Molecular line study of the very young protostar IRAM 04191 in Taurus:: infall, rotation, and outflow

We present a detailed millimeter spectroscopic study of the circumstellar environment of the low-luminosity Class 0 protostar IRAM 04191 + 1522 in the Taurus molecular cloud. Molecular line observations with the IRAM 30 m telescope demonstrate that the similar to14 000 AU radius protostellar envelope is undergoing both extended infall and fast, differential rotation. Radiative transfer modeling of multitransition CS and (CS)-S-34 maps indicate an infall velocity v(inf) similar to0.15 km s(-1) at r similar to 1500 AU and v(inf) similar to0.1 km s(-1) up to r similar to11 000 AU, as well as a rotational angular velocity Omega similar to 3.9 x 10(-13) rad s(-1), strongly decreasing with radius beyond 3500 AU down to a value Omega similar to 1.5-3 x 10(14) rad s(-1) at similar to11 000 AU. Two distinct regions, which differ in both their infall and their rotation properties, therefore seem to stand out: the inner part of the envelope (r less than or similar to 2000-4000 AU) is rapidly collapsing and rotating, while the outer part undergoes only moderate infall / contraction and slower rotation. These contrasted features suggest that angular momentum is conserved in the collapsing inner region but efficiently dissipated due to magnetic braking in the slowly contracting outer region. We propose that the inner envelope is in the process of decoupling from the ambient cloud and corresponds to the effective mass reservoir (similar to0.5 M-.) from which the central star is being built. Comparison with the rotational properties of other objects in Taurus suggests that IRAM 04191 is at a pivotal stage between a prestellar regime of constant angular velocity enforced by magnetic braking and a dynamical, protostellar regime of nearly conserved angular momentum. The rotation velocity profile we derive for the inner IRAM 04191 envelope should thus set some constraints on the distribution of angular momentum on the scale of the outer Solar system at the onset of protostar disk formation.