The uncertainty in Galactic parameters

We reanalyse the measurements of parallax, proper motion and line-of-sight velocity for 18 masers in high-mass star-forming regions presented by Reid et al. We use a likelihood analysis to investigate the distance of the Sun from the Galactic Centre, R o , the rotational speed of the local standard of rest, v 0 , and the peculiar velocity of the Sun, v ⊙ , for various models of the rotation curve and models which allow for a typical peculiar motion of the high-mass star-forming regions. We find that these data are best fit by models with non-standard values for v ⊙ or a net peculiar motion of the high-mass star-forming regions. We argue that a correction to v ⊙ is much more likely and these data support the conclusion of Binney that V ⊙ should be revised upwards from 5.2 to 11 km s ―1 . We find that the values of R o and v 0 that we determine are heavily dependent on the model we use for rotation curve, with model-dependent estimates of R 0 ranging from 6.7 ± 0.5 to 8.9 ± 0.9 kpc, and those of v 0 ranging from 200 ± 20 to 279 ± 33 km s ―1 . We argue that these data cannot be thought of as implying any particular values of R o or v 0 . However, we find that v 0 /R 0 is better constrained, lying in the range 29.9 -31.6 km s ―1 kpc ―1 for all models but one.