What is a protostellar disk, or 'proplyd'?

A protostellar disk is a rotating disk of dense gas and dust that surrounds a newly formed star. It is the raw material from which planets, moons, asteroids, and comets eventually form.

Why is the Orion Nebula an important place to study star formation?

The Orion Nebula is the closest massive star-forming region to Earth. Its relative proximity and large population of young stars provide an unparalleled opportunity to study the birth of stars and planetary systems in detail and in a variety of environments.

What is the 'snow line' in a protostellar disk?

The snow line is the distance from the central star where it becomes cold enough for volatile compounds, like water, to freeze into solid ice grains. Its location is a critical factor in determining the type of planets that can form in a solar system.

What does this research tell us about how our own solar system formed?

By identifying the chemical building blocks and physical conditions in these young disks, we can create better models of our own solar system's formation 4.6 billion years ago. The isotopic ratios of elements like water, for example, provide clues about the origin of Earth's oceans.

Chemical Analysis of Orion's Planet-Forming Disks

The study of planet formation has entered a revolutionary era, driven by new observational capabilities. Protostellar disks, the rotating structures of gas and dust around young stars, are the literal birthplaces of planetary systems. The Orion Nebula Cluster (ONC), as the nearest region of massive star formation, offers a unique laboratory to study these disks in a high-radiation environment similar to that in which many stars, possibly including our Sun, were born. This publication presents the findings of a high-resolution spectroscopic survey of ten protostellar disks within the ONC, aimed at creating a detailed census of their chemical composition and gas kinematics.

Observational Campaign and Data Reduction

Data for this survey were obtained using the James Webb Space Telescope's Near-Infrared Spectrograph (NIRSpec) and Mid-Infrared Instrument (MIRI). We employed the Integral Field Unit (IFU) mode for both instruments, which provides spatially-resolved spectra across the entirety of each targeted disk. This technique allows us to create 2D maps of chemical abundances and gas velocities. The raw data were processed through a custom pipeline to perform background subtraction, correct for instrumental effects, and extract high-fidelity spectra, achieving a spectral resolution (R) of ~3000.

Kinematic Analysis: Tracing Gas Motion and Protoplanet Signatures

By measuring the Doppler shift of bright carbon monoxide (CO) emission lines, we mapped the velocity field of each disk. Eight of the ten disks in our sample exhibit clear Keplerian rotation, allowing for a precise dynamical measurement of their central protostar's mass. However, two disks, SNOI-3 and SNOI-7, show significant deviations from smooth Keplerian motion. These localized velocity perturbations, or 'kinematic kinks,' are consistent with theoretical models of gravitational disturbances caused by embedded, Jupiter-mass protoplanets, providing compelling circumstantial evidence of ongoing planet formation.

Astrochemical Analysis: The Building Blocks of Planets

The high sensitivity of JWST allowed for an unprecedented chemical inventory of the gas-phase components of these disks.

Mapping Water Vapor and the Snow Line

We successfully detected and mapped multiple emission lines of water vapor (H₂O) in the inner regions of all ten disks. By tracing the sharp drop-off in the water signal, we were able to directly locate the water snow line in each disk. We find that the snow lines are located farther from the central star than predicted by models that neglect the external heating from the Orion Nebula's massive central stars, a key finding for understanding water delivery to forming terrestrial planets.
Detection of Key Organic Molecules

Our spectra reveal the presence of a suite of simple organic molecules crucial for prebiotic chemistry, including carbon dioxide (CO₂), methane (CH₄), and formaldehyde (H₂CO). The abundance ratios of these molecules vary significantly from disk to disk, suggesting that the initial chemical cocktail available for planet formation is not universal and may be influenced by the local environment of each star.
Isotopic Ratios and the Origin of Earth's Water

For the three brightest disks, we measured the deuterium-to-hydrogen (D/H) ratio in their water vapor. The measured D/H ratios are consistent with those found in comets within our own Oort cloud. This finding lends strong support to the theory that a significant fraction of Earth's water was delivered by cometary impacts during the early history of our solar system.

Discussion: The Impact of Photoevaporation on Planet Formation

A key feature of the Orion environment is the intense far-ultraviolet (FUV) radiation from the central Trapezium stars. Our data show clear evidence of this radiation's impact. We observe strong emission from ionized neon ([Ne II]), a tracer of photoevaporative winds, at the outer edges of the disks. This indicates that the disks are actively being stripped of their gas by the harsh external environment. This finding suggests that planet formation in such massive star clusters may operate on an accelerated timescale, as planets must form before their natal disk is completely dissipated.

Conclusion and Future Directions

This high-resolution spectroscopic survey of ten protostellar disks in the Orion Nebula has provided a detailed snapshot of the physical and chemical conditions in the cradles of planet formation. We confirm the widespread presence of water and simple organic molecules, find kinematic evidence for nascent protoplanets, and quantify the destructive impact of the local environment on disk longevity. The building blocks for planets are abundant, but they exist in a dynamic and challenging setting. Future work will focus on expanding this survey to a larger sample of disks and utilizing even longer integration times to search for more complex, prebiotic molecules.