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mkspiral - uniform-density test disk plus spiral in spherical potential


mkspiral out=snapshot [parameters=values ...]


mkspiral sets up a cold disk of test particles orbiting clockwise in the gravitational potential of a user-supplied potential (in potential(5NEMO) format). In addition a spiral is added with constant width in degrees along a galactic circle. For given ratio of spiral/disk perturbation a and width w (now measured in radians) the fraction of particles in the disk will be about pi / (pi + a.w.sqrt(2.pi)) for not too large w. The inverse wavelenght of the spiral (related to the pitch angle) will be positive for a trailing spiral pattern.

snapmass(1NEMO) and snapcenter(1NEMO) have to be used if for some reason the user want finite mass particles and a perfectly centered set of particles.

Note this particle distribution is only useful for display, e.g. spiral structure in a Longitude-Velocity diagram, and should not be integrated with programs like hackcode3(1NEMO) .


The following parameters are recognized in any order if the keyword is also given:
The data are written to this file in SnapShot(5NEMO) format [no default].
Number of disk particles [default: 2048]
name of the potential, see $NEMO/obj/potential for the current repository. The user can supply his own, see potential(5NEMO) . [default: plummer].
Paramaters to the user supplied potential. The number of parameters depends on the potential supplied, the first parameter is reserved for a pattern speed in future implementation. [default: not supplied, parameters as defined by potential(5) ].
Data_file(s) to the user supplied potential. The number of files depends on the potential supplied. [default: not supplied, datafile(s) defined by potential(5) ].
Inner cutoff radius of test-particle disk. [Default: 0.0].
Outer cutoff radius of test-particle disk. [Default: 1.0].
Ratio of maximum spiral perturbation w.r.t. base disk. [default: 1].
Wavenumber of the spiral pattern, such that 2.pi.k.r is the phase of the spiral density maximum. Positive means trailing spiral pattern. (note that this is defined in term of sign=, see below) [default: 1].
Width of the spiral arm along a galactic circle. Depending on the next keyword this width is either is degrees along a galactic circle if that is to be constant, or in length units if the width of the spiral is to be constant. [default: 15].
Is the width of the spiral defined in angular terms, in which case the physical real width linearly increases with distance from the galactic center, or is is constant as a function of radius. [default: t, i.e. defined in angular terms].
Use random number seed. A zero seed will generate a system-time dependant seed. [default: 0].
Sign of the angular moment vector. Positive means counter-clock wise rotation. Not
Velocity dispersion (isotropic) in the X-Y plane, plus optionally an offset and exponential dropoff: s = s1 + s2 * exp(-r/s3). Default: 0
The ratio of the tangential to radial velocity dispersion. By default 1, but if you need realistic values, our local solar neighbor measures about 0.5 but the best self-consistent (despite these models are NOT) estimate can be set by using the value -1, in which case 0.5*kappa/omega is used. Default: 1
Number of output models. [Default: 1].
Should the spiral be linear or logarithmic. Although spiral galaxies are mostly logarithmic, for historic reasons the default here is linear. [Default: t]
Text headline for output file [default: not used].


This program has been used as benchmark to test writing >2GB files (40 models of 1,000,000 particles creates a file just over this magic limit):
    % time mkspiral s000 1000000 nmodel=40
Here is a sample on some 2002-class machines:
  U-sparc IIi/440 MHz:  161.30u 42.59s 5:04.85 66.8%  
  PentiumIV 2.2 GHz:    29.54u 10.68s 2:40.14 25.1% 
  PentiumIV 2.4 GHz:    26.46u 10.14s 1:37.71 37.4%  
  G5 2.0GHz:            28.400u 24.660s 1:05.41 81.1% (at no load of 0.0)
                        33.460u 44.950s 13:01.31 10.0%  (at full load of 2.0)
  P4/1.6 GHz:           69.940u 18.470s 4:50.64 30.4% (laptop)
  P4/2.6 GHz + SATA     27.400u 16.320s 0:52.13 83.8%   0+0k 0+0io 300pf+0w
(at no load of 0.0)
                        27.790u 20.020s 1:52.22 42.6%   0+0k 0+0io 300pf+0w
sum: 79.35u  3.72s 2:17.64 60.3%   P2400
tsf:  0.00u  7.59s 1:59.46  6.3%   P2400
sum: 13.610u 4.130s 1:10.12 25.2%  P2600/SATA
tsf:  0.010u 7.800s 1:09.74 11.1%  P2600/SATA
sum: 13.334u 2.915s 0:57.67 28.1%  P2800/SATA (Peter’s asus )
tsf:  0.003u 4.696s 0:44.36 10.5%  P2800/SATA
sum: 68.63u 22.34s 2:31.54 60.0%  U440
tsf: 21.85u 30.73s 2:32.06 34.5%  U440

See Also

mkdisk(1NEMO) , snapmass(1NEMO) , snapcenter(1NEMO) , hackcode3(1NEM0)

Update History

13-mar-89    V1.0  created                       PJT
6-apr-89    V1.1 seed=0 implemented              PJT
7-apr-89    V1.2 angular keyword introduced      PJT
26-feb-93    V1.4 new potential keywrds    PJT
16-sep-95    V1.5 added nmodel=, sign= ; radii now random    PJT
26-may-02    added benchmark data for >2GB files    PJT
3-aug-09    V1.8 and documented forgotten stuff        PJT
11-aug-09    V1.9 added sigmator=    PJT

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