Filamentary structures have been found at almost all size scales in the Galaxy. Massive, long filamentary dark clouds are commonly found inside giant molecular clouds (GMCs; e.g. Bergin & Tafalla 2007; André et al. 2014, and references therein), such as the dark clouds L1495 in the Taurus cloud complex (e.g. Chapman et al. 2011) and the Serpens South cloud in the Serpens region (e.g. Dhabal et al. 2018). Filamentary clouds of 4–6 pc length are common, and possibly longer than 10 pc. Some of these clouds are dark at infrared wavelengths. The line–width size relation observed for molecular gas indicates that the thermal Mach number would exceed 10 at such size scales. The long-term survival of these filamentary structures requires a reinforcing mechanism. As shown in the ideal magnetohydrodynamical (MHD) simulations of Li & Klein (2019), a moderately strong, large-scale magnetic field (Alfv$\acute{\rm e}$n Mach number, ${{\cal M}_{\rm A}}\sim 1$) can provide such a mechanism. In the weak-field model with ${{\cal M}_{\rm A}}=10$, the appearance of molecular clouds is clumpy, rather than the long and slender filamentary clouds seen in moderately strong field models. High-resolution images of massive molecular clouds from the Herschel space telescope reveal complex filamentary substructures (e.g. André et al. 2014). The characteristic inner width of molecular filaments found with Herschel is about ∼0.1 pc (Arzoumanian 2011; Arzoumanian et al. 2019). Dense cores, where stars form, are located along or at the intersections of some of these fine substructures (e.g. Könyves et al. 2015; Tafalla & Hacar 2015). From these observations of molecular cloud structures at different size scales, one can visualize an evolutionary sequence of star formation starting from highly supersonic, magnetized GMCs, continuing on to filamentary dark clouds that form within them, and then on to finer filamentary substructures. Fragmentation of these filamentary structures and substructures leads to the clumps and dense cores that form protostellar clusters and protostars. Knowing the physical conditions inside filamentary clouds would provide crucial information on the formation of filamentary substructures and dense cores, and on the origin of the initial mass function (IMF) and the star formation rate. Particularly important is the characterization of the physical properties of transcritical filamentary structures whose mass per unit length is within a factor of ∼2 of the critical line mass ${M_{\rm crit,\, th,\, \ell }}=2\, c_{\rm s}^2/G$ of nearly isothermal cylindrical filaments (e.g. Ostriker 1964; Inutsuka & Miyama 1997), where cs is the isothermal sound speed. Indeed, Herschel observations suggest that transcritical filamentary structures dominate the mass function of star-forming filaments and that their fragmentation may set the peak of the prestellar core mass function and perhaps ultimately the peak of the IMF (André et al. 2019). In this paper, we report the results of polarimetric observations of the pristine section B211 of one such transcritical filament, the Taurus B211/B213 filament, using the High-resolution Airborne Wideband Campera plus (HAWC+) onboard Stratospheric Observatory For Infrared Astronomy (SOFIA). We determine the magnetic field structure inside a filamentary cloud with filamentary substructures.
Citation: Tafalla & Hacar 2015
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