Cuppen & Herbst . from both UV and X-ray photons

however, cosmic rays are able to penetrate the entire disk. As expected, the disk surface is heavily irradiated with the UV and X-ray fluxes reaching a value

∼ 10-2 erg cm-2 s-1

an order of magnitude stronger than the integrated interstellar UV flux (1.6 × 10-3 erg cm-2 s-1

We present results from five different models which increase incrementally in complexity from Model 1 through to Model 5

Model 1 is the most simple and includes gas-phase chemistry, freezeout onto dust grains, and thermal desorption. In Model 2, we add cosmic-ray-induced thermal desorption and photodesorption by internal and external UV photons and X-ray photons

Article number, page 8 of 34

Catherine : Complex organic molecules in protoplanetary disks

In Model 3, we include grain-surface chemistry and in Model

4, we also add the cosmic-ray, X-ray, and UV photoprocessing of ice mantle material. The most complex model, Model 5, also includes reactive desorption (see Sect. 2.2)

In Figs. 3 and 4 we present the fractional abundance (relative to H nuclei number density) as a function of disk height, Z, at a radius, R = 305 AU, of a selection of gas-phase and grainsurface (ice) COMs, respectively10

Note that we have used the same scale for the abundance of each gas-phase molecule and analogous grain-surface species to ease the comparison between plots

3.2.1. Model 1: Freezeout and thermal desorption

In Model 1 (red lines in Figs. 3 and 4), where we include freezeout and thermal desorption only, a handful of molecules achieve an appreciable fractional abundance ( 10-11

in the disk molecular layer: H2CO, HC3N, CH3CN, and CH3NH2. These species are depleted in the disk midplane below a height of ≈ 100 AU due to efficient freezeout onto dust grains. Higher in the disk, gas-phase formation replenishes molecules lost via freezeout onto grain surfaces. These species generally retain a similar peak fractional abundance as that achieved under dark cloud conditions. The exception to this is acetonitrile (CH3CN) which increases from an initial fractional abundance of ∼ 10-12 to reach a peak value of ∼ 10-10 at Z ≈ 200 AU. The fractional abundances decrease towards the disk surface due to increasing photodestruction. None of the other gas-phase species achieve significant peak fractional abundances in the molecular layer ( 10-11

even those which begin with an appreciable initial abundance, i.e., CH3OH and CH3CCH. The fractional abundance of gasphase CH3OH remains 10-14 throughout the disk height. For these species, freezeout onto dust grains wins over gas-phase formation. The more complex molecules, which cannot form in the gas-phase under dark cloud conditions, are also unable to form under the conditions in the outer disk, in the absence of grainsurface chemistry

Regarding the grain-surface results for Model 1 (Fig. 4), due to the higher binding energies of most species, thermal desorption alone is unable to remove significant fractions of the grain mantle. Hence, the ice remains abundant throughout the vertical extent of the disk with most species retaining their initial fractional abundance. We see enhancements in the fractional abundances of s-H2CO, s-HCOOH, s-HC3N and s-CH3CN around

AU. All four species have gas-phase routes to formation

however, under the conditions beyond Z ≈ 100 AU, additional molecules created in the gas phase can accrete onto dust grains thereby increasing their abundance on the grain mantle

3.2.2. Model 2: Non-thermal desorption

In Model 2 (green lines in Figs. 3 and 4), we have added cosmicray-induced thermal desorption and photodesorption due to external and internal UV photons and X-ray photons. Non-thermal desorption has a powerful effect on both the gas-phase and grainsurface abundances as we also concluded in our previous work

(WMN10). There are several noticeable effects: (i) the gas-phase abundances of many molecules are enhanced towards the disk midplane, relative to the results for Model 1, due to cosmic-rayinduced thermal desorption and photodesorption, (ii) the abundances of grain-surface molecules drop significantly towards the disk surface due to photodesorption by external UV photons, 10

The data used to plot Figs. 3 to 7 are available upon request. and (iii) there is a shift in the position of the gas-phase ‘molecular layer’ towards the midplane. This latter effect is due to a combination of non-thermal desorption and enhanced gas-phase formation lower in the disk, and enhanced destruction higher in the disk due to the release of a significant fraction of the grain mantle back into the gas phase. Non-thermal desorption effectively ‘seeds’ or replenishes the gas with molecules that otherwise would remain bound to the grain, e.g., H2O and its protonated form, H3O+

which then go on to take part in gas-phase reactions which can form (destroy) molecules which would otherwise be depleted (abundant)

The fractional abundance of gas-phase H2CO is enhanced to ∼ 10-12 in the disk midplane. However, its fractional abundance in the molecular layer and disk surface reaches values similar to that in Model 1 (∼ 10-10