Context. Determining the gas density and temperature structures of protoplanetary disks is a fundamental task to constrain planet formation theories. This is a challenging procedure and most determinations are based on model-dependent assumptions. Aims. We attempt a direct determination of the radial and vertical temperature structure of the Flying Saucer disk, thanks to its favorable inclination of 90 degrees. Methods. We present a method based on the tomographic study of an edge-on disk. Using ALMA, we observe at 0.5 resolution the Flying Saucer in CO J=2-1 and CS J=5-4. This edge-on disk appears in silhouette against the CO J=2-1 emission from background molecular clouds in ρ Oph. The combination of velocity gradients due to the Keplerian rotation of the disk and intensity variations in the CO background as a function of velocity provide a direct measure of the gas temperature as a function of radius and height above the disk mid-plane. Results. The overall thermal structure is consistent with model predictions, with a cold (< 15 − 12 K), CO-depleted mid-plane, and a warmer disk atmosphere. However, we find evidence for CO gas along the mid-plane beyond a radius of about 200 au, coincident with a change of grain properties. Such a behavior is expected in case of efficient rise of UV penetration re-heating the disk and thus allowing CO thermal desorption or favoring direct CO photo-desorption. CO is also detected up to 3-4 scale heights while CS is confined around 1 scale height above the mid-plane. The limits of the method due to finite spatial and spectral resolutions are also discussed. Conclusions. This method appears to be very promising to determine the gas structure of planet-forming disks, provided that the molecular data have an angular resolution which is high enough, of the order of 0.3 − 0.1 at the distance of the nearest star forming regions.

The NO(XΠ) + N(S) reaction which occurs entirely in the triplet manifold of NO is investigated using quasiclassical trajectories and quantum simulations. Fully-dimensional potential energy surfaces for the A' andA'' states are computed at the MRCI+Q level of theory and are represented using a reproducing kernel Hilbert space. The N-exchange and N-formation channels are followed by using the multi-state adiabatic reactive molecular dynamics method. Up to 5000 K these reactions occur predominantly on the NO A'' surface. However, for higher temperatures the contributions of theA' and A'' states are comparable and the final state distributions are far from thermal equilibrium. From the trajectory simulations a new set of thermal rate coefficients of up to 20 000 K is determined. Comparison of the quasiclassical trajectory and quantum simulations shows that a classical description is a good approximation as determined from the final state analysis.

We present a new method taking explicitly into account the coupling between rotation and bending of a nonlinear triatomic molecule colliding with an atom. This approach based on a rigid-bender treatment of the triatomic molecule was originally developed for the case of triatomic molecule linear at equilibrium. It is here extended to the case of a colliding bent triatomic molecule at equilibrium and applied to the case of the para-H2 + H2O inelastic collision using a new H2O-para-H2 adiabatically reduced 4D potential. The results of the method for purely rotational transitions are compared to those of rigid-rotor calculations while vibrational quenching rates of the first exited bending level are calculated for the first time at the close-coupling level.

We present a new theoretical method to treat atom-rigid bender inelastic collisions at the Close Coupling (RB-CC) level in the space fixed frame. The coupling between rotation and bending is treated exactly within the rigid bender approximation and we obtain the cross section for the rotational transition between levels belonging to different bending levels. The results of this approach are compared with those obtained when using the rigid bender averaged approximation (RBAA) introduced in our previous work dedicated to this system. We discuss the validity of this approximation and of the previous studies based on rigid linear HCN. We find that l-type transitions cross sections have to be calculated at the RB-CC level for the He-HCN collision while pure rotational transitions cross sections may be calculated accurately at the RBAA level.