2,495 research outputs found
Thermal instability of an expanding dusty plasma with equilibrium cooling
We present an analysis of radiation induced instabilities in an expanding
plasma with considerable presence of dust particles and equilibrium cooling. We
have shown that the equilibrium expansion and cooling destabilize the radiation
condensation modes and the presence of dust particles enhances this effect. We
have examined our results in the context of ionized, dusty-plasma environments
such as those found in planetary nebulae (PNe). We show that due to the
non-static equilibrium and finite equilibrium cooling, small-scale localized
structures formed out of thermal instability, become transient, which agrees
with the observational results. The dust-charge fluctuation is found to heavily
suppress these instabilities, though in view of non-availability of convincing
experimental data, a definitive conclusion could not be made.Comment: 23 pages, 14 figure
Stellar parameters for stars of the CoRoT exoplanet field
Aims:To support the computation and evolutionary interpretation of periods
associated with the rotational modulation, oscillations, and variability of
stars located in the CoRoT fields, we are conducting a spectroscopic survey for
stars located in the fields already observed by the satellite. These
observations allow us to compute physical and chemical parameters for our
stellar sample. Method: Using spectroscopic observations obtained with UVES/VLT
and Hydra/Blanco, and based on standard analysis techniques, we computed
physical and chemical parameters (, , ,
, , , and ) for a large
sample of CoRoT targets. Results: We provide physical and chemical parameters
for a sample comprised of 138 CoRoT targets. Our analysis shows the stars in
our sample are located in different evolutionary stages, ranging from the main
sequence to the red giant branch, and range in spectral type from F to K. The
physical and chemical properties for the stellar sample are in agreement with
typical values reported for FGK stars. However, we report three stars
presenting abnormal lithium behavior in the CoRoT fields. These parameters
allow us to properly characterize the intrinsic properties of the stars in
these fields. Our results reveal important differences in the distributions of
metallicity, , and evolutionary status for stars belonging to
different CoRoT fields, in agreement with results obtained independently from
ground-based photometric surveys. Conclusions: Our spectroscopic catalog, by
providing much-needed spectroscopic information for a large sample of CoRoT
targets, will be of key importance for the successful accomplishment of several
different programs related to the CoRoT mission, thus it will help further
boost the scientific return associated with this space mission.Comment: 43 pages, 17 figures, accepted for publication in A&
Nature of the constant factor in the relation between radial breathing mode frequency and tube diameter for single-wall carbon nanotubes
Resonance Raman scattering is used to determine the radial breathing mode (RBM) frequency (ωRBM) dependence on tube diameter (dt) for single-wall carbon nanotubes (SWNTs). We establish experimentally the ωRBM=227.0/dt as the fundamental relation for pristine SWNTs. All the other RBM values found in the literature can be explained by an upshift in frequency due mostly to van der Waals interaction between SWNTs and environment
Analysis of flight parameters and georeferencing of images with different control points obtained by RPA
ArticleNew techniques for analysing the earth's surface have been explored, such as the use
of remotely piloted aircraft (RPA) to obtain aerial images. However, one of the obstacles of
photogrammetry is the reliability of the scenes, because in some cases, considerable geometric
errors are generated, thus necessitating adjustments. Some parameters used in these adjustments
are image overlaps and control points, which generate uncertainties about the amount and
arrangement of these points in an area. The aim of this study was to test the potential of a
commercial RPA for monitoring and its applicability in the management of and decision-making
about coffee crops with two different overlaps and to evaluate geometric errors by applying four
grids of georeferenced points. The study area is located in an experimental Arabica coffee
plantation measuring 0.65 ha. To capture the images, the flight altitude was standardized to a
30 m altitude from the ground, and a constant travel speed of 3 m s
-1 was used. The treatments
studied were two combinations of image overlap, namely, 80/80% and 70/60%. Six points were
tracked through Global Navigation Satellite System (GNSS) receivers and identified with signs,
followed by an RPA flight for image collection. The obtained results indicated distinct residual
error rates pointing to larger errors along Cartesian axis Y, demonstrating that the point
distribution directly affects the residual errors. The use of control points is necessary for image
adjustments, but to optimize their application, it is necessary to consider the shape of the area to
be studied and to distribute the points in a non-biased way relative to the coordinate axes. It is
concluded that the lower overlap can be recommended for use in the flight plan due to the high
resolution of the orthomosaic and the shorter processing time
Optically Thick Radio Cores of Narrow-Waist Bipolar Nebulae
We report our search for optically thick radio cores in sixteen narrow-waist
bipolar nebulae. Optically thick cores are a characteristic signature of
collimated ionized winds. Eleven northern nebulae were observed with the Very
Large Array (VLA) at 1.3 cm and 0.7 cm, and five southern nebulae were observed
with the Australia Telescope Compact Array (ATCA) at 6 cm and 3.6 cm. Two
northern objects, 19W32 and M 1-91, and three southern objects, He 2-25, He
2-84 and Mz 3, were found to exhibit a compact radio core with a rising
spectrum consistent with an ionized jet. Such jets have been seen in M 2-9 and
may be responsible for shaping bipolar structure in planetary nebulae.Comment: 29 pages, accepted for publication in Ap
Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating
We model the photoelectric emission from and charging of interstellar dust
and obtain photoelectric gas heating efficiencies as a function of grain size
and the relevant ambient conditions. Using realistic grain size distributions,
we evaluate the net gas heating rate for various interstellar environments, and
find less heating for dense regions characterized by R_V=5.5 than for diffuse
regions with R_V=3.1. We provide fitting functions which reproduce our
numerical results for photoelectric heating and recombination cooling for a
wide range of interstellar conditions. In a separate paper we will examine the
implications of these results for the thermal structure of the interstellar
medium. Finally, we investigate the potential importance of photoelectric
heating in H II regions, including the warm ionized medium. We find that
photoelectric heating could be comparable to or exceed heating due to
photoionization of H for high ratios of the radiation intensity to the gas
density. We also find that photoelectric heating by dust can account for the
observed variation of temperature with distance from the galactic midplane in
the warm ionized medium.Comment: 50 pages, including 18 figures; corrected title and abstract field
Doses de nitrogênio sobre o acúmulo de nitrogênio na parte aérea e raiz de mudas de cafeeiro (Coffea arábica).
bitstream/item/69000/1/047-maciel-doses.pdfPublicado também no Cadernos de Agroecologia, v. 7, n.2, 2012
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