11 research outputs found
A Four-Stokes-Parameter Spectral Line Polarimeter at the Caltech Submillimeter Observatory
We designed and built a new Four-Stokes-Parameter spectral line Polarimeter
(FSPPol) for the Caltech Submillimeter Observatory (CSO). The simple design of
FSPPol does not include any mirrors or optical components to redirect or
re-image the radiation beam and simply transmits the beam to the receiver
through its retarder plates. FSPPol is currently optimized for observation in
the 200-260 GHz range and measures all four Stokes parameters, I, Q, U, and V.
The very low level of instrument polarization makes it possible to obtain
reliable measurements of the Goldreich-Kylafis effect in molecular spectral
lines. Accordingly, we measured a polarization fraction of a few percent in the
spectral line wings of ^{12}\mathrm{CO} (J=2\rightarrow1) in Orion KL/IRc2,
which is consistent with previous observations. We also used FSPPol to study
the Zeeman effect in the N=2\rightarrow1 transition of CN in DR21(OH) for the
first time. At this point we cannot report a Zeeman detection, but more
observations are ongoing
Non-Zeeman Circular Polarization of Molecular Rotational Spectral Lines
We present measurements of circular polarization from rotational spectral
lines of molecular species in Orion KL, most notably 12CO (J=2 - 1), obtained
at the Caltech Submillimeter Observatory with the Four-Stokes-Parameter Spectra
Line Polarimeter. We find levels of polarization of up to 1 to 2% in general,
for 12CO (J=2 - 1) this level is comparable to that of linear polarization also
measured for that line. We present a physical model based on resonant
scattering in an attempt to explain our observations. We discuss how slight
differences in scattering amplitudes for radiation polarized parallel and
perpendicular to the ambient magnetic field, responsible for the alignment of
the scattering molecules, can lead to the observed circular polarization. We
also show that the effect is proportional to the square of the magnitude of the
plane of the sky component of the magnetic field, and therefore opens up the
possibility of measuring this parameter from circular polarization measurements
of Zeeman insensitive molecules.Comment: 30 pages, 7 figures; accepted for publication in the Ap
Submillimeter Polarimetry with PolKa, a reflection-type modulator for the APEX telescope
Imaging polarimetry is an important tool for the study of cosmic magnetic
fields. In our Galaxy, polarization levels of a few up to 10\% are
measured in the submillimeter dust emission from molecular clouds and in the
synchrotron emission from supernova remnants. Only few techniques exist to
image the distribution of polarization angles, as a means of tracing the
plane-of-sky projection of the magnetic field orientation. At submillimeter
wavelengths, polarization is either measured as the differential total power of
polarization-sensitive bolometer elements, or by modulating the polarization of
the signal. Bolometer arrays such as LABOCA at the APEX telescope are used to
observe the continuum emission from fields as large as \sim0\fdg2 in
diameter. %Here we present the results from the commissioning of PolKa, a
polarimeter for Here we present PolKa, a polarimeter for LABOCA with a
reflection-type waveplate of at least 90\% efficiency. The modulation
efficiency depends mainly on the sampling and on the angular velocity of the
waveplate. For the data analysis the concept of generalized synchronous
demodulation is introduced. The instrumental polarization towards a point
source is at the level of \%, increasing to a few percent at the
db contour of the main beam. A method to correct for its effect in
observations of extended sources is presented. Our map of the polarized
synchrotron emission from the Crab nebula is in agreement with structures
observed at radio and optical wavelengths. The linear polarization measured in
OMC1 agrees with results from previous studies, while the high sensitivity of
LABOCA enables us to also map the polarized emission of the Orion Bar, a
prototypical photon-dominated region
Non-Zeeman circular polarization of CO rotational lines in SNR IC 443
Context. We study interstellar magnetic fields by measuring the polarization in molecular spectral lines and thermal emission of dust.
Aims. We report detection of non-Zeeman circular polarization and linear polarization levels of up to 1% in the ^(12)CO spectral line emission in a shocked molecular clump around the supernova remnant (SNR) IC 443. We examine our polarization results to confirm that the circular polarization signal in CO lines is caused by a conversion of linear to circular polarization, consistent with anisotropic resonant scattering. In this process background, linearly polarized CO emission interacts with similar foreground molecules aligned with the ambient magnetic field and scatters at a transition frequency. The difference in phase shift between the orthogonally polarized components of this scattered emission can cause a transformation of linear to circular polarization.
Methods. We compared linear polarization maps from the dust continuum, which were obtained with PolKa at APEX, and ^(12)CO (J = 2 → 1) and (J = 1 → 0) from the IRAM 30-m telescope. We found no consistency between the two sets of polarization maps. We then reinserted the measured circular polarization signal in the CO lines across the source to the corresponding linear polarization signal to test whether the linear polarization vectors of the CO maps were aligned with those of the dust before this linear to circular polarization conversion.
Results. After the flux correction for the two transitions of the CO spectral lines, the new polarization vectors for both CO transitions aligned with the dust polarization vectors, establishing that the non-Zeeman CO circular polarization is due to a linear to circular polarization conversion
Observational Determination of the Turbulent Ambipolar Diffusion Scale and Magnetic Field Strength in Molecular Clouds
We study the correlation of the velocity dispersion of the coexisting
molecules H13CN and H13CO+ and the turbulent energy dissipation scale in the
DR21(OH) star-forming region. The down-shift of the H13CO+ spectrum relative to
H13CN is consistent with the presence of ambipolar diffusion at dissipation
length scales that helps the process of turbulent energy dissipation, but at a
different cut-off for ions compared to the neutrals. We use our observational
data to calculate a turbulent ambipolar diffusion length scale L'\simeq17 mpc
and a strength of B_{pos}\simeq1.7 mG for the plane of the sky component of the
magnetic field in DR21(OH)
Study on the properties of piezoelectric materials and manganese-based oxide perovskites /
Perovskite type piezoelectric and manganese oxide materials have gained a lot of attention
in the field of device engineering. Lead zirconium titananium oxide (PbZri.iTiiOa or PZT)
is a piezoelectric material widely used as sensors and actuators. Miniaturization of PZTbased
devices will not only perfect many existing products, but also opens doors to new
applications. Lanthanum manganese oxides Lai-iAiMnOa (A-divalent alkaline earth such
as Sr, Ca or Ba) have been intensively studied for their colossal magnetoresistance (CMR)
properties that make them applicable in memory cells, magnetic and pressure sensors. In
this study, we fabricate PZT and LSMO(LCMO) heterostructures on SrTiOa substrates
and investigate their temperature dependency of resistivity and magnetization as a function
of the thickness of LSMO(LCMO) layer. The microstructure of the samples is analysed
through TEM. In another set of samples, we study the effect of application of an electric
field across the PZT layer that acts as an external pressure on the manganite layer. This
verifies the correlation of lattice distortion with transport and magnetic properties of the
CMR materials
Ambipolar diffusion and turbulent magnetic fields in molecular clouds
We review the introduction and development of a novel method for the characterization of magnetic fields in star-forming regions. The technique is based on the comparison of spectral line profiles from coexistent neutral and ion molecular species commonly detected in molecular clouds, sites of star formation. Unlike other methods used to study magnetic fields in the cold interstellar medium, this ion/neutral technique is not based on spin interactions with the field. Instead, it relies on and takes advantage of the strong cyclotron coupling between the ions and magnetic fields, thus exposing what is probably the clearest observational manifestation of magnetic fields in the cold, weakly ionized gas that characterizes the interior of molecular clouds. We will show how recent development and modeling of the ensuing ion line narrowing effect leads to a determination of the ambipolar diffusion scale involving the turbulent component of magnetic fields in star-forming regions, as well as the strength of the ordered component of the magnetic field