Molecular Ions in L1544. I. Kinematics

We have mapped the dense dark core L1544 in H13CO+(1-0), DCO+(2-1),DCO+(3-2), N2H+(1-0), NTH+(3-2), N2D+(2-1), N2D+(3-2), C18O(1-0), and C17O(1-0)using the IRAM 30-m telescope. We have obtained supplementary observations ofHC18O+(1-0), HC17O+(1-0), and D13CO+(2-1). Many of the observed maps show ageneral correlation with the distribution of dust continuum emission incontrast to C18O(1-0) and C17O(1-0) which give clear evidence for depletion ofCO at positions close to the continuum peak. In particular N2D+(2-1) and (3-2)and to a lesser extent N2H+(1-0) appear to be excellent tracers of the dustcontinuum. We find that the tracers of high density gas (in particular N2D+)show a velocity gradient along the minor axis of the L1544 core and that thereis evidence for larger linewidths close to the dust emission peak. We interpretthis using the model of the L1544 proposed by Ciolek & Basu (2000) and bycomparing the observed velocities with those expected on the basis of theirmodel. The results show reasonable agreement between observations and model inthat the velocity gradient along the minor axis and the line broadening towardthe center of L1544 are predicted by the model. This is evidence in favour ofthe idea that amipolar diffusion across field lines is one of the basicprocesses leading to gravitational collapse. However, line widths aresignificantly narrower than observed and are better reproduced by the Myers &Zweibel (2001) model which considers the quasistatic vertical contraction of alayer due to dissipation of its Alfvenic turbulence, indicating the importanceof this process for cores in the verge of forming a star.Comment: 24 pages, 9 figures, to be published in Ap