Consistent dust and gas models for protoplanetary disks

Abstract

Aims. We define a small and large chemical network which can be used for the quantitative simultaneous analysis of molecular emission from the near-IR to the submm. We revise reactions of excited molecular hydrogen, which are not included in UMIST, to provide a homogeneous database for future applications. Methods. We use the thermo-chemical disk modeling code ProDiMo and a standard T Tauri disk model to evaluate the impact of various chemical networks, reaction rate databases and sets of adsorption energies on a large sample of chemical species and emerging line fluxes from the near-IR to the submm wavelength range. Results. We find large differences in the masses and radial distribution of ice reservoirs when considering freeze-out on bare or polar ice coated grains. Most strongly the ammonia ice mass and the location of the snow line (water) change. As a consequence molecules associated to the ice lines such as N 2 H + change their emitting region; none of the line fluxes in the sample considered here changes by more than 25% except CO isotopologues, CN and N 2 H + lines. The three-body reaction N+H 2 +M plays a key role in the formation of water in the outer disk. Besides that, differences between the UMIST 2006 and 2012 database change line fluxes in the sample considered here by less than a factor 2 (a subset of low excitation CO and fine structure lines stays even within 25%); exceptions are OH, CN, HCN, HCO + and N 2 H + lines. However, different networks such as OSU and KIDA 2011 lead to pronounced differences in the chemistry inside 100 au and thus affect emission lines from high excitation CO, OH and CN lines. H 2 is easily excited at the disk surface and state-to-state reactions enhance the abundance of CH + and to a lesser extent HCO + . For sub-mm lines of HCN, N 2 H + and HCO + , a more complex larger network is recommended. Conclusions. More work is required to consolidate data on key reactions leading to the formation of water, molecular ions such as HCO + and N 2 H + as well as the nitrogen chemistry. This affects many of the key lines used in the interpretation of disk observations. Differential analysis of various disk models using the same chemical input data will be more robust than the interpretation of absolute fluxes.

Warm dust surface chemistry

“…Reaction rate coefficients that are not explicitly discussed in this paper are taken from UMIST2012 (McElroy et al 2013) The adsorption energies are mixed from various sources (Aikawa et al 1996), Garrod & Herbst (2006), and UMIST2012 (McElroy et al 2013). The network was complemented by reactions relevant to high temperature conditions (Kamp et al 2017). We used the chemistry solver in the ProDiMo code in a zerodimensional model.…”

Why does ammonia not freeze out in the centre of pre-stellar cores?

“…Our fiducial value of 5500 K is taken from Collings et al (2004). However, lower values of the order of 3000 K have been suggested (see Kamp et al 2017, and references therein). We consider both values in our model grid.…”

Observing the gas component of circumplanetary disks around wide-orbit planet-mass companions in the (sub)mm regime

“…For the chemistry, we used a chemical network including 86 chemical species and 1148 chemical reactions, including gasphase chemistry, H 2 formation on grains, and ice chemistry (freeze-out; thermal, cosmic ray, and photo desorption). The chemical network is identical to the so-called small network of Kamp et al (2017) except for the X-ray chemistry, which we did not include as we do not expect strong X-ray radiation from the planet. The gas-phase chemical reactions are based on the UMIST 2012 database (McElroy et al 2013).…”