Abstract
Abstract We investigate the interplay between magnetic (B) field, gravity, and turbulence in the fragmentation process of cores within the filamentary infrared dark cloud G34.43+00.24. We observe the magnetic field morphology across G34.43, traced with thermal dust polarization at 350 μm with an angular resolution of 10″ (0.18 pc), and compare with the kinematics obtained from N2H+ across the filament. We derive local velocity gradients from N2H+, tracing motion in the plane of sky, and compare with the observed local B field orientations in the plane of sky. The B field orientations are found to be perpendicular to the long axis of the filament toward the MM1 and MM2 ridge, suggesting that the B field can guide material toward the filament. Toward MM3, the B field orientations appear more parallel to the filament and aligned with the elongated core of MM3, indicating a different role of the B field. In addition to a large-scale east–west velocity gradient, we find a close alignment between local B field orientations and local velocity gradients toward the MM1/MM2 ridge. This local correlation in alignment suggests that gas motions are influenced by the B field morphology or vice versa. Additionally, this alignment seems to become even closer with increasing integrated emission in N2H+, possibly indicating that a growing gravitational pull alignes the B field and gas motion more and more. We analyze and quantify B field, gravity, turbulence, and their relative importance toward the MM1, MM2, and MM3 regions with various techniques over two scales, a larger clump area at 2 pc scale and the smaller core area at 0.6 pc scale. While gravitational energy, B field, and turbulent pressure all grow systematically from large to small scale, the ratios among the three constituents clearly develop differently over scale. We propose that this varying relative importance between B field, gravity, and turbulence over scale drives and explains the different fragmentation types seen at subparsec scale (no fragmentation in MM1; aligned fragmentation in MM2; clustered fragmentation in MM3). We discuss uncertainties, subtleties, and the robustness of our conclusion, and we stress that a multiscale joint analysis is required to understand the dynamics in these systems.
Highlights
Recent Herschel results show that molecular clouds are mostly filamentary (André et al 2013)
As the derived parameters can be sensitive to the selected area, we consider two different representative scales, namely the smaller core area at a scale of 0.6 pc and the larger clump area at a scale of 2 pc
In the previous sections we have presented observational facts that characterize the region from the outer filamentary zones to the inner clump/core regions, namely (1) a mostly uniform large-scale B field perpendicular to the filament is observed toward MM1 and MM2, while a bending B field is seen closely aligned with the MM3 major axis; (2) the velocity gradients are closely aligned with the B field toward MM1 and MM2, while they show systematically larger misalignements in MM3; (3) different values in the dispersion of the B field orientations are observed, being smallest in MM2, more than twice as large in MM3, and intermediate in MM1
Summary
Recent Herschel results show that molecular clouds are mostly filamentary (André et al 2013). One mechanism that forms filamentary structures is through compressive flows, where the filaments appear at the interfaces where flows collide (e.g., Ballesteros-Paredes et al 2007; Inutsuka et al 2015). Molecular clouds are both turbulent and threaded by magnetic (B) fields, which possibly explains the observed low star formation rate (e.g., Vázquez-Semadeni 2015, and the references therein). B fields are recognized as one of the key components in star formation theories (e.g., McKee & Ostriker 2007) Their exact role in the formation and evolution of molecular clouds is still a matter of debate in the literature.
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