Submillimeter and Far-Infrared Polarimetric Observations of Magnetic Fields in Star-Forming Regions

Observations of star-forming regions by the current and upcoming generation of submillimeter polarimeters will shed new light on the evolution of magnetic fields over the cloud-to-core size scales involved in the early stages of the star formation process. Recent wide-area and high-sensitivity polarization observations have drawn attention to the challenges of modeling magnetic field structure of star forming regions, due to variations in dust polarization properties in the interstellar medium. However, these observations also for the first time provide sufficient information to begin to break the degeneracy between polarization efficiency variations and depolarization due to magnetic field sub-beam structure, and thus to accurately infer magnetic field properties in the star-forming interstellar medium. In this article we discuss submillimeter and far-infrared polarization observations of star-forming regions made with single-dish instruments. We summarize past, present and forthcoming single-dish instrumentation, and discuss techniques which have been developed or proposed to interpret polarization observations, both in order to infer the morphology and strength of the magnetic field, and in order to determine the environments in which dust polarization observations reliably trace the magnetic field. We review recent polarimetric observations of molecular clouds, filaments, and starless and protostellar cores, and discuss how the application of the full range of modern analysis techniques to recent observations will advance our understanding of the role played by the magnetic field in the early stages of star formation.Comment: 29 pages, 12 figures, 1 table, published in Frontiers in Astronomy and Space Sciences. Open-access, available here: https://www.frontiersin.org/articles/10.3389/fspas.2019.00015/ful

On the origin of de-polarization in CB54

We present polarimetric observations of the Bok globule CB54 in the far-infrared via SOFIA/HAWC+ bands D (154 (Formula presented.) m) and E (214 (Formula presented.) m). We detect polarization with mean polarization degrees of (Formula presented.) (Formula presented.) 5.4 (Formula presented.) and (Formula presented.) (Formula presented.) 5.4 (Formula presented.) at 154 and 214 (Formula presented.) m, respectively. The polarization degree decreases toward the inner region of CB54, revealing a “polarization hole”. This finding can be explained by the impact of dichroic absorption counteracting the effect of polarized emission. The same effect allows us to explain the observed wavelength-dependent orientation of the linear polarization in the dense, central region of CB54. The polarization pattern appears uniform at core scales but un-ordered at larger scales—similar to what was found in previous studies of this object. The mean polarization angle amounts to (Formula presented.) = 62.4 (Formula presented.) 44.5° and (Formula presented.) = (Formula presented.) 60.0° at 154 and 214 (Formula presented.) m, respectively

The JCMT Nearby Galaxies Legacy Survey: SCUBA-2 observations of nearby galaxies

We present 850$\mu$m observations of a sample of 8 nearby spiral galaxies, made using the SCUBA-2 camera on the James Clerk Maxwell Telescope (JCMT) as part of the JCMT Nearby Galaxies Legacy Survey (NGLS). We corrected our data for the presence of the $^{12}$CO $J=3\to 2$ line in the SCUBA-2 850$\mu$m bandwidth using NGLS HARP data, finding a typical $^{12}$CO contribution of $\sim 20$%. We measured dust column densities, temperatures and opacity indices by fitting spectral energy distributions constructed from SCUBA-2 and archival Herschel observations, and used archival GALEX and Spitzer data to make maps of surface density of star formation ($\Sigma_{\rm SFR}$). Typically, comparing SCUBA-2-derived H$_2$ surface densities ($\Sigma_{\rm H_2}$) to $\Sigma_{\rm SFR}$ gives shallow star formation law indices within galaxies, with SCUBA-2-derived values typically being sublinear and Herschel-derived values typically being broadly linear. This difference is likely due to the effects of atmospheric filtering on the SCUBA-2 data. Comparing the mean values of $\Sigma_{\rm H_2}$ and $\Sigma_{\rm SFR}$ of the galaxies in our sample returns a steeper star formation law index, broadly consistent with both the Kennicutt-Schmidt value of 1.4 and linearity. Our results show that a SCUBA-2 detection is a good predictor of star formation. We suggest that Herschel emission traces gas in regions which will form stars on timescales $\sim 5-100$ Myr, comparable to the star formation timescale traced by GALEX and Spitzer data, while SCUBA-2 preferentially traces the densest gas within these regions, which likely forms stars on shorter timescales.Comment: Accepted for publication in MNRAS. 15 pages, 12 figures, 8 tables (plus 15 pages of appendices, with 31 figures

Magnetic fields in the Horsehead Nebula

We present the first polarized dust emission measurements of the Horsehead Nebula, obtained using the POL-2 polarimeter on the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) camera on the James Clerk Maxwell Telescope (JCMT). The Horsehead Nebula contains two sub-millimeter sources, a photodissociation region (PDR; SMM1) and a starless core (SMM2). We see well-ordered magnetic fields in both sources. We estimated plane-of-sky magnetic field strengths of 56$\pm$9 and 129$\pm$21 $\mu$G in SMM1 and SMM2, respectively, and obtained mass-to-flux ratios and Alfv\'en Mach numbers of less than 0.6, suggesting that the magnetic field can resist gravitational collapse and that magnetic pressure exceeds internal turbulent pressure in these sources. In SMM2, the kinetic and gravitational energies are comparable to one another, but less than the magnetic energy. We suggest a schematic view of the overall magnetic field structure in the Horsehead Nebula. Magnetic field lines in SMM1 appear have been compressed and reordered during the formation of the PDR, while the likely more-embedded SMM2 may have inherited its field from that of the pre-shock molecular cloud. The magnetic fields appear to currently play an important role in supporting both sources.Comment: Accepted to the Astronomical Journa

First Observations of the Magnetic Field inside the Pillars of Creation : Results from the BISTRO Survey

We present the first high-resolution, submillimeter-wavelength polarimetric observations of—and thus direct observations of the magnetic field morphology within—the dense gas of the Pillars of Creation in M16. These 850 μm observations, taken as part of the B-Fields in Star-forming Region Observations Survey (BISTRO) using the POL-2 polarimeter on the Submillimeter Common-User Bolometer Array 2 (SCUBA-2) camera on the James Clerk Maxwell Telescope (JCMT), show that the magnetic field runs along the length of the Pillars, perpendicular to and decoupled from the field in the surrounding photoionized cloud. Using the Chandrasekhar–Fermi method we estimate a plane-of-sky magnetic field strength of 170–320 μG in the Pillars, consistent with their having been formed through the compression of gas with initially weak magnetization. The observed magnetic field strength and morphology suggests that the magnetic field may be slowing the Pillars' evolution into cometary globules. We thus hypothesize that the evolution and lifetime of the Pillars may be strongly influenced by the strength of the coupling of their magnetic field to that of their parent photoionized cloud—i.e., that the Pillars' longevity results from magnetic support

Magnetic fields and outflows in CB 54

We have observed the large Bok globule CB 54 in 850-μm polarized light using the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We find that the magnetic field in the periphery of the globule shows a significant, ordered deviation from the mean-field direction in the globule centre. This deviation appears to correspond with the extended but relatively weak 12CO outflow emanating from the Class 0 sources at the centre of the globule. Energetics analysis suggests that if the outflow is reshaping the magnetic field in the globule’s periphery, then we can place an upper limit of 0.1 pc

Magnetic fields and outflows in the large Bok globule CB 54

We have observed the large Bok globule CB 54 in 850 μm polarised light using the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We find that the magnetic field in the periphery of the globule shows significant, ordered deviation from the mean field direction in the globule centre. This deviation appears to correspond with the extended but relatively weak 12CO outflow emanating from the Class 0 sources at the centre of the globule. Energetics analysis suggests that if the outflow is reshaping the magnetic field in the globule’s periphery, then we can place an upper limit of 0.1 pc

Submillimetre observations of the two-component magnetic field in M82

We observed the starburst galaxy M82 in 850 µm polarized light with the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We interpret our observed polarization geometry as tracing a two-component magnetic field: a poloidal component aligned with the galactic ‘superwind’, extending to a height ∼350 pc above and below the central bar; and a spiral-arm-aligned, or possibly toroidal, component in the plane of the galaxy, which dominates the 850 µm polarized light distribution at galactocentric radii ≳2 kpc. Comparison of our results with recent High-resolution Airborne Wideband Camera Plus (HAWC+) measurements of the field in the dust entrained by the M82 superwind suggests that the superwind breaks out from the central starburst at ∼350 pc above the plane of the galaxy