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. (2009). We use a likelihood analysis to investigate the distance of the Sun from the Galactic centre, R0, the rotational speed of the local standard of rest, v0, and the peculiar velocity of the Sun, v⊙, for various models of the rotation curve, and models in 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 change in v⊙ is much more likely, and that these data support the conclusion of Binney (2009) that V⊙ should be revised upwards from 5.2 km s −1 to 11 km s. We find that the values of R0 and v0 that we determine are heavily dependent on the model we use for rotation curve, with model-dependent estimates of R0 ranging from 6.7 ± 0.5 kpc to 8.9 ± 0.9 kpc, and those of v0 ranging from 200 ± 20 km s −1 to 279 ± 33 km s. We conclude that these data do not provide evidence for any particular values of R0 or v0 within these ranges. However, we find that v0/R0 is better constrained, lying in the range 29.9− 31.6 km s kpc for all models but one.