The role of OH in the chemical evolution of protoplanetary disks

Abstract

Context. We present a method for including gas extinction of cosmic-ray-generated UV photons in chemical models of the midplane of protoplanetary disks, focusing on its implications on ice formation and chemical evolution. Aims. Our goal is to improve on chemical models by treating cosmic rays, the main source of ionization in the midplane of the disk, in a way that is consistent with current knowledge of the gas and grain environment present in those regions. We trace the effects of cosmic rays by identifying the main chemical reaction channels and also the main contributors to the gas opacity to cosmic-ray-induced UV photons. This information is crucial in implementing gas opacities for cosmic-ray-induced reactions in full 2D protoplanetary disk models. Methods. We considered time-dependent chemical models within the range 1-10 AU in the midplane of a T Tauri disk. The extinction of cosmic-ray-induced UV photons by gaseous species was included in the calculation of photorates at each timestep. We integrated the ionization and dissociation cross sections of all atoms/molecules over the cosmic-ray-induced UV emission spectrum of H 2 . By analyzing the relative contribution of each gas phase species over time, we were able to identify the main contributors to the gas opacity in the midplane of protoplanetary disks. Results. At 1 AU the gas opacity contributes up to 28.2% of the total opacity, including the dust contribution. At 3-5 AU the gas contribution is 14.5% of the total opacity, and at 7-8 AU it reaches a value of 12.2%. As expected, at 10-15 AU freeze-out of species causes the gas contribution to the total opacity to be very low (6%). The main contributors to the gas opacity are CO, CO 2 , S, SiO, and O 2 . OH also contributes to the gas opacity, but only at 10-15 AU.

Disk Evolution, Element Abundances and Cloud Properties of Young Gas Giant Planets

“…At later epochs ( t ≳ 10 Myrs) additional, very stable ices with rare gaseous counterparts are formed, in particular NH 2 #, NH 3 #, C 2 H 2 #, CH 3 OH#, CH 3 # and CH 4 #. In fact, the simple ices formed in the first place are now slowly converted into these “late ices” (see also [ 11 , 18 ]). Since our model has only very limited surface chemistry [ 38 ], this conversion requires to temporarily evaporate the simple ices (by cosmic ray desorption), to convert the respective molecules in the gas phase into different ones by cosmic-ray chemistry, and then to freeze out the new molecules.…”

“…The ices play an essential role in the dust growth process as “glue” or “cement” during planet formation [ 10 ]. The dust particles have been in contact with the gas in the disk for > 10 6 years, which is certainly long enough to cause most of the gaseous oxygen in the disk midplane to form H 2 O ice outside the water “ice-line” and for most of the gaseous carbon to form CO ice outside the CO “ice-line” [ 11 ]. The elements bound in those ices should then rather follow the dust than the gas dynamical evolution.…”

“…Gaseous C/O ratios close to unity between the water and CO ice-lines, mainly driven by the formation of water, CO and CO 2 ices [ 17 ]. The time-dependent ice composition in the midplane of T Tauri disks has been studied by [ 11 , 18 ], who found agreement with measured chemical compositions of comets in their model, after long integration times (10 Myrs) at 10 AU, using a relatively high cosmic ray ionization rate of 5 × 10 −17 s −1 and a sophisticated treatment of the secondary cosmic ray induced photo reactions, the rates of which are enhanced due to an increased ratio of UV gas absorption with respect to dust absorption, driven by dust growth with respect to interstellar conditions.…”