191 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
Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR21(OH)
We present a new method for the simultaneous calculation of the cosmic ray
ionization rate, zeta(H2), and the ionization fraction, chi(e), in dense
molecular clouds. A simple network of chemical reactions dominant in the
creation and destruction of HCNH+ and HCO+ is used in conjunction with observed
pairs of rotational transitions of several molecular species in order to
determine the electron abundance and the H3+ abundance. The cosmic ray
ionization rate is then calculated by taking advantage of the fact that, in
dark clouds, it governs the rate of creation of H3+. We apply this technique to
the case of the star-forming region DR21(OH), where we successfully detected
the (J=3-2) and (J=4-3) rotational transitions of HCNH+. We also determine the
C and O isotopic ratios in this source to be 12C/13C=63+-4 and 16O/18O=318+-64,
which are in good agreement with previous measurements in other clouds. The
significance of our method lies in the ability to determine N(H3+) and chi(e)
directly from observations, and estimate zeta(H2) accordingly. Our results,
zeta(H2)=3.1x10^(-18) 1/s and chi(e)=3.2x10^(-8), are consistent with recent
determinations in other objects.Comment: 22 pages, including 3 figure
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)
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
Dispersion of Magnetic Fields in Molecular Clouds. III
We apply our technique on the dispersion of magnetic fields in molecular
clouds to high spatial resolution Submillimeter Array polarization data
obtained for Orion KL in OMC-1, IRAS 16293, and NGC 1333 IRAS 4A. We show how
one can take advantage of such high resolution data to characterize the
magnetized turbulence power spectrum in the inertial and dissipation ranges.
For Orion KL we determine that in the inertial range the spectrum can be
approximately fitted with a power law k^-(2.9\pm0.9) and we report a value of
9.9 mpc for {\lambda}_AD, the high spatial frequency cutoff presumably due to
turbulent ambipolar diffusion. For the same parameters we have \sim
k^-(1.4\pm0.4) and a tentative value of {\lambda}_AD \simeq 2.2 mpc for NGC
1333 IRAS 4A, and \sim k^-(1.8\pm0.3) with an upper limit of {\lambda}_AD < 1.8
mpc for IRAS 16293. We also discuss the application of the technique to
interferometry measurements and the effects of the inherent spatial filtering
process on the interpretation of the results.Comment: 25 pages, 9 figures; accepted for publication in The Astrophysical
Journa
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
Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
Observations of H3+ in the Galactic diffuse interstellar medium (ISM) have
led to various surprising results, including the conclusion that the cosmic-ray
ionization rate (zeta_2) is about 1 order of magnitude larger than previously
thought. The present survey expands the sample of diffuse cloud sight lines
with H3+ observations to 50, with detections in 21 of those. Ionization rates
inferred from these observations are in the range (1.7+-1.3)x10^-16
s^-1<zeta_2<(10.6+-8.2)x10^-16 s^-1 with a mean value of
zeta_2=(3.5^+5.3_-3.0)x10^-16 s^-1. Upper limits (3 sigma) derived from
non-detections of H3+ are as low as zeta_2<0.4x10^-16 s^-1. These low
upper-limits, in combination with the wide range of inferred cosmic-ray
ionization rates, indicate variations in zeta_2 between different diffuse cloud
sight lines. A study of zeta_2 versus N_H (total hydrogen column density) shows
that the two parameters are not correlated for diffuse molecular cloud sight
lines, but that the ionization rate decreases when N_H increases to values
typical of dense molecular clouds. Both the difference in ionization rates
between diffuse and dense clouds and the variation of zeta_2 among diffuse
cloud sight lines are likely the result of particle propagation effects. The
lower ionization rate in dense clouds is due to the inability of low-energy
(few MeV) protons to penetrate such regions, while the ionization rate in
diffuse clouds is controlled by the proximity of the observed cloud to a site
of particle acceleration.Comment: 48 pages, 19 figures, 4 tables, accepted for publication in Ap
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