In our SCUBA survey of Perseus, we find that the fraction of protostellar
cores increases towards higher masses and the most massive cores are all
protostellar. In this paper we consider the possible explanations of this
apparent mass dependence in the evolutionary status of these cores, and the
implications for protostellar evolution and the mapping of the embedded core
mass function (CMF) onto the stellar IMF. We consider the following potential
causes: dust temperature; selection effects in the submillimetre and in the
mid-infrared observations used for pre/protostellar classification; confusion
and multiplicity; transient cores; and varying evolutionary timescales. We
develop Core Mass Evolution Diagrams (CMEDs) to investigate how the mass
evolution of individual cores maps onto the observed CMF. Two physical
mechanisms -- short timescales for the evolution of massive cores, and
continuing accumulation of mass onto protostellar cores -- best explain the
relative excess of protostars in high mass cores and the rarity of massive
starless cores. In addition, confusion both increases the likelihood that a
protostar is identified within a core, and increases mass assigned to a core.
The observed pre/protostellar mass distributions are consistent with faster
evolution and a shorter lifetime for higher-mass prestellar cores. We rule out
longer timescales for higher-mass prestellar cores. The differences in the
prestellar and protostellar mass distributions imply that the prestellar CMF
(and possibly the combined pre+protostellar CMF) should be steeper than the
IMF. A steeper prestellar CMF can be reconciled with the observed similarity of
the CMF and the IMF in some regions if a second opposing effect is present,
such as the fragmentation of massive cores into multiple systems.Comment: 11 pages, 5 figures. Accepted by A&
