L1544: A starless dense core with extended inward motions

We present a multiline study of the dense core L1544 in the Taurus molecula r complex. Although L1544 does not harbor an embedded star, it presents sev eral characteristics of cores that have already undergone star formation, s uggesting that it may be rather advanced in its evolution toward becoming a star-forming core. The spectral lines from L1544 present an interesting di chotomy, with the thick dense gas tracers suffering very strong self absorp tion while CO and its isotopes are not being absorbed at all. The presence of the self absorptions allows us to study both the density structure and k inematics of the gas in detail. A simple analysis shows that the core is al most isothermal and that the self absorptions are due to very subthermal ex citation of the dense gas tracers in the outer layers. The density has to d ecrease outward rapidly, and a detailed radiative transfer calculation that simultaneously fits three isotopes of CO and two of CS shows that the dens ity approximately follows a r(-1.5) power law. The self absorptions, in add ition, allow us to measure the relative velocity between the inner and oute r layers of the core, and we find that there is a global pattern of inward motions (background and foreground approaching each other). The relative sp eed between the foreground and background changes with position, and we use a simple two-layer model to deduce that while the foreground gas has a con stant velocity, the background material presents systematic velocity change s that we interpret as arising from two velocity components. We explore the origin of the inward motions by comparing our observations with models of gravitational collapse. A model in which the infall starts at the center an d propagates outward las in the inside-out collapse of Shu) is inconsistent with the large extension of the absorption (that suggests an advanced age) and the lack of a star at the core center (that suggests extreme youth). A mbipolar diffusion seems also ruled out because of the large amount of the inward speed (up to 0.1 km s(-1)) and the fact that ionized species move wi th speeds similar to those of the neutrals. Other infall models seem also t o have problems fitting the data, so if L1544 is infalling, it seems to be doing so in a manner not contemplated by the standard theories of star form ation. Our study of L1544 illustrates how little is still known about the p hysical conditions that precede star formation and how detailed studies of starless cores are urgently needed.