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-0, the rotational speed of the local standard of rest, v(0), and the peculiar velocity of the Sun, v(circle dot), 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(circle dot) or a net peculiar motion of the high-mass star-forming regions. We argue that a correction to v(circle dot) is much more likely and these data support the conclusion of Binney that V-circle dot should be revised upwards from 5.2 to 11 km s(-1). We find that the values of R-0 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-0 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.