Science: Webb illuminates the structure of interstellar ice

MADRID, (EUROPA PRESS). – The Webb Space Telescope has made it possible to investigate deep into the cores of dense molecular clouds, revealing details of interstellar ice that could not be observed before.

The study focused on the Chamaeleon I region, using the NIRCam instrument to measure spectroscopic lines toward hundreds of stars behind the cloud. For the first time, faint spectroscopic features known as “pendant OH” have been detected, indicating that water molecules are not completely bound to ice.

These features could trace the porosity and modification of ice grains as they evolve from molecular clouds to protoplanetary disks. This discovery improves our understanding of the structure of ice grains and their role in planet formation, according to the authors.

Thanks to the sensitivity of the Webb telescope, ices can be investigated deep within the cores of dense clouds, where extinction is so high that it eluded previous observatories. These lines of sight are the missing link between the initial formation of ices on the surfaces of dust grains in molecular clouds and the aggregation of ice grains into icy planetesimals, a poorly understood process that occurs in the protoplanetary disk surrounding a new star. Looking deep into the birthplace of stars will provide new clues about these ice grain modifications.

In the Ice Age program, which focuses on the Chamaeleon I region, a dense cloud region close to us in the Milky Way, observations of the densest part of the cloud with the NIRCam instrument on the Webb telescope have allowed simultaneous spectroscopic measurements of lines of sight to hundreds of stars behind the cloud. The light emitted by these stars interacts with ice grains as it passes through the cloud before being captured by the large mirror on JWST and detected.

Until now, the main absorption features associated with the main ice species, namely water, carbon dioxide, carbon monoxide, methanol and ammonia, have been measured. Thanks to the large size of the telescope mirror, much weaker features can now be measured. In-depth studies of the positions and profiles of weak spectroscopic features reveal some of the physical conditions of the object.

Thus, a team led by the Max Planck Institute for Extraterrestrial Physics achieved the first detection of a particular set of very faint bands linked to only a small fraction of the water molecules in the ice. The spectroscopic features, called “pendant OH” by laboratory astrophysicists who have measured them in laboratory ices for decades, correspond to water molecules that are not completely bound to the ice and could trace surfaces and interfaces within the icy grains, or when water is intimately mixed with other molecular species in the ice.

The “pendant OH” features lie in a spectral region that is inaccessible from the ground, and so while they have been actively sought since the 1990s, previous space-based observatories covering that spectral range lacked the combination of spectral resolution and sensitivity required to detect them, providing only upper limits.

These signals can be used to track the modification of ice grains on the path to planet formation. It has long been anticipated that, if detected, these signals could be used to track the porosity of ices – that is, their presence would indicate “fluffy” grains with high porosity, while their absence would indicate compaction and aggregation. Although this simple interpretation is still up for debate, the successful detection of these signals now means that we can look 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 ice evolves under different conditions.

“The detection of the characteristic hanging water bonds in the ice sheets demonstrates the importance of laboratory astrophysics in interpreting the James Webb data,” said Barbara Michela Giuliano, one of the authors, in a statement.

This study shows that potentially “fluffy” ice grains are present in the cloud, which affects the chemistry that can occur in these regions and therefore the degree of chemical complexity that can accumulate. The discovery also opens a new window in the study of planet formation as these spectral features ultimately allow us to 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 and to planets.