tokens: conclude that the errors assigned by Reid et al. (2014) to and are conservative, and there do not appear to be any bias, given the available maser samples so far. For the rotation curve, the situation is somewhat different. Although the values found for are very close to the initial values adopted, the statistical spread is larger than expected. Reid et al. (2014) reported an error of 0.4 in the rotation curve which is optimistic compared with our findings. From our simulations, we would constrain the rotation curve value as given the BeSSeL data sample selection. The larger error possibly indicates that our assumed velocity distributions are too wide. Fig. 10 Marginalized posterior probability density distributions for correlated circular velocity parameters from 100 simulated galaxies mimicking the BeSSeL sample. Left panel : circular orbital speed of the Sun. Middle panel : orbital angular solar speed. Right panel : difference between the circular solar motion and average source peculiar motions. When more sources are added to the sample selection (sets A and B), Fig. 8 shows an initial improvement in the accuracy of the fundamental Galactic parameters. For set A, the errors in , , and can improve up to , , and , respectively, when more northern hemisphere sources are added. In contrast when southern hemisphere sources are also added (set B), the errors can decrease to , , and , respectively. Further improvements would require much better astrometry for weak sources, requiring for example much more sensitive observations. 4.2.2. Solar motion and average source peculiar motions Figure 7 shows the distribution obtained for solar velocity components (, , and ). The uncertainties derived were around 2 for all solar velocity components with respect to the initial parameters. For , the standard deviation found was 1.2 , which compares well to the results published by Reid et al. (2014) (i.e., ). In addition, the spread for and are narrower with values of 2.4 and 0.5 , respectively. Compared to the BeSSeL results (i.e., and ), we can affirm that the solar motion results published by Reid et al. (2014) have conservative estimates. For the radial and tangential average peculiar motions ( and ), Table 4 shows that indeed these peculiar velocities can be fitted with high accuracy using the simulations; however, the relative spreads are high compared with other parameters (see Fig. 7 ). Even so, compared to the BeSSeL results (i.e., and ), the errors in the simulated sample selection are still lower (i.e., and ). The reason for higher dispersions for and could be related to the number of parameters that must fit independently. The parameters and have largely the same effect on the observations for nearby sources and therefore are directly correlated (see Table 5 ). For , the correlation is high with two components (, ), which affects the fitting and hence the estimated accuracy. 4.2.3. Parameter correlations Pearson product-moment correlation
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