1,142 research outputs found
Far-infrared polarimetry from the Stratospheric Observatory for Infrared Astronomy
Multi-wavelength imaging polarimetry at far-infrared wavelengths has proven
to be an excellent tool for studying the physical properties of dust, molecular
clouds, and magnetic fields in the interstellar medium. Although these
wavelengths are only observable from airborne or space-based platforms, no
first-generation instrument for the Stratospheric Observatory for Infrared
Astronomy (SOFIA) is presently designed with polarimetric capabilities. We
study several options for upgrading the High-resolution Airborne Wideband
Camera (HAWC) to a sensitive FIR polarimeter. HAWC is a 12 x 32 pixel bolometer
camera designed to cover the 53 - 215 micron spectral range in 4 colors, all at
diffraction-limited resolution (5 - 21 arcsec). Upgrade options include: (1) an
external set of optics which modulates the polarization state of the incoming
radiation before entering the cryostat window; (2) internal polarizing optics;
and (3) a replacement of the current detector array with two state-of-the-art
superconducting bolometer arrays, an upgrade of the HAWC camera as well as
polarimeter. We discuss a range of science studies which will be possible with
these upgrades including magnetic fields in star-forming regions and galaxies
and the wavelength-dependence of polarization.Comment: 12 pages, 5 figure
Statistical Assessment of Shapes and Magnetic Field Orientations in Molecular Clouds through Polarization Observations
We present a novel statistical analysis aimed at deriving the intrinsic
shapes and magnetic field orientations of molecular clouds using dust emission
and polarization observations by the Hertz polarimeter. Our observables are the
aspect ratio of the projected plane-of-the-sky cloud image, and the angle
between the mean direction of the plane-of-the-sky component of the magnetic
field and the short axis of the cloud image. To overcome projection effects due
to the unknown orientation of the line-of-sight, we combine observations from
24 clouds, assuming that line-of-sight orientations are random and all are
equally probable. Through a weighted least-squares analysis, we find that the
best-fit intrinsic cloud shape describing our sample is an oblate disk with
only small degrees of triaxiality. The best-fit intrinsic magnetic field
orientation is close to the direction of the shortest cloud axis, with small
(~24 deg) deviations toward the long/middle cloud axes. However, due to the
small number of observed clouds, the power of our analysis to reject
alternative configurations is limited.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Design and Initial Performance of SHARP, a Polarimeter for the SHARC-II Camera at the Caltech Submillimeter Observatory
We have developed a fore-optics module that converts the SHARC-II camera at
the Caltech Submillimeter Observatory into a sensitive imaging polarimeter at
wavelengths of 350 and 450 microns. We refer to this module as "SHARP". SHARP
splits the incident radiation into two orthogonally polarized beams that are
then re-imaged onto opposite ends of the 32 x 12 pixel detector array in
SHARC-II. A rotating half-wave plate is used just upstream from the
polarization-splitting optics. The effect of SHARP is to convert SHARC-II into
a dual-beam 12 x 12 pixel polarimeter. A novel feature of SHARP's design is the
use of a crossed grid in a submillimeter polarimeter. Here we describe the
detailed optical design of SHARP and present results of tests carried out
during our first few observing runs. At 350 microns, the beam size (9
arcseconds), throughput (75%), and instrumental polarization (< 1%) are all
very close to our design goals.Comment: submitted to Applied Optic
Hertz: an imaging polarimeter
The University of Chicago polarimeter, Hertz, is designed for observations at the Caltech Submillimeter Observatory in the 350 ”m atmospheric window. Initial observations with this instrument, the first array polarimeter for submillimeter observations, have produced over 700 measurements at 3Ï or better. This paper summarizes the characteristics of the instrument, presents examples of its performance including polarization maps of molecular clouds and regions near the Galactic center, and outlines the opportunities for improvements with emphasis on requirements for mapping widely extended sources
Anthracnose: The sophisticated rot
The mold fungus Colletotrichum graminicola causes anthracnose, one of the most economically damaging corn diseases worldwide. Anthracnose can occur either as a stalk rot (ASR), or a leaf blight (ALB) (4; 27). The leaf blight phase is generally insignificant in North America as a cause of yield loss, although in the tropics and subtropics it is much more important. Resistance to ASR is usually not correlated with resistance to ALB, complicating efforts to breed resistant corn varieties (2; 4). Resistance to ASR and ALB is mostly quantitative, although sources of major gene resistance have been described (10; 29). Hybrids containing some of these major-gene resistance sources are likely to become available for management of ASR in the near future
A \u3cem\u3eColletotrichum graminicola\u3c/em\u3e Mutant Deficient in the Establishment of Biotrophy Reveals Early Transcriptional Events in the Maize Anthracnose Disease Interaction
Background: Colletotrichum graminicola is a hemibiotrophic fungal pathogen that causes maize anthracnose disease. It progresses through three recognizable phases of pathogenic development in planta: melanized appressoria on the host surface prior to penetration; biotrophy, characterized by intracellular colonization of living host cells; and necrotrophy, characterized by host cell death and symptom development. A âMixed Effectsâ Generalized Linear Model (GLM) was developed and applied to an existing Illumina transcriptome dataset, substantially increasing the statistical power of the analysis of C. graminicola gene expression during infection and colonization. Additionally, the in planta transcriptome of the wild-type was compared with that of a mutant strain impaired in the establishment of biotrophy, allowing detailed dissection of events occurring specifically during penetration, and during early versus late biotrophy. Results: More than 2000 fungal genes were differentially transcribed during appressorial maturation, penetration, and colonization. Secreted proteins, secondary metabolism genes, and membrane receptors were over-represented among the differentially expressed genes, suggesting that the fungus engages in an intimate and dynamic conversation with the host, beginning prior to penetration. This communication process probably involves reception of plant signals triggering subsequent developmental progress in the fungus, as well as production of signals that induce responses in the host. Later phases of biotrophy were more similar to necrotrophy, with increased production of secreted proteases, inducers of plant cell death, hydrolases, and membrane bound transporters for the uptake and egress of potential toxins, signals, and nutrients. Conclusions: This approach revealed, in unprecedented detail, fungal genes specifically expressed during critical phases of host penetration and biotrophic establishment. Many encoded secreted proteins, secondary metabolism enzymes, and receptors that may play roles in host-pathogen communication necessary to promote susceptibility, and thus may provide targets for chemical or biological controls to manage this important disease. The differentially expressed genes could be used as âlandmarksâ to more accurately identify developmental progress in compatible versus incompatible interactions involving genetic variants of both host and pathogen
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