A study of optically featureless objects from the EC Survey: searching for new southern BL Lac objects     

R. R. Sefako  I. S. Glass  D. Kilkenny  O. C. De Jager  R. S. Stobie  D. O'Donoghue  C. Koen 《Monthly notices of the Royal Astronomical Society》1999,309(4):1043-1050

In an attempt to discover new southern BL Lac objects, 14 optically featureless objects from the Edinburgh–Cape Survey were selected. Optical polarimetry and spectroscopy, radio and IR observations were carried out in order to improve their classifications. The 14 objects were examined according to special criteria that are described. Their UBV and JHK colour–colour distributions, spectrograms, radio observations and visible-region polarimetry were utilized to conclude that the selected objects are not BL Lacs. Most of them are apparently not extragalactic objects. It is suggested that four of the 14 candidates are DC white dwarfs, one is a QSO, three are DA white dwarfs or sdB subdwarfs, one is a cataclysmic variable, one is a DAO/sdO and another is possibly a DA+dM binary (composite system with a 'primary' hot white dwarf and a 'secondary' M-type main-sequence dwarf) or a cataclysmic variable. One object is likely to be a subdwarf, while two remain unclassifiable.  相似文献   

Eight new δ Scuti stars     

Chris Koen  Ruby Van Rooyen  Francois Van Wyk  Freddy Marang 《Monthly notices of the Royal Astronomical Society》1999,309(4):1051-1062

HD 23194, a member of the Pleiades, was found to pulsate with a period of about 30 min. The literature on the star is reviewed, and it is concluded that it may be a marginal Am star in a binary system. HD 95321 is an evolved Am ( ρ Puppis) star with a 5.1-h periodicity. Mode identification of its pulsation, based on multicolour photometry, suggests that the oscillation is probably non-radial with ℓ=2. We also report on the discovery of six other new δ Scuti stars, some of which may be pulsating in gravity modes.  相似文献   

Long-term waves on Cepheid pulsation period trends     

L. N. Berdnikov  V. V. Ignatova  John A. R. Caldwell  C. Koen 《New Astronomy》2000,4(8):587-639

The secular evolution of the pulsation period of δ Cephei, η Aquilae, and ζ Geminorum during a cumulative total of 577 years suggests wave-like departures about the smooth time trends.  相似文献   

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Koen A. van Toorenenburg  Marinus E. Donselaar  Gert Jan Weltje 《地球表面变化过程与地形》2018,43(11):i-i

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Post-fire soil erosion mitigation at the scale of swales using forest logging residues at a reduced application rate     

Sergio A. Prats  Óscar González-Pelayo  Flavio C. Silva  Koen J. Bokhorst  Jantiene E.M. Baartman  Jan J. Keizer 《地球表面变化过程与地形》2019,44(14):2837-2848

Mulching with forest residues has proved to be highly effective in reducing post-fire soil losses at the plot scale. However, its effectiveness has not been quantified at the application rates that are typically used in operational post-fire land management (2–3 Mg ha^-1 using straw), as well as at scales larger than 100 m². The present study compared post-fire erosion rates for six convergent hillslopes or swales of 500 to 800 m², three of which were left untreated while the other three were mulched immediately after the fire with shredded eucalypt bark at a rate of 2.4 Mg ha^-1. Erosion rates were monitored at irregular intervals during the first three post-fire years, whilst ground cover was assessed yearly. Selected topsoil properties (0–2 cm) such as organic matter content and aggregate stability were determined at a single occasion – two years after the wildfire, for three micro-environments separately: bare soil, and under mulch/litter and vegetation. Soil losses on the untreated swales decreased with post-fire year from 2.2 to 0.4 and 0.11 Mg ha^-1 yr^-1 (respectively for the first, second and third post-fire years), while the mulched swales produced 84%, 77% and 38% less soil losses than the untreated swales. Soil losses also depended on slope aspect, with the north-facing swales producing less erosion than the west-facing ones. This could be linked to their significant differences in bare soil, vegetation and stone cover, or a combination thereof. The type of micro-environment also played a significant role in topsoil properties (stone content, bulk density, resistance to penetration/shear stress, porosity and organic matter content). The present results add to the increasing evidence that forest residues should be duly considered for operational post-fire land management. Forest residues were highly effective in reducing erosion from swales at application rates as low as the typical 2 Mg ha^-1 of post-fire straw mulch. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.  相似文献   

Ecology and fluvial dynamics of an Early Holocene medium‐sized European lowland river valley (Upper Scheldt,northern Belgium)     

Annelies Storme  Jan Bastiaens  Philippe Cromb  Sofie Debruyne  Stephen Louwye  Koen Deforce 《Boreas: An International Journal of Quaternary Research》2019,48(3):678-694

The fluvial environment of Early Holocene small‐ to middle‐sized lowland rivers in northwest Europe is mostly unstudied due to a lack of preserved and accessible deposits. A rescue excavation in the Scheldt valley in northern Belgium offered the opportunity to study a Boreal alluvial succession in detail. The results of palaeoecological and sedimentological analyses (diatoms, pollen, botanical macro‐remains, molluscs, grain size) characterize the biotic and physical environment in the middle reach of this medium‐sized river system. Although the Early Holocene in the Scheldt Basin has often been portrayed as a period of fluvial stability with marshy conditions and diffuse discharge, this study showed evidence of point bar formation by a small, low‐energy meandering river between ~9.5 and ~8.8 cal. ka BP. The point bar was at least temporarily vegetated and shows a shift from herbaceous riparian vegetation to an open willow‐dominated alluvial forest. This evidence points to a more open vegetation and a more energetic environment than traditionally described for rivers of this size and age. A link to the 9.3 ka BP cooling event is suggested and possible reasons for the scarcity of records of this type of deposits are discussed.  相似文献   

Modelling the rotation period distribution of M dwarfs in the <Emphasis Type="Italic">Kepler</Emphasis> field     

Chris Koen 《Astrophysics and Space Science》2018,363(1):11

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. It is shown here that the data can be modelled as a mixture of four subpopulations, three of which are overlapping log-normal distributions. The fourth subpopulation has a power law distribution, with \(P^{-1/2}\). It is also demonstrated that the bulk of the longer periods, representing the two major subpopulations, could be drawn from a single subpopulation, but with a period-dependent probability of observing half the true period.  相似文献   

Control of the repeatability of high frequency multibeam echosounder backscatter by using natural reference areas     

Marc Roche  Koen Degrendele  Christophe Vrignaud  Sophie Loyer  Tim Le Bas  Jean-Marie Augustin  Xavier Lurton 《Marine Geophysical Researches》2018,39(1-2):89-104

The increased use of backscatter measurements in time series for environmental monitoring necessitates the comparability of individual results. With the current lack of pre-calibrated multibeam echosounder systems for absolute backscatter measurement, a pragmatic solution is the use of natural reference areas for ensuring regular assessment of the backscatter measurement repeatability. This method mainly relies on the assumption of a sufficiently stable reference area regarding its backscatter signature. The aptitude of a natural area to provide a stable and uniform backscatter response must be carefully considered and demonstrated by a sufficiently long time-series of measurements. Furthermore, this approach requires a strict control of the acquisition and processing parameters. If all these conditions are met, stability check and relative calibration of a system are possible by comparison with the averaged backscatter values for the area. Based on a common multibeam echosounder and sampling campaign completed by available bathymetric and backscatter time series, the suitability as a backscatter reference area of three different candidates was evaluated. Two among them, Carré Renard and Kwinte, prove to be excellent choices, while the third one, Western Solent, lacks sufficient data over time, but remains a valuable candidate. The case studies and the available backscatter data on these areas prove the applicability of this method. The expansion of the number of commonly used reference areas and the growth of the number of multibeam echosounder controlled thereon could greatly contribute to the further development of quantitative applications based on multibeam echosounder backscatter measurements.  相似文献   

Correcting CCD photometry of stars for seeing effects     

Chris Koen 《Monthly notices of the Royal Astronomical Society》2009,395(1):531-536

Systematic variability in stellar magnitudes, as derived from profile fitting to CCD images, may in some instances be due to variable seeing. It is suggested that this happens in cases where the stars are unresolved pairs, typically with sub-arcsecond separation between the components. It is shown that the fitting of suitable Generalised Additive Models to time series photometry can disentangle intrinsic stellar variability and seeing-induced brightness changes. It is possible that there will be a fixed seeing response associated with a given star which exhibits the effect: estimation of this response from several long photometric runs is demonstrated.  相似文献   

A geological constraint on relative sea level in Marine Isotope Stage 3 in the Larsemann Hills, Lambert Glacier region, East Antarctica (31 366–33 228 cal yr BP)     

Dominic A. Hodgson  Elie Verleyen  Wim Vyverman  Koen Sabbe  Melanie J. Leng  Matthew D. Pickering  Brendan J. Keely 《Quaternary Science Reviews》2009,28(25-26):2689-2696

In this paper we present geological evidence from the Larsemann Hills (Lambert Glacier – Amery Ice Shelf region, East Antarctica) of marine sediments at an altitude of c. 8 m a.s.l., as revealed by diatom, pigment and geochemical proxies in a lake sediment core. The sediments yielded radiocarbon dates between c. 26 650 and 28 750 ¹⁴C yr BP (31 366–33 228 cal yr BP). This information can be used to constrain relative sea level adjacent to the Lambert Glacier at the end of Marine Isotope Stage 3. These data are compared with the age and altitude of Marine Isotope Stage 3 marine deposits elsewhere in East Antarctica and discussed with reference to late Quaternary ice sheet history and eustatic sea-level change.  相似文献