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Register a new userFirst observations of the magnetic field inside the Pillars of Creation: Results from the BISTRO survey
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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$,mu$m observations, taken as part of the BISTRO (B-Fields in Star-forming Region Observations) Survey using the POL-2 polarimeter on the 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 $170-320,mu$G in the Pillars, consistent with their having been formed through 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
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We present the first results from the B-fields In STar-forming Region Observations (BISTRO) survey, using the Sub-millimetre Common-User Bolometer Array 2 (SCUBA-2) camera, with its associated polarimeter (POL-2), on the James Clerk Maxwell Telescope (JCMT) in Hawaii. We discuss the surveys aims and objectives. We describe the rationale behind the survey, and the questions which the survey will aim to answer. The most important of these is the role of magnetic fields in the star formation process on the scale of individual filaments and cores in dense regions. We describe the data acquisition and reduction processes for POL-2, demonstrating both repeatability and consistency with previous data. We present a first-look analysis of the first results from the BISTRO survey in the OMC 1 region. We see that the magnetic field lies approximately perpendicular to the famous integral filament in the densest regions of that filament. Furthermore, we see an hour-glass magnetic field morphology extending beyond the densest region of the integral filament into the less-dense surrounding material, and discuss possible causes for this. We also discuss the more complex morphology seen along the Orion Bar region. We examine the morphology of the field along the lower-density north-eastern filament. We find consistency with previous theoretical models that predict magnetic fields lying parallel to low-density, non-self-gravitating filaments, and perpendicular to higher-density, self-gravitating filaments.
We present the POL-2 850 $mu$m linear polarization map of the Barnard 1 clump in the Perseus molecular cloud complex from the B-fields In STar-forming Region Observations (BISTRO) survey at the James Clerk Maxwell Telescope. We find a trend of decreasing polarization fraction as a function of total intensity, which we link to depolarization effects towards higher density regions of the cloud. We then use the polarization data at 850 $mu$m to infer the plane-of-sky orientation of the large-scale magnetic field in Barnard 1. This magnetic field runs North-South across most of the cloud, with the exception of B1-c where it turns more East-West. From the dispersion of polarization angles, we calculate a turbulence correlation length of $5.0 pm 2.5$ arcsec ($1500$ au), and a turbulent-to-total magnetic energy ratio of $0.5 pm 0.3$ inside the cloud. We combine this turbulent-to-total magnetic energy ratio with observations of NH$_3$ molecular lines from the Green Bank Ammonia Survey (GAS) to estimate the strength of the plane-of-sky component of the magnetic field through the Davis-Chandrasekhar-Fermi method. With a plane-of-sky amplitude of $120 pm 60$ $mu$G and a criticality criterion $lambda_c = 3.0 pm 1.5$, we find that Barnard 1 is a supercritical molecular cloud with a magnetic field nearly dominated by its turbulent component.
We determine the magnetic field strength in the OMC 1 region of the Orion A filament via a new implementation of the Chandrasekhar-Fermi method using observations performed as part of the James Clerk Maxwell Telescope (JCMT) B-Fields In Star-Forming Region Observations (BISTRO) survey with the POL-2 instrument. We combine BISTRO data with archival SCUBA-2 and HARP observations to find a plane-of-sky magnetic field strength in OMC 1 of $B_{rm pos}=6.6pm4.7$ mG, where $delta B_{rm pos}=4.7$ mG represents a predominantly systematic uncertainty. We develop a new method for measuring angular dispersion, analogous to unsharp masking. We find a magnetic energy density of $sim1.7times 10^{-7}$ Jm$^{-3}$ in OMC 1, comparable both to the gravitational potential energy density of OMC 1 ($sim 10^{-7}$ Jm$^{-3}$), and to the energy density in the Orion BN/KL outflow ($sim 10^{-7}$ Jm$^{-3}$). We find that neither the Alfv{e}n velocity in OMC 1 nor the velocity of the super-Alfv{e}nic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the $sim$500-year lifetime of the outflow. Hence, we propose that the hour-glass field morphology in OMC 1 is caused by the distortion of a primordial cylindrically-symmetric magnetic field by the gravitational fragmentation of the filament and/or the gravitational interaction of the BN/KL and S clumps. We find that OMC 1 is currently in or near magnetically-supported equilibrium, and that the current large-scale morphology of the BN/KL outflow is regulated by the geometry of the magnetic field in OMC 1, and not vice versa.
Measurement of magnetic field strengths in a molecular cloud is essential for determining the criticality of magnetic support against gravitational collapse. In this paper, as part of the JCMT BISTRO survey, we suggest a new application of the Davis-Chandrasekhar-Fermi (DCF) method to estimate the distribution of magnetic field strengths in the OMC-1 region. We use observations of dust polarization emission at 450 $mu$m and 850 $mu$m, and C$^{18}$O (3-2) spectral line data obtained with the JCMT. We estimate the volume density, the velocity dispersion and the polarization angle dispersion in a box, 40$$ $times$ 40$$ (5$times$5 pixels), which moves over the OMC-1 region. By substituting three quantities in each box to the DCF method, we get magnetic field strengths over the OMC-1 region. We note that there are very large uncertainties in inferred field strengths, as discussed in detail in this paper. The field strengths vary from 0.8 to 26.4 mG and their mean value is about 6 mG. Additionally, we obtain maps of the mass-to-flux ratio in units of a critical value and the Alfv$acute{e}$n mach number. The central parts of the BN-KL and South (S) clumps in the OMC-1 region are magnetically supercritical, so the magnetic field cannot support the clumps against gravitational collapse. However, the outer parts of the region are magnetically subcritical. The mean Alfv$acute{e}$n mach number is about 0.4 over the region, which implies that the magnetic pressure exceeds the turbulent pressure in the OMC 1 region.
We present the first 850 $mu$m polarization observations in the most active star-forming site of the Rosette Molecular Cloud (RMC, $dsim$1.6 kpc) in the wall of the Rosette Nebula, imaged with the SCUBA-2/POL-2 instruments of the JCMT, as part of the B-Fields In Star-Forming Region Observations 2 (BISTRO-2) survey. From the POL-2 data we find that the polarization fraction decreases with the 850 $mu$m continuum intensity with $alpha$ = 0.49 $pm$ 0.08 in the $p propto I^{rm -alpha}$ relation, which suggests that some fraction of the dust grains remain aligned at high densities. The north of our 850 $mu$m image reveals a gemstone ring morphology, which is a $sim$1 pc-diameter ring-like structure with extended emission in the head to the south-west. We hypothesize that it might have been blown by feedback in its interior, while the B-field is parallel to its circumference in most places. In the south of our SCUBA-2 field the clumps are apparently connected with filaments which follow Infrared Dark Clouds (IRDCs). Here, the POL-2 magnetic field orientations appear bimodal with respect to the large-scale Planck field. The mass of our effective mapped area is $sim$ 174 $M_odot$ that we calculate from 850 $mu$m flux densities. We compare our results with masses from large-scale emission-subtracted Herschel 250 $mu$m data, and find agreement within 30%. We estimate the POS B-field strength in one typical subregion using the Davis-Chandrasekhar-Fermi (DCF) technique and find 80 $pm$ 30 $mu$G toward a clump and its outskirts. The estimated mass-to-flux ratio of $lambda$ = 2.3 $pm$ 1.0 suggests that the B-field is not sufficiently strong to prevent gravitational collapse in this subregion.
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