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mkgalorbit - Make a galactic orbit with from given initial conditions


mkgalorbit out=orbit-file [parameter=value]


Unlike mkorbit(1NEMO) where an arbitrary orbit can be initialized, mkgalorbit(1NEMO) is specifically intended to initialize orbits in a galactic potential w.r.t. an assumed solar motion (LSR). Input coordinates are galactic.

The sun is assumed to be along the negative X axis, i.e. at (-R0,0,0) and rotate clock wise, i.e. (0,V0,0). See comments below on this COORDINATE SYSTEM.


The following parameters are recognized in any order if the keyword is also given:
output filename (an orbit) [???]
Longitude (in degrees) []
Latitude (in degrees) []
Distance (in kpc). []
Proper Motion in Longitude (mas/yr) []
Proper Motion in Latitude (mas/yr) []
Radial velocity (in km/s) []
LSR distance (in kpc) [8.0]
LSR clockwise velocity (in km/s) [220.0]
Use LSR correction [f]
U,V,W space velocities of the SUN w.r.t. LSR in a right-handed coordinate system. [Default: 10.00,5.25,7.17]
lon/lat coordinate system: equ=RA/DEC gal=GLON/GLAT [gal]
time [0.0]
optional potential(5NEMO) []
random verbiage []


Here’s an orbit representing the LSR
   % mkgalorbit orb1    0 0 0   0 0 0
   pos: -8.000000 0.000000 0.000000
   vel: 0.000000 220.000000 0.000000
and solar motion
   % mkgalorbit orb2    0 0 0   0 0 0   lsr=t
   pos: -8.000000 0.000000 0.000000
   vel: 9.000000 232.000000 7.000000 
and the benchmark case, the example from the gal_uvw IDL script:
   HD 6755 from the Hipparcos catalog:
   RA=1h 9m 43.3s  DEC=61d 32m 49.5s  Distance=144 pc  mu_ra=627.89 mas/yr
 mu_dec=77.84 mas/yr  VRAD=-321.4 km/s
   % mkgalorbit . "(1+(9+42.3/60)/60)*15" "61+(32+49.5/60)/60" 0.144 627.89
77.84 -321.4 coordsys=equ lsr=t V0=0
   pos: -8.082794 0.117776 -0.003135
   vel: -153.932343 -493.094568 97.359254
    (V0=0 was set to get the local UVW’s, to get the numbers from IDL below)
   % idl
   IDL> .run gal_uvw
   IDL> ra = ten(1,9,42.3)*15.    & dec = ten(61,32,49.5)
   IDL> pmra = 627.89  &  pmdec = 77.84 
   IDL> dis = 144    &  vrad = -321.4
   IDL> gal_uvw,u,v,w,ra=ra,dec=dec,pmra=pmra,pmdec=pmdec,vrad=vrad,dis=dis,/lsr
   IDL> print,-u,v,w
         -153.932     -493.095      97.3592
To use a particular galaxy model in an external potential, we first need to compute the phase space coordinates of the LSR in that model. snapgalview(1NEMO) can help viewing a snapshot(5NEMO) .

Coordinate Systems

In galactic astronomy you will find papers that use a right handed or a left handed coordinate system for the UVW space velocities. It seems more natural to use the more common right-handed coordinate system (e.g. as argued by Johnson & Soderblom 1987), which we also adopt internally to the program (also UVW match our defintion of vx,vy-V0,vz). This is the system where U is positive for inwards radial motions, and V points along the galactic rotation and W points towards the galactic north pole (and this oppostive that of the angular moment vector, which is mathematically negative). In order for (u,v,w) to be compatible with the (vx,vy,vz) we use in NEMO, the sun must be placed at (-R,0,0) and since the galaxy is rotating clock wise, with a velocity of (0,V,0). If you want something else, you can always use snaprotate(1NEMO)

Solar Motion

Several systems of the peculiar solar motion have been practiced.

20 km/s towards of ra,dec=18h,30d (18:03:50.27,30:00:16.8 in J2000), is the common convention since 1960, and is referred to as the kinematic LSR.

19.5 km/s towards 18:02:36,+29:56:41 (J2000), or l,b=56d,23d

15.4 km/s towards 17:55:36,+25:36:38 (J2000), or l,b=51d,23d

16.5 km/s towards 17:49:53,+28:00:02 (J2000), or l,b=53d,25d (canonical value)

18.0 km/s (11.1 +/- 0.74, 12.24 +/- 0.47, 7.25 +/-0.37) - Shoenrich, Binney & Dehnen - 2010

Kinematic: 20.0 km/s towards B1900 RA/Dec direction of (270,30) degrees.
Dynamical: (9,12,7) km/s in galactic coordinates, or 16.552945 towards l,b
= 53.13, +25.02 deg.

CASA supported coordinate systems:

REST -- Rest frequency
LSRD -- Local Standard of Rest (J2000) -- as the dynamical definition (IAU,
[9,12,7] km/s in galactic coordinates)
LSRK -- LSR as a kinematical (radio) definition -- 20.0 km/s in direction ra,dec
= [270,+30] deg (B1900.0)
BARY -- Barycentric (J2000)
GEO --- Geocentric
TOPO -- Topocentric
GALACTO -- Galacto centric (with rotation of 220 km/s in direction l,b =
[90,0] deg.
LGROUP -- Local group velocity -- 308km/s towards l,b = [105,-7] deg (F. Ghigo)
CMB -- CMB velocity -- 369.5km/s towards l,b = [264.4, 48.4] deg (F. Ghigo)

See Also

mkorbit(1NEMO) , potcode(1NEMO) , snapgalview(1NEMO) , snaprotate(1NEMO) , potential(5NEMO) - used the opposite
U definition
Johnson & Soderblom (1987, AJ, 93,864)
Mignard [2002, A&A, 354, 522]
Olling & Dehnen [2003ApJ...599..275]
Hipparcos catalog - introduction contains the J2000 transformation matrix
(ra,dec) -> (glon,glat)  (Reid et al.)
Kerr & Lynden-Bell 1986 MNRAS 221, 1023.
Reid et al. 2009 -
And an online example from NEMO’s webpage:


Peter Teuben

Update History

18-Apr-05    V0.1 Created    PJT
20-apr-05    V0.6 got the math right     PJT
3-jun-05    V0.7a   uses modern Hipparchos for solar motion     PJT

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