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mkop73 - set up a Mestel disk using 1973 Ostriker & Peebles’ method
mkop73 sets up a Mestel disk,
with an optional static background potential, in close virial equilibrium.
The procedure how exactly the initial velocities are computed is involved,
and details can be found in the original Ostriker & Peebles 1973 paper or
as detailed in the code. Although the disk is flat, there will be an initial
velocity dispersion in Z, which will quickly evolve into a finite thickness
These disks are known to be dynamically unstable against bar formation
in the absence of a sizeable halo, and are fun to play with.
defaults are set such as to as closely as possible reproduce (apart from
the random number generation) the ‘‘standard’’ model 1 in Ostriker & Peebles.
Some parameters below are related; so be aware that changing one parameter
in the OP73 notation, may require you changing two in this program, for
example scaling the masses: nbody, mdisk and potpars(2)
are related, and
scaling the lengths: rcut, c, and potpars(3)
following parameters are recognized in any order if the keyword is also
Some parameters are related, and the program may not always deal
well with cases where they deviate from OP73. For example, changing the
disk-radius (rcut) has dubious side-effects on a variety of length-scale
parameters (c, potpars(3)
- output file name, will be in snapshot(5NEMO)
format. No default.
- Number of particles in the disk [Default: 300].
disk mass. Normally each particle has unity mass, hence mdisk should be
the same as nbody [Default: 300.0].
- outer cutoff radius of the disk
(the halo cutoff radius is determined with the appropriate paramter in
the potpars= series of parameters. [Default: 1].
scaling parameter sigma for velocity dispersion in Toomre’s Q parameter
(see eq. (7) and (8)
in OP73). Toomre’s Q=1 corresponds to sigma=0.378. [Default:
- number of particles per ring; should divide nbody. Each ring
will be divided into nring segments, and one particle will be placed randomly
within that segment. There will be nbody/nring rings. [Default: 10].
- number of measurements used to determine the circular velocity. Note that
OP73 do not quote the exact value of this parameter. Although particles
are randomly rotated to compute the average circular velocity, the first
measurement is not, as to be able to compute the potential and forces in
case options= would want to output them. [Default: 10].
- number of
rings (zones) used to determine dv/dr (used to calculate the velocity dispersion).
Should probably be an integer fraction of nbody/nring. [Default: 10].
- usual random number seed [Default: 0].
- softening for force calculation,
called c in OP73. [Default: 0.05].
- Center the snapshot? Note this
option was not mentioned in OP73. [Default: t].
- Creation mode: 1=OP73
method. 2=theoretical Mestel disk rotation curve. [Default: 1]. mode=2 not
- Options for additional snapshot output (phi,acc,rot,sig).
phi and acc output potential and forces in their usual Phi and Acc slots,
whereas sig stores the three velocity dispersions (radial, tangential and
vertical) in the Acc slots, and rot stores the pre-dispersion rotational
velocities in the Aux slot (see snapshot(5NEMO)
for an explanation on these
slots). Default: no extra output.
- External static potential, in
format. [Default: op73, i.e. the standard OP73 potential].
- Potential parameters. Default: 0,0, i.e. with 0 mass
(the first 0 is the pattern speed of the potential, and is not used in
this application. The standard order of the OP73 potential parameters are:
omega [0.0], mass [1.0], core radius [0.1] and outer cutoff radius [1.0].
In general the default value depends on potname=.
- Optional datafile
name for potential. OP73 doesn’t use this parameter. In general the default
value depends on potname=.
- Additional text for output file. Default:
The original OP73 paper is vague or unclear
in a few places in the exact procedures implemented in the program. See
source code for more comments.
~src/orbit/potential/data/op73.c The ’OP73’
code ~src/nbody/init/mkop73.c Nbody initializers
J.P. Ostriker & P.J.E. Peebles
- ApJ 186, 467-480, 1973. A numerical study of the stability of flattened
galaxies: or, can cold galaxies survive? (OP73)
Jeffrey Newman, Peter
xx-jul-93 V0.4 Created as MKOP JAN
3-nov-93 V0.9 Fixed bugs, added more features and CTEX notes PJT
4-nov-93 V1.0 formal realase as MKOP73 PJT
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