χ² distribution from the Keplerian fit (see Fig. 10) as a function of the free parameters YSO mass and position (left panel), and YSO V_LSR and mass (right panel). To produce these plots, the third free parameter of the Keplerian fit, that is, the YSO V_LSR and position in the left and right plot, respectively, is taken equal to the best-fit value. The color scale at the top of each panel gives the value of the χ² in a logarithmic scale. In each of the two plots, the position of the minimum χ², = 6.6, is labeled, and the white contour indicates the level +3.5 = 10.1, which corresponds to the 1-σ confidence level for three free parameters (Lampton et al. 1976).