771 research outputs found
Possible magnetic field variability during the 6.7 GHz methanol maser flares of G09.62+0.20
(Abridged) Recently, the magnetic field induced Zeeman splitting was measured
for the strongest known 6.7 GHz methanol maser, which arises in the massive
star forming region G09.62+0.20. This maser is one of a handful of periodically
flaring methanol masers. The 100-m Effelsberg telescope was used to monitor the
6.7 GHz methanol masers of G09.62+0.20. With the exception of a two week period
during the peak of the maser flare, we measure a constant magnetic field of
B_||~11+-2 mG in the two strongest maser components of G09.62+0.20 that are
separated by over 200 AU. In the two week period that coincides exactly with
the peak of the maser flare of the strongest maser feature, we measure a sharp
decrease and possible reversal of the Zeeman splitting. The exact cause of both
maser and polarization variability is still unclear, but it could be related to
either background amplification of polarized emission or the presence of a
massive protostar with a close-by companion. Alternatively, the polarization
variability could be caused by non-Zeeman effects related to the radiative
transfer of polarized maser emission.Comment: 4 pages, 3 figures, accepted for publication Astronomy and
Astrophysic
The effect of canopy position on growth and mortality in mixed sapling communities during self-thinning
This research investigates how species in the sapling phase differ in growth and survival depending on light availability (as estimated by canopy position) by means of tree-ring analysis and modelling mortality. We harvested 120 live and 158 dead saplings in self-thinning communities consisting of Silver birch (Betula pendula Roth.), Scots pine (Pinus sylvestris L.), Japanese larch (Larix kaempferi Carr.) and Douglas fir (Pseudotsuga menziesii Mirb. Franco) in the Netherlands. Results are evaluated within the framework of a trade-off between high-light growth and low-growth survival. Radial growth, measured at ground level, generally declined over time. In addition, a decreasing light availability further reduced growth in all species except Douglas fir. Trees died when radial growth was reduced to about 0.5 mm year¿1. Mortality in all species except Scots pine was significantly related to recent growth, but mortality curves were not different. The light-demanding Silver birch and Japanese larch differed from the shade-tolerant Douglas fir in both high-light growth and low-growth mortality, in line with a growth-survival trade-off. The light-demanding Scots pine did not fit this pattern as it was unable to transfer high radial growth into height gain, leaving it in suppressed canopy positions. This indicates the importance of height growth in the growth-survival trade-off. Differences in mortality probabilities affect the potential for coexistence, however, in all species also fast-growing individuals died suggesting additional factors causing mortality during self-thinning, other than direct competition for ligh
Maintenance of memory CD8 T cells: Divided over division
Once generated during an infection, memory CD8(+) T cells can provide long-lasting protection against reinfection with an intracellular pathogen, but the longevity of this defense depends on the ability of these pathogen-specific memory cells to be maintained. It is generally believed that the bone marrow plays an important role in this respect, where memory CD8 T cells receive reinvigorating signals from cytokines that induce homeostatic proliferation. However, in the current issue of the European Journal of Immunology, Siracusa et al. (Eur. J. Immunol. 2017. 47: 1900-1905) argue against this dogma, as they provide evidence that CD8 memory T cells in murine bone marrow are not proliferating, but largely quiescent, which protects them from elimination by the cytostatic drug Cyclophosphamide. Interestingly, this is in sharp contrast to the proliferating cell counterparts in the spleen, which are eliminated by this treatment. Here, we will discuss the impact of these results, how they relate to opposing findings by others in the field, and what the relevance of these findings is for humans and clinical applications
VLBA imaging of a periodic 12.2 GHz methanol maser flare in G9.62+0.20E
The class II methanol maser source G9.62+0.20E undergoes periodic flares at
both 6.7 and 12.2 GHz. The flare starting in 2001 October was observed at seven
epochs over three months using the VLBA at 12.2 GHz. High angular resolution
images (beam size 1.7 x 0.6 mas) were obtained, enabling us to observe
changes in 16 individual maser components. It was found that while existing
maser spots increased in flux density, no new spots developed and no changes in
morphology were observed. This rules out any mechanism which disturbs the
masing region itself, implying that the flares are caused by a change in either
the seed or pump photon levels. A time delay of 1--2 weeks was observed between
groups of maser features. These delays can be explained by light travel time
between maser groups. The regularity of the flares can possibly be explained by
a binary system.Comment: 11 pages, accepted for publication in MNRA
Growth and Lipid Content of Osmo-Tolerant and Sensitive Algae
Drought tolerance in a number of crop species has been correlated with changes in and composition of cell membrane lipids. Since membrane stability is a function of the physical properties of the lipid constituents, recent studies have focused on lipid changes in response to natural and artifically-induced water stress. Such studies have clearly shown that lipid adjustment and composition differs in drought-resistant and -tolerant species.
A difficulty in working with higher plant systems is that vascular plants make structural and functional level adjustments which aid in drought resistance and which affect the process of adjustment at the cellular level. For this reason, in vitro cell culture techniques have many advantages in studying stress responses. Thus, the objectives of this research were to determine the effects of polyethylene glycol (PEG) -induced osmotic stress on the lipid and sterol composition of osmotic tolerant and susceptible species of unicellular algae belonging to the genus Chlorella
PlotXpress, a webtool for normalization and visualization of reporter expression data [version 1; peer review: 3 approved]
In molecular cell biology, reporter assays are frequently used to investigate gene expression levels. Reporter assays employ a gene that encodes a light-emitting protein, of which the luminescence is quantified as a proxy of gene expression. Commercial parties provide reporter assay kits that include protocols and specialized detection machinery. However, downstream analysis of the output data and their presentation are not standardized. We have developed plotXpress to fill this gap, providing a free, open-source platform for the semi-automated analysis and standardized visualisation of experimental gene reporter data. Users can upload raw luminescence data acquired from a reporter gene assay with an internal control. In plotXpress, the data is corrected for sample variation with the internal control and the average for each condition is calculated. When a reference condition is selected the fold change is calculated for all other conditions, based on the selected reference. The results are shown as dot plots with a statistical summary, which can be adjusted to create publication-grade plots without requiring coding skills. Altogether, plotXpress is an open-source, low-threshold, web-based tool, that promotes a standardized and reproducible analysis while providing an appealing visualization of reporter data. The webtool can be accessed at: https://huygens.science.uva.nl/PlotXpress
Periodic class II methanol masers in G9.62+0.20E
We present the light curves of the 6.7 and 12.2 GHz methanol masers in the
star forming region G9.62+0.20E for a time span of more than 2600 days. The
earlier reported period of 244 days is confirmed. The results of monitoring the
107 GHz methanol maser for two flares are also presented. The results show that
flaring occurs in all three masing transitions. It is shown that the average
flare profiles of the three masing transitions are similar. The 12.2 GHz masers
are the most variable of the three masers with the largest relative amplitude
having a value of 2.4. The flux densities for the different masing transitions
are found to return to the same level during the low phase of the masers,
suggesting that the source of the periodic flaring is situated outside the
masing region, and that the physical conditions in the masing region are
relatively stable. On the basis of the shape of the light curve we excluded
stellar pulsations as the underlying mechanism for the periodicity. It is
argued that a colliding wind binary can account for the observed periodicity
and provide a mechanism to qualitatively explain periodicity in the seed photon
flux and/or the pumping radiation field. It is also argued that the dust
cooling time is too short to explain the decay time of about 100 days of the
maser flare. A further analysis has shown that for the intervals from days 48
to 66 and from days 67 to 135 the decay of the maser light curve can be
interpreted as due to the recombination of a thermal hydrogen plasma with
densities of approximately and respectively.Comment: 11 pages, 9 figuer
Why and how teachers use nature of science in teaching quantum physics:Research on the use of an ecological teaching intervention in upper secondary schools
Students at upper secondary and college level in many countries are introduced to quantum physics (QP) in a mostly mathless course. Research shows that addressing epistemological and philosophical aspects would be beneficial for novice students’ conceptual understanding. However, physics teachers seldom address these nature of science (NOS) aspects in their lessons. We take the view that teachers only implement these aspects if this serves their goals. This study explores whether experienced Dutch high school teachers, who are not trained for NOS teaching, address NOS in their QP lessons when provided with NOS-infused teaching resources. We based our framework on literature about pedagogic content knowledge and on the principles of the practicality of educational innovations. Teacher interviews (N=10) supported by classroom observations provided insights into how and why teachers use specific elements from the resources. Our research reveals teachers’ perspectives on teaching QP in secondary schools and why they think NOS aspects can be helpful to reach their teaching goals. Our findings support the view that conceptual QP is valuable for all students because an informed NOS view is vital for everybody in today’s society. Additionally, we expect that an ecological intervention that supports teachers and at the same time recognizes their professionality and environment has potential implications for other fields of science education and could have a significant positive impact in classrooms
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