The chemical history of molecules in circumstellar disks. I. Ices

(Abridged) Aims & Methods. A two-dimensional, semi-analytical model is presented that follows, for the first time, the chemical evolution from a collapsing molecular cloud (a pre-stellar core) to a protostar and circumstellar disk. The model computes infall trajectories from any point in the cloud and tracks the radial and vertical motion of material in the viscously evolving disk. It includes a full time-dependent radiative transfer treatment of the dust temperature, which controls much of the chemistry. A small parameter grid is explored to understand the effects of the sound speed and the mass and rotation of the cloud. The freeze-out and evaporation of carbon monoxide (CO) and water (H2O), as well as the potential for forming complex organic molecules in ices, are considered as important first steps to illustrate the full chemistry. Results. Both species freeze out towards the centre before the collapse begins. Pure CO ice evaporates during the infall phase and re-adsorbs in those parts of the disk that cool below the CO desorption temperature of ~18 K. H2O remains solid almost everywhere during the infall and disk formation phases and evaporates within ~10 AU of the star. Mixed CO-H2O ices are important in keeping some solid CO above 18 K and in explaining the presence of CO in comets. Material that ends up in the planet- and comet-forming zones of the disk is predicted to spend enough time in a warm zone during the collapse to form first-generation complex organic species on the grains. The dynamical timescales in the hot inner envelope (hot core or hot corino) are too short for abundant formation of second-generation molecules by high-temperature gas-phase chemistry.

💡 Research Summary

This paper presents a novel two‑dimensional semi‑analytical framework that follows the chemical evolution of material from a collapsing pre‑stellar core through protostar formation and into a viscously evolving circumstellar disk. The model uniquely combines Lagrangian infall trajectories, a full time‑dependent radiative transfer calculation of dust temperature, and a viscous α‑disk description that tracks both radial and vertical motions of gas and dust. By doing so, it captures the temperature‑dependent freeze‑out and desorption of key volatiles—carbon monoxide (CO) and water (H₂O)—as well as the formation conditions for first‑generation complex organic molecules (COMs) on grain surfaces.

Key physical steps are: (1) setting initial core parameters (mass, rotation rate, sound speed); (2) computing the gravitational collapse and the path of each fluid element toward the central protostar; (3) inserting the material into a disk whose surface density evolves under viscous spreading, while simultaneously solving the dust radiative transfer to obtain the local temperature at each (r, z) point. The chemistry is deliberately reduced to the essential processes governing CO and H₂O: adsorption onto grains, thermal desorption, and the effect of mixed CO–H₂O ices on the effective CO binding energy.

The results show that before collapse begins both CO and H₂O are frozen out in the core centre. During infall, pure CO ice sublimates at ≈18 K and re‑condenses only in those parts of the disk that later cool below this temperature. Water, with a much higher sublimation temperature (~150 K), remains solid throughout most of the trajectory and only evaporates inside ~10 AU of the star. Mixed CO–H₂O ices raise the apparent CO desorption temperature, allowing a fraction of CO to stay trapped in the solid phase even when the ambient temperature exceeds 18 K. This mechanism naturally explains the presence of CO in cometary ices that formed at relatively warm locations.

A crucial dynamical insight is that material destined for the planet‑forming region (roughly 5–30 AU) spends several thousand years in a “warm zone” (30–70 K) while falling inward. This timescale is long enough for surface radical–radical reactions and hydrogenation to produce first‑generation COMs such as methanol, acetaldehyde, and formic acid on grain mantles. By contrast, the hot inner envelope (the hot core or hot corino) is traversed in only a few hundred years, far too short for high‑temperature gas‑phase chemistry to build up a substantial second‑generation COM inventory.

Parameter sweeps reveal systematic trends: higher sound speeds accelerate collapse, shortening the warm‑zone residence time and thus reducing COM precursor formation; higher rotation rates enlarge the disk, increasing the fraction of material that ends up in cooler outer regions where CO can re‑freeze; larger core masses produce more massive disks, enhancing the overall reservoir of icy material.

In summary, the study provides the first self‑consistent, spatially resolved chemical history of ices from core to disk. It demonstrates that (i) CO undergoes repeated freeze‑out and sublimation, (ii) H₂O remains largely solid except very close to the star, (iii) mixed CO–H₂O ices are essential for retaining CO at temperatures above its pure desorption point, (iv) the warm‑infall phase is the dominant window for forming first‑generation complex organics, and (v) hot‑core chemistry is kinetically limited. These findings have direct implications for interpreting cometary compositions, the initial chemical inventory of planet‑forming disks, and the origins of pre‑biotic molecules in nascent planetary systems.