1,424 research outputs found
A photodissociation model for the morphology of the HI near OB associations in M33
We present an approach for analysing the morphology and physical properties
of Hi features near giant OB asso- ciations in M33, in the context of a model
whereby the Hi excess arises from photodissociation of the molecular gas in
remnants of the parent Giant Molecular Clouds (GMCs). Examples are presented
here in the environs of NGC604 and CPSDPZ204, two prominent Hii regions in M33.
These are the first results of a detailed analysis of the environs of a large
number of OB associations in that galaxy. We present evidence for "diffusion"
of the far-UV radiation from the OB association through a clumpy remnant GMC,
and show further that enhanced CO(1-0) emission appears preferentially
associated with GMCs of higher volume density.Comment: Accepted to Ap
A new method for obtaining the star formation law in galaxies
We present a new observational method to evaluate the star formation law as
formulated by Schmidt: the power-law expression assumed to relate the rate of
star formation in a volume of space to the local total gas volume density.
Volume densities in the clouds surrounding an OB association are determined
with a simple model which considers atomic hydrogen as a photodissociation
product on cloud surfaces. The photodissociating flux incident on the cloud is
computed from the far-UV luminosity of the OB association and the geometry. We
have applied this "PDR Method" to a sample of star-forming regions in M33 using
VLA 21-cm data for the HI and GALEX imagery in the far-UV. It provides an
estimate of the total volume density of hydrogen (atomic + molecular) in the
gas clouds surrounding the young star cluster. A logarithmic graph of the
cluster UV luminosity versus the surrounding gas density is a direct measure of
the star formation law. However, this plot is severely affected by
observational selection, rendering large areas of the diagram inaccessible to
the data. An ordinary least-squares regression fit therefore gives a strongly
biased result. Its slope primarily reflects the boundary defined when the 21-cm
line becomes optically thick, no longer reliably measuring the HI column
density. We use a maximum-likelihood statistical approach which can deal with
truncated and skewed data, taking into account the large uncertainties in the
derived total gas densities. The exponent we obtain for the Schmidt law in M33
is 1.4 \pm 0.2.Comment: Accepted for publication in Ap
Ultrafast absorption kinetics of NADH in folded and unfolded conformations
The non-radiative energy transfer is shown to occur on a ~3ps time scale for NADH in the folded form in H2O. Addition of methanol thermodynamically favours the open form, for which energy transfer does not occur
Atomic Hydrogen produced in M33 Photodissociation Regions
We derive total (atomic + molecular) hydrogen densities in giant molecular
clouds (GMCs) in the nearby spiral galaxy M33 using a method that views the
atomic hydrogen near regions of recent star formation as the product of
photodissociation. Far-UV photons emanating from a nearby OB association
produce a layer of atomic hydrogen on the surfaces of nearby GMCs. Our approach
provides an estimate of the total hydrogen density in these GMCs from
observations of the excess far-UV emission that reaches the GMC from the OB
association, and the excess 21-cm radio HI emission produced after these far-UV
photons convert H2 into HI on the GMC surface. The method provides an
alternative approach to the use of CO emission as a tracer of H2 in GMCs, and
is especially sensitive to a range of density well below the critical density
for CO(1-0) emission. We describe our "PDR method" in more detail and apply it
using GALEX far-UV and VLA 21-cm radio data to obtain volume densities in a
selection of GMCs in the nearby spiral galaxy M33. We have also examined the
sensitivity of the method to the linear resolution of the observations used;
the results obtained at 20 pc are similar to those for the larger set of data
at 80 pc resolution. The cloud densities we derive range from 1 to 500 cm-3,
with no clear dependence on galactocentric radius; these results are generally
similar to those obtained earlier in M81, M83, and M101 using the same method.Comment: Accepted for publication in MNRAS. 25 figures, 16 tables, including
online-only material
Quantified H i Morphology VII: The Morphology of Extended Disks in UV and H i
Extended UltraViolet (xuv) disks have been found in a substantial fraction of
late-type --S0, spiral and irregular-- galaxies. Similarly, most late-type
spirals have an extended gas disk, observable in the 21cm radio line (HI). The
morphology of galaxies can be quantified well using a series of scale-invariant
parameters; Concentration- Asymmetry-Smoothness (CAS), Gini, M20, and GM
parameters. In this paper, we compare the quantified morphology and effective
radius (R50) of the Westerbork observations of neutral Hydrogen in Irregular
and Spiral galaxies Project (WHISP) HI maps to those of far-and
near-ultraviolet images obtained with galex, to explore how close the
morphology and scales of HI and UV in these disks correlate. We find that xuv
disks do not stand out by their effective radii in UV or HI. However, the
concentration index in FUV appears to select some xuv disks. And known xuv
disks can be identified via a criterion using Asymmetry and M20; 80% of xuv
disks are included but with 55% contamination. This translates into 61
candidate xuv disk out of our 266 galaxies, (23%) consistent with previous
findings. We consider three scenarios; tidal features from major mergers, the
typical extended Hi disk is a photo- dissociation product of the xuv regions
and both Hi and UV features originate in cold flows fueling the main galaxy. We
define extended HI and UV disks based on their concentration (CHI > 5 and CFUV
> 4 respectively), but note that these two subsamples never overlap in the
WHISP sample. This appears to discount a simple photo-dissociation origin of
the outer HI disk. Previously, we identified the morphology space occupied by
ongoing major mergers. Known xuv disks rarely reside in the merger dominated
part of HI morphology space but those that do are Type 1. This suggests cold
flows as the origin for the xuv complexes and their surrounding HI structures.Comment: 18 pages, 13 figures, 2 table
The volume densities of giant molecular clouds in M83
Using observed GALEX far-ultraviolet (FUV) fluxes and VLA images of the 21-cm
HI column densities, along with estimates of the local dust abundances, we
measure the volume densities of a sample of actively star-forming giant
molecular clouds (GMCs) in the nearby spiral galaxy M83 on a typical resolution
scale of 170 pc. Our approach is based on an equilibrium model for the cycle of
molecular hydrogen formation on dust grains and photodissociation under the
influence of the FUV radiation on the cloud surfaces of GMCs. We find a range
of total volume densities on the surface of GMCs in M83, namely 0.1 - 400 cm-3
inside R25, 0.5 - 50 cm-3 outside R25 . Our data include a number of GMCs in
the HI ring surrounding this galaxy. Finally, we discuss the effects of
observational selection, which may bias our results.Comment: 9 pages, 11 figure
Neue Methoden zur Bekämpfung des Echten Mehltaus im Ökologischen Landbau
Blattoberflächen-modifizierende Salzapplikationen sollten als Bekämpfungsmittel gegen Echten Mehltau an Tomaten und Gurken getestet werden hinsichtlich folgender Eigenschaften:
- Wirkung verschiedener Salze
- Blattverträglichkeit
- Untersuchung der unterschiedlichen Wirkungsmechanismen mittels REM
- Übertragbarkeit in praxisnahe Versuche.
Die verschiedenen Salzlösungen und Kombinationen aus diesen Salzen unterschieden sich deutlich in ihrer Wirkung auf Gurkenmehltau. Die beste Wirkung ergab sich mit allenfalls geringer Schädigung des Blattes mit 0,1% MnCl2, 1 % Patentkali (K2SO4/MgSO4) und 1% Knöterichextrakt. MnCl2 liegt auf Grund seines niedrigen Deliqueszenzpunktes ständig gelöst auf der Blattoberfläche vor, wogegen sich Knöterichextrakt und Patentkali auf der Blattoberfläche in einer Art „Salzkriechen“ verteilen durch häufigen Wechsel zwischen Trocknen und Wiederverflüssigung. Salzschäden ließen sich verringern durch Zusatz von Netzmitteln zur Sprühlösung.
Rasterelektronische Visualisierung zeigte für alle untersuchten Lösungen einen direkt toxischen Effekt auf die Mehltausporen und ihre Hyphen in jeder Entwicklungsphase des Pilzes. Eine Verbesserung des Nährstoffstatus der Pflanzen scheidet als Wirkungsmechanismus aus, da alle Pflanzen optimal ernährt waren und die Behandlungen zu keinen signifikanten Veränderungen der Nährstoffgehalte in den Blättern führten.
Besonders chloridische Salze führten z.T. zu erheblicher Nekrotisierung der Blätter. Da diese Salzlösungen jedoch auch in geringer Konzentration in der Lage waren, die Ausbreitung des Echten Mehltau effektiv zu verhindern, sollten Möglichkeiten zur Mikrodosierung dieser Salze geprüft werden.
Extreme Witterungsbedingungen im Sommer 2003 führten dazu, dass Praxistests nicht ausgeführt oder normal beendet werden konnten. Es gilt jedoch als sicher, dass einige der untersuchten Salzmischungen in geringen Konzentrationen (zw. 0,1 und 1%) den Befall mit Echten Mehltaupilzen effektiv reduzieren können, ohne Blattoberflächen zu schädigen
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