Abstract
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 and 850
μ
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″ × 40″ (5×5 pixels), which moves over the OMC-1 region. By substituting three quantities in each box with the DCF method, we get magnetic field strengths over the OMC-1 region. We note that there are very large uncertainties in the 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 Alfvén 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 Alfvén 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 compare the directions of molecular outflows of 62 low-mass Class 0 and I protostars in nearby (<450 pc) star-forming regions with the mean orientations of the magnetic fields on 0.05-0.5 pc ...scales in the dense cores/clumps where they are embedded. The magnetic field orientations were measured using the JCMT POL-2 data taken by the BISTRO-1 survey and from the archive. The outflow directions were observed with interferometers in the literature. The observed distribution of the angles between the outflows and the magnetic fields peaks between 15° and 35°. After considering projection effects, our results could suggest that the outflows tend to be misaligned with the magnetic fields by 50° 15° in three-dimensional space and are less likely (but not ruled out) randomly oriented with respect to the magnetic fields. There is no correlation between the misalignment and the bolometric temperatures in our sample. In several sources, the small-scale (1000-3000 au) magnetic field is more misaligned with the outflow than the large-scale magnetic field, suggesting that the small-scale magnetic field has been twisted by the dynamics. In comparison with turbulent MHD simulations of core formation, our observational results are more consistent with models in which the energy densities in the magnetic field and the turbulence of the gas are comparable. Our results also suggest that the misalignment alone cannot sufficiently reduce the efficiency of magnetic braking to enable formation of the observed number of large Keplerian disks with sizes larger than 30-50 au.
ABSTRACT
Polarized dust emission is a key tracer in the study of interstellar medium and of star formation. The observed polarization, however, is a product of magnetic field structure, dust grain ...properties, and grain alignment efficiency, as well as their variations in the line of sight, making it difficult to interpret polarization unambiguously. The comparison of polarimetry at multiple wavelengths is a possible way of mitigating this problem. We use data from HAWC+ /SOFIA and from SCUBA-2/POL-2 (from the BISTRO survey) to analyse the NGC 2071 molecular cloud at 154, 214, and 850 $\mu$m. The polarization angle changes significantly with wavelength over part of NGC 2071, suggesting a change in magnetic field morphology on the line of sight as each wavelength best traces different dust populations. Other possible explanations are the existence of more than one polarization mechanism in the cloud or scattering from very large grains. The observed change of polarization fraction with wavelength, and the 214-to-154 $\mu$m polarization ratio in particular, are difficult to reproduce with current dust models under the assumption of uniform alignment efficiency. We also show that the standard procedure of using monochromatic intensity as a proxy for column density may produce spurious results at HAWC+wavelengths. Using both long-wavelength (POL-2, 850 $\mu$m) and short-wavelength (HAWC+, $\lesssim 200\, \mu$m) polarimetry is key in obtaining these results. This study clearly shows the importance of multi-wavelength polarimetry at submillimetre bands to understand the dust properties of molecular clouds and the relationship between magnetic field and star formation.
Abstract
We investigate the internal 3D magnetic structure of dense interstellar filaments within NGC 1333 using polarization data at 850
μ
m from the
B
-fields In STar-forming Region Observations ...survey at the James Clerk Maxwell Telescope. Theoretical models predict that the magnetic field lines in a filament will tend to be dragged radially inward (i.e., pinched) toward the central axis due to the filament’s self-gravity. We study the cross-sectional profiles of the total intensity (
I
) and polarized intensity (PI) of dust emission in four segments of filaments unaffected by local star formation that are expected to retain a pristine magnetic field structure. We find that the filaments’ FWHMs in PI are not the same as those in
I
, with two segments being appreciably narrower in PI (FWHM ratio ≃0.7–0.8) and one segment being wider (FWHM ratio ≃1.3). The filament profiles of the polarization fraction (
P
) do not show a minimum at the spine of the filament, which is not in line with an anticorrelation between
P
and
I
normally seen in molecular clouds and protostellar cores. Dust grain alignment variation with density cannot reproduce the observed
P
distribution. We demonstrate numerically that the
I
and PI cross-sectional profiles of filaments in magnetohydrostatic equilibrium will have differing relative widths depending on the viewing angle. The observed variations of FWHM ratios in NGC 1333 are therefore consistent with models of pinched magnetic field structures inside filaments, especially if they are magnetically near-critical or supercritical.
This paper presents our observational attempts to precisely measure the central mass of a proto-brown dwarf candidate, L328-IRS, in order to investigate whether L328-IRS is in the substellar mass ...regime. Observations were made for the central region of L328-IRS with the dust continuum and CO isotopologue line emission at Atacama Large Millimeter/submillimeter Array (ALMA) band 6, discovering the detailed outflow activities and a deconvolved disk structure of a size of ∼87 × 37 au. We investigated the rotational velocities as a function of the disk radius, finding that its motions between 130 and 60 au are partially fitted with a Keplerian orbit by a stellar object of ∼0.30 M , while the motions within 60 au do not follow any Keplerian orbit at all. This makes it difficult to lead a reliable estimation of the mass of L328-IRS. Nonetheless, our ALMA observations were useful enough to well constrain the inclination angle of the outflow cavity of L328-IRS as ∼66°, enabling us to better determine the mass accretion rate of ∼8.9 × 10−7 M yr−1. From assumptions that the internal luminosity of L328-IRS is mostly due to this mass accretion process in the disk, or that L328-IRS has mostly accumulated the mass through this constant accretion rate during its outflow activity, its mass was estimated to be ∼0.012-0.023 M , suggesting L328-IRS to be a substellar object. However, we leave our identification of L328-IRS as a proto-brown dwarf to be tentative because of various uncertainties, especially regarding the mass accretion rate.
We present ~1'' resolution 1.3 mm dust continuum and spectral line (12CO and 13CO J = 2-1) observations of the transitional disk system HD 135344B obtained with the Submillimeter Array. The disk ...shows a Keplerian rotation pattern with an inclination of ~11?, based on the spatially and spectrally resolved 12CO and 13CO emission. The data show clear evidence for both dust and gas surface density reductions in the inner region of the disk (radius 50 AU) from the continuum and 13CO J = 2-1 data, respectively. The presence of this inner cavity in both the dust and gas is more consistent with clearing by giant planet formation than by photoevaporation or by grain growth. There is also an indication of global CO gas depletion in the disk, as the mass estimated from 13CO emission (~3.8 X 10--4 M ) is about two orders of magnitude lower than that derived from the 1.3 mm continuum (~2.8 X 10--2 M ).
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 camera, with its associated polarimeter ...(POL-2), on the James Clerk Maxwell Telescope in Hawaii. We discuss the survey's aims and objectives. We describe the rationale behind the survey, and the questions that 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 "hourglass" 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 northeastern 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 new observations of the active star formation region NGC 1333 in the Perseus molecular cloud complex from the James Clerk Maxwell Telescope B-Fields In Star-forming Region Observations ...(BISTRO) survey with the POL-2 instrument. The BISTRO data cover the entire NGC 1333 complex (∼1.5 pc × 2 pc) at 0.02 pc resolution and spatially resolve the polarized emission from individual filamentary structures for the first time. The inferred magnetic field structure is complex as a whole, with each individual filament aligned at different position angles relative to the local field orientation. We combine the BISTRO data with low- and high- resolution data derived from Planck and interferometers to study the multiscale magnetic field structure in this region. The magnetic field morphology drastically changes below a scale of ∼1 pc and remains continuous from the scales of filaments (∼0.1 pc) to that of protostellar envelopes (∼0.005 pc or ∼1000 au). Finally, we construct simple models in which we assume that the magnetic field is always perpendicular to the long axis of the filaments. We demonstrate that the observed variation of the relative orientation between the filament axes and the magnetic field angles are well reproduced by this model, taking into account the projection effects of the magnetic field and filaments relative to the plane of the sky. These projection effects may explain the apparent complexity of the magnetic field structure observed at the resolution of BISTRO data toward the filament network.
We present the results of dust emission polarization measurements of Ophiuchus-B (Oph-B) carried out using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) camera with its associated ...polarimeter (POL-2) on the James Clerk Maxwell Telescope in Hawaii. This work is part of the B-fields in Star-forming Region Observations survey initiated to understand the role of magnetic fields in star formation for nearby star-forming molecular clouds. We present a first look at the geometry and strength of magnetic fields in Oph-B. The field geometry is traced over ∼0.2 pc, with clear detection of both of the sub-clumps of Oph-B. The field pattern appears significantly disordered in sub-clump Oph-B1. The field geometry in Oph-B2 is more ordered, with a tendency to be along the major axis of the clump, parallel to the filamentary structure within which it lies. The degree of polarization decreases systematically toward the dense core material in the two sub-clumps. The field lines in the lower density material along the periphery are smoothly joined to the large-scale magnetic fields probed by NIR polarization observations. We estimated a magnetic field strength of 630 410 G in the Oph-B2 sub-clump using a Davis-Chandrasekhar-Fermi analysis. With this magnetic field strength, we find a mass-to-flux ratio λ = 1.6 1.1, which suggests that the Oph-B2 clump is slightly magnetically supercritical.
We present the 850 m polarization observations toward the IC 5146 filamentary cloud taken using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) and its associated polarimeter (POL-2), ...mounted on the James Clerk Maxwell Telescope, as part of the B-fields In STar forming Regions Observations. This work is aimed at revealing the magnetic field morphology within a core-scale ( 1.0 pc) hub-filament structure (HFS) located at the end of a parsec-scale filament. To investigate whether the observed polarization traces the magnetic field in the HFS, we analyze the dependence between the observed polarization fraction and total intensity using a Bayesian approach with the polarization fraction described by the Rice likelihood function, which can correctly describe the probability density function of the observed polarization fraction for low signal-to-noise ratio data. We find a power-law dependence between the polarization fraction and total intensity with an index of 0.56 in AV ∼ 20-300 mag regions, suggesting that the dust grains in these dense regions can still be aligned with magnetic fields in the IC 5146 regions. Our polarization maps reveal a curved magnetic field, possibly dragged by the contraction along the parsec-scale filament. We further obtain a magnetic field strength of 0.5 0.2 mG toward the central hub using the Davis-Chandrasekhar-Fermi method, corresponding to a mass-to-flux criticality of ∼1.3 0.4 and an Alfvénic Mach number of <0.6. These results suggest that gravity and magnetic field are currently of comparable importance in the HFS and that turbulence is less important.
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