Molecular Asymmetries in Disks

Background

Protoplanetary disks - the circumstellar environments where planets form - exhibit complex physical and chemical structures that directly influence the composition of emerging planetary systems. Recent high-resolution observations from facilities like ALMA have revealed that these disks often display significant asymmetries in their molecular emission, challenging our traditional understanding of disk chemistry.

This is demonstrated in the Figure below, which shows CS J=2-1 emission from five protoplanetary disks observed by the ALMA program 'Molecules with ALMA at Planet-Forming Scales' (MAPS). Each disk exhibits significant brightness asymmetries, where molecular emission is substantially stronger on one side compared to the other.

Figure 1: CS J=2-1 emission from five disks surveyed as part of the large ALMA program 'Molecules with ALMA at Planet-Forming Scales' (adapted from LeGal et al. 2021)

Case study: HD 100546

To understand the origin of such asymmetries, we conducted detailed observations of HD 100546, a young star system where planets are currently forming. Using ALMA, we observed the emission from two key molecular tracers: CS (carbon monosulfide) and SO (sulfur monoxide). As shown below, both molecules display dramatic spatial asymmetries across the disk. Most intriguingly, CS and SO appear brightest in opposite regions of the disk - a clue that would prove crucial to understanding the underlying chemistry.

Figure 2: Moment 0 emission maps for SO (left) and CS (right). The position of the star is denoted by the green 'x'. The molecules appear brightest on opposite sides of the disk.

Through detailed thermochemical modeling, we discovered that these asymmetries arise from a fundamental variation in the disk's chemical composition. Most of the disk maintains a 'Solar-like' carbon-to-oxygen ratio (C/O ≈ 0.5), creating oxygen-rich conditions that favour SO production. However, a distinct region on the western side of the disk (right side of the image) shows an elevated C/O ratio > 1. In this carbon-rich environment, CS production is favoured while SO formation is suppressed.

A Shadow's Influence?

What could cause such a stark chemical division within a single disk? Our analysis points to a physical mechanism involving temperature variations and molecular freeze-out. We propose that a local concentration of dust, possibly linked to a forming planet, casts a shadow on part of the disk. Within this shadowed region, three key processes occur: 1. The temperature drops significantly 2. Water molecules freeze onto dust grains more readily 3. The removal of water from the gas raises the C/O ratio

As shown in the Figure below, this shadowing effectively creates two distinct chemical environments within the same system.

Figure 3: Schematic showing our proposed shadowing mechanism

Implications for Planet Formation

This discovery fundamentally changes our understanding of protoplanetary disk chemistry. Previously, we knew that chemical composition could vary with distance from the central star, primarily due to temperature differences. Our finding that chemistry can also vary dramatically at the same orbital distance opens new possibilities for understanding planetary diversity.

These azimuthal variations could significantly impact planet formation: - Planets forming at the same distance from their star might incorporate vastly different chemical ingredients - A planet's final composition may depend on its location around the disk as well as its distance from the star - Changes in these chemical patterns over time could create complex compositional layering in forming planets

Looking Forward

Our findings raise exciting questions for future research: - How common are such chemical asymmetries in other disks? - How do these asymmetries evolve as planets form? - What role do these variations play in determining the final composition of planets?

Answering these questions will help us better understand the connection between disk chemistry and the remarkable diversity of planetary systems we observe.

This research was originally published in Nature Astronomy (Keyte et al. 2023; "Azimuthal C/O variations in a planet-forming disk")