Time-dependent accretion and ejection implied by pre-stellar density profiles.

A recent homogeneous study of outflow activity in low-mass embedded young stellar objects (YSOs) (Bontemps et al. 1996) suggests that mass ejection and mass accretion both decline signicantly with time during protostellar evolution. In the present paper, we propose that this rapid decay of accre- tion/ejection activity is a direct result of the non-singular den- sity proles characterizing pre-collapse clouds. Submillimeter dustcontinuummappingindicatesthattheradialprolesofpre- stellar cores flatten out near their centers, being much flatter than (r) / r 2 at radii less than a few thousand AU (Ward- Thompson et al. 1994). In some cases, sharp edges are observed at a nite core radius. Here we show, through Lagrangian ana- lytical calculations, that the supersonic gravitational collapse of pre-stellar cloud cores with such centrally peaked, but flattened densityprolesleadstoatransitoryphaseofenergeticaccretion immediately following the formation of the central hydrostatic protostar. Physically, the collapse occurs in various stages. The rst stage corresponds to the nearly isothermal, dynamical col- lapse of the pre-stellar flat inner region, which ends with the formation of a nite-mass stellar nucleus. This phase is essen- tiallynon-existentinthe'standard'singularmodeldevelopedby Shuandco-workers.Inasecondstage,theremainingcloudcore materialaccretessupersonicallyontoanon-zeropointmass.Be- cause of the signicant infall velocity eld achieved during the rst collapse stage, the accretion rate is initially higher than in the Shu model. This enhanced accretion persists as long as the gravitational pull of the initial point mass remains signicant. Theaccretionratethenquicklyconvergestowardsthecharacter- istic value a 3 =G (where a is the sound speed), which is also the constant rate found by Shu (1977). If the model pre-stellar core has a nite outer boundary, there is a terminal decline of theaccretionrateatlatetimesduetothenitereservoirofmass. We suggest that the initial epoch of vigorous accretion pre- dicted by our non-singular model coincides with Class 0 proto- stars, which would explain their unusually powerful jets com- Send offprint requests to: P. Andre ? (andre@sapvxg.saclay.cea.fr) pared to the more evolved Class I YSOs. We use a simple two-component power-law model to t the diagrams of out- flow power versus envelope mass observed by Bontemps et al. (1996), and suggest that Taurus and Ophiuchi YSOs follow different accretion histories because of differing initial condi- tions. While the isolated Class I sources of Taurus are relatively well explained by the standard Shu model, most of the Class I objects of the Oph cluster may be effectively in their terminal accretion phase.