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Search Results: 1 - 10 of 196291 matches for " John E. Vaillancourt "
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Observations of Polarized Dust Emission at Far-infrared through Millimeter Wavelengths
John E. Vaillancourt
Physics , 2009,
Abstract: Interstellar polarization at far-infrared through millimeter wavelengths (0.1 - 1 mm) is primarily due to thermal emission from dust grains aligned with magnetic fields. This mechanism has led to studies of magnetic fields in a variety of celestial sources, as well as the physical characteristics of the dust grains and their interaction with the field. Observations have covered a diverse array of sources, from entire galaxies to molecular clouds and proto-stellar disks. Maps have been generated on a wide range of angular scales, from surveys covering large fractions of the sky, down to those with arcsecond spatial resolution. Additionally, the increasing availability of observations at multiple wavelengths in this band allows empirical tests of models of grain alignment and cloud structure. I review some of the recent work in this field, emphasizing comparisons of observations on multiple spatial scales and at multiple wavelengths.
Complex Impedance as a Diagnostic Tool for Characterizing Thermal Detectors
John E. Vaillancourt
Physics , 2004, DOI: 10.1063/1.1889427
Abstract: The complex ac impedance of a bolometer or microcalorimeter detector is easily measured and can be used to determine thermal time constants, thermal resistances, heat capacities, and sensitivities. Accurately extracting this information requires an understanding of the electrical and thermal properties of both the detector and the measurement system. We show that this is a practical method for measuring parameters in detectors with moderately complex thermal systems.
Submillimeter Polarization of Galactic Clouds: A Comparison of 350 micron and 850 micron Data
John E. Vaillancourt,Brenda C. Matthews
Physics , 2012, DOI: 10.1088/0067-0049/201/2/13
Abstract: The Hertz and SCUBA polarimeters, working at 350 micron and 850 micron respectively, have measured the polarized emission in scores of Galactic clouds. Of the clouds in each dataset, 17 were mapped by both instruments with good polarization signal-to-noise ratios. We present maps of each of these 17 clouds comparing the dual-wavelength polarization amplitudes and position angles at the same spatial locations. In total number of clouds compared, this is a four-fold increase over previous work. Across the entire data-set real position angle differences are seen between wavelengths. While the distribution of \phi(850)-\phi(350) is centered near zero (near-equal angles), 64% of data points with high polarization signal-to-noise (P >= 3\sigma_p) have |\phi(850)-\phi(350)| > 10 degrees. Of those data with small changes in position angle (<= 10 degrees) the median ratio of the polarization amplitudes is P(850)/P(350) = 1.7 +/- 0.6. This value is consistent with previous work performed on smaller samples and models which require mixtures of different grain properties and polarization efficiencies. Along with the polarization data we have also compiled the intensity data at both wavelengths; we find a trend of decreasing polarization with increasing 850-to-350 micron intensity ratio. All the polarization and intensity data presented here (1699 points in total) are available in electronic format.
A Far-Infrared Observational Test of the Directional Dependence in Radiative Grain Alignment
John E. Vaillancourt,B-G Andersson
Physics , 2015, DOI: 10.1088/2041-8205/812/1/L7
Abstract: The alignment of interstellar dust grains with magnetic fields provides a key method for measuring the strength and morphology of the fields. In turn, this provides a means to study the role of magnetic fields from diffuse gas to dense star-forming regions. The physical mechanism for aligning the grains has been a long-term subject of study and debate. The theory of radiative torques, in which an anisotropic radiation field imparts sufficient torques to align the grains while simultaneously spinning them to high rotational velocities, has passed a number of observational tests. Here we use archival polarization data in dense regions of the Orion molecular cloud (OMC-1) at 100, 350, and $850\,\mu$m to test the prediction that the alignment efficiency is dependent upon the relative orientations of the magnetic field and radiation anisotropy. We find that the expected polarization signal, with a 180-degree period, exists at all wavelengths out to radii of 1.5 arcminutes centered on the BNKL object in OMC-1. The probabilities that these signals would occur due to random noise are low ($\lesssim$1\%), and are lowest towards BNKL compared to the rest of the cloud. Additionally, the relative magnetic field to radiation anisotropy directions accord with theoretical predictions in that they agree to better than 15 degrees at $100\,\mu$m and 4 degrees at $350\,\mu$m.
Dispersion of Magnetic Fields in Molecular Clouds. III
Martin Houde,Ramprasad Rao,John E. Vaillancourt,Roger H. Hildebrand
Physics , 2011, DOI: 10.1088/0004-637X/733/2/109
Abstract: 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.
Dispersion of Magnetic Fields in Molecular Clouds. II
Martin Houde,John E. Vaillancourt,Roger H. Hildebrand,Shadi Chitsazzadeh,Larry Kirby
Physics , 2009, DOI: 10.1088/0004-637X/706/2/1504
Abstract: We expand our study on the dispersion of polarization angles in molecular clouds. We show how the effect of signal integration through the thickness of the cloud as well as across the area subtended by the telescope beam inherent to dust continuum measurements can be incorporated in our analysis to correctly account for its effect on the measured angular dispersion and inferred turbulent to large-scale magnetic field strength ratio. We further show how to evaluate the turbulent magnetic field correlation scale from polarization data of sufficient spatial resolution and high enough spatial sampling rate. We apply our results to the molecular cloud OMC-1, where we find a turbulent correlation length of approximately 16 mpc, a turbulent to large-scale magnetic field strength ratio of approximately 0.5, and a plane-of-the-sky large-scale magnetic field strength of approximately 0.76 mG.
The Galactic Magnetic Field's Effect in Star-Forming Region
Ian W. Stephens,Leslie W. Looney,C. Darren Dowell,John E. Vaillancourt,Konstantinos Tassis
Physics , 2010, DOI: 10.1088/0004-637X/728/2/99
Abstract: We investigate the effect of the Milky Way's magnetic field in star forming regions using archived 350 micron polarization data on 52 Galactic star formation regions from the Hertz polarimeter module. The polarization angles and percentages for individual telescope beams were combined in order to produce a large-scale average for each source and for complexes of sources. In more than 80% of the sources, we find a meaningful mean magnetic field direction, implying the existence of an ordered magnetic field component at the scale of these sources. The average polarization angles were analyzed with respect to the Galactic coordinates in order to test for correlations between polarization percentage, polarization angle, intensity, and Galactic location. No correlation was found, which suggests that the magnetic field in dense molecular clouds is decoupled from the large-scale Galactic magnetic field. Finally, we show that the magnetic field directions in the complexes are consistent with a random distribution on the sky.
Characterizing Magnetized Turbulence in M51
Martin Houde,Andrew Fletcher,Rainer Beck,Roger H. Hildebrand,John E. Vaillancourt,Jeroen M. Stil
Physics , 2013, DOI: 10.1088/0004-637X/766/1/49
Abstract: We use previously published high-resolution synchrotron polarization data to perform an angular dispersion analysis with the aim of charactering magnetized turbulence in M51. We first analyze three distinct regions (the center of the galaxy, and the northwest and southwest spiral arms) and can clearly discern the turbulent correlation length scale from the width of the magnetized turbulent correlation function for two regions and detect the imprint of anisotropy in the turbulence for all three. Furthermore, analyzing the galaxy as a whole allows us to determine a two-dimensional Gaussian model for the magnetized turbulence in M51. We measure the turbulent correlation scales parallel and perpendicular to the local mean magnetic field to be, respectively, delta_{para} = 98 +/- 5 pc and delta_{perp} = 54 +/- 3 pc, while the turbulent to ordered magnetic field strength ratio is found to be Bt/B0 = 1.01 +/- 0.04. These results are consistent with those of Fletcher et al. (2011), who performed a Faraday rotation dispersion analysis of the same data, and our detection of anisotropy is consistent with current magnetized turbulence theories.
Magnetic Fields and Infall Motions in NGC 1333 IRAS 4
Michael Attard,Martin Houde,Giles Novak,Hua-bai Li,John E. Vaillancourt,C. Darren Dowell,Jacqueline Davidson,Hiroko Shinnaga
Physics , 2009, DOI: 10.1088/0004-637X/702/2/1584
Abstract: We present single-dish 350 micron dust continuum polarimetry as well as HCN and HCO+ J=4-3 rotational emission spectra obtained on NGC 1333 IRAS 4. The polarimetry indicates a uniform field morphology over a 20" radius from the peak continuum flux of IRAS 4A, in agreement with models of magnetically supported cloud collapse. The field morphology around IRAS 4B appears to be quite distinct however, with indications of depolarization observed towards the peak flux of this source. Inverse P-Cygni profiles are observed in the HCN J=4-3 line spectra towards IRAS 4A, providing a clear indication of infall gas motions. Taken together, the evidence gathered here appears to support the scenario that IRAS 4A is a cloud core in a critical state of support against gravitational collapse.
350 micron Polarimetry from the Caltech Submillimeter Observatory
Jessie L. Dotson,John E. Vaillancourt,Larry Kirby,C. Darren Dowell,Roger H. Hildebrand,Jacqueline A. Davidson
Physics , 2010, DOI: 10.1088/0067-0049/186/2/406
Abstract: We present a summary of data obtained with the 350 micron polarimeter, Hertz, at the Caltech Submillimeter Observatory. We give tabulated results and maps showing polarization vectors and flux contours. The summary includes over 4300 individual measurements in 56 Galactic sources and 2 galaxies. Of these measurements, 2153 have P >= 3\sigma_p statistical significance. The median polarization of the entire data set is 1.46%.
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