676 research outputs found
Spitzer/MIPS Imaging of NGC 650: Probing the History of Mass Loss on the Asymptotic Giant Branch
We present the far-infrared (IR) maps of a bipolar planetary nebula (PN), NGC
650, at 24, 70, and 160 micron taken with the Multiband Imaging Photometer for
Spitzer (MIPS) on-board the Spitzer Space Telescope. While the two-peak
emission structure seen in all MIPS bands suggests the presence of a near
edge-on dusty torus, the distinct emission structure between the 24 micron map
and the 70/160 micron maps indicates the presence of two distinct emission
components in the central torus. Based on the spatial correlation of these two
far-IR emission components with respect to various optical line emission, we
conclude that the 24 micron emission is largely due to the [O IV] line at 25.9
micron arising from highly ionized regions behind the ionization front, whereas
the 70 and 160 micron emission is due to dust continuum arising from
low-temperature dust in the remnant asymptotic giant branch (AGB) wind shell.
The far-IR nebula structure also suggests that the enhancement of mass loss at
the end of the AGB phase has occurred isotropically, but has ensued only in the
equatorial directions while ceasing in the polar directions. The present data
also show evidence for the prolate spheroidal distribution of matter in this
bipolar PN. The AGB mass loss history reconstructed in this PN is thus
consistent with what has been previously proposed based on the past optical and
mid-IR imaging surveys of the post-AGB shells.Comment: 9 pages in the emulated ApJ format with 6 figures, to appear in Ap
The extinction and dust-to-gas structure of the planetary nebula NGC 7009 observed with MUSE
The large field and wavelength range of MUSE is well suited to mapping
Galactic planetary nebulae (PN). The bright PN NGC 7009 was observed with MUSE
on the VLT during the Science Verification of the instrument in seeing of 0.6".
Emission line maps in hydrogen Balmer and Paschen lines were formed from
analysis of the MUSE cubes. The measured electron temperature and density from
the MUSE cube were employed to predict the theoretical hydrogen line ratios and
map the extinction distribution across the nebula. After correction for the
interstellar extinction to NGC 7009, the internal dust-to-gas ratio (A_V/N_H)
has been mapped for the first time in a PN. The extinction map of NGC 7009 has
considerable structure, broadly corresponding to the morphological features of
the nebula. A large-scale feature in the extinction map, consisting of a crest
and trough, occurs at the rim of the inner shell. The nature of this feature
was investigated and instrumental and physical causes considered; no convincing
mechanisms were identified to produce this feature, other than mass loss
variations in the earlier asymptotic giant branch phase. The dust-to-gas ratio
A_V/N_H increases from 0.7 times the interstellar value to >5 times from the
centre towards the periphery of the ionized nebula. The integrated A_V/N_H is
about 2 times the mean ISM value. It is demonstrated that extinction mapping
with MUSE provides a powerful tool for studying the distribution of PN internal
dust and the dust-to-gas ratio. (Abridged.)Comment: 10 pages, 7 figures. Accepted by A&
Westbrook's Molecular Gun: Discovery of Near-IR Micro-Structures in AFGL 618
We present high-sensitivity near-IR images of a carbon-rich proto-planetary
nebula, AFGL 618, obtained with the Subaru Telescope. These images have
revealed ``bullets'' and ``horns'' extending farther out from the edges of the
previously known bipolar lobes. The spatial coincidence between these near-IR
micro-structures and the optical collimated outflow structure, together with
the detection of shock-excited, forbidden IR lines of atomic species, strongly
suggests that these bullets and horns represent the locations from which
[\ion{Fe}{2}] IR lines arise. We have also discovered CO clumps moving at km s at the positions of the near-IR bullets by re-analyzing the
existing CO interferometry data. These findings indicate that
the near-IR micro-structures represent the positions of shocked surfaces at
which fast-moving molecular clumps interface with the ambient circumstellar
shell.Comment: 2 figures. To appear in the ApJ Letter
Influência de matéria orgânica, pH e umidade sobre a população microbiana de solos incubados com atrazina e 2,4-D.
Foram coletadas mensalmente em 1996 amostras de solo de 0-20 e 80-90 cm de profundidade em area de cana-de-acucar na regiao de Ribeirao Preto para a avaliacao do efeito sazonal do teor de materia organica, umidade, pH e interacoes com microrganismos, 2,4-D e atrazina. Detectou-se tres diferentes tipos de solos na area. Os resultados mostraram teores mais elevados de umidade em solos argilosos que arenosos, tanto em meses de seca ou de chuva. Observou-se um pico de elevacao do teor de materia organica entre maio e agosto, (meses mais secos), com variacoes anuais de 0,8 a 1,5% para solos arenosos e 1,5 a 3,5% para argilosos. Na profundidade de 80-90 cm o teor de materia organica foi menor. Coincidindo com os meses de elevacao do teor de materia organica, maio e agosto, observou-se a acidificacao dos solos. O comportamento dos microrganismos ao tratamento com atrazina e 2,4-D mostrou correlacao com a variacao de pH e materia organica. O crescimento dos microorganismos nos solos coletados de fevereiro a maio, meses com menos materia organica e maior pH, foi inibido por 2,4-D em ate 90% em comparacao com a atrazina. Ao inves, nos meses de maior materia organica e menor pH, junho e julho, o crescimento foi inibido por atrazina. Os resultados sugerem que houve forte influencia do teor de materia organica e pH no estimulo ou inibicao do crescimento dos microorganismos pelos herbicidas. Com o aumento de materia organica e acidificacao do solo, 2,4-D estimulou o crescimento microbiano, enquanto atrazina inibiu. Nos meses que precedeu o aumento de materia organica o efeito foi oposto, com 2,4-D inibindo fortemente o crescimento microbiano
2-Dust : a Dust Radiative Transfer Code for an Axisymmetric System
We have developed a general purpose dust radiative transfer code for an
axisymmetric system, 2-Dust, motivated by the recent increasing availability of
high-resolution images of circumstellar dust shells at various wavelengths.
This code solves the equation of radiative transfer following the principle of
long characteristic in a 2-D polar grid while considering a 3-D radiation field
at each grid point. A solution is sought through an iterative scheme in which
self-consistency of the solution is achieved by requiring a global luminosity
constancy throughout the shell. The dust opacities are calculated through Mie
theory from the given size distribution and optical properties of the dust
grains. The main focus of the code is to obtain insights on (1) the global
energetics of dust grains in the shell (2) the 2-D projected morphologies that
are strongly dependent on the mixed effects of the axisymmetric dust
distribution and inclination angle of the shell. Here, test models are
presented with discussion of the results. The code can be supplied with a
user-defined density distribution function, and thus, is applicable to a
variety of dusty astronomical objects possessing the axisymmetric geometry.Comment: To be published in ApJ, April 2003 issue; 13 pages, 4 tables, 17
figures, 5-page appendix (no figures for the main text included in this
preprint). For the complete preprint and code distribution, contact the
author
Caracterização sazonal do efeito de atrazina e 2,4-D sobre microrganismos de solos da microbacia do córrego do Espraiado, Ribeirão Preto-SP.
Com o objetivo de estudar o efeito sazonal de atrazina e 2,4-D sobre os microrganismos do solo e se isolar aqueles com potencial para degradar os herbicidas, foram coletadas mensalmente, durante o ano de 1996, amostras de solo de 9 pontos a profundidade de 0-20 cm em area de cana-de-acucar da microbacia do Espraiado, regiao de Ribeirao Preto. Os solos variaram de argiloso a arenoso. Suspensoes de solo, (5 g/50 ml), recem coletadas foram tratadas com atrazina (1 mg/ml) e 2,4-D (0,5 mg/ml) e incubadas a 30oC por 21 dias, juntamente com a testemunha. A populacao microbiana foi monitorada semanalmente por plaqueamento, contagem, fotografia e isolamento de colonias. Os resultados mostraram variacao sazonal significativa no comportamento quali e quantitativo da populacao microbiana. O crescimento dos microrganismos nas amostras de fevereiro a maio foi fortemente inibido por 2,4-D. No mes de junho, a atrazina inibiu o crescimento, ao passo que em agosto, 2,4-D passou a exercer forte efeito estimulante, com populacoes de ate 15 vezes maior que a testemunha. Em outubro, ambos herbicidas estimularam de maneira geral o crescimento. O tipo de solo apresentou pequena influencia. Os tratamentos promoveram crescimento diferenciado de populacoes homogeneas tipicas e caracteristicas para cada um dos herbicidas com a predominancia de bacterias para ambos, apesar de tambem serem detectados fungos e leveduras. Atrazina e 2,4-D na maioria das vezes agiram antagonicamente, sendo o 2,4-D inibidor mais potente sobre a populacao de solos arenosos, quando a inibiu. O tratamento com os herbicidas reduziu o grau de biodiversidade sendo comumente encontrado um unico tipo celular nas placas, distinto para atrazina e 2,4-D
Herschel spectral-mapping of the Helix Nebula (NGC 7293): Extended CO photodissociation and OH+ emission
The Helix Nebula (NGC 7293) is the closest planetary nebulae. Therefore, it
is an ideal template for photochemical studies at small spatial scales in
planetary nebulae. We aim to study the spatial distribution of the atomic and
the molecular gas, and the structure of the photodissociation region along the
western rims of the Helix Nebula as seen in the submillimeter range with
Herschel. We use 5 SPIRE FTS pointing observations to make atomic and molecular
spectral maps. We analyze the molecular gas by modeling the CO rotational lines
using a non-local thermodynamic equilibrium (non-LTE) radiative transfer model.
For the first time, we have detected extended OH+ emission in a planetary
nebula. The spectra towards the Helix Nebula also show CO emission lines (from
J= 4 to 8), [NII] at 1461 GHz from ionized gas, and [CI] (2-1), which together
with the OH+ lines, trace extended CO photodissociation regions along the rims.
The estimated OH+ column density is (1-10)x1e12 cm-2. The CH+ (1-0) line was
not detected at the sensitivity of our observations. Non-LTE models of the CO
excitation were used to constrain the average gas density (n(H2)=(1-5)x1e5
cm-3) and the gas temperature (Tk= 20-40 K). The SPIRE spectral-maps suggest
that CO arises from dense and shielded clumps in the western rims of the Helix
Nebula whereas OH+ and [CI] lines trace the diffuse gas and the UV and X-ray
illuminated clumps surface where molecules reform after CO photodissociation.
[NII] traces a more diffuse ionized gas component in the interclump medium.Comment: Accepted for publication in Astronomy and Astrophysic
On the Green's Function of the almost-Mathieu Operator
The square tight-binding model in a magnetic field leads to the
almost-Mathieu operator which, for rational fields, reduces to a
matrix depending on the components , of the wave vector in the
magnetic Brillouinzone. We calculate the corresponding Green's function without
explicit knowledge of eigenvalues and eigenfunctions and obtain analytical
expressions for the diagonal and the first off-diagonal elements; the results
which are consistent with the zero magnetic field case can be used to calculate
several quantities of physical interest (e. g. the density of states over the
entire spectrum, impurity levels in a magnetic field).Comment: 9 pages, 3 figures corrected some minor errors and typo
Sharp lines in the absorption edge of EuTe and PbEuTe in high magnetic fields
The optical absorption spectra in the region of the \fd transition energies
of epitaxial layers of of EuTe and \PbEuTe, grown by molecular beam epitaxy,
were studied using circularly polarized light, in the Faraday configuration.
Under \sigmam polarization a sharp symmetric absorption line (full width at
half-maximum 0.041 eV) emerges at the low energy side of the band-edge
absorption, for magnetic fields intensities greater than 6 T. The absorption
line shows a huge red shift (35 meV/T) with increasing magnetic fields. The
peak position of the absorption line as a function of magnetic field is
dominated by the {\em d-f} exchange interaction of the excited electron and the
\Euion spins in the lattice. The {\em d-f} exchange interaction energy was
estimated to be eV. In \PbEuTe the same absorption line
is detected, but it is broader, due to alloy disorder, indicating that the
excitation is localized within a finite radius. From a comparison of the
absorption spectra in EuTe and \PbEuTe the characteristic radius of the
excitation is estimated to be \AA.Comment: Journal of Physics: Condensed Matter (2004, at press
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