ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
tokens: data. We assumed radial expansion, one of the simplest laws for the velocity. Radial expansion is a simple definition and the expected case if launching takes place along the force lines of a (locally) radial magnetic field. A linear variation of the absolute value is in general expected if most accelerations take place at the beginning of the process; in our case, variation depending on the latitude and – (as for the density, see Appendix A.1) leads to somewhat better predictions than dependence on the distance to the center. We can exclude some other simple cases, such as constant velocity modulus, expansion in the axial direction, and expansion parallel to the equator, for which the predictions do not match the observations. However, other velocity laws, such as that found for the outflowing gas in Red Rectangle by Bujarrabal et al. (2016), which is more complex, would lead to results compatible with the data (although involving a higher number of parameters). We stress that the launching mechanism itself is obviously very uncertain and its possible nature is a weak argument to support any velocity field; a theoretical discussion on such processes is beyond the scope of this letter. The total size, general shape, and overall velocity of the outflowing gas are however relatively well constrained, since they are given by the observed structure at several km s from the central velocity and the total velocity extent of the emission. As mentioned in Sect. 3.1, the assumption of LTE level population simplifies a lot our analysis. Nevertheless, we must keep in mind that the rotational temperatures we use in our calculations may only represent -averaged excitation temperatures and not true kinetic temperatures. Since high- levels can obviously be less populated in reality than for LTE, these rotational temperatures are probably a lower limit to the actual ones in the outflowing, relatively diffuse gas. Another consequence of the assumption of thermalized level populations is that it is not possible to distinguish the effects of the density and the relative CO abundances, (CO, CO). Fortunately, seems relatively well constrained in the best studied objects, particularly (CO), which is the basic parameter to determine the total density and mass because of the lower opacity of CO lines. Our previous works always yielded (CO) ranging between 10 and , both for post-AGB disks and for young PNe in general. We will adopt (CO) to ease the comparison with previous results on this object (Bujarrabal et al. 2013a ). In order to match the relatively low CO/CO intensity ratio, we need a relatively low abundance ratio. We adopt (CO) 10; similar low ratios are often found in similar objects. CO is weaker than CO , confirming that the CO emission is not very opaque. Our model yields typical values of the optical depth (CO ) 3 in the disk (for the representative velocities), while we typically find (CO ) 0.2 in the outflow emission. For CO , we find optical