JCMT POL-2 and BISTRO Survey observations of magnetic fields in the L1689 molecular cloud

Abstract

We present 850µm 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 µG in L1689N, 284 ± 34 µG in SMM-16, and 72 ± 33 µG in L1689B, for our fiducial value of dust opacity. These values indicate that all three regions are likely to be magnetically trans-critical 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 P lanck 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 P lanck, suggesting that it may be unable to efficiently accrete further material from its surroundings.

The JCMT BISTRO-2 Survey: The Magnetic Field in the Center of the Rosette Molecular Cloud

“…Given that the diffuse ISM shows a well-defined median magnetic field strength of 6.0 ± 1.8 µG (Heiles & Troland 2005), our field is at least an order of magnitude stronger. The B-field strength in this clump and its surroundings seems to be comparable to that of the nearby starless core L1689B (Pattle et al 2020a), and also to those in the northern and southern parts of the G34 IRDC at a distance of ∼ 3.7 kpc (Soam et al 2019). The configuration of our selected subregion for the B POS calculations looks more similar to the geometry of the northern part of the elongated G34 IRDC in that they both contain dense core(s), which are probably already protostellar, as well their surrounding environment.…”

“…We then separated the non-thermal component ∆v NT in km s −1 using a similar relation to eqn. B8 of Pattle et al (2020a).…”

“…This method assumes small-scale non-thermal motions, thus should not be applied under super-Alfvénic turbulent conditions, i.e., when M A 1, where M A is the Alfvén Mach number. Following Pattle et al (2020a), it can be expressed as…”

“…angle dispersion, σ θ 15.43 The uncertainties on the column densities, and so on the mass and volume densities, were taken to be 20% in these calculation, in order to avoid the propagation of the typical factor of about 2 systematic errors mainly due to the uncertainties in the dust opacity law. For a more subtle treatment of these systematic errors in the DCF analysis see Pattle et al (2020a). With other derived properties we use the quadratic addition of errors.…”

B-fields in Star-Forming Region Observations (BISTRO): Magnetic Fields in the Filamentary Structures of Serpens Main

“…We applied the 'August 2019' instrumental polarization (IP) model 3 to the data, with our final total intensity map serving as the model for estimating artificial polarized emission to be corrected for. In addition to the typical POL-2 procedures (e.g., Pattle et al 2020a), additional steps were taken to combine the SE and NW fields. Data from the two pointings were initially reduced separately to make initial total intensity maps.…”