Rotation of the pre-stellar core L1689B

Abstract

The search for the onset of star formation in pre-stellar cores has focused on the identification of an infall signature in the molecular line profiles of tracer species. The classic infall signature is a double-peaked line profile with an asymmetry in the strength of the peaks such that the blue peak is stronger. L1689B is a pre-stellar core and infall candidate, but new James Clerk Maxwell Telescope (JCMT) HCO+ line profile data, presented here, confirm that both blue and red asymmetric line profiles are present in this source. Moreover, a dividing line can be drawn between the locations where each type of profile is found. It is argued that it is unlikely that the line profiles can be interpreted with simple models of infall or outflow, and that rotation of the inner regions is the most likely explanation. A rotational model is developed in detail with a new three-dimensional molecular line transport code, and it is found that the best type of model is one in which the rotational velocity profile is in between solid-body and Keplerian. It is first shown that red and blue asymmetric line profiles can be generated with a rotation model entirely in the absence of any infall motion. The model is then quantitively compared with the JCMT data, and an iteration over a range of parameters is performed to minimize the difference between the data and the model. The results indicate that rotation can dominate the line profile shape even before the onset of infall.