We do not find convincing evidence for rotation in our data. We also identify several problems with the pure infall interpretation proposed by Hayashi et al. (1993). The required central mass would be at least for , which seems uncomfortably large. In addition, the detailed velocity structure and the asymmetry between blueshifted and redshifted gas are not well explained. Hence, infall alone cannot reproduce all of our observations. We derive an upper limit to the free-fall rate toward HL Tau of yr.
The problems encountered by an infall model, together with the known presence of a molecular outflow from HL Tau, lead us to propose that kinematics in the remnant envelope around HL Tau are heavily affected by entrained outflow motions. The distinctive blue/red asymmetric structure in our PdBI maps (blueshifted emission is weaker and mostly confined in the system midplane, while redshifted emission is stronger and more closely follows the jet axis) is then naturally accounted for, as the same asymmetry is observed in the large-scale bipolar outflow from HL Tau.
The action of jet bow shocks, or the steady-state entrainment of circumstellar gas, seems able to explain the transverse extent of the perturbed gas and its overall kinematics. The net molecular mass outflow rate is large ( M yr ), far exceeding the present disk accretion rate, and at least comparable to the envelope infall rate. Envelope clearing by jet entrainment could then be an important process regulating the inner disk accretion rate in HL Tau, as well as the transition to the fully optically revealed T Tauri stage.
If our estimates of outflow and infall rates are correct, the modest remnant
mass inferred from our IRAM 30-m observations
indicates that HL Tau has already accumulated most of its final stellar
mass, and that it is in a relatively short-lived (
yr) phase of energetic mass ejection, leading to envelope dispersal.