Filamentary Structure and Helical Magnetic Fields in the Environment of a Starless Dense Core

Abstract

The environment of L1512, a starless dense core, has been mapped at high angular resolution in the 12CO (J \ 2È1) line over more than 1 pc, with a few positions observed in the 12CO (J \ 3È2) and (J \ 4È3) lines. The gas outside the dense core is structured in several Ðlaments, roughly 1 pc long and D0.1 pc thick, converging at the dense core position. Small longitudinal (D1 km s~1 pc~1) but large transverse (up to 8 km s~1 pc~1) velocity gradients are observed. Remarkably, the transverse gradients can be seen to change sign periodically, along at least one of the Ðlaments. Thus, there are oscillations in the toroidal velocity within the Ðlaments, which may be a signature of a magnetohydrodynamic instability developing in Ðlaments permeated by a helical magnetic Ðeld. In the case of L1512, according to the analysis of Fiege & Pudritz, the growth rate of the instability is low, corresponding to a timescale of the order of 1 Myr. We deduce from the wavelength of the oscillations that the toroidal component of the magnetic Ðeld dominates the poloidal component. The toroidal component helps conÐne the Ðla-ments, which are not otherwise conÐned by self-gravity by the pressure of the galactic H I (m/m vir D 0.2), layer, or by external turbulent pressure. We Ðnd that the velocity gradients in the vicinity of the dense core provide an estimate for an upper limit to the accretion rate onto the dense core of M 0 \ 4 ] 10~6 yr~1. For the gas characteristics in the Ðlaments, we Ðnd that a broad range of density and tem-M _ perature is allowed for the gas, from cm~3 for the coldest case K) down to n H2 \ 2 ] 103 (T k \ 20 n H2 \ 180 cm~3 for the warmest K). (T k \ 250