Setting the volatile composition of (exo)planet-building material

Abstract

Context. The atmospheres of extrasolar planets are thought to be built largely through accretion of pebbles and planetesimals. Such pebbles are also the building blocks of comets. The chemical composition of their volatiles are usually taken to be inherited from the ices in the collapsing cloud. However, chemistry in the protoplanetary disk midplane can modify the composition of ices and gases. Aims. To investigate if and how chemical evolution affects the abundances and distributions of key volatile species in the midplane of a protoplanetary disk in the 0.2-30 AU range. Methods. A disk model used in planet population synthesis models is adopted, providing temperature, density and ionisation rate at different radial distances in the disk midplane. A full chemical network including gas-phase, gas-grain interactions and grain-surface chemistry is used to evolve chemistry in time, for 1 Myr. Both molecular (inheritance from the parent cloud) and atomic (chemical reset) initial conditions are investigated. Results. Great diversity is observed in the relative abundance ratios of the main considered species: H 2 O, CO, CO 2 , CH 4 , O 2 , NH 3 and N 2 . The choice of ionisation level, the choice of initial abundances, as well as the extent of chemical reaction types included are all factors that affect the chemical evolution. The only exception is the inheritance scenario with a low ionisation level, which results in negligible changes compared with the initial abundances, regardless of whether grain-surface chemistry is included. The grain temperature plays an important role, especially in the critical 20-28 K region where atomic H no longer sticks long enough to the surface to react, but atomic O does. Above 28 K, efficient grain-surface production of CO 2 ice is seen, as well as O 2 gas and ice under certain conditions, at the expense of H 2 O and CO. H 2 O ice is produced on grain surfaces only below 28 K. For high ionisation levels at intermediate disk radii, CH 4 gas is destroyed and converted into CO and CO 2 (in contrast with previous models), and similarly NH 3 gas is converted into N 2 . At large radii around 30 AU, CH 4 ice is enhanced leading to a low gaseous CO abundance. As a result, the overall C/O ratios for gas and ice change significantly with radius and with model assumptions. For high ionisation levels, chemical processing becomes significant after a few times 10 5 yrs. Conclusions. Chemistry in the disk midplane needs to be considered in the determination of the volatile composition of planetesimals. In the inner <30 AU disk, interstellar ice abundances are preserved only if the ionisation level is low, or if these species are included in larger bodies within 10 5 yrs.

Candidate Water Vapor Lines to Locate the H2O Snowline Through High-dispersion Spectroscopic Observations. II. The Case of a Herbig Ae Star

“…A similar distribution of gas-phase H 2 O in the midplane of a Herbig Ae disk is reported in Figure 1 of Woitke et al (2009b). Here we also note that Eistrup et al (2016) calculated the chemical evolution of a disk midplane under both molecular and atomic initial conditions as initial chemical abundances. They showed that in the latter atomic conditions, the abundance of H 2 O gas and ice around the H 2 O snowline (∼ 10 −6 ) is smaller than that for molecular initial abundances (∼ 10 −4 ).…”

CO2 infrared emission as a diagnostic of planet-forming regions of disks

“…The CO 2 ice content in the outer disk can be orders of magnitude higher. Both chemical models and measurements of comets show that the CO 2 content in ices can be more than 20% of the total ice content (Le Roy et al 2015;Eistrup et al 2016), with CO 2 ice even becoming more abundant than H 2 O ice in some models of outer disk chemistry (Drozdovskaya et al 2016). This translates into an abundance up to a few × 10 −5 .…”

Lupus disks with faint CO isotopologues: low gas/dust or high carbon depletion?

“…This result has been confirmed by physical-chemical modeling of the source, which was able to reproduce, among other lines, spatially resolved ALMA data and atomic carbon lines Schwarz et al 2016). This result is interpreted as chemical evolution, that is, carbon has been turned from CO into more complex species either in the gas or in the ice (Aikawa et al 1996;Bergin et al 2014;Drozdovskaya et al 2015;Eistrup et al 2016), or alternatively, as a grain growth effect, carbon has been locked up in large icy bodies that no longer participate in the gas-phase chemistry (Du et al 2015;Kama et al 2016), resulting in less bright CO lines. This hints at the possible existence of a class of disks where CO is not the main carbon reservoir.…”