Spectral signatures of water-dominated interstellar ice reveal details of how icy grains form and grow in their journey to planets.

Thanks to the unprecedented sensitivity of the JWST, we are able to probe ices deep within dense cloud cores, where extinction is so high that they eluded previous observatories. These lines of sight are the missing link between the initial formation of ices on dust grain surfaces in molecular clouds and the aggregation of icy grains into icy planetesimals, a still little-understood process that occurs in the protoplanetary disk surrounding a new star. Peeking deep into the birthplace of stars will give new clues  to these modifications of icy grains.

In the Ice Age program targeting the Chamaeleon I region, a dense cloud region close to us in the Galaxy, observations of the densest part of the cloud with JWST’s NIRCam instrument have allowed simultaneous spectroscopic measurements of lines of sight towards hundreds of stars behind the cloud. The light emitted by these stars interacts with icy grains as it crosses the cloud before being captured by the JWST’s large mirror and detected. Up until now, we have been able to measure the major, intense absorption features linked with major species in the ice, namely water, carbon dioxide, carbon monoxide, methanol, and ammonia. Thanks to the large size of the telescope’s mirror, we can now measure much weaker features. For example, the detection of the molecule OCS and the molecular ion OCN^- in dense cloud ices were made by the Ice Age team last year (McClure+ 2023).  Now, more in-depth studies of the positions and profiles of weak spectroscopic features have revealed some of the physical conditions in the object. Here, we have made the first detection of a particular set of very weak bands linked to only a small fraction of the water molecules in the ice (Noble+ 2024). The spectroscopic features, named ‘dangling OH’ by laboratory astrophysicists who have measured them in laboratory ices for decades, correspond to water molecules that are not fully bound into the ice, and could trace surfaces and interfaces within the icy grains, or when the water is intimately mixed with other molecular species in the ice.

The ‘dangling OH’ features lie in a spectral region that is inaccessible from the ground and so, while they have been actively searched for since the 1990s, the previous space observatories covering that spectral range lacked the combination of spectral resolution and sensitivity required to detect them providing only upper limits. Now in the JWST era, we can use these signatures to trace icy grain modification on the journey to planet formation. It has long been anticipated that, if detected, these signatures could be used to trace the porosity of the ices, i.e. their presence would signal ‘fluffy’ grains with high porosity while their absence would signal compaction and aggregation. Although this simple interpretation remains under debate, the successful detection of these signatures now means that we can search for them in different environments and at different times during the star formation process to determine whether or not they can be used as a tracer of how the ice evolves under different conditions.

This comes on the heels of the use of spectral signatures from the highly sensitive JWST instruments to answer another key question in the evolution of icy grains. The same team of scientists from the JWST Ice Age program published another study earlier this year where they looked at the growth mechanisms of grains and the question of whether grains grow by accumulating icy mantles or whether they grow by accretion of small grains (Dartois+2024). By observing how the profile of the spectroscopic features is morphed by the scattering of light along the line of sight, it is possible to determine how big the grains must be. The analogy with the visible spectrum that human eyes are sensitive to is that the scattering of mainly blue light by particles in the Earth’s atmosphere is the reason why the sky is blue, why sunsets are red. The size of the particles in Earth’s atmosphere and the wavelength of the visible light coming from the sun are similar enough that the scattering process impacts the visible part of the spectrum. Observing the same phenomenon in the infrared part of the spectrum means that the particles are around the same size as the wavelength of the light i.e. the micrometre (10^-6 m). This suggests that icy grains have grown much more than previously believed by the time the cloud collapses to form a star, and thus that grains can’t grow solely by accumulating icy mantles, but must also begin to accrete together.

Combining the information revealed by these spectroscopic probes of the physical structure of icy grains (the ‘dangling OH’ features and the profile warping from scattering) will give astronomers clues as to how the physics of ices and icy grains impacts the star formation process. These studies show that grain evolution in the cloud provides larger, potentially more ‘fluffy’ aggregate, icy grains for the start of the planet formation process in protoplanetary disks as well as impacting the chemistry that can occur in these regions and thus the degree of chemical complexity that can build up. This new information can be fed into models to better constrain the missing link at the onset of disk formation. These discoveries open a new window on studying planet formation since, ultimately, by comparing these spectral features from one environment to another we can build up an idea of the spatial distribution and variation of ices as well as how they evolve on their journey from molecular clouds to protoplanetary disks to planets.

Image:

© SWRI, AJ Galaviz III, JA Noble, D Qasim. Noble et al. Nature Astronomy 2024, doi: 10.1038/s41550-024-02307-7.

Link to the published article:

https://www.nature.com/articles/s41550-024-02307-7

Contact:

Jenny Noble jennifer.noble@univ-amu.fr

References:

Noble et al. ‘Detection of the elusive dangling OH ice features at  ∼ 2.7 μm in Chamaeleon I with JWST NIRCam,’ Nature Astronomy 2024, doi: 10.1038/s41550-024-02307-7

McClure et al Nature Astronomy 2023, 7, 431. doi: 10.1038/s41550-022-01875-w

Dartois et al. Nature Astronomy 2024, doi: 10.1038/s41550-023-02155-x