Witnessing the fragmentation of a filament into prestellar cores in Orion B/NGC 2024?

Abstract

Recent Herschel observations of nearby clouds have shown that filamentary structures are ubiquitous and that most prestellar cores form in filaments. Probing the density (n) and velocity (V) structure of filaments is crucial for the understanding of the star formation process. To characterize both the n and the V field of a fragmenting filament, we mapped NGC2024. 13CO, C18O, and H13CO+ trace the filament seen in the NH2 data. The radial profile from the NH2 data shows DHP~0.081 pc, which is similar to the Herschel findings. The DHP from 13CO and C18O are broader, while the DHP from H13CO+ is narrower, than DHP from Herschel. These results suggest that 13CO and C18O trace only the outer part of the filament and H13CO+ only the inner part. The H13CO+ Vcentroid map reveals V gradients along both filament axis, as well as V oscillations with a period λ~0.2 pc along the major axis. Comparison between the V and the n distribution shows a tentative λ/4 shift in H13CO+ or C18O. This λ/4 shift is not simultaneously observed for all cores in any single tracer but is tentatively seen in either H13CO+ or C18O. We produced a toy model taking into account a transverse V gradient, a longitudinal V gradient, and a longitudinal oscillation mode caused by fragmentation. Examination of synthetic data shows that the oscillation component produces an oscillation pattern in the velocity structure function (VSF) of the model. The H13CO+ VSF shows an oscillation pattern, suggesting that our observations are partly tracing core-forming motions and fragmentation. We also found that the mean Mcore corresponds to the effective MBE in the filament. This is consistent with a scenario in which higher-mass cores form in higher line-mass filaments.