1,272 research outputs found
On the polarization mechanism in the R Mon/NGC 2261 complex
The detection of circular polarization in R Mon and NGC 2261 is reported. This detection implies that the mechanism responsible for the linear and circular polarization is most likely multiple scattering in a flattened distribution. It replaces the previously suggested scenario where dichroic extinction by elongated dust grains aligned by a toroidal magnetic field was producing the polarization. The multiple scattering interpretation of linear polarization maps also means that these maps now provide direct evidence for a circumstellar disk around R Mon and possibly around many other young stellar objects
Complex resonance frequencies of a finite, circular radiating duct with an infinite flange
Radiation by solid or fluid bodies can be characterized by resonance modes.
They are complex, as well as resonance frequencies, because of the energy loss
due to radiation. For ducts, they can be computed from the knowledge of the
radiation impedance matrix. For the case of a flanged duct of finite length
radiating on one side in an infinite medium, the expression of this matrix was
given by Zorumski, using a decomposition in duct modes. In order to calculate
the resonance frequencies, the formulation used in Zorumski's theory must be
modified as it is not valid for complex frequencies. The analytical development
of the Green's function in free space used by Zorumski depends on the integrals
of Bessel functions which become divergent for complex frequencies. This paper
proposes first a development of the Green's function which is valid for all
frequencies. Results are applied to the calculation of the complex resonance
frequencies of a flanged duct, by using a formulation of the internal pressure
based upon cascade impedance matrices. Several series of resonance modes are
found, each series being shown to be related to a dominant duct mode. Influence
of higher order duct modes and the results for several fluid densities is
presented and discussed
Polarization of stars with debris disks: comparing observations with models
The Space telescope carried out an unprecedented survey of nearby
stars for debris disks. The dust present in these debris disks scatters and
polarizes stellar light in the visible part of the spectrum. We explore what
can be learned with aperture polarimetry and detailed radiative transfer
modelling about stellar systems with debris disks. We present a polarimetric
survey, with measurements from the literature, of candidate stars observed by
DEBRIS and DUNES surveys. We perform a statistical analysis of the
polarimetric data with the detection of far-infrared excess by and
with a sample of 223 stars. Monte Carlo simulations were performed to
determine the effects of various model parameters on the polarization level and
find the mass required for detection with current instruments. Eighteen stars
were detected with a polarization per cent and
, but only two of them have a debris disk. No statistically
significant difference is found between the different groups of stars, with,
without, and unknown status for far-infrared excess, and presence of
polarization. The simulations show that the integrated polarization is rather
small, usually per cent for typical masses detected by their
far-infrared excess for hot and most warm disks. Masses observed in cold disks
can produce polarization levels above per cent since there is usually
more dust in them than in closer disks. We list five factors which can explain
the observed low-polarization detection rate. Observations with high-precision
polarimeters should lead to additional constraints on models of unresolved
debris disks.Comment: Corrected some quotations and typos and deleted superfluous
references. 20 pages, 5 figure
The Alignment of the Magnetic Field and Collimated Outflows in Star-forming Regions: the Case of NGC 2071
The magnetic field is believed to play a crucial role in the process of star
formation. From the support it provides during the initial collapse of
molecular clouds to the creation of strong collimated jets responsible for
large mass losses, current theories predict its importance in many different
stages during the formation of stars. Here we report on observational evidence
which tests one aspect that can be inferred from these theories: the alignment
between the local magnetic field and collimated bipolar outflows in such
environments. There is good evidence of an alignment in the case of NGC 2071.Comment: 11 pages, 4 figure
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