1,374 research outputs found
The chemical identity of intervessel pit membranes in Acer challenges hydrogel control of xylem hydraulic conductivity.
Ion-mediated enhancement of the hydraulic conductivity of xylem tissue (i.e., the ionic effect) has been reported for various angiosperm species. One explanation of the ionic effect is that it is caused by the swelling and shrinking of intervessel pit membranes due to the presence of pectins and/or other cell wall matrix polymers such as heteroxylans or arabinogalactan-proteins (AGPs) that may contain acidic sugars. Here, we examined the ionic effect for six Acer species and their pit membrane chemistry using immunocytochemistry, including antibodies against glycoproteins. Moreover, anatomical features related to the bordered pit morphology and vessel dimensions were investigated using light and electron microscopy. The ionic effect varied from 18% (± 9) to 32% (± 13). Epitopes of homogalacturonan (LM18) and xylan (LM11) were not detected in intervessel pit membranes. Negative results were also obtained for glycoproteins (extensin: LM1, JIM20; AGP glycan: LM2), although AGP (JIM13) related epitopes were detected in parenchyma cells. The mean vessel length was significantly correlated with the magnitude of the ionic effect, unlike other pit or vessel related characteristics.Our results suggest that intervessel pit membranes of Acer are unlikely to contain pectic or other acidic polysaccharides. Therefore, alternative explanations should be tested to clarify the ionic effect
Simultaneous X-ray spectroscopy of YY Gem with Chandra and XMM-Newton
We report on a detailed study of the X-ray spectrum of the nearby eclipsing
spectroscopic binary YY Gem. Observations were obtained simultaneously with
both large X-ray observatories, XMM-Newton and Chandra. We compare the
high-resolution spectra acquired with the Reflection Grating Spectrometer
onboard XMM-Newton and with the Low Energy Transmission Grating Spectrometer
onboard Chandra, and evidence in direct comparison the good performance of both
instruments in terms of wavelength and flux calibration. The strongest lines in
the X-ray spectrum of YY Gem are from oxygen. Oxygen line ratios indicate the
presence of a low-temperature component (1-4 MK) with density n_e < 2 10^{10}
cm^-3. The X-ray lightcurve reveals two flares and a dip corresponding to the
secondary eclipse. An increase of the density during phases of high activity is
suggested from time-resolved spectroscopy. Time-resolved global fitting of the
European Photon Imaging Camera CCD spectrum traces the evolution of temperature
and emission measure during the flares. These medium-resolution spectra show
that temperatures > 10^7 K are relevant in the corona of YY Gem although not as
dominant as the lower temperatures represented by the strongest lines in the
high-resolution spectrum. Magnetic loops with length on the order of 10^9 cm,
i.e., about 5 % of the radius of each star, are inferred from a comparison with
a one-dimensional hydrodynamic model. This suggests that the flares did not
erupt in the (presumably more extended) inter-binary magnetosphere but are
related to one of the components of the binary.Comment: 15 pages, accepted for publication in A&
Observation of enhanced X-ray emission from the CTTS AA Tau during a transit of an accretion funnel
AA Tau was observed for about 5h per XMM orbit (2 days) over 8 successive
orbits, which covers two optical eclipse periods (8.2 days). The XMM optical/UV
monitor simultaneously provided UV photometry with a ~15 min sampling rate.
Some V-band photometry was also obtained from the ground during this period in
order to determine the dates of the eclipses. Two X-ray and UV measurements
were secured close to the center of the eclipse. The UV flux is the highest
just before the eclipse starts and the lowest towards the end of it. We model
the UV flux variations with a weekly modulation (inner disk eclipse), plus a
daily modulation, which suggests a non-steady accretion. No eclipses are
detected in X-rays. For one measurement, the X-ray count rate was nearly 50
times stronger than the minimum observed level, and the plasma temperature
reached 60 MK, i.e., a factor of 2-3 higher than in the other observations.
This X-ray event, observed close to the center of the optical eclipse, is
interpreted as an X-ray flare. We identify the variable column density with the
low-density accretion funnel flows blanketing the magnetosphere. The lack of
X-ray eclipses indicates that X-ray emitting regions are located at high
latitudes. Furthermore, the occurrence of a strong X-ray flare near the center
of the optical eclipse suggests that the magnetically active areas are closely
associated with the base of the high-density accretion funnel flow. We
speculate that the impact of this free falling accretion flow onto the strong
magnetic field of the stellar corona may boost the X-ray emission (abridged).Comment: 17 pages and 9 Figures. Accepted by A&
Exposure pathways matter: Aquatic phototrophic communities respond differently to agricultural run-off exposed via sediment or water
1. Small shallow ponds are widespread but understudied water bodies in agricultural landscapes. Agricultural run-off (ARO) transports pesticides and nutrients into adjacent aquatic ecosystems where they occur dissolved in the water column or are bound to sediments. Consequently, aquatic communities are affected by ARO via different exposure pathways. We hypothesize that sediment-bound ARO mainly affects submerged rooted macrophytes, while phytoplankton and periphyton are more prone to ARO in water. These primary producers compete for resources resulting in a regime shift between alternative stable states of macrophyte or phytoplankton dominance. We hypothesize that warming increases nutrient release from sediments and thereby facilitates the occurrence of phytoplankton dominance.
2. Using a full-factorial microcosm design, we exposed aquatic primary producers to either sediment or water application of a mixture of common pesticides (terbuthylazine, pirimicarb, tebuconazole and copper) and nitrate at two concentrations and two temperatures (22°C and 26°C) for 4 weeks. Initial and final concentrations of pesticides and nitrate, final biomass of macrophytes, periphyton and phytoplankton, pesticide accumulation in macrophytes and changes in carbon, nitrogen and phosphorus content and selected exoenzyme activities in the sediment were measured.
3. We found lower final macrophyte biomass for both ARO treatments compared to controls, indicating a prevalence of negative effects by herbicides and competition for light with other phototrophs. In contrast, phytoplankton and periphyton biomass increased, but only when exposed to ARO via the water column, indicating a prevalence of positive effects by nutrient supply. Microbial carbon and nutrient cycling in sediments was not affected by ARO. Higher temperature mitigated ARO-related effects on macrophytes under sediment exposure.
4. Synthesis and application. ARO poses a strong risk of submerged macrophyte loss and establishment of turbid conditions with phytoplankton dominance in aquatic ecosystems. In conclusion, exposure pathways as well as indirect and interacting effects of multiple stressors need to be considered when designing appropriate mitigation measures. Under climate change, we suggest to prioritize local measures as buffer strips a reduced use of pesticides and fertilizers, and sediment removal as appropriate measures to protect these vulnerable but widespread aquatic systems, which are highly relevant for biodiversity in agricultural landscapes
XMM-Newton survey of the low-metallicity open cluster NGC 2516
We present the first results of an XMM-Newton EPIC survey of NGC 2516, a
southern low-metallicity open cluster with an age close to the Pleiades. The
attained limiting sensitivity is of ~ 2.4 10^-15 erg sec^-1} cm^-2 in the
0.1--4.0 keV bandpass. This has been achieved by summing the data of the MOS
and PN cameras of two distinct observations for a total exposure time of ~ 33
ks and by analyzing the summed data set with the wavelet detection algorithm
developed at Osservatorio Astronomico di Palermo (OAPA) that has yielded over
200 X-ray detections. Using data just from a single exposure or from a single
camera would have reduced by a factor 2--4 our limiting sensitivity and would
have resulted in 25--40% less X-ray detections. To date, 129 detections have as
counterparts one or more of the 540 photometrically selected cluster members in
the surveyed region, for a total of 147 likely detected members, with unique
identification in 112 cases. We derive the X-ray luminosity functions (XLF) of
NGC 2516 members of different spectral types and compare them with those of the
more metal rich, approximately coeval Pleiades cluster, finding the NGC 2516
photometrically selected dG and dK stars less luminous than the Pleiades. The
XLFs of the NGC 2516 and of the Pleiades dM stars are indistinguishable. We
compare the XMM-Newton results with those recently obtained with Chandra.Comment: 8 pages, 5 figures (in 11 postscript files
The European Photon Imaging Camera on XMM-Newton: The MOS Cameras
The EPIC focal plane imaging spectrometers on XMM-Newton use CCDs to record
the images and spectra of celestial X-ray sources focused by the three X-ray
mirrors. There is one camera at the focus of each mirror; two of the cameras
contain seven MOS CCDs, while the third uses twelve PN CCDs, defining a
circular field of view of 30 arcmin diameter in each case. The CCDs were
specially developed for EPIC, and combine high quality imaging with spectral
resolution close to the Fano limit. A filter wheel carrying three kinds of
X-ray transparent light blocking filter, a fully closed, and a fully open
position, is fitted to each EPIC instrument. The CCDs are cooled passively and
are under full closed loop thermal control. A radio-active source is fitted for
internal calibration. Data are processed on-board to save telemetry by removing
cosmic ray tracks, and generating X-ray event files; a variety of different
instrument modes are available to increase the dynamic range of the instrument
and to enable fast timing. The instruments were calibrated using laboratory
X-ray beams, and synchrotron generated monochromatic X-ray beams before launch;
in-orbit calibration makes use of a variety of celestial X-ray targets. The
current calibration is better than 10% over the entire energy range of 0.2 to
10 keV. All three instruments survived launch and are performing nominally in
orbit. In particular full field-of-view coverage is available, all electronic
modes work, and the energy resolution is close to pre-launch values. Radiation
damage is well within pre-launch predictions and does not yet impact on the
energy resolution. The scientific results from EPIC amply fulfil pre-launch
expectations.Comment: 9 pages, 11 figures, accepted for publication in the A&A Special
Issue on XMM-Newto
X-ray emission from the young brown dwarfs of the Taurus Molecular Cloud
The XMM-Newton Extended Survey of the TMC (XEST) is a large program designed
to systematically investigate the X-ray properties of young stellar/substellar
objects in the TMC. In particular, the area surveyed by 15 XMM-Newton pointings
(of which three are archival observations), supplemented with one archival
Chandra observation, allows us to study 17 BDs with M spectral types. Half of
this sample (9 out of 17 BDs) is detected; 7 BDs are detected here for the
first time in X-rays. We observed a flare from one BD. We confirm several
previous findings on BD X-ray activity: a log-log relation between X-ray and
bolometric luminosity for stars (with L*<10 Lsun) and BDs detected in X-rays; a
shallow log-log relation between X-ray fractional luminosity and mass; a
log-log relation between X-ray fractional luminosity and effective temperature;
a log-log relation between X-ray surface flux and effective temperature. We
find no significant log-log correlation between the X-ray fractional luminosity
and EW(Halpha). Accreting and nonaccreting BDs have a similar X-ray fractional
luminosity. The median X-ray fractional luminosity of nonaccreting BDs is about
4 times lower than the mean saturation value for rapidly rotating low-mass
field stars. Our TMC BDs have higher X-ray fractional luminosity than BDs in
the Chandra Orion Ultradeep Project. The X-ray fractional luminosity declines
from low-mass stars to M-type BDs, and as a sample, the BDs are less efficient
X-ray emitters than low-mass stars. We thus conclude that while the BD
atmospheres observed here are mostly warm enough to sustain coronal activity, a
trend is seen that may indicate its gradual decline due to the drop in
photospheric ionization degree (abridged).Comment: 20 pages and 19 Figures. Accepted by A&A, to appear in a special
section/issue dedicated to the XMM-Newton Extended Survey of the Taurus
Molecular Cloud (XEST). Preprint with higher resolution figures is available
at http://hal.ccsd.cnrs.fr/ccsd-0009049
A multi-wavelength study of the young star V1118 Orionis in outburst
Abriged version for astroph: The young late-type star V1118 Orionis was in
outburst from 2005 to 2006. We followed the outburst with optical and
near-infrared photometry; the X-ray emission was further probed with
observations taken with XMM-Newton and Chandra during and after the outburst.
In addition, we obtained mid-infrared photometry and spectroscopy with Spitzer
at the peak of the outburst and in the post-outburst phase. The spectral energy
distribution of V1118 Ori varied significantly over the course of the outburst.
The optical flux showed the largest variations, most likely due to enhanced
emission by a hot spot. The latter dominated the optical and near-infrared
emission at the peak of the outburst, while the disk emission dominated in the
mid-infrared. The X-ray flux correlated with the optical and infrared fluxes,
indicating that accretion affected the magnetically active corona and the
stellar magnetosphere. The thermal structure of the corona was variable with
some indication of a cooling of the coronal temperature in the early phase of
the outburst with a gradual return to normal values. Color-color diagrams in
the optical and infrared showed variations during the outburst, with no obvious
signature of reddening due to circumstellar matter. Using MC realizations of
star+disk+hotspot models to fit the SED in ``quiescence'' and at the peak of
the outburst, we determined that the mass accretion rate varied from about
2.5E-7 Msun/yr to 1E-6 Msun/yr; in addition the fractional area of the hotspot
increased significantly as well. The multi-wavelength study of the V1118 Ori
outburst helped us to understand the variations in spectral energy
distributions and demonstrated the interplay between the disk and the stellar
magnetosphere in a young, strongly accreting star.Comment: Accepted in A&A, Tables will be published onlin
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