Magnetic field is one of the key agents that play a crucial role in shaping molecular clouds and regulating star formation, yet the complete information on the magnetic field is not well constrained ...due to the limitations in observations. We study the magnetic field in the massive infrared dark cloud G035.39-00.33 from dust continuum polarization observations at 850 \(\micron\) with SCUBA-2/POL-2 at JCMT. The magnetic field tends to be perpendicular to the densest part of the main filament (F\(_{M}\)), whereas it has a less defined relative orientation in the rest of the structure, where it tends to be parallel to some diffuse regions. A mean plane-of-the-sky magnetic field strength of \(\sim\)50 \(\mu\)G for F\(_{M}\) is obtained using Davis-Chandrasekhar-Fermi method. Based on \(^{13}\)CO (1-0) line observations, we suggest a formation scenario of F\(_{M}\) due to large-scale (\(\sim\)10 pc) cloud-cloud collision. Using additional NH\(_3\) line data, we estimate that F\(_{M}\) will be gravitationally unstable if it is only supported by thermal pressure and turbulence. The northern part of F\(_{M}\), however, can be stabilized by a modest additional support from the local magnetic field. The middle and southern parts of F\(_{M}\) are likely unstable even if the magnetic field support is taken into account. We claim that the clumps in F\(_{M}\) may be supported by turbulence and magnetic fields against gravitational collapse. Finally, we identified for the first time a massive (\(\sim\)200 M\(_{\sun}\)), collapsing starless clump candidate, "c8", in G035.39-00.33. The magnetic field surrounding "c8" is likely pinched, hinting at an accretion flow along the filament.
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 (JCMT) in Hawaii. This work is part of the B-fields In Star-forming Region Observations (BISTRO) 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 \(\sim\)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 towards 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\(\pm\)410 \(\mu\)G in the Oph-B2 sub-clump using a Davis-Chandeasekhar-Fermi analysis. With this magnetic field strength, we find a mass-to-flux ratio \(\lambda\)= 1.6\(\pm\)1.1, which suggests that the Oph-B2 clump is slightly magnetically supercritical.
We present 850 $\mu$m imaging polarimetry data of the $\rho$ Oph-A core taken
with the Submillimeter Common-User Bolometer Array-2 (SCUBA-2) and its
polarimeter (POL-2), as part of our ongoing survey ...project, BISTRO (B-fields In
STar forming RegiOns). The polarization vectors are used to identify the
orientation of the magnetic field projected on the plane of the sky at a
resolution of 0.01 pc. We identify 10 subregions with distinct polarization
fractions and angles in the 0.2 pc $\rho$ Oph A core; some of them can be part
of a coherent magnetic field structure in the $\rho$ Oph region. The results
are consistent with previous observations of the brightest regions of $\rho$
Oph-A, where the degrees of polarization are at a level of a few percents, but
our data reveal for the first time the magnetic field structures in the fainter
regions surrounding the core where the degree of polarization is much higher
($> 5 \%$). A comparison with previous near-infrared polarimetric data shows
that there are several magnetic field components which are consistent at
near-infrared and submillimeter wavelengths. Using the
Davis-Chandrasekhar-Fermi method, we also derive magnetic field strengths in
several sub-core regions, which range from approximately 0.2 to 5 mG. We also
find a correlation between the magnetic field orientations projected on the sky
with the core centroid velocity components.
Most protostars have luminosities that are fainter than expected from steady accretion over the protostellar lifetime. The solution to this problem may lie in episodic mass accretion -- prolonged ...periods of very low accretion punctuated by short bursts of rapid accretion. However, the timescale and amplitude for variability at the protostellar phase is almost entirely unconstrained. In "A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions", we are monitoring monthly with SCUBA-2 the sub-mm emission in eight fields within nearby (<500 pc) star forming regions to measure the accretion variability of protostars. The total survey area of ~1.6 sq.deg. includes ~105 peaks with peaks brighter than 0.5 Jy/beam (43 associated with embedded protostars or disks) and 237 peaks of 0.125-0.5 Jy/beam (50 with embedded protostars or disks). Each field has enough bright peaks for flux calibration relative to other peaks in the same field, which improves upon the nominal flux calibration uncertainties of sub-mm observations to reach a precision of ~2-3% rms, and also provides quantified confidence in any measured variability. The timescales and amplitudes of any sub-mm variation will then be converted into variations in accretion rate and subsequently used to infer the physical causes of the variability. This survey is the first dedicated survey for sub-mm variability and complements other transient surveys at optical and near-IR wavelengths, which are not sensitive to accretion variability of deeply embedded protostars.
We present the results of simultaneous 450 \(\mu\)m and 850 \(\mu\)m polarization observations toward the massive star forming region NGC 2071IR, a target of the BISTRO (B-fields in Star-Forming ...Region Observations) Survey, using the POL-2 polarimeter and SCUBA-2 camera mounted on the James Clerk Maxwell Telescope. We find a pinched magnetic field morphology in the central dense core region, which could be due to a rotating toroidal disk-like structure and a bipolar outflow originating from the central young stellar object, IRS 3. Using the modified Davis-Chandrasekhar-Fermi method, we obtain a plane-of-sky magnetic field strength of 563\(\pm\)421 \(\mu\)G in the central \(\sim\)0.12 pc region from 850 \(\mu\)m polarization data. The corresponding magnetic energy density of 2.04\(\times\)10\(^{-8}\) erg cm\(^{-3}\) is comparable to the turbulent and gravitational energy densities in the region. We find that the magnetic field direction is very well aligned with the whole of the IRS 3 bipolar outflow structure. We find that the median value of polarization fractions, 3.0 \%, at 450 \(\mu\)m in the central 3 arcminute region, which is larger than the median value of 1.2 \% at 850 \(\mu\)m. The trend could be due to the better alignment of warmer dust in the strong radiation environment. We also find that polarization fractions decrease with intensity at both wavelengths, with slopes, determined by fitting a Rician noise model, of \(0.59 \pm 0.03\) at 450 \(\mu\)m and \(0.36 \pm 0.04\) at 850 \(\mu\)m, respectively. We think that the shallow slope at 850 \(\mu\)m is due to grain alignment at the center being assisted by strong radiation from the central young stellar objects.
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 survey's 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 report the first high spatial resolution measurement of magnetic fields surrounding LkH\(\alpha\) 101, a part of the Auriga-California molecular cloud. The observations were taken with the POL-2 ...polarimeter on the James Clerk Maxwell Telescope within the framework of the B-fields In Star-forming Region Observations (BISTRO) survey. Observed polarization of thermal dust emission at 850 \(\mu\)m is found to be mostly associated with the red-shifted gas component of the cloud. The magnetic field displays a relatively complex morphology. Two variants of the Davis-Chandrasekhar-Fermi method, unsharp masking and structure function, are used to calculate the strength of magnetic fields in the plane of the sky, yielding a similar result of \(B_{\rm POS}\sim 115\) \(\mathrm{\mu}\)G. The mass-to-magnetic-flux ratio in critical value units, \(\lambda\sim0.3\), is the smallest among the values obtained for other regions surveyed by POL-2. This implies that the LkH\(\alpha\) 101 region is sub-critical and the magnetic field is strong enough to prevent gravitational collapse. The inferred \(\delta B/B_0\sim 0.3\) implies that the large scale component of the magnetic field dominates the turbulent one. The variation of the polarization fraction with total emission intensity can be fitted by a power-law with an index of \(\alpha=0.82\pm0.03\), which lies in the range previously reported for molecular clouds. We find that the polarization fraction decreases rapidly with proximity to the only early B star (LkH\(\alpha\) 101) in the region. The magnetic field tangling and the joint effect of grain alignment and rotational disruption by radiative torques are potential of explaining such a decreasing trend.
Based on the 850 \(\mu\)m dust continuum data from SCUBA-2 at James Clerk Maxwell Telescope (JCMT), we compare overall properties of Planck Galactic Cold Clumps (PGCCs) in the \(\lambda\) Orionis ...cloud to those of PGCCs in the Orion A and B clouds. The Orion A and B clouds are well known active star-forming regions, while the \(\lambda\) Orionis cloud has a different environment as a consequence of the interaction with a prominent OB association and a giant Hii region. PGCCs in the \(\lambda\) Orionis cloud have higher dust temperatures (\(Td=16.13\pm0.15\) K) and lower values of dust emissivity spectral index (\( \beta=1.65\pm0.02\)) than PGCCs in the Orion A (Td=13.79\(\pm 0.21\)K, \(\beta=2.07\pm0.03\)) and Orion B (\(Td=13.82\pm0.19\)K, \(\beta=1.96\pm0.02\)) clouds. We find 119 sub-structures within the 40 detected PGCCs and identify them as cores. Of total 119 cores, 15 cores are discovered in the \(\lambda\) Orionis cloud, while 74 and 30 cores are found in the Orion A and B clouds, respectively. The cores in the \(\lambda\) Orionis cloud show much lower mean values of size R=0.08 pc, column density N(H2)=\((9.5\pm1.2) \times 10^{22}\) cm\(^{-2}\), number density n(H2)=\((2.9 \pm 0.4)\times10^{5}\) cm\(^{-3}\), and mass \(M_{core}\)=\(1.0\pm0.3\) M\(_{\odot}\) compared to the cores in the Orion A (R=0.11pc, \(N(H2)=(2.3\pm0.3) \times 10^{23}\) cm\(^{-2}\), n(H2)=\((3.8\pm0.5) \times 10^{5}\)cm\(^{-3}\), and \(M_{core}\)=\(2.4 \pm 0.3\) M\(_{\odot}\)) and Orion B (R=0.16pc, N(H2)=\((3.8 \pm 0.4) \times 10^{23}\)cm\(^{-2}\), n(H2)=\((15.6\pm1.8)\times10^{5}\) cm\(^{-3}\), and \(M_{core}\)= \(2.7\pm0.3\) M\(_{\odot}\)) clouds. These core properties in the \(\lambda\) Orionis cloud can be attributed to the photodissociation and external heating by the nearby Hii region, which may prevent the PGCCs from forming gravitationally bound structures and eventually disperse them. These results support the idea of negative stellar feedback on core formation.
We report 850~\(\mu\)m dust polarization observations of a low-mass (\(\sim\)12 \(M_{\odot}\)) starless core in the \(\rho\) Ophiuchus cloud, Ophiuchus C, made with the POL-2 instrument on the James ...Clerk Maxwell Telescope (JCMT) as part of the JCMT B-fields In STar-forming Region Observations (BISTRO) survey. We detect an ordered magnetic field projected on the plane of sky in the starless core. The magnetic field across the \(\sim\)0.1~pc core shows a predominant northeast-southwest orientation centering between \(\sim\)40\(^\circ\) to \(\sim\)100\(^\circ\), indicating that the field in the core is well aligned with the magnetic field in lower-density regions of the cloud probed by near-infrared observations and also the cloud-scale magnetic field traced by Planck observations. The polarization percentage (\(P\)) decreases with an increasing total intensity (\(I\)) with a power-law index of \(-\)1.03 \(\pm\) 0.05. We estimate the plane-of-sky field strength (\(B_{\mathrm{pos}}\)) using modified Davis-Chandrasekhar-Fermi (DCF) methods based on structure function (SF), auto-correlation (ACF), and unsharp masking (UM) analyses. We find that the estimates from the SF, ACF, and UM methods yield strengths of 103 \(\pm\) 46 \(\mu\)G, 136 \(\pm\) 69 \(\mu\)G, and 213 \(\pm\) 115 \(\mu\)G, respectively. Our calculations suggest that the Ophiuchus C core is near magnetically critical or slightly magnetically supercritical (i.e. unstable to collapse). The total magnetic energy calculated from the SF method is comparable to the turbulent energy in Ophiuchus C, while the ACF method and the UM method only set upper limits for the total magnetic energy because of large uncertainties.
We present 850 \(\mu\)m imaging polarimetry data of the \(\rho\) Oph-A core taken with the Submillimeter Common-User Bolometer Array-2 (SCUBA-2) and its polarimeter (POL-2), as part of our ongoing ...survey project, BISTRO (B-fields In STar forming RegiOns). The polarization vectors are used to identify the orientation of the magnetic field projected on the plane of the sky at a resolution of 0.01 pc. We identify 10 subregions with distinct polarization fractions and angles in the 0.2 pc \(\rho\) Oph A core; some of them can be part of a coherent magnetic field structure in the \(\rho\) Oph region. The results are consistent with previous observations of the brightest regions of \(\rho\) Oph-A, where the degrees of polarization are at a level of a few percents, but our data reveal for the first time the magnetic field structures in the fainter regions surrounding the core where the degree of polarization is much higher (\(> 5 \%\)). A comparison with previous near-infrared polarimetric data shows that there are several magnetic field components which are consistent at near-infrared and submillimeter wavelengths. Using the Davis-Chandrasekhar-Fermi method, we also derive magnetic field strengths in several sub-core regions, which range from approximately 0.2 to 5 mG. We also find a correlation between the magnetic field orientations projected on the sky with the core centroid velocity components.
