A B S T R A C TWe study the equilibrium of pressure truncated, filamentary molecular clouds that are threaded by rather general helical magnetic fields. We first apply the virial theorem to filamentary molecular clouds, including the effects of non-thermal motions and the turbulent pressure of the surrounding ISM. When compared with the data, we find that many filamentary clouds have a mass per unit length that is significantly reduced by the effects of external pressure, and that toroidal fields play a significant role in squeezing such clouds.We also develop exact numerical MHD models of filamentary molecular clouds with more general helical field configurations than have previously been considered. We examine the effects of the equation of state by comparing`isothermal' filaments, with constant total (thermal plus turbulent) velocity dispersion, with equilibria constructed using a logatropic equation of state.Our theoretical models involve three parameters: two to describe the mass loading of the toroidal and poloidal fields, and a third that describes the radial concentration of the filament. We thoroughly explore our parameter space to determine which choices of parameters result in models that agree with the available observational constraints. We find that both equations of state result in equilibria that agree with the observational results. Moreover, we find that models with helical fields have more realistic density profiles than either unmagnetized models or those with purely poloidal fields; we find that most isothermal models have density distributions that fall off as r 21.8 to r 22 , while logatropes have density profiles that range from r 21 to r 21.8 . We find that purely poloidal fields produce filaments with steep radial density gradients that are not allowed by the observations.

We present λ 1.3 mm Combined Array for Research in Millimeter-wave Astronomy observations of dust polarization toward 30 star-forming cores and eight star-forming regions from the TADPOL survey. We show maps of all sources, and compare the ∼2. 5 resolution TADPOL maps with ∼20 resolution polarization maps from single-dish submillimeter telescopes. Here we do not attempt to interpret the detailed B-field morphology of each object. Rather, we use average B-field orientations to derive conclusions in a statistical sense from the ensemble of sources, bearing in mind that these average orientations can be quite uncertain. We discuss three main findings. (1) A subset of the sources have consistent magnetic field (B-field) orientations between large (∼20 ) and small (∼2. 5) scales. Those same sources also tend to have higher fractional polarizations than the sources with inconsistent large-to-small-scale fields. We interpret this to mean that in at least some cases B-fields play a role in regulating the infall of material all the way down to the ∼1000 AU scales of protostellar envelopes. (2) Outflows appear to be randomly aligned with B-fields; although, in sources with low polarization fractions there is a hint that outflows are preferentially perpendicular to small-scale B-fields, which suggests that in these sources the fields have been wrapped up by envelope rotation. (3) Finally, even at ∼2. 5 resolution we see the so-called polarization hole effect, where the fractional polarization drops significantly near the total intensity peak. All data are publicly available in the electronic edition of this article.

We present results of λ1.3 mm dust polarization observations toward 16 nearby, low-mass protostars, mapped with ∼2.5 resolution at CARMA. The results show that magnetic fields in protostellar cores on scales of ∼1000 AU are not tightly aligned with outflows from the protostars. Rather, the data are consistent with scenarios where outflows and magnetic fields are preferentially misaligned (perpendicular), or where they are randomly aligned. If one assumes that outflows emerge along the rotation axes of circumstellar disks, and that the outflows have not disrupted the fields in the surrounding material, then our results imply that the disks are not aligned with the fields in the cores from which they formed.

In this paper we present the first observations of the Ophiuchus molecular cloud performed as part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey (GBS) with the SCUBA-2 instrument. We demonstrate methods for combining these data with previous HARP CO, Herschel, and IRAM N 2 H + observations in order to accurately quantify the properties of the SCUBA-2 sources in Ophiuchus. We produce a catalogue of all of the sources found by SCUBA-2. We separate these into protostars and starless cores. We list all of the starless cores and perform a full virial analysis, including external pressure. This is the first time that external pressure has been included in this level of detail. We find that the majority of our cores are either bound or virialised. Gravitational energy and external pressure are on average of a similar order of magnitude, but with some variation from region to region. We find that cores in the Oph A region are gravitationally bound prestellar cores, while cores in the Oph C and E regions are pressure-confined. We determine that N 2 H + is a good tracer of the bound material of prestellar cores, although we find some evidence for N 2 H + freezeout at the very highest core densities. We find that non-thermal linewidths decrease substantially between the gas traced by C 18 O and that traced by N 2 H + , indicating the dissipation of turbulence at higher densities. We find that the critical Bonnor-Ebert stability criterion is not a good indicator of the boundedness of our cores. We detect the pre-brown dwarf candidate Oph B-11 and find a flux density and mass consistent with previous work. We discuss regional variations in the nature of the cores and find further support for our previous hypothesis of a global evolutionary gradient across the cloud from southwest to northeast, indicating sequential star formation across the region.

We model the sub-millimetre polarization patterns that are expected for filamentary clouds that are threaded by helical magnetic fields. We previously developed a three parameter model of such clouds (Fiege & Pudritz 2000a), which are described by a concentration parameter C, and two flux to mass ratios Γ z and Γ φ to specify the mass loading of the poloidal and toroidal field lines respectively. Our models provide a simple and purely geometric explanation for the well-known "polarization hole" effect, in which the sub-millimetre polarization percentage decreases toward the regions of peak intensity. This occurs because of a cancellation between contributions to the polarization from the "backbone" of poloidal flux along the filament's axis and its surrounding envelope, which is dominated by the toroidal field component. A systematic exploration of our parameter space allows us to classify the polarization patterns due to filaments aligned approximately perpendicular to the plane of the sky into three basic types. The polarization vectors are parallel to filaments when B z,S /B φ,S < ∼ 0.1, where B z,S and B φ,S are respectively the poloidal and toroidal magnetic field components at the outer surface of the filament. The polarization vectors are perpendicular to filaments when B z,S /B φ,S > ∼ 0.33. Intermediate cases result in polarization patterns that contain 90 • flips in the orientation of the polarization vectors. The flips are symmetric about the central axis for filaments oriented parallel to the plane of the sky, but more complicated asymmetric patters result from filaments that are inclined at some angle.