# Do cores with sharp transitions to coherence correspond to velocity-coherent fibers?---the case of Perseus B5

Abstract

Hacar et al. (2013) found “velocity-coherent fibers” in Taurus, by applying Friends-of-Friends (FoF) algorithm on the fitted peaks of FCRAO C$$^{18}$$O (1-0) line emission. Ever since, the velocity-coherent fibers are more than often related to the coherent cores defined by a sharp transition from supersonic velocity dispersion outside the core to subsonic velocity dispersion inside (Goodman et al., 1998). For example, the elongated coherent core found in Perseus B5 (Pineda et al., 2010) is often treated as a velocity-coherent fiber. Here we examine the C$$^{18}$$O (2-1) line emission in Perseus B5 from IRAM 30m telescope, and compare to the GBT NH$$_3$$ emission from Pineda et al. (2010). We find that, even though the coherent core defined by the sharp transition in velocity dispersion overlaps in the position-position-velocity (PPV) space with a velocity-coherent fiber found by applying the FoF algorithm on the C$$^{18}$$O (2-1) emission, the two structures do not match in the PPV space and have different kinematics. The result indicates that the coherent core does not always correspond to the velocity-coherent fiber, and that the structures defined by the sharp transition in velocity dispersion (coherent cores) and those defined by connection of velocity centroids in the PPV space (velocity-coherent fiber) are not the same.