Ammonium Salt Formation and Abundance in Protoplanetary Disks

Ammonium salts may represent an important reservoir of volatile species in solar system primitive bodies, but the question of how and when these salts can form during the star formation process remains unknown. In this paper, we use thermochemical models to study the formation of ammonium salts during the protoplanetary disk stage. We show that ammonium salts form efficiently in the inner disk midplane (i.e., r ≲ 50 au), inside the comet-forming region. In this region, our model predicts that almost all the available nitrogen is in the form of salts (i.e., mainly in ammonium cyanate) at the surface of grains after evolving for 10 Myr. For sulfur, we show that almost all the available S is in the form of ammonium hydrosulfide in the inner disk midplane. We show that, inside r ∼ 30 au, ammonium salt formation is enhanced by a cosmic-ray-driven sink effect that progressively converts gas-phase CO and N 2 into carbon dioxide and salts, respectively, at the surface of grains on a timescale ≳1 Myr. This impacts the location of the CO and N 2 radial snowlines, both of which shift closer to the star as a function of time.