721 research outputs found
Enhanced effects of starlight on the interstellar medium
The photodesorption of molecules and atoms from the surfaces of interstellar grains can be an important source of heating for the interstellar medium and the origin of instabilities which may separate grains and gas. For low densities, the force exerted on the grains is proportional to the gas density and independent of the radiation intensity; for high densities, it is proportional to the radiative flux and independent of the gas density. This force may act differently on grains of different sizes. The photoelectric effect may also be an efficient mechanism for the separation of gas and dust in diffuse clouds
Theory of the continuous spectrum of gaseous nebula of pure hydrogen
It has been studied the formation of the continuum of nebular hydrogen when ionization is due to radiation and to collisions. Two photon emission and continuous emissivity in the reaction of formation of H- have been included. The continuum results extremely sensitive to the relative amount of the two types of thermal excitation.Asociación Argentina de Astronomí
Theory of the continuous spectrum of gaseous nebula of pure hydrogen
It has been studied the formation of the continuum of nebular hydrogen when ionization is due to radiation and to collisions. Two photon emission and continuous emissivity in the reaction of formation of H- have been included. The continuum results extremely sensitive to the relative amount of the two types of thermal excitation.Asociación Argentina de Astronomí
Comment Regarding the Functional Form of the Schmidt Law
Star formation rates on the galactic scale are described phenomenologically
by two distinct relationships, as emphasized recently by Elmegreen (2002). The
first of these is the Schmidt law, which is a power-law relation between the
star formation rate and the column density. The other relationship is that
there is a cutoff in the gas density below which star formation shuts off.
The purpose of this paper is to argue that 1) these two relationships can be
accommodated by a single functional form of the Schmidt law, and 2) this
functional form is motivated by the hypothesis that star formation is a
critical phenomenon, and that as a corollary, 3) the existence of a sharp
cutoff may thus be an emergent property of galaxies, as was argued by Seiden
(1983), as opposed to the classical view that this cutoff is due to an
instability criterion.Comment: 14 pages, 3 figures, in press, New Astronomy. Figs provided in
original (png) format as well as ps format for ps/pdf generatio
OSHI - Open Source Hybrid IP/SDN networking (and its emulation on Mininet and on distributed SDN testbeds)
The introduction of SDN in IP backbones requires the coexistence of regular
IP forwarding and SDN based forwarding. The former is typically applied to best
effort Internet traffic, the latter can be used for different types of advanced
services (VPNs, Virtual Leased Lines, Traffic Engineering...). In this paper we
first introduce the architecture and the services of an "hybrid" IP/SDN
networking scenario. Then we describe the design and implementation of an Open
Source Hybrid IP/SDN (OSHI) node. It combines Quagga for OSPF routing and Open
vSwitch for OpenFlow based switching on Linux. The availability of tools for
experimental validation and performance evaluation of SDN solutions is
fundamental for the evolution of SDN. We provide a set of open source tools
that allow to facilitate the design of hybrid IP/SDN experimental networks,
their deployment on Mininet or on distributed SDN research testbeds and their
test. Finally, using the provided tools, we evaluate key performance aspects of
the proposed solutions. The OSHI development and test environment is available
in a VirtualBox VM image that can be downloaded.Comment: Final version (Last updated August, 2014
Impact of ubiquitous inhibitors on the GUS gene reporter system: evidence from the model plants Arabidopsis, tobacco and rice and correction methods for quantitative assays of transgenic and endogenous GUS
<p>Abstract</p> <p>Background</p> <p>The β-glucuronidase (GUS) gene reporter system is one of the most effective and employed techniques in the study of gene regulation in plant molecular biology. Improving protocols for GUS assays have rendered the original method described by Jefferson amenable to various requirements and conditions, but the serious limitation caused by inhibitors of the enzyme activity in plant tissues has thus far been underestimated.</p> <p>Results</p> <p>We report that inhibitors of GUS activity are ubiquitous in organ tissues of Arabidopsis, tobacco and rice, and significantly bias quantitative assessment of GUS activity in plant transformation experiments. Combined with previous literature reports on non-model species, our findings suggest that inhibitors may be common components of plant cells, with variable affinity towards the <it>E. coli </it>enzyme. The reduced inhibitory capacity towards the plant endogenous GUS discredits the hypothesis of a regulatory role of these compounds in plant cells, and their effect on the bacterial enzyme is better interpreted as a side effect due to their interaction with GUS during the assay. This is likely to have a bearing also on histochemical analyses, leading to inaccurate evaluations of GUS expression.</p> <p>Conclusions</p> <p>In order to achieve reliable results, inhibitor activity should be routinely tested during quantitative GUS assays. Two separate methods to correct the measured activity of the transgenic and endogenous GUS are presented.</p
Pellicle Ultrastructure of Some Euglena Species
SUMMARYThe pellicle of five species of Euglena, belonging to three different groups (Catilliferae, Radiatae and Serpentes), was studied by transmission electron microscopy, in order to see if pellicular morphology could be taken as a valuable character to differentiate the groups. The data demonstrate that pellicular ultrastructure is not constant within the groups, and can therefore be considered a differential character only at the level of species
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