Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Protostellar Collapse Induced by Compression   We present numerical simulations of the evolution of low-mass, isothermal,
molecular cores which are subjected to an increase in external pressure $P\xt$.
If $P\xt$ increases very slowly, the core approaches instability quite
quasistatically. However, for larger (but still quite modest) $dP\xt/dt$ a
compression wave is driven into the core, thereby triggering collapse from the
outside in. If collapse of a core is induced by increasing $P\xt$, this has a
number of interesting consequences. (i) The density profile is approximately
flat in the centre during the prestellar phase (i.e. before the compression
wave converges on the centre creating the central protostar). (ii) During the
prestellar phase there are (subsonic) inward velocities in the outer layers of
the core, whilst the inner parts are still approximately at rest. (iii) There
is an initial short phase of rapid accretion (notionally the Class 0 phase),
followed by a longer phase of slower accretion (the Class I phase). All these
features accord well with observation, but are at variance with the predictions
of the standard theory of star formation based on the inside-out collapse of a
singular isothermal sphere. We note that the setting up of a coherent inward
velocity field appears to be a generic feature of compression waves; and we
speculate that interactions and interference between such velocity fields may
play a crucial r\^ole in initiating the fragmentation of cores and the genesis
of multiple star systems.

23
The categories of this paper are: solv-int
5
The categories of this paper are: gr-qc
37
The categories of this paper are: supr-con
20
The categories of this paper are: acc-phys
35
The categories of this paper are: mtrl-th
17
The categories of this paper are: q-fin
27
The categories of this paper are: comp-gas
0
The categories of this paper are: hep-ph
15
The categories of this paper are: nucl-ex
1
The categories of this paper are: cs
30
The categories of this paper are: q-alg
36
The categories of this paper are: cmp-lg
34
The categories of this paper are: chem-ph
19
The categories of this paper are: eess
9
The categories of this paper are: nlin
25
The categories of this paper are: patt-sol