Galactic space velocity (UVW)

PyAstronomy.pyasl.gal_uvw(ra, dec, pmra, pmdec, distance, vrad, lsr=None)

Calculate the Galactic space velocity (U,V,W)

Note

In contrast to the IDL implementation, U is taken positive toward the galactic center.

Parameters
ra, decfloat or array

Position in degrees

pmra, pmdecfloat or array

Proper motion in mas/yr

distancefloat or array

Distance in pc

vradfloat or array

Radial velocity in km/s

lsrthree-float tuple, optional

If given, the output UVW velocities are corrected for the solar motion (U,V,W)_Sun to the local standard of rest (LSR). Use, e.g., lsr = (8.5, 13.38, 6.49) from Coșkunoǧlu et al. 2011, MNRAS 412, 1237-1245. Be aware that the value of the solar motion through the LSR remains poorly determined.

Returns
U, V, Wfloat or arrays

U: Velocity (km/s) positive toward the Galactic center V: Velocity (km/s) positive in the direction of Galactic rotation W: Velocity (km/s) positive toward the North Galactic Pole

Notes

Note

This function was ported from the IDL Astronomy User’s Library.

IDL - Documentation

NAME:

GAL_UVW

PURPOSE:

Calculate the Galactic space velocity (U,V,W) of star

EXPLANATION:

Calculates the Galactic space velocity U, V, W of star given its (1) coordinates, (2) proper motion, (3) distance (or parallax), and (4) radial velocity.

CALLING SEQUENCE:
GAL_UVW [/LSR, RA=, DEC=, PMRA= ,PMDEC=, VRAD= , DISTANCE=

PLX= ]

OUTPUT PARAMETERS:

U - Velocity (km/s) positive toward the Galactic *anti*center V - Velocity (km/s) positive in the direction of Galactic rotation W - Velocity (km/s) positive toward the North Galactic Pole

REQUIRED INPUT KEYWORDS:

User must supply a position, proper motion,radial velocity and distance (or parallax). Either scalars or vectors can be supplied.

  1. Position: RA - Right Ascension in Degrees Dec - Declination in Degrees

  2. Proper Motion PMRA = Proper motion in RA in arc units (typically milli-arcseconds/yr) PMDEC = Proper motion in Declination (typically mas/yr)

  3. Radial Velocity VRAD = radial velocity in km/s

  4. Distance or Parallax DISTANCE - distance in parsecs

    or

    PLX - parallax with same distance units as proper motion measurements

    typically milliarcseconds (mas)

OPTIONAL INPUT KEYWORD:
/LSR - If this keyword is set, then the output velocities will be

corrected for the solar motion (U,V,W)_Sun = (-8.5, 13.38, 6.49) (Coskunoglu et al. 2011 MNRAS) to the local standard of rest. Note that the value of the solar motion through the LSR remains poorly determined.

EXAMPLE:

  1. Compute the U,V,W coordinates for the halo star HD 6755. Use values from Hipparcos catalog, and correct to the LSR

ra = ten(1,9,42.3)*15. & dec = ten(61,32,49.5) pmra = 627.89 & pmdec = 77.84 ;mas/yr dis = 144 & vrad = -321.4 gal_uvw,u,v,w,ra=ra,dec=dec,pmra=pmra,pmdec=pmdec,vrad=vrad,dis=dis,/lsr

===> u=154 v = -493 w = 97 ;km/s

(2) Use the Hipparcos Input and Output Catalog IDL databases (see http://idlastro.gsfc.nasa.gov/ftp/zdbase/) to obtain space velocities for all stars within 10 pc with radial velocities > 10 km/s

dbopen,’hipparcos,hic’ ;Need Hipparcos output and input catalogs list = dbfind(‘plx>100,vrad>10’) ;Plx > 100 mas, Vrad > 10 km/s dbext,list,’pmra,pmdec,vrad,ra,dec,plx’,pmra,pmdec,vrad,ra,dec,plx ra = ra*15. ;Need right ascension in degrees GAL_UVW,u,v,w,ra=ra,dec=dec,pmra=pmra,pmdec=pmdec,vrad=vrad,plx = plx forprint,u,v,w ;Display results

METHOD:

Follows the general outline of Johnson & Soderblom (1987, AJ, 93,864) except that U is positive outward toward the Galactic *anti*center, and the J2000 transformation matrix to Galactic coordinates is taken from the introduction to the Hipparcos catalog.

REVISION HISTORY:

Written, W. Landsman December 2000 fix the bug occuring if the input arrays are longer than 32767 and update the Sun velocity Sergey Koposov June 2008 vectorization of the loop – performance on large arrays is now 10 times higher Sergey Koposov December 2008

Example

from __future__ import print_function
from PyAstronomy import pyasl

ra, dec = 017.4294147291650, +61.5473037149441
pmra, pmdec = 627.89, 77.84
d = 144
rv = -321.4

print("U, V, W [kms/s]: % 5.3f, % 5.3f, % 5.3f " % \
    pyasl.gal_uvw(ra, dec, pmra, pmdec, d, rv))
print("U, V, W (LSR corrected) [kms/s]:  % 5.3f, % 5.3f, % 5.3f" % \
    pyasl.gal_uvw(ra, dec, pmra, pmdec, d, rv, lsr=(8.5, 13.38, 6.49) ) )