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density_centre - find the global density maximum; center snapshots
density_centre [parameter=value]
requires NEMO version
3.0.13 or higher, see http://www.astro.umd.edu/nemo.
density_centre
finds the position x_c at which
1 |x_c-x_i|
(1) rho_h(x_c) = ----- Sum m_i W( --------- )
h^3 i h
has a global maximum. The kernel W(r) is proportional to (1-r^2)^3
for r<1 and W=0 for r>1 (Ferrers n=3 sphere), while the smoothing
length h is determined such that a fixed number of bodies contribute to
the density estimate.
The routine first builds an oct-tree with final cells containing less
then Ncen/4 bodies and then uses the center of mass of the densest
final cell as the initial guess for x_c, which is iteratively
improved. This process is somewhat slow, but reliable, i.e. really
finds even a weak density peak. (If you think you have a better/faster
way to do it, I would like to test it on some snapshots which my older
versions failed.)
parameters in=file Input snapshots are read from file, which must be in
NEMO snapshot format and contain at least masses and positions. If file=-,
input is expected from stdin, useful for piping data from another program.
Default: required out=file If given, snapshots are first centred on the
position and velocity of the density centre found and then written in NEMO
snapshot format to file with same body data as the input, unless give specifies
otherwise. No default. give=IOstring If given, only body data indicated by
IOstring (and present in the input) are written out with the snapshot output.
No default. step=val If snapshots are written out, the outputs are made
every val simulation time units. If val=0, every snapshot processed is
written out. Default: 0. times=time range(s) Range(s) of simulation times,
for which snapshots shall be processed. Default: all. Ncen=num The smoothing
length h is set such that num bodies contribute to the density estimate
(1). Default: 200 centrefile=file Simulation time, position x_c, velocity
v_c (if body velocities are given), and density rho_h(x_c) of the centre
are written to file for every snapshot processed. If file=-, output is made
to stdout, while if file=. no output is made. Default: - . centerfile=file
If given, this superseeds the file given with centrefile. (to please American
spellers) remarkUseful for isolated stellar systems only. files
falcON/doc/user_guide.pdf User Guide for falcON
Walter Dehnen walter.dehnen@astro.le.ac.uk
update history of this man page
18-May-2004 first version of manual WD
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