THE FORMATION AND EVOLUTION OF LOW-MASS PROTOSTARS

A review is presented of the earliest stages of protostellar evolution . Observations of pre-stellar cores, which are believed to represent t he initial conditions for protostellar collapse, depart significantly from the scale-free density distribution which is usually taken as the starting point for the formation of a low-mass protostar. Pre-stellar cores are observed to have radial density profiles which have hat inn er regions, steepening towards their edges. This is seen to qualitativ ely match the predictions of the Bonnor-Ebert stability criterion for pressure-bounded self-gravitating gas clouds. From these initial condi tions, theoretical modelling of cores threaded by magnetic fields pred icts that quasi-static evolution by the process of ambipolar diffusion will lead to a significantly different starting point for collapse th an the static singular isothermal sphere. This departure from a scale- free density distribution for the initial conditions has recently been shown to produce an ensuing protostellar collapse which has a non-con stant accretion rate. Recently published observations of low-mass prot ostars in the rho Ophiuchi cluster are demonstrated to be consistent w ith such a non-constant protostellar mass accretion rate, contrary to the standard protostellar collapse model. Instead, the data appear con sistent with an initially high accretion rate, which subsequently deca ys. The initial phase of high accretion rate is labelled the 'main acc retion phase', during which greater than or equal to 50 per cent of th e circumstellar envelope mass is accreted in similar to 10 per cent of the total accretion time, and which is equated observationally with C lass 0 objects. The subsequent phase with roughly an order of magnitud e lower accretion rate is labelled the 'late accretion phase', during which the remainder of the envelope mass is accreted in the remaining similar to 90 per cent of the total accretion time, at an order of mag nitude lower accretion rate, and which is equated observationally with Class I objects. The growth of circumstellar discs begins in the Clas s 0 stage, and proceeds through the Class I and II stages. Published d ata of the Taurus star-forming region currently available appear also to be consistent with this scenario.