More on SPECX velocity scales - January 1993

Following publication of the last issue of the Newsletter, Pat Wallace (author of the SLALIB package) wrote to point out that my use of the phrase ``the LSR'' may have been confusing. The ``local standard of rest'' is strictly a defined frame, but is meant approximately to represent the basic solar motion with respect to the neighbouring stars. This motion is established by measuring the radial velocities of the stars in the solar neighbourhood. The number you get depends on the depth of the sample (how far out you go), and the spectral classes of the stars you use. Thus there are several determinations of the kinematic solar motion, which differ by a few kilometres per second, and by a few degrees in direction. Spectral line radio astronomers however traditionally use the defined ``LSR'', which has a velocity of 20km/s in the direction of 18h,+30d(B1900), and differs slightly from the most modern determinations of the solar motion.

Pat has recently altered the SLALIB routine SLA_RVLSR to provide a better estimate of the dynamical solar motion -- i.e., the motion of the Sun with respect to the appropriate circular orbit around the galactic centre. This also makes SLA_RVLSR consistent with the routine SLA_RVGALC, but unfortunately there is now a velocity difference of up to $\pm 3$km/s between the velocities calculated by SLA_RVLSR and those used by radio astronomers (and SPECX). I propose not to implement this new definition in SPECX, as I suspect it will only cause confusion if Orion starts to come out with "an lsr velocity" of 6km/s. Pat Wallace's program RV, which is included in the JCMT utilities, is based on the SLALIB routines, but the older version is being retained to prevent undue confusion arising as a result of the change in the SLALIB routines.

One further note: SPECX and the JCMT control system have both in fact been using a velocity of 20km/s towards an apex of 18h,+30d(B1950), as used in the original Bonn software. For consistency I will change the epoch used in SPECX to B1900 in the next release, but the difference in velocities will be very small («0.1km/s), so shouldn't be observable for AOSC data. The apex used in the telescope control software will also be changed to B1900 as soon as practicable.

My thanks to Pat Wallace, Chris Mayer and Per Friberg for helping me to sort out what was actually going on here.