ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N₂H⁺RING IN PROTOPLANETARY DISKS

Abstract

We investigate the chemistry of ion molecules in protoplanetary disks, motivated by the detection of the N 2 H + ring around TW Hya. While the ring inner radius coincides with the CO snow line, it is not apparent why N 2 H + is abundant outside the CO snow line in spite of the similar sublimation temperatures of CO and N 2 . Using the full gas-grain network model, we reproduced the N 2 H + ring in a disk model with millimeter grains. The chemical conversion of CO and N 2 to less volatile species (sink effect hereinafter) is found to affect the N 2 H + distribution. Since the efficiency of the sink depends on various parameters such as activation barriers of grain-surface reactions, which are not well constrained, we also constructed the no-sink model; the total (gas and ice) CO and N 2 abundances are set constant, and their gaseous abundances are given by the balance between adsorption and desorption. Abundances of molecular ions in the no-sink model are calculated by analytical formulae, which are derived by analyzing the full-network model. The N 2 H + ring is reproduced by the no-sink model, as well. The 2D (R-Z) distribution of N 2 H + , however, is different among the full-network model and no-sink model. The column density of N 2 H + in the no-sink model depends sensitively on the desorption rate of CO and N 2 and the cosmic-ray flux. We also found that N 2 H + abundance can peak at the temperature slightly below the CO sublimation, even if the desorption energies of CO and N 2 are the same.

Stellar energetic particle ionization in protoplanetary disks around T Tauri stars

“…Such a strong impact of CR ionization on N N 2 H + is also reported by Aikawa et al (2015) and Cleeves et al (2014). Higher X-ray luminosities can compensate for low CR ionization only to some extent.…”

“…Differently to Cleeves et al (2014), in our model this dip is not predominantly due to the erosion of CO by reactions with He + ("sink effect" e.g. Aikawa et al 1997;Bergin et al 2014;Furuya & Aikawa 2014;Aikawa et al 2015), but mainly due to the interplay of CO freeze-out and non-thermal desorption in the outer disk. The CO sink effect is also active in our model but less efficient (see Appendix B.2.2).…”

“…Aikawa et al 2015). Consequently the N 2 H + abundance peaks in regions where CO is depleted and N 2 , the precursor of N 2 H + , is still in the gas phase.…”

The Gas Disk: Evolution and Chemistry

“…The resulting ring like structure visible in images of the N 2 H + line emissions is also, at least qualitatively, reproduced by thermo-chemical disk models (e.g. Cleeves et al 2015;Aikawa et al 2015). The situation is illustrated in Fig.…”

“…10 for our generic T Tauri disk model. However, Aikawa et al (2015) discuss other relevant chemical processes influencing the resulting N 2 H + distribution. One example is the so called sink effect, where CO is converted to less volatile species in regions with temperatures exceeding the sublimation temperature of CO (e.g.…”