John E. Vaillancourt

A Primer on Far‐Infrared Polarimetry

ABSTRACT We present an introduction to observing procedures and principles of analysis used in far‐infrared polarimetry. The observing procedures are those for single‐dish observations of thermal emission from aligned dust grains. We discuss techniques for removing backgrounds and for reducing and evaluating errors. The principles of analysis are those required for interpreting polarization maps and polarization spectra in terms of opacity, field structure, and variations in temperature and polarizing efficiency.

Next generation of silicon-based x-ray microcalorimeters

After the design of the calorimeter array for the high-resolution x-ray spectrometer (XRS) on the original Astro-E was frozen, new fabrication techniques became available and our understanding of these devices continually increased. We are now able to complete the optimization of this technology and, potentially, to increase the capability of new XRS instrument for Astro-E2, our on-going sounding recket experiments, and possible further applications. The most significant improvement comes from greatly reducing the excess noise of the ion-implanted thermistors by increasing the thickness of the implanted region.

Complex impedance as a diagnostic tool for characterizing thermal detectors

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.

Hale: a multi-wavelength far-infrared polarimeter for SOFIA

A far-infrared polarimeter, Hale, will be proposed for the next round of instruments for SOFIA. Key features are: simultaneous detection of two components of polarization; detector arrays providing >4000 pixels on the sky; and four passbands between 53 μm and 215 μm, a range characterized by strong dependence of polarization on wavelength. At 53 μm the diffraction-limited resolution, 1.2 λ/D, will be 5.2 arcsec. In all passbands the systematic errors in polarization will be Δ(P) < 0.2%, Δθ< 2 °.

Microelectronic Fabrication of Transition Edge Sensors

Achieving optimum spectral energy resolution in conventional scanning electron microscopes (SEM) can be accomplished by using microcalorimeters. Improvements in device design are being studied, although many research groups have developed fabrication techniques that produce consistent results [1]. Here, we discuss the fabrication method that we are using to produce microcalorimeters using superconducting transition edge sensors (TES) as thermometers, and some of the roadblocks we are observing.

Dust Grain Alignment in the Interstellar Medium

The first observations of interstellar polarization at visible wavelengths over 60 years ago were quickly attributed to the net alignment of irregular dust grains with local magnetic fields. This mechanism provides a method to measure the topology and strength of the magnetic field and to probe the physical characteristics of the dust (e.g., material, size, and shape). However, to do so with confidence, the physics and variability of the alignment mechanism(s) must be quantitatively understood. The description of the physical alignment mechanism has a long history with key contributions spanning decades; the last 15 years have seen major advances in both the theoretical and observational understanding of the problem. For example, it is now clear that the canonical process of paramagnetic relaxation, in which grain rotational components perpendicular to the magnetic field are damped out, is inadequate to align grains on the necessary timescales (compared to damping via collisions) for typical interstellar medium conditions. However, the modern theory of radiative alignment has been more successful; in this theory grains are aligned with respect to the magnetic field via photon-grain interactions that impart the necessary torques to the rotation axes of grains. Here we highlight key observational tests of these alignment mechanisms, especially those involving spectropolarimetry of both dust extinction at near-optical wavelengths and dust emission at far-infrared through millimeter wavelengths. Observations in both these regimes can place limits on such grain aspects as their size and temperature. To date, most observations of the polarized emission have been in the densest regions of the interstellar medium where interpretation in terms of grain alignment models is complicated by regions containing embedded stars and a wide range of temperatures. Additionally, direct comparison of the optical extinction polarization (AV . 10 magnitudes) with dust emission polarization (AV . 25 magnitudes) has not been possible. Future observations with increased sensitivity and spectral coverage in the far-infrared may reach the lower extinction levels necessary to allow more definitive tests of grain alignment models.

Mapping Magnetic Fields in the Cold Dust at the Galactic Center

We report the measurement of 158 new 350 μm polarimetry vectors in the central 30 parsecs of the Galactic center. These data were obtained at the Caltech Submillimeter Observatory using Hertz. Morphologically, these results show a consistency with previously published far-infrared and submillimeter results. We find that the angle of the magnetic field inferred from these observations is related to the 350 μm flux as obtained by SHARC/CSO in the following way. At low fluxes, the magnetic field angle is consistent with that of a poloidal field as seen in nearby features such as the Galactic Center Radio Arc and the Northern and Southern Threads. At high fluxes, the magnetic field is oriented parallel to the plane of the Galaxy. This relationship suggests a model in which an initially poloidal field is sheared out in dense regions which are dominated by gravity. If this model is correct, it implies a characteristic field strength for the region of 3 mG.

Tracing the Magnetic Field in Molecular Clouds

We discuss how the combination of polarimetry and ion-to-neutral molecular line width ratio measurements permits the determination of the orientation of the magnetic field in the weakly ionized parts of molecular clouds. Polarimetry measurements give the field orientation in the plane of the sky, and the ion-to-neutral molecular line width ratio determines the angle between the magnetic field and the line of sight. We present the results obtained with this technique on the M17 and Orion A star-forming regions using Hertz 350 μm polarimetry maps and HCO^(+)-to-HCN molecular line width ratios to provide the first view of the spatial orientation of the magnetic field these molecular clouds.