discovery of δ Scuti pulsation in the most evolved Ap star known     

C. Koen  D.W. Kurtz  R.O. Gray  D. Kilkenny  G. Handler  F. Van Wyk  F. Marang  H. Winkler 《Monthly notices of the Royal Astronomical Society》2001,326(1):387-396

Time-series photometry of the Hipparcos variable stars HD 199434 and 21190 is reported. Both stars are pulsators of the δ Scuti type. Reclassifications of the MK types of the stars, based on new spectrograms, are given. HD 21190 is found to be F2III SrEuSi:, making it the most evolved Ap star known. Its Strömgren photometric indices support the peculiar spectral type. It is also one of the most evolved δ Scuti stars known. Its combined Ap– δ Scuti nature makes it an important test of models of pulsation in peculiar stars recently developed by Turcotte et al., although it is more extreme than any model they examined. Physical parameters of both stars are estimated from Strömgren and H β photometry, and Hipparcos absolute magnitudes. We attempt mode identifications based on amplitude ratios and phase differences from our photometry. The dominant pulsation of HD 21190 may be an overtone radial mode. The model fits for HD 199434 are even less satisfactory, but favour an ℓ=2 mode. Given the good quality and wavelength coverage of our data, the poor results from the application of the photometric theory of mode identification may call into question the use of that technique.  相似文献   

Another look at theR-modes     

Chris Koen 《Astrophysics and Space Science》1987,135(2):393-398

Possible oscillation modes for a rotating star are listed. The only assumption made is that oscillations are adiabatic and that rotation is uniform. It is found that two modes not present for non-rotating stars are possible. Oscillation frequencies of these modes are rather different from those given in the literature for so-calledr-modes.  相似文献   

Evidence for rapid evolution of periodic variations in an ultracool dwarf     

C. Koen 《Monthly notices of the Royal Astronomical Society》2006,367(4):1735-1738

The 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   I_C   band amplitude which decreased from 27 to 11 mmag over the three nights. The amplitude in the   R_C   band appeared to be substantially smaller than in the   I_C   band.  相似文献   

The Nyquist frequency for irregularly spaced time-series: a calculation formula     

Chris Koen 《Monthly notices of the Royal Astronomical Society》2006,371(3):1390-1394

Eyer & 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/Δ on this frequency, where Δ is the best accuracy with which time is recorded.  相似文献   

Three new pulsating sdB stars from the Edinburgh–Cape survey     

D. Kilkenny  R. S. Stobie  D. O'Donoghue  C. Koen  N. Hambly  H. MacGillivray  A. E. Lynas-Gray 《Monthly notices of the Royal Astronomical Society》2006,367(4):1603-1608

We report the discovery of very rapid pulsations in three sdB stars from the Edinburgh–Cape blue object survey. The short periods, small amplitudes and multi-periodicity clearly establish these stars as members of the EC 14026 class. EC 11583−2708 has pulsation periods near 149, 144 and 114 s, though evidence is presented that the 149-s period is resolved into two periods at 148.87 and 148.55 s by the full photoelectric data set. The amplitudes of the detected variations are in the range 0.002–0.006 mag. The light variation of EC 20338−1925 is dominated by a period near 147 s with a very large amplitude for a variable sdB star (0.025 mag), though four other frequencies are detected with periods near 168, 151, 141 and 135 s and amplitudes in the range 0.002–0.005 mag. The third star, EC 09582−1137, displays a light curve which is virtually a textbook example of frequency beating, being produced by two pulsations of almost equal amplitude (∼0.008 mag) and periods near 136.0 and 151.2 s.  相似文献   

A photometric and spectroscopic study of the hottest pulsating extreme helium star, V2076 Oph (HD 160641)     

D. J. Wright  A. E. Lynas-Gray  D. Kilkenny  P. L. Cottrell  R. R. Shobbrook  C. Koen  F. W. van Wyk  P. M. Kilmartin  P. Martinez  A. C. Gilmore 《Monthly notices of the Royal Astronomical Society》2006,369(4):2049-2058

We present results from a three-site photometric and high-resolution spectroscopic campaign on the hottest known extreme helium star V2076 Oph (HD 160641). A core programme of intensive observations covered two weeks and a much lower sampling rate extended over another two months. Despite the fact that the data seem to indicate periodicity near half a day (though the light curves are clearly not formed by a single periodicity), conventional Fourier analysis of the data fails to reveal coherent frequencies. Furthermore, we are unable to recover frequencies which were apparently clear in an earlier campaign on the star. Evidence of monotonic pulsation amplitude changes is seen at the higher frequencies from a wavelet analysis, but more data are needed before this study can be extended to lower frequencies. The application of linear stochastic differential equation (LSDE) methods indicates that the observed light variations could be a result of random variations giving the appearance of intermittent periodicity. High-resolution spectroscopic observations were obtained during the campaign and additional observations were made three years later. Complex line profile variations were observed. It is proposed that the different behaviour of the emission line studied may indicate it is associated with a stellar wind or resident circumstellar material. The frequencies that are extracted from the velocity data do not conform to those identified in the current or previous photometric campaigns.  相似文献   

Climate-induced vegetation dynamics and the Bantu Expansion: Evidence from Bantu names for pioneer trees (Elaeis guineensis,Canarium schweinfurthii,and Musanga cecropioides)     

Koen Bostoen  Rebecca Grollemund  Joseph Koni Muluwa 《Comptes Rendus Geoscience》2013,345(7-8):336-349

The present article examines whether Late Holocene climate-induced vegetation changes in the Central African forest block may have facilitated the Bantu Expansion. This is done through a body of evidence that is not commonly used for the reconstruction of vegetation dynamics, i.e. language data. The article focuses on common Bantu vocabulary for three pioneer species abundantly present in the Central African pollen record between ca. 2500 and 2000 BP: Musanga cecropioides, Elaeis guineensis, and Canarium schweinfurthii. The geographical distribution patterns of the vernacular names for these pioneer trees add weight to the hypothesis according to which the rainforest contraction that emerged in the first millennium BC had an impact on the way Bantu languages dispersed.  相似文献   

Reduction of bend scour with an air-bubble screen-morphology and flow patterns     

Violaine DUGU  Koen BLANCKAERT  Qiuwen CHEN  Anton J.SCHLEISS 《国际泥沙研究》2013,28(1):15-23

The interplay between streamwise flow,curvature-induced secondary flow,sediment transport and bed morphology leads to the formation of a typical bar-pool bed morphology in open-channel bends.The associated scour at the outer bank and deposition at the inner bank may endanger the outer bank’s stability or reduce the navigable width of the channel.Previous preliminary laboratory experiments in a sharply curved flume with a fixed horizontal bed have shown that a bubble screen located near the outer bank can generate an additional secondary flow located between the outer bank and the curvature-induced secondary flow and with a sense of rotation opposite to the latter.This bubble-induced secondary flow redistributes velocities and bed shear stresses.The reported study investigates the implications of a bubble screen on the flow and the morphology in configurations with mobile bed.Velocity measurements show that the bubble-induced secondary flow shifts the curvature-induced secondary flow in inwards direction and reduces its strength.The bubble screen considerably reduces morphological gradients.Maximum bend scour is reduced by about 50%and occurs further away from the outer bank where it does not endanger the bank stability anymore.The location of maximum scour coincides with the junction of the curvature-induced and bubble-induced secondary flows.At this same location,the maximum streamwise velocities and maximum vertical velocities impinging on the bed also occur,which indicates their importance with respect to the formation of bend scour.The bubble screen also substantially reduced deposition at the inner bank.These preliminary experiments show the potential of a bubble screen to influence and modify the bed morphology.  相似文献   

Retrieval of Suspended Sediment from Advanced Hyperspectral Sensor Data in the Scheldt Estuary at Different Stages in the Tidal Cycle     

Sindy Sterckx  Els Knaeps  Mark Bollen  Koen Trouw  Rik Houthuys 《Marine Geodesy》2013,36(1-2):97-108

Hyperspectral airborne remote sensing images and in-situ data are combined to assess the spatial and temporal sediment dynamics in the tidal Scheldt river. A log-linear empirical relationship has been developed between a near-infrared reflectance difference and total suspended matter. The relationship was shown to be relatively insensitive to the varying cirrus cloud cover occurring during data acquisition. The produced sediment maps show good agreement with known variations of turbidity over the tidal cycle: maximum turbidity around high water, gradual settling of the sediment in the succeeding slack water and resuspension at the onset of the ebb flow stage.  相似文献   

Comprehensive Parametrization of Surface-Layer Transfer Coefficients for Use in Atmospheric Numerical Models   总被引：1，自引：0，他引：1  

Hendrik Wouters  Koen De Ridder  Nicole P. M. van Lipzig 《Boundary-Layer Meteorology》2012,145(3):539-550

A new non-iterative bulk parametrization for surface-layer transfer coefficients for momentum and heat is presented. It is applicable for a wide range of aerodynamic and thermal roughness lengths, and includes the effect of the roughness sublayer. As a consequence, the non-iterative method is suitable for every surface type, especially for urban surfaces for which existing non-iterative parametrizations fail. The analytical approximation compares very well with an iterative approach. Our method can be easily implemented in atmospheric numerical models that already employ a non-iterative approach.  相似文献