Space Forum / Astronomy / April 2007

I have been calculating the solar rotation rate from the population of
halo stars. Halo stars are thought to have no net rotation, so by
finding an average V-velocity one finds the orbital speed of the Sun.

I took the metal poor database of Beers and restricted to stars inside
2kpc, to keep errors under control while having a large enough
population for the statistical analysis. To study the halo I restricted
to metalicities [Fe/H] < -1.6. I subdivided the population into three,
Giants, Dwarfs (or main sequence stars) and RRLyrae.

I am interested in systematic radial velocity errors and systematic
distance errors, so I subdivided each population again with a cone of
semi-angle 65deg from the direction, V, of rotation. For each population
I calculate a much higher mean V-velocity for the sun from the stars
within the cone, and I found much higher V-velocities from RR Lyrae than
the other populations. This appears indicative of the systematic errors
I am looking for.

One might try to account for that by supposing that distances are
understated. By increasing distances to dwarves by 30%, to RRLyrae by
20% and to Giants by 50%, I got a reasonable match between the stars
inside and outside the cone, but did not completely eliminate the
difference. These increases gives a mean velocity of about 270-280 from
each population and leads to a distance to SgrA* of 9kpc or more, which
is too large by other measurements.

I then applied the teleconnection correction to radial velocity (about
23%). That brought things under more control, but to achieve the most
consistent result I had to reduce distances to RRLyrae by 5%, increase
distances to dwarfs by 7% and increase distances to giants by 21%. With
these corrections all populations agree with a radial velocity of the
sun as 225km/s +-12 statistical +-15 systematic. In conjunction with
Reid's proper motion measurement that gives a distance to SgrA* of
7.5+-0.4 +- 0.5 kpc, in line with recent measurements:

H20 masers      7.2 +- 0.7      (Reid, 1993),

infrared photometry of bulge red clump stars, 7.52+-0.10 (stat) +-0.35
(sys) (Nishiyama et. al, 2006)

Globular clusters 7.2 +- 0.30 kpc (Bica et al 2006),

Keplerian motions, 7.62 +- 0.32 kpc (Eisenhauer et al, 2005),

Regards