New constraints on a triaxial model of the Galaxy

We determine the most likely values of the free parameters of an N ‐body model for the Galaxy developed by Fux via a discrete–discrete comparison with the positions on the sky and line‐of‐sight velocities of an unbiased, homogeneous sample of OH/IR stars. Via Monte Carlo simulation, we find the plausibility of the best‐fitting models, as well as the errors on the determined values. The parameters that are constrained best by these projected data are the total mass of the model and the viewing angle of the central bar, although the distribution of the latter has multiple maxima. The other two free parameters, the size of the bar and the (azimuthal) velocity of the Sun, are less well‐constrained. The best model has a viewing angle of ∼ 44°, a semimajor axis of 2.5 kpc (corotation radius 4.5 kpc, pattern speed 46 km s −1  kpc −1 ), a bar mass of 1.7×10 10  M ⊙ and a tangential velocity of the local standard of rest of 171 km s −1 . We argue that the lower values that are commonly found from stellar data for the viewing angle (∼25°) arise when too few coordinates are available, when the longitude range is too narrow or when low latitudes are excluded from the fit. The new constraints on the viewing angle of the Galactic bar from stellar line‐of‐sight velocities decrease further the ability of the distribution of the bar to account for the observed microlensing optical depth toward Baade’s window: our model reproduces only half the observed value. The signal of triaxiality diminishes quickly with increasing latitude, fading within approximately 1 scaleheight (≲3°). This suggests that Baade’s window is not a very appropriate region in which to sample bar properties.