摘要：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: H2O, CO, CO2, CH4, O2, NH3 and N2. 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 or not 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 CO2 ice is seen, as well as O2 gas and ice under certain
conditions, at the expense of H2O and CO. H2O ice is produced on grain surfaces only below 28 K.
For high ionisation levels at intermediate disk radii, CH4 gas is destroyed and
converted into CO and CO2 (in contrast with previous models), and similarly
NH3 gas is
converted into N2.
At large radii around 30 AU, CH4 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 105 yr.
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 105 yr.
关键词：astrochemistry;planets and satellites: formation;protoplanetary disks;planets and satellites: atmospheres;molecular processes
