Protoplanetary discs: sensitivity of the chemical composition to various model parameters

Abstract

Protoplanetary disks are challenging objects for astrochemical models due to strong density and temperature gradients and due to the UV photons 2D propagation. In this paper, we have studied the importance of several model parameters on the predicted column densities of observed species. We considered: 1) 2-phase (gas and homogeneous grains) or 3-phase (gas, surface, and bulk of grains) models, 2) several initial compositions, 3) grain growth and dust settling, and 4) several cosmic-ray ionization rates. Our main result is that dust settling is the most crucial parameter. Including this effect renders the computed column densities sensitive to all the other model parameters, except cosmic-ray ionization rate. In fact, we found almost no effect of this parameter for radii larger than 10 au (the minimum radius studied here) except for N 2 H + . We also compared all our models with all the column densities observed in the protoplanetary disk around DM Tau and were not able to reproduce all the observations despite the studied parameters. N 2 H + seems to be the most sensitive species. Its observation in protoplanetary disks at large radius could indicate enough N 2 in the gas-phase (inhibited by the 3-phase model, but boosted by the settling) and a low electron abundance (favored by low C and S elemental abundances).

Chemical signatures of a warped protoplanetary disc

“…Abundances of CO and HCN from the aligned model (Fig 4) match well with the base chemical model of Kamp et al (2017). The CO column density is in excellent agreement with the models presented by Wakelam et al (2019). The column density of SO at 10 au on the illuminated side of the disc (Fig.…”

The first multidimensional view of mass loss from externally FUV irradiated protoplanetary discs

“…At deeper, colder, denser layers within the disc that are extremely optically thick to the UV the chemistry becomes much more complicated and is not captured by our model (see e.g. Walsh et al 2010;Woitke et al 2016;Cleeves et al 2017;Kamp et al 2017;Wakelam et al 2019). Although this is unimportant for computing the dynamical evolution of the disc (since such zones are cold regardless of the chemical complexity) we do not attempt to properly capture the composition deep within the disc.…”

“…Ionisation and dissociation contributing to the chemistry of the disc itself is therefore more sensitive to the cosmic ray ionisation rate than the ambient UV field (for more information on disc chemistry including assessments of the cosmic ray contribution see e.g. Cleeves et al 2014Cleeves et al , 2015Wakelam et al 2019). Though as we will shortly see parts of the wind are molecular, so may be identified as part of the disc in molecular line observations and certainly would be influenced by the ambient UV field.…”

A Three-phase Approach to Grain Surface Chemistry in Protoplanetary Disks: Gas, Ice Surfaces, and Ice Mantles of Dust Grains

“…less than few 10 −14 with respect to n H ). Wakelam et al (2019) recently presented a parametric study of disk physical properties using a gas grain model which our network is also based upon, although we have substantially updated the network as described in the appendices. They find that the initial chemical composition of the disk, dust settling and grain growth all have strong effects on the disk chemical structure.…”