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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Browsing by Author "Kilkenny, David"
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Item Detection of an increasing orbital period in the subdwarf B eclipsing system NSVS 14256825(Oxford University Press, 2012) Kilkenny, David; Koen, ChrisNew timings of eclipses made in 2010 and 2011 are presented for the hot subdwarf B (sdB) eclipsing binary NSVS 14256825. Composed of an sdB star and a much cooler companion, with a period near 0.1104 days, this system is very similar to the prototype sdB eclipsing binary HW Vir. The new observations show that the binary period of NSVS 14256825 is rapidly increasing at a rate of about 12 × 10−12 days orbit−1.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 HE 0230−4323 revisited: a new rapidly pulsating sdB star(Oxford University Press, 2010) Kilkenny, David; Koen, Chris; Worters, H.HE 0230−4323 is a hot sdB star in a binary system. An earlier work demonstrated that the light curve of the system shows a strong (∼4 per cent) reflection effect and also appears to exhibit photometric variations of the type associated with the slowly pulsating class of sdB star (multiple periods in the range ∼1–2 h). In this paper, we show that HE 0230−4323 is, in fact, a rapidly pulsating sdB with at least five frequencies between 3227 and 3532 μHz (periods between 310 and 283 s). The long periods previously claimed were the result of undersampling the light curve at a time interval very close to that of the short periods. The interpretation of the very slow variation (∼0.45 d) as a reflection effect in a close binary is unaffected by these new results.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 A search for p-mode pulsations in white dwarf stars using the Berkeley Visible Imaging Tube detector(Oxford University Press, 2014) 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.Item Two new variable sdB stars, HE 0218−3437 and LB 1516(Oxford University Press, 2010) Koen, Chris; Kilkenny, David; Pretorius, M.L.; Frew, D.J.We present photometry which shows that two known hot subdwarf stars, HE 0218−3437 and LB 1516, are variable. LB 1516 exhibits several frequencies in the range 12–25 cycles d−1 (periods of about 1–2 h) with amplitudes less than about 0.003 mag and appears to be a typical slowly pulsating sdB star. Results from a multisite campaign on HE 0218−3437 show the presence of two frequencies only, the lower amplitude variation an apparent subharmonic of the higher amplitude periodicity. It is likely that the star is in a binary system, and that the variability is due to ellipsoidal deformation of the primary star.Item UBV(RI)C JHK observations of Hipparcos-selected nearby stars(Oxford University Press, 2010) Koen, Chris; Kilkenny, David; van Wyk, F.; Marang, F.We present homogeneous, standardized UBV(RI)C photometry for over 700 nearby stars selected on the basis of Hipparcos parallaxes. Additionally, we list JHK photometry for about half of these stars, as well as L photometry for 86 of the brightest. A number of stars with peculiar colours or anomalous locations in various colour–magnitude diagrams are discussed.