Browsing Faculty of Natural Sciences by Author "Koen, Chris"
Now showing items 21-40 of 62
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IC and RC band time-series observations of some bright ultracool dwarfs
Koen, Chris (Oxford University Press, 2005)The 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. ... -
Identifying multiple periodicities in sparse photon event time series
Koen, Chris (Oxford University Press, 2016)The data considered are event times (e.g. photon arrival times, or the occurrence of sharp pulses). The source is multiperiodic, or the data could be multiperiodic because several unresolved sources contribute to the time ... -
Improved SAAO–2MASS photometry transformations
Koen, Chris; Marang, F.; Kilkenny, David; Jacobs, C. (Oxford University Press, 2007)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 ... -
Is there evolution in the infrared Tully–Fisher relation? Comparing two linear regressions
Koen, Chris; Lombard, Fred (Oxford University Press, 2009)In a recent paper, Puech and co-workers compared K-band Tully–Fisher relations derived for nearby and distant galaxies, respectively. They concluded that the two relations differ, and deduced that there is evolution in ... -
JHKs time-series observations of a few ultracool dwarfs
Koen, Chris; Tanabe, T.; Tamura, M.; Kusakabe, N. (Oxford University Press, 2005)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 ... -
JL 82: a slowly pulsating hot subdwarf star in a close binary system
Koen, Chris (Oxford University Press, 2009)JL 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 ... -
KPD 0629–0016, a slowly pulsating hot subdwarf star
Koen, Chris; Green, E.M. (Oxford University Press, 2007)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 ... -
Linking multi-epoch CCD photometry of partially overlapping fields
Koen, Chris (Oxford University Press, 2013)A common problem in CCD photometry is to combine measurements obtained at different epochs. This is especially challenging if the fields only overlap partially, and are sparse, so that not all calibrating stars are ... -
Modelling the rotation period distribution of M dwarfs in the Kepler field
Koen, Chris (Springer Verlag, 2018)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. ... -
More on the nature of the variable star UNSW-V-760
Koen, Chris (Oxford University Press, 2012)The results of two multicolour time-series photometric observing runs, separated by about two months, are presented. The same periodicities previously detected in photometry of UNSWV- 760 are still present, albeit at ... -
Multicolour time series photometry of 2MASS 1207−3932
Koen, Chris (Oxford University Press, 2008)Time 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 ... -
Multicolour time series photometry of four short-period weak-lined T Tauri stars
Koen, Chris (Oxford University Press, 2015)The paper describes continuous photometric monitoring of four pre-main-sequence stars, probable members of young stellar associations. Measurements, covering at least four nights per star, were obtained by cycling through ... -
Multicolour time series photometry of the T Tauri star CVSO 30
Koen, Chris (Oxford University Press, 2015)Five consecutive runs of at least five hours, and two shorter runs, of V(RI)C time series photometry of CVSO 30 are presented. No evidence could be seen for planetary transits, previously claimed in the literature for ... -
Multicolour time series photometry of the variable star 1SWASP J234401.81−212229.1
Koen, Chris (Oxford University Press, 2014)1SWASP J234401.81-212229.1 may be one of a handful of contact binaries comprising two Mdwarfs. Modelling of the available observations is complicated by the fact that the radiation of the eclipsing system is dominated ... -
Multicolour time series photometry of three periodic ultracool dwarfs
Koen, Chris (Oxford University Press, 2013)Photometry in I, or contemporaneously in I and R, of the known variable ultracool dwarfs Kelu-1 and 2MASS J11553952−3727350 is presented. The nature of the variability of Kelu-1 appears to evolve on time-scales of a day ... -
Multifilter observations of the complex periodic variations in eight pre-main sequence stars
Koen, Chris (Royal Astronomical Society, 2023)Follow up time series photometry through various combinations of the BVRCIC filter set is presented for eight young stars. The complex periodic variability of seven of the stars was first discovered in TESS photometry, ... -
Multifilter time-series observations of eleven blue short-period atlas variable stars
Koen, Chris; Schaffenroth, Veronika; Kniazev, Alexei (IOP Publishing, 2023)Eleven periodic variable stars were observed photometrically through two to four filters from the set UBVRCIC. Phase-folded data for each star cover full cycles of variation. Spectral energy distributions, based on ... -
Multiple regression of GRB luminosity on light-curve properties
Koen, Chris (Oxford University Press, 2009)Statistical relations between the luminosity L of gamma-ray burster (GRBs) and several of their other observable properties have been discovered. Four of these properties are considered here: the spectral lag; a measure ... -
Multivariate comparisons of the period–light-curve shape distributions of Cepheids in five galaxies
Koen, Chris; Siluyele, I. (Oxford University Press, 2007)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. ... -
Near-infrared time-series photometry of six fields in the young open cluster IC 2391
Koen, Chris; Ishihara, Akika (Oxford University Press, 2006)Fields 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 ...