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تسجيل مستخدم جديدThe role of the magnetic field in the fragmentation process: the case of G14.225-0.506
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B-fields are predicted to play a role in the formation of filamentary structures and their fragmentation process. We aim at investigating the role of the B-field in the process of core fragmentation toward the hub-filament systems in the IRDC G14.2, which present different fragmentation level. We performed observations of the thermal dust polarization at 350 {mu}m using the CSO toward the hubs. We applied the polarization--intensity-gradient method to estimate the significance of the B-field over the G-force. The B-field in Hub-N shows a uniform structure along the E-W orientation, perpendicular to the major axis of the hub-filament system. The I-gradient in Hub-N displays a local minimum coinciding with the dust core MM1a detected with interferometric observations. The B-field orientation is perturbed when approaching the dust core. Hub-S shows 2 local minima, reflecting the bimodal distribution of the B-field. In Hub-N, both E and W of the hub-filament system, the I-gradient and the B-field are parallel whereas they tend to be perpendicular when penetrating the filaments and hub. The analysis of the {delta}- and {Sigma} B-maps indicate that, the B-field cannot prevent the collapse, suggesting that the B-field is initially dragged by the infalling motion and aligned with it, or is channeling material toward the central ridge from both sides. Values of {Sigma} B > 1 are found toward a N-S ridge encompassing the dust emission peak, indicating that in this region B-field dominates over G-force, or that with the current angular resolution we cannot resolve an hypothetical more complex structure. We estimated the B-field strength, the MtF ratio and the A-M number, and found differences between the 2 hubs. The different levels of fragmentation observed in these 2 hubs could arise from the differences in the B-field properties rather than from different intensity of the G-field.
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We present observations of the 1.3 mm continuum emission toward hub-N and hub-S of the infrared dark cloud G14.225-0.506 carried out with the Submillimeter Array, together with observations of the dust emission at 870 and 350 microns obtained with AP
EX and CSO telescopes. The large scale dust emission of both hubs consists of a single peaked clump elongated in the direction of the associated filament. At small scales, the SMA images reveal that both hubs fragment into several dust condensations. The fragmentation level was assessed under the same conditions and we found that hub-N presents 4 fragments while hub-S is more fragmented, with 13 fragments identified. We studied the density structure by means of a simultaneous fit of the radial intensity profile at 870 and 350 microns and the spectral energy distribution adopting a Plummer-like function to describe the density structure. The parameters inferred from the model are remarkably similar in both hubs, suggesting that density structure could not be responsible in determining the fragmentation level. We estimated several physical parameters such as the level of turbulence and the magnetic field strength, and we found no significant differences between these hubs. The Jeans analysis indicates that the observed fragmentation is more consistent with thermal Jeans fragmentation compared with a scenario that turbulent support is included. The lower fragmentation level observed in hub-N could be explained in terms of stronger UV radiation effects from a nearby HII region, evolutionary effects, and/or stronger magnetic fields at small scales, a scenario that should be further investigated.
We present the results of combined NH3(1,1) and (2,2) line emission observed with the Very Large Array and the Effelsberg 100m telescope of the Infrared Dark Cloud G14.225-0.506. The NH3 emission reveals a network of filaments constituting two hub-fi
lament systems. Hubs are associated with gas of rotational temperature Trot sim 25 K, non-thermal velocity dispersion ~1.1 km/s, and exhibit signs of star formation, while filaments appear to be more quiescent (Trot sim 11 K, non-thermal velocity dispersion ~0.6 km/s). Filaments are parallel in projection and distributed mainly along two directions, at PA sim 10 deg and 60 deg, and appear to be coherent in velocity. The averaged projected separation between adjacent filaments is between 0.5 pc and 1pc, and the mean width of filaments is 0.12 pc. Cores within filaments are separated by ~0.33 pc, which is consistent with the predicted fragmentation of an isothermal gas cylinder due to the sausage-type instability. The network of parallel filaments observed in G14.225-0.506 is consistent with the gravitational instability of a thin gas layer threaded by magnetic fields. Overall, our data suggest that magnetic fields might play an important role in the alignment of filaments, and polarization measurements in the entire cloud would lend further support to this scenario.
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