613 research outputs found
How Planting Density Affects Number and Yield of Potato Minitubers in a Commercial Glasshouse Production System
Commercial potato minituber production systems aim at high tuber numbers per plant. This study investigated by which mechanisms planting density (25.0, 62.5 and 145.8 plants/m2) of in vitro derived plantlets affected minituber yield and minituber number per plantlet. Lowering planting density resulted in a slower increase in soil cover by the leaves and reduced the accumulated intercepted radiation (AIR). It initially also reduced light use efficiency (LUE) and harvest index, and thus tuber weights per m2. At the commercial harvest 10 weeks after planting (WAP), LUE tended to be higher at lower densities. This compensated for the lower AIR and led to only slightly lower tuber yields. Lowering planting density increased tuber numbers per (planted) plantlet in all grades. It improved plantlet survival and increased stem numbers per plant. However, fewer stolons were produced per stem, whereas stolon numbers per plant were not affected. At lower densities, more tubers were initiated per stolon and the balance between initiation and later resorption of tubers was more favourable. Early interplant competition was thought to reduce the number of tubers initiated at higher densities, whereas later-occurring interplant competition resulted in a large fraction of the initiated tubers being resorbed at intermediate planting densities. At low planting densities, the high number of tubers initiated was also retained. Shortening of the production period could be considered at higher planting densities, because tuber number in the commercial grade > 9 mm did not increase any more after 6 WA
RXTE observations of the dipping low-mass X-ray binary 4U 1624-49
We analyse ~ 360 ks of archival data from the Rossi X-Ray Timing Explorer
(RXTE) of the 21 hr orbital period dipping low-mass X-ray binary 4U 1624-49. We
find that outside the dips the tracks in the colour-colour and
hardness-intensity diagrams (CDs and HIDs) are reminiscent of those of atoll
sources in the middle and upper parts of the banana branch. The tracks show
secular shifts up to ~ 10%. We study the power spectrum of 4U 1624-49 as a
function of the position in the CD. This is the first time power spectra of
this source are presented. No quasi-periodic oscillations (QPOs) are found. The
power spectra are dominated by very low frequency noise (VLFN), characteristic
for atoll sources in the banana state, and band limited noise (BLN) which is
not reliably detected but may, uncharacteristically, strengthen and increase in
frequency with spectral hardness. The VLFN fits to a power law, which becomes
steeper when the source moves to the harder part of the CD. We conclude that 4U
1624-49 is an atoll source which in our observations is in the upper banana
branch. Combining this with the high (0.5-0.7 L_Edd) luminosity, the long-term
flux stability of the source as seen with the RXTE All-Sky Monitor (ASM), and
with the fact that it is an X-ray dip source, we conclude that 4U 1624-49 is
most likely a GX atoll source such as GX 3+1 and GX 9+9, but seen edge on.Comment: 8 pages, 5 figures, 2 tables, accepted for publication in A&A. This
version: a few typos correcte
Detectie van tulpengalmijt voor gerichte galmijtbestrijding
mijtenval doorlopen moet worden duurt echter te lang voor toepassing in de praktijk. De doelstelling van dit onderzoek was om een alternatieve, snelle en betrouwbare detectie-methode te ontwikkelen om tulpengalmijt (vroegtijdig) in partijen tulp aan te kunnen tonen. De effectiviteit van een teentje knoflook en van een tulpenbol die behandeld is met ethyleen, is vergeleken met de effectiviteit van de mijtenval
Pulsar timing array observations of gravitational wave source timing parallax
Pulsar timing arrays act to detect gravitational waves by observing the
small, correlated effect the waves have on pulse arrival times at Earth. This
effect has conventionally been evaluated assuming the gravitational wave
phasefronts are planar across the array, an assumption that is valid only for
sources at distances , where is physical extent of
the array and the radiation wavelength. In the case of pulsar timing
arrays (PTAs) the array size is of order the pulsar-Earth distance (kpc) and
is of order pc. Correspondingly, for point gravitational wave sources
closer than ~Mpc the PTA response is sensitive to the source parallax
across the pulsar-Earth baseline. Here we evaluate the PTA response to
gravitational wave point sources including the important wavefront curvature
effects. Taking the wavefront curvature into account the relative amplitude and
phase of the timing residuals associated with a collection of pulsars allows us
to measure the distance to, and sky position of, the source.Comment: 11 pages, 2 figures, submitted to MNRA
Extending gravitational wave burst searches with pulsar timing arrays
Pulsar timing arrays (PTAs) are being used to search for very low frequency
gravitational waves. A gravitational wave signal appears in pulsar timing
residuals through two components: one independent of and one dependent on the
pulsar's distance, called the 'Earth term' (ET) and 'pulsar term' (PT),
respectively. The signal of a burst (or transient) gravitational wave source in
pulsars' residuals will in general have the Earth and pulsar terms separated by
times of the order of the time of flight from the pulsar to the Earth.
Therefore, both terms are not observable over a realistic observation span, but
the ETs observed in many pulsars should be correlated. We show that pairs (or
more) of pulsars can be aligned in such a way that the PTs caused by a source
at certain sky locations can arrive at Earth within a time window short enough
to be captured during a realistic observation span. We find that for the
pulsars within the International Pulsar Timing Array (IPTA) ~67 per cent of the
sky produces such alignments for pulsars terms separated by less than 10 years.
We compare estimates of the source event rate that would be required to observe
one signal in the IPTA if searching for the correlated ETs, or in searching via
the PTs, and find that event rates would need to be about two orders of
magnitude higher to observe an event with the PTs than the ETs. We also find
that an array of hundreds of thousands of pulsars would be required to achieve
similar numbers of observable events in PT or ET searches. This disfavours PTs
being used for all-sky searches, but they could potentially be used target
specific sources and be complementary to ET only searches.Comment: version accepted for MNRA
Potato Yield and Yield Components as Affected by Positive Selection During Several Generations of Seed Multiplication in Southwestern Uganda
Potato (Solanum tuberosum L.) is an important crop in Uganda but production is low. There is not a well-functioning official seed system and farmers use potato tubers from a previous harvest as seed. This study investigated how effectively the seed technology positive selection enhanced yield and underlying crop characteristics across multiple seasons, compared to the farmers’ selection method. Positive selection is selecting healthy plants during crop growth for harvesting seed potato tubers to be planted in the next season. Farmers’ selection involves selection of seed tubers from the bulk of the ware potato harvest. Positive selection was compared to farmers’ seed selection for up to three seasons in three field trials in different locations in southwestern Uganda using seed lots from different origins. Across all experiments, seasons and seed lots, yields were higher under positive selection than under farmers’ selection. The average yield increase resulting from positive selection was 12%, but yield increases were variable, ranging from − 5.7% to + 36.9%, and in the individual experiments often not significant. These yield increases were due to higher yields per plant, and mostly higher weights per tuber, whereas the numbers of tubers per plant were not significantly different. Experimentation and yield assessment were hampered by a varying number of plants that could not be harvested because plants had to be rogued from the experimental plots because of bacterial wilt (more frequent under farmers’ selection than under positive selection), plants disappeared from the experimental field and sometimes plants did not emerge. Nevertheless, adoption of positive selection should be encouraged due to a higher production and less virus infection of seed tubers in positive selected plants, resulting in a lower degeneration rate of potato seed tubers.</p
Search for Memory and Inspiral Gravitational Waves from Super-Massive Binary Black Holes with Pulsar Timing Arrays
The merger of a super-massive binary black hole (SBBH) is one of the most
extreme events in the universe with a huge amount of energy released by
gravitational radiation. Although the characteristic gravitational wave (GW)
frequency around the merger event is far higher than the nHz regime optimal for
pulsar timing arrays (PTAs), nonlinear GW memory might be a critical smoking
gun of the merger event detectable with PTAs. In this paper, basic aspects of
this interesting observation are discussed for SBBHs, and the detection numbers
of their memory and inspiral GWs are estimated for ongoing and planned PTAs. We
find that the expected detection number would be smaller than unity for the
two-types of signals even with the Square Kilometer Array. We also provide
various scaling relations that would be useful to study detection probabilities
of GWs from individual SBBHs with PTAs.Comment: 5 pages, 2 figure
Ongevleugelde lieveheersbeestjes
Bladluizen in stadsbomen zorgen soms voor grote overlast. Vooral onder lindebomen is er jaarlijks wel een periode van honingdauwoverlast. Sommige steden gaan het probleem te lijf met het uitzetten van tweestippelige lieveheersbeestjes in de lindebomen. Het resultaat is niet altijd bevredigend. Nu is het effect van deze maatregel voor het eerst onderzocht. Voor dit onderzoek is echter speciaal gebruikgemaakt van een in de natuur voorkomend ongevleugeld type van dit lieveheersbeestje. De verwachting is dat dit langer in de boom blijft en daardoor beter is in bladluisbestrijding
Grain growth signatures in the protoplanetary discs of Chamaeleon and Lupus
We present ATCA results of a 3 and 7 mm continuum survey of 20 T Tauri stars
in the Chamaeleon and Lupus star forming regions. This survey aims to identify
protoplanetary discs with signs of grain growth. We detected 90% of the sources
at 3 and 7 mm, and determined the spectral slopes, dust opacity indices and
dust disc masses. We also present temporal monitoring results of a small
sub-set of sources at 7, 15 mm and 3+6 cm to investigate grain growth to cm
sizes and constrain emission mechanisms in these sources. Additionally, we
investigated the potential correlation between grain growth signatures in the
infrared (10 \mu m silicate feature) and millimetre (1-3 mm spectral slope,
{\alpha}). Eleven sources at 3 and 7 mm have dominant thermal dust emission up
to 7 mm, with 7 of these having a 1-3 mm dust opacity index less than unity,
suggesting grain growth up to at least mm sizes. The Chamaeleon sources
observed at 15 mm and beyond show the presence of excess emission from an
ionised wind and/or chromo- spheric emission. Long-timescale monitoring at 7 mm
indicated that cm-sized pebbles are present in at least four sources.
Short-timescale monitoring at 15 mm suggests the excess emission is from
thermal free-free emission. Finally, a weak correlation was found between the
strength of the 10 \mum feature and {\alpha}, suggesting simultaneous dust
evolution of the inner and outer parts of the disc. This survey shows that
grain growth up to cm-sized pebbles and the presence of excess emission at 15
mm and beyond are common in these systems, and that temporal monitoring is
required to disentangle these emission mechanisms.Comment: 31 pages, 13 figures, 11 tables, accepted for publication in MNRA
Orbital modulation of X-ray emission lines in Cygnus X-3
We address the problem where the X-ray emission lines are formed and
investigate orbital dynamics using Chandra HETG observations, photoionizing
calculations and numerical wind-particle simulations.The observed Si XIV (6.185
A) and S XVI (4.733 A) line profiles at four orbital phases were fitted with P
Cygni-type profiles consisting of an emission and a blue-shifted absorption
component. In the models, the emission originates in the photoionized wind of
the WR companion illuminated by a hybrid source: the X-ray radiation of the
compact star and the photospheric EUV-radiation from the WR star. The emission
component exhibits maximum blue-shift at phase 0.5 (when the compact star is in
front), while the velocity of the absorption component is constant (around -900
km/s). The simulated FeXXVI Ly alpha line (1.78 A) from the wind is weak
compared to the observed one. We suggest that it originates in the vicinity of
the compact star, with a maximum blue shift at phase 0.25 (compact star
approaching). By combining the mass function derived with that from the
infrared HeI absorption (arising from the WR companion), we constrain the
masses and inclination of the system. Both a neutron star at large inclination
(over 60 degrees) and a black hole at small inclination are possible solutions.Comment: 8 pages, accepted 19/04/2009 for publication in Astronomy and
Astrophysic
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