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 mG, where 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 J m−3 in OMC 1, comparable both to the gravitational potential energy density of OMC 1 (∼10−7 J m−3) and to the energy density in the Orion BN/KL outflow (∼10−7 J m−3). We find that neither the Alfvén velocity in OMC 1 nor the velocity of the super-Alfvénic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the ∼500 yr lifetime of the outflow. Hence, we propose that the hourglass 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.
ABSTRACT
We study the prospects of detecting magnetic helicity in galaxies by observing the dust polarization of the edge-on galaxy NGC 891. Our numerical results of mean-field dynamo calculations ...show that there should be a large-scale component of the rotationally invariant parity-odd B polarization that we predict to be negative in the first and third quadrants, and positive in the second and fourth quadrants. The large-scale parity-even E polarization is predicted to be negative near the axis and positive further away in the outskirts. These properties are shown to be mostly a consequence of the magnetic field being azimuthal and the polarized intensity being maximum at the centre of the galaxy and are not a signature of magnetic helicity.
The dependence of the polarization fraction p on total intensity I in polarized submillimeter emission measurements is typically parameterized as p ∝ I− ( ≤ 1) and used to infer dust grain alignment ...efficiency in star-forming regions, with an index = 1 indicating near-total lack of alignment of grains with the magnetic field. In this work, we demonstrate that the non-Gaussian noise characteristics of the polarization fraction may produce apparent measurements of ∼ 1 even in data with significant signal-to-noise in Stokes Q, U, and I emission, and so with robust measurements of polarization angle. We present a simple model demonstrating this behavior and propose a criterion by which well-characterized measurements of the polarization fraction may be identified. We demonstrate that where our model is applicable, can be recovered by fitting the p-I relationship with the mean of the Rice distribution without statistical debiasing of the polarization fraction. We apply our model to JCMT BISTRO Survey POL-2 850 m observations of three clumps in the Ophiuchus molecular cloud, finding that in the externally illuminated Oph A region, 0.34, while in the more isolated Oph B and C, despite their differing star formation histories, ∼ 0.6-0.7. Our results thus suggest that dust grain alignment in dense gas is more strongly influenced by the incident interstellar radiation field than by star formation history. We further find that grains may remain aligned with the magnetic field at significantly higher gas densities than has previously been believed, thus allowing investigation of magnetic field properties within star-forming clumps and cores.
We present 850 m polarization observations of the L1689 molecular cloud, part of the nearby Ophiuchus molecular cloud complex, taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope ...(JCMT). We observe three regions of L1689: the clump L1689N which houses the IRAS 16293-2433 protostellar system, the starless clump SMM-16, and the starless core L1689B. We use the Davis-Chandrasekhar-Fermi method to estimate plane-of-sky field strengths of 366 55 G in L1689N, 284 34 G in SMM-16, and 72 33 G in L1689B, for our fiducial value of dust opacity. These values indicate that all three regions are likely to be magnetically transcritical with sub-Alfvénic turbulence. In all three regions, the inferred mean magnetic field direction is approximately perpendicular to the local filament direction identified in Herschel Space Telescope observations. The core-scale field morphologies for L1689N and L1689B are consistent with the cloud-scale field morphology measured by the Planck Space Observatory, suggesting that material can flow freely from large to small scales for these sources. Based on these magnetic field measurements, we posit that accretion from the cloud onto L1689N and L1689B may be magnetically regulated. However, in SMM-16, the clump-scale field is nearly perpendicular to the field seen on cloud scales by Planck, suggesting that it may be unable to efficiently accrete further material from its surroundings.
Abstract
Understanding the initial conditions of star formation requires both observational studies and theoretical works taking into account the magnetic field, which plays an important role in star ...formation processes. Herein, we study the young nearby dense cloud core L1521 F n(H2) ∼104−6 cm−3 in the Taurus Molecular Cloud. This dense core hosts a 0.2 M⊙ protostar, categorized as a very low luminosity object with complex velocity structures, particularly in the vicinity of the protostar. To trace the magnetic field within the dense core, we conducted high-sensitivity submillimeter polarimetry of the dust continuum at λ = 850 μm and 450 μm using the POL-2 polarimeter situated in front of the SCUBA-2 submillimeter bolometer camera on the James Clerk Maxwell Telescope. This was compared with millimeter polarimetry taken at λ = 3.3 mm with ALMA. The magnetic field was detected at λ = 850 μm in the peripheral region, which is threaded in a north–south direction, while the central region traced at λ = 450 μm shows a magnetic field with an east–west direction, i.e., orthogonal to that of the peripheral region. Magnetic field strengths are estimated to be ∼70 μG and 200 μG in the peripheral and central regions, respectively, using the Davis–Chandrasekhar–Fermi method. The resulting mass-to-flux ratio of three times larger than that of magnetically critical state for both regions indicates that L 1521 F is magnetically supercritical, i.e., gravitational forces dominate over magnetic turbulence forces. Combining observational data with magnetohydrodynamic simulations, detailed parameters of the morphological properties of this puzzling object are derived for the first time.
Abstract
Optical stellar polarimetry in the Perseus molecular cloud direction is known to show a fully mixed bimodal distribution of position angles across the cloud. We study the Gaia trigonometric ...distances to each of these stars and reveal that the two components in position angles trace two different dust clouds along the line of sight. One component, which shows a polarization angle of −37.°6 ± 35.°2 and a higher polarization fraction of 2.0 ± 1.7 %, primarily traces the Perseus molecular cloud at a distance of 300 pc. The other component, which shows a polarization angle of +66.°8 ± 19.°1 and a lower polarization fraction of 0.8 ± 0.6 %, traces a foreground cloud at a distance of 150 pc. The foreground cloud is faint, with a maximum visual extinction of ≤1 mag. We identify that foreground cloud as the outer edge of the Taurus molecular cloud. Between the Perseus and Taurus molecular clouds, we identify a lower-density ellipsoidal dust cavity with a size of 100–160 pc. This dust cavity is located at
l
= 170°,
b
= −20°, and
d
= 240 pc, which corresponds to an H
I
shell generally associated with the Per OB2 association. The two-component polarization signature observed toward the Perseus molecular cloud can therefore be explained by a combination of the plane-of-sky orientations of the magnetic field both at the front and at the back of this dust cavity.
ABSTRACT
We present ALMA Band 7 polarization observations of the OMC-1 region of the Orion molecular cloud. We find that the polarization pattern observed in the region is likely to have been ...significantly altered by the radiation field of the >104 L⊙ high-mass protostar Orion Source I. In the protostar’s optically thick disc, polarization is likely to arise from dust self-scattering. In material to the south of Source I – previously identified as a region of ‘anomalous’ polarization emission – we observe a polarization geometry concentric around Source I. We demonstrate that Source I’s extreme luminosity may be sufficient to make the radiative precession time-scale shorter than the Larmor time-scale for moderately large grains ($\gt 0.005\!-\!0.1\, \mu$m), causing them to precess around the radiation anisotropy vector (k-RATs) rather than the magnetic field direction (B-RATs). This requires relatively unobscured emission from Source I, supporting the hypothesis that emission in this region arises from the cavity wall of the Source I outflow. This is one of the first times that evidence for k-RAT alignment has been found outside of a protostellar disc or AGB star envelope. Alternatively, the grains may remain aligned by B-RATs and trace gas infall on to the Main Ridge. Elsewhere, we largely find the magnetic field geometry to be radial around the BN/KL explosion centre, consistent with previous observations. However, in the Main Ridge, the magnetic field geometry appears to remain consistent with the larger-scale magnetic field, perhaps indicative of the ability of the dense Ridge to resist disruption by the BN/KL explosion.
Abstract
Using the ALMA archival data of both 12CO (6–5) line and 689-GHz continuum emission towards the archetypical Seyfert galaxy, NGC 1068, we identified a distinct continuum peak separated by 15 ...pc from the nuclear radio component S1 in projection. The continuum flux gives a gas mass of ∼2 × 105 M⊙ and bolometric luminosity of ∼108 L⊙, leading to a star formation rate of ∼0.1 M⊙ yr−1. Subsequent analysis on the line data suggest that the gas cloud has a size of ∼10 pc, yielding to a mean H2 number density of ∼105 cm−3. We therefore refer to the gas as a “massive dense gas cloud”: the gas density is high enough to form a “protostar cluster” with a stellar mass of ∼104 M⊙. We found that the gas stands at a unique position between galactic and extraglactic clouds in the diagrams of start formation rate (SFR) vs. gas mass proposed by Lada et al. (2012, ApJ, 745, 190) and surface density of gas vs. SFR density by Krumholz and McKee (2005, ApJ, 630, 250). All the gaseous and star-formation properties may be understood in terms of the turbulence-regulated star formation scenario. Since there are two stellar populations with ages of 300 Myr and 30 Myr in the 100 pc scale circumnulear region, we discuss that NGC 1068 has experienced at least three episodic star-formation events with the likelihood that the inner star-forming region is the younger. Together with several lines of evidence that the dynamics of the nuclear region is decoupled from that of the entire galactic disk, we discuss that the gas inflow towards the nuclear region of NGC 1068 may be driven by a past minor merger.
To shed light on the early phase of a low-mass protostar formation process, we conducted interferometric observations toward protostar GF 9-2 using the CARMA and SMA. The observations were carried ...out in the 12CO line and the continuum emission at wavelengths of 3.3 mm, 1.1 mm, and 850 m with a spatial resolution of 400 au. All of the continuum images detected a single point-like source with a beam-deconvolved effective radius of 250 80 au at the center of the previously known 1.1-4.5 molecular cloud core. Compact emission is detected toward the object at the Spitzer MIPS and IRAC bands, as well as the four bands at the Wide-field Infrared Survey Explorer. Our spectroscopic imaging of the CO line revealed that the continuum source is driving a 1000 au scale molecular outflow, including a pair of lobes where a collimated "higher"-velocity (∼10 with respect to the velocity of the cloud) red lobe exists inside a poorly collimated "lower"-velocity (∼5 ) red lobe. These lobes are rather young (dynamical timescales of ∼500-2000 yr) and the least powerful (momentum rates of yr−1 ) ones so far detected. A protostellar mass of was estimated using an upper limit of the protostellar age of τ* (4 1) × 103 yr and an inferred nonspherical steady mass accretion rate of ∼1 × 10−5 yr−1. Together with the results from an SED analysis, we discuss that the outflow system is driven by a protostar with a surface temperature of ∼3000 K, and that the natal cloud core is being dispersed by the outflow.
Theory predicts and observations confirm that low-mass stars (like the Sun) in their early life grow by accreting gas from the surrounding material. But for stars ∼10 times more massive than the Sun ...(∼10M ), the powerful stellar radiation is expected to inhibit accretion and thus limit the growth of their mass. Clearly, stars with masses >10M exist, so there must be a way for them to form. The problem may be solved by non-spherical accretion, which allows some of the stellar photons to escape along the symmetry axis where the density is lower. The recent detection of rotating disks and toroids around very young massive stars has lent support to the idea that high-mass ( 8M ) stars could form in this way. Here we report observations of an ammonia line towards a high-mass star forming region. We conclude that the gas is falling inwards towards a very young star of ∼20M , in line with theoretical predictions of non-spherical accretion.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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