Consistency of Ambipolar Diffusion Models with Infall in the L1544 Protostellar Core

Abstract

Recent high-resolution studies of the L1544 protostellar core by Tafalla et al. and Williams et al. reveal the structure and the kinematics of the gas. The observations of this prestellar core provide a natural test for theoretical models of core formation and evolution. Based on their results, the above authors claim a discrepancy with the implied infall motions from ambipolar di usion models. In this paper, we reexamine the earlier ambipolar di usion models and conclude that the L1544 core can be understood to be a magnetically supercritical core undergoing magnetically diluted collapse. We also present a new ambipolar di usion model speciÐcally designed to simulate the formation and evolution of the L1544 core. This model, which uses reasonable input parameters, yields mass and radial density dis-tributions, as well as neutral and ion infall speed proÐles, that are in very good agreement with physical values deduced by observations. The lifetime of the core is also in good agreement with mean prestellar core lifetimes estimated from statistics of an ensemble of cores. The observational input can act to con-strain other currently unobserved quantities, such as the degree of ionization and the background mag-netic Ðeld strength and orientation near the L1544 core. Subject headings : di usion È ISM : clouds È ISM : individual (L1544) È ISM : kinematics and dynamics È ISM : magnetic Ðelds È MHD È stars : formation