282 research outputs found
VLA OH and H I Zeeman Observations of the NGC 6334 Complex
We present OH and H I Zeeman observations of the NGC 6334 complex taken with
the Very Large Array. The OH absorption profiles associated with the complex
are relatively narrow (del-v_FWHM ~ 3 km s^1) and single-peaked over most of
the sources. The H I absorption profiles contain several blended velocity
components. One of the compact continuum sources in the complex (source A) has
a bipolar morphology. The OH absorption profiles toward this source display a
gradient in velocity from the northern continuum lobe to the southern continuum
lobe; this velocity gradient likely indicates a bipolar outflow of molecular
gas from the central regions to the northern and southern lobes. Magnetic
fields of the order of 200 microG have been detected toward three discrete
continuum sources in the complex. Virial estimates suggest that the detected
magnetic fields in these sources are of the same order as the critical magnetic
fields required to support the molecular clouds associated with the sources
against gravitational collapse.Comment: 14 pages, 9 postscript figures, accepted for publication in the
Astrophysical Journal (ApJ), tentatively scheduled for vol. 533, Apr. 20,
2000; also available at
http://www.pa.uky.edu/~sarma/RESEARCH/aps_research.htm
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
SCUBA Polarization Measurements of the Magnetic Field Strengths in the L183, L1544, and L43 Prestellar Cores
We have mapped linearly polarized dust emission from L183 with the JCMT SCUBA
polarimeter and have analyzed these and our previously published data for the
prestellar cores L183, L1544, and L43 in order to estimate magnetic field
strengths in the plane of the sky, . The analysis used the
Chandrasekhar-Fermi technique, which relates the dispersion in polarization
position angles to . We have used these estimates of the field
strengths (neglecting the unmeasured line-of-sight component) to find the
mass-to-magnetic flux ratios (in units of the critical ratio for
magnetic support). Results are G and for L183, G and for
L1544, and G and for L43.
Hence, without correction for geometrical biases, for all three cores the
mass-to-flux ratios are supercritical by a factor of , and magnetic
support cannot prevent collapse. However, a statistical mean correction for
geometrical bias may be up to a factor of three; this correction would reduce
the individual 's to , 0.8, and 0.6,
respectively; these values are approximately critical or slightly subcritical.
These data are consistent with models of star formation driven by ambipolar
diffusion in a weakly turbulent medium, but cannot rule out models of star
formation driven by turbulence.Comment: Version 2 has minor revisions to reflect referee comments. Paper
accepted for ApJ publicatio
High Resolution Millimeter-Wave Mapping of Linearly Polarized Dust Emission: Magnetic Field Structure in Orion
We present 1.3 and 3.3 mm polarization maps of Orion-KL obtained with the
BIMA array at approximately 4 arcsec resolution. Thermal emission from
magnetically aligned dust grains produces the polarization. Along the Orion
``ridge'' the polarization position angle varies smoothly from about 10 degrees
to 40 degrees, in agreement with previous lower resolution maps. In a small
region south of the Orion ``hot core,'' however, the position angle changes by
90 degrees. This abrupt change in polarization direction is not necessarily the
signpost of a twisted magnetic field. Rather, in this localized region
processes other than the usual Davis-Greenstein mechanism might align the dust
grains with their long axes parallel with the field, orthogonal to their normal
orientation.Comment: AAS preprint:14 pages, 2 figures (3mm.eps and 1mm.eps); requires
aaspp4.sty To be published in Astrophysical Journal Letter
Orion's Veil: Magnetic field strengths and other properties of a PDR in front of the Trapezium Cluster
We present an analysis of physical conditions in the Orion Veil, a largely
atomic PDR that lies just in front (about 2 pc) of the Trapezium stars. We have
obtained 21 cm HI and 18 cm OH VLA Zeeman effect data. These data yield images
of the line-of-sight magnetic field strength Blos in atomic and molecular
regions of the Veil. We find Blos is typically -50 to -75 microgauss in the
atomic gas across much of the Veil (25" resolution); Blos is -350 microgauss at
one position in the molecular gas (40" resolution). The Veil has two principal
HI velocity components. Magnetic and kinematical data suggest a close
connection between these components. They may represent gas on either side of a
shock wave preceding a weak-D ionization front. Magnetic fields in the Veil HI
components are 3-5 times stronger than they are elsewhere in the ISM where N(H)
and n(H) are comparable. The HI components are magnetically subcritical
(magnetically dominated), like the CNM, although they are about 1 dex denser.
Strong fields in the Veil HI components may have resulted from low turbulence
conditions in the diffuse gas that gave rise to OMC-1. Strong fields may also
be related to magnetostatic equilibrium that has developed in the Veil since
star formation. We consider the location of the Orion-S molecular core,
proposing a location behind the main Orion H+ region.Comment: 44 pages, 11 figures, 4 tables, accepted by Ap
Far infrared observations of pre-protostellar sources in Lynds 183
Using ISOPHOT maps at 100 and 200um and raster scans at 100, 120, 150 and
200um we have detected four unresolved far-infrared sources in the high
latitude molecular cloud L183. Two of the sources are identified with 1.3mm
continuum sources found by Ward-Thompson et al. and are located near the
temperature minimum and the coincident column density maximum of dust
distribution. For these two sources, the ISO observations have enabled us to
derive temperatures (about 8.3 K) and masses (about 1.4 and 2.4 solar masses).
They are found to have masses greater than or comparable to their virial masses
and are thus expected to undergo gravitational collapse. We classify them as
pre-protostellar sources. The two new sources are good candidates for
pre-protostellar sources or protostars within L183.Comment: 12 pages, 7 Postscript figures, 1 JPEG figure. Accepted for
publication in Astronomy & Astrophysic
Spatially-resolved Thermal Continuum Absorption against the Supernova Remnant W49B
We present sub-arcminute resolution imaging of the Galactic supernova remnant
W49B at 74 MHz (25") and 327 MHz (6"), the former being the lowest frequency at
which the source has been resolved. While the 327 MHz image shows a shell-like
morphology similar to that seen at higher frequencies, the 74 MHz image is
considerably different, with the southwest region of the remnant almost
completely attenuated. The implied 74 MHz optical depth (~ 1.6) is much higher
than the intrinsic absorption levels seen inside two other relatively young
remnants, Cas A and the Crab Nebula, nor are natural variations in the
relativistic electron energy spectra expected at such levels. The geometry of
the absorption is also inconsistent with intrinsic absorption. We attribute the
absorption to extrinsic free-free absorption by a intervening cloud of thermal
electrons. Its presence has already been inferred from the low-frequency
turnover in the integrated continuum spectrum and from the detection of radio
recombination lines toward the remnant. Our observations confirm the basic
conclusions of those measurements, and our observations have resolved the
absorber into a complex of classical HII regions surrounded either partially or
fully by low-density HII gas. We identify this low-density gas as an extended
HII region envelope (EHE), whose statistical properties were inferred from low
resolution meter- and centimeter-wavelength recombination line observations.
Comparison of our radio images with HI and H_2CO observations show that the
intervening thermal gas is likely associated with neutral and molecular
material as well.Comment: 18 pages, LaTeX with AASTeX-5, 5 figures in 7 PostScript files;
accepted for publication in the Ap
Submillimeter Studies of Prestellar Cores and Protostars: Probing the Initial Conditions for Protostellar Collapse
Improving our understanding of the initial conditions and earliest stages of
protostellar collapse is crucial to gain insight into the origin of stellar
masses, multiple systems, and protoplanetary disks. Observationally, there are
two complementary approaches to this problem: (1) studying the structure and
kinematics of prestellar cores observed prior to protostar formation, and (2)
studying the structure of young (e.g. Class 0) accreting protostars observed
soon after point mass formation. We discuss recent advances made in this area
thanks to (sub)millimeter mapping observations with large single-dish
telescopes and interferometers. In particular, we argue that the beginning of
protostellar collapse is much more violent in cluster-forming clouds than in
regions of distributed star formation. Major breakthroughs are expected in this
field from future large submillimeter instruments such as Herschel and ALMA.Comment: 12 pages, 9 figures, to appear in the proceedings of the conference
"Chemistry as a Diagnostic of Star Formation" (C.L. Curry & M. Fich eds.
Interchanging Interactive 3-d Graphics for Astronomy
We demonstrate how interactive, three-dimensional (3-d) scientific
visualizations can be efficiently interchanged between a variety of mediums.
Through the use of an appropriate interchange format, and a unified interaction
interface, we minimize the effort to produce visualizations appropriate for
undertaking knowledge discovery at the astronomer's desktop, as part of
conference presentations, in digital publications or as Web content. We use
examples from cosmological visualization to address some of the issues of
interchange, and to describe our approach to adapting S2PLOT desktop
visualizations to the Web.
Supporting demonstrations are available at
http://astronomy.swin.edu.au/s2plot/interchange/Comment: 10 pages, 7 figures, submitted to Publications of the Astronomical
Society of Australia. v2. Revised title, revised figure 1, fixed typos, minor
additions to future work sectio
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