277 research outputs found
The Submillimeter Array Polarimeter
We describe the Submillimeter Array (SMA) Polarimeter, a polarization
converter and feed multiplexer installed on the SMA. The polarimeter uses
narrow-band quarter-wave plates to generate circular polarization sensitivity
from the linearly-polarized SMA feeds. The wave plates are mounted in rotation
stages under computer control so that the polarization handedness of each
antenna is rapidly selectable. Positioning of the wave plates is found to be
highly repeatable, better than 0.2 degrees. Although only a single polarization
is detected at any time, all four cross correlations of left- and
right-circular polarization are efficiently sampled on each baseline through
coordinated switching of the antenna polarizations in Walsh function patterns.
The initial set of anti-reflection coated quartz and sapphire wave plates
allows polarimetry near 345 GHz; these plates have been have been used in
observations between 325 and 350 GHz. The frequency-dependent
cross-polarization of each antenna, largely due to the variation with frequency
of the retardation phase of the single-element wave plates, can be measured
precisely through observations of bright point sources. Such measurements
indicate that the cross-polarization of each antenna is a few percent or
smaller and stable, consistent with the expected frequency dependence and very
small alignment errors. The polarimeter is now available for general use as a
facility instrument of the SMA.Comment: To appear in Proc. SPIE 7020, 'Millimeter and Submillimeter Detectors
and Instrumentation'. Uses spie.cl
Shaping a high-mass star-forming cluster through stellar feedback. The case of the NGC 7538 IRS 1-3 complex
Context: NGC 7538 IRS 1-3 is a high-mass star-forming cluster with several
detected dust cores, infrared sources, (ultra)compact H regions,
molecular outflows, and masers. In such a complex environment, important
interactions and feedback among the embedded objects are expected to play a
major role in the evolution of the region. Aims: We study the dust, kinematic,
and polarimetric properties of the NGC 7538 IRS 1-3 region to investigate the
role of the different forces interplaying in the formation and evolution of
high-mass star-forming clusters. Methods: We perform SMA high angular
resolution observations at 880 m with the compact configuration. We
develop the RATPACKS code to generate synthetic velocity cubes from models of
choice to be compared to the observational data. We develop the "mass balance"
analysis to quantify the stability against gravitational collapse accounting
for all the energetics at core scales. Results: We detect 14 dust cores from
3.5 to 37 arranged in two larger scale structures: a
central bar and a filamentary spiral arm. The spiral arm presents large scale
velocity gradients in HCO 4-3 and CO 3-2, and magnetic field
segments well aligned to the dust main axis. The velocity gradient is well
reproduced by a spiral arm expanding at 9 km s with respect to the
central core MM1, which is known to power a large precessing outflow. The
energy of the outflow is comparable with the spiral arm kinetic energy, which
is dominant over gravitational and magnetic energies. In addition, the
dynamical ages of the outflow and spiral arm are comparable. ... (Full abstract
in the pdf version)Comment: 15 pages, 9 figures, 4 tables. Accepted for publication in A&
What is the Hidden Depolarization Mechanism in Low Luminosity AGN?
Millimeter wavelength polarimetry of accreting black hole systems can provide
a tomographic probe of the accretion flow on a wide range of linear scales. We
searched for linear polarization in two low luminosity active galactic nuclei
(LLAGN), M81 and M84, using the Combined Array for Millimeter Astronomy (CARMA)
and the Submillimeter Array (SMA). We find upper limits of
averaging over the full bandwidth and with a rotation measure (RM) synthesis
technique. These low polarization fractions, along with similar low values for
LLAGN M87 and 3C84, suggest that LLAGN have qualitatively different
polarization properties than radio-loud sources and Sgr A*. If the sources are
intrinsically polarized and then depolarized by Faraday rotation then we place
lower limits on the RM of a few times for the full
bandwidth case and for the RM synthesis
analysis. These limits are inconsistent with or marginally consistent with
expected accretion flow properties. Alternatively, the sources may be
depolarized by cold electrons within a few Schwarzschild radii from the black
hole, as suggested by numerical models.Comment: Accepted for publication in ApJ
The magnetic field in the NGC 2024 FIR 5 dense core
We used the Submillimeter Array (SMA) to observe the thermal polarized dust
emission from the protostellar source NGC 2024 FIR 5. The polarized emission
outlines a partial hourglass morphology for the plane-of-sky component of the
core magnetic field. Our data are consistent with previous BIMA maps, and the
overall magnetic field geometries obtained with both instruments are similar.
We resolve the main core into two components, FIR 5A and FIR 5B. A possible
explanation for the asymmetrical field lies in depolarization effects due to
the lack of internal heating from FIR 5B source, which may be in a prestellar
evolutionary state. The field strength was estimated to be 2.2 mG, in agreement
with previous BIMA data. We discuss the influence of a nearby H{\sc ii} region
over the field lines at scales of pc. Although the hot component is
probably compressing the molecular gas where the dust core is embedded, it is
unlikely that the radiation pressure exceeds the magnetic tension. Finally, a
complex outflow morphology is observed in CO (3 2) maps. Unlike
previous maps, several features associated with dust condensations other than
FIR 5 are detected.Comment: 48 pages, 12 figures, accepted for publication in The Astrophysical
Journa
Magnetic Fields in the Formation of Sun-Like Stars
We report high-angular-resolution measurements of polarized dust emission
toward the low-mass protostellar system NGC 1333 IRAS 4A. We show that in this
system the observed magnetic field morphology is in agreement with the standard
theoretical models of the formation of Sun-like stars in magnetized molecular
clouds at scales of a few hundred astronomical units; gravity has overcome
magnetic support, and the magnetic field traces a clear hourglass shape. The
magnetic field is substantially more important than turbulence in the evolution
of the system, and the initial misalignment of the magnetic and spin axes may
have been important in the formation of the binary system.Comment: 8 pages including 2 figures, accepted draft for publication in
Scienc
Detection of 40-48 GHz dust continuum linear polarization towards the Class 0 young stellar object IRAS 16293-2422
We performed the new JVLA full polarization observations at 40-48 GHz
(6.3-7.5 mm) towards the nearby ( 1473.4 pc) Class 0 YSO IRAS
16293-2422, and compare with the previous SMA observations reported by Rao et
al. (2009; 2014). We observed the quasar J1407+2827 which is weakly polarized
and can be used as a leakage term calibrator for 9 GHz observations, to
gauge the potential residual polarization leakage after calibration. We did not
detect Stokes Q, U, and V intensities from the observations of J1407+2827, and
constrain (3-) the residual polarization leakage after calibration to
be 0.3\%. We detect linear polarization from one of the two binary
components of our target source, IRAS\,16293-2422\,B. The derived polarization
position angles from our observations are in excellent agreement with those
detected from the previous observations of the SMA, implying that on the
spatial scale we are probing (50-1000 au), the physical mechanisms for
polarizing the continuum emission do not vary significantly over the wavelength
range of 0.88-7.5 mm. We hypothesize that the observed polarization
position angles trace the magnetic field which converges from large scale to an
approximately face-on rotating accretion flow. In this scenario, magnetic field
is predominantly poloidal on 100 au scales, and becomes toroidal on smaller
scales. However, this interpretation remains uncertain due to the high dust
optical depths at the central region of IRAS\,16293-2422\,B and the uncertain
temperature profile. We suggest that dust polarization at wavelengths
comparable or longer than 7\,mm may still trace interstellar magnetic field.
Future sensitive observations of dust polarization in the fully optically thin
regime will have paramount importance for unambiguously resolving the magnetic
field configuration.Comment: 14 pages, 7 figures, accepted to A&A. Comments are welcom
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