Changing disc compositions via internal photoevaporation
arxiv(2024)
摘要
The chemical evolution of protoplanetary discs is not fully understood,
several factors influence the final distribution of disc material. One such
factor are inward drifting and evaporating pebbles that enrich the inner disc
with vapour. In particular, it is first enriched with water vapour, resulting
in a low C/O ratio, before carbon-rich gas from the outer disc is transported
inwards elevating the C/O ratio again. However, it is unclear how internal
photoevaporation, which carries away gas and opens gaps that block inward
drifting pebbles, affects the chemical composition of the disc.
We aim to study these effects in discs around solar-like stars, where we
especially focus on the C/O ratio and the water content.
The simulations are carried out using a semi-analytical 1D disc model. Our
code chemcomp includes viscous evolution and heating, pebble growth and drift,
pebble evaporation and condensation, and a simple chemical partitioning model.
We show that internal photoevaporation plays a major role in the (chemical)
evolution of protoplanetary discs: As it opens a gap, inward drifting pebbles
are stopped and cannot contribute to the volatile content any more. In
addition, gas from the outer disc is carried away by photoevaporative winds.
Consequently, the C/O ratio in the inner disc is low. In contrast, gaps opened
by giant planets allow the gas to pass, resulting in an elevated C/O ratio,
similar to viscous discs without internal photoevaporation. This will enable us
to distinguish observationally between these two scenarios when measuring the
C/O ratio, implying that we can infer the cause of gap structures in disc
observations. In the case of a photoevaporative disc, we additionally find an
elevated water content in the inner disc as the water vapour and ice undergo a
cycle of evaporation/re-condensation, preventing its inward accretion onto the
star.
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