Decoding Galactic Deuterium: Insights from Protostellar Outflows Using JWST

The paper by Francis et al. examines the abundance of deuterium-to-hydrogen ([D/H]) across the Galactic disk using data from the James Webb Space Telescope (JWST). This ratio serves as a crucial tracer of the universe's chemical evolution. The study utilizes observations of both high- and low-mass protostellar outflows, combining advanced spectroscopy with rotational line analysis of molecular hydrogen (H₂) and its deuterated form (HD).

Introduction

Deuterium is a rare isotope formed during the Big Bang and gradually destroyed in stars. Its abundance ([D/H]) provides a measure of Galactic chemical evolution. However, variations in observed [D/H] values have puzzled astronomers, possibly due to deuterium locking into dust grains. Previous measurements using ultraviolet (UV) absorption were limited in spatial scope, prompting the need for mid-infrared studies using JWST's advanced instruments.

Observations and Methods

The researchers analyzed data from the JOYS program, targeting 10 protostellar sources across the Galactic disk. By observing molecular hydrogen and HD emissions in these outflows, the study employed JWST's Mid-Infrared Instrument (MIRI) to derive column densities and [D/H] ratios. The team applied rotational diagram analysis, corrected for effects like chemical conversion of HD to atomic deuterium and non-local thermodynamic equilibrium (non-LTE) conditions.

Results

The team observed significant spatial variations in [D/H], particularly among low-mass protostars, with some measurements up to four times lower than expected based on chemical evolution models. HD emissions strongly correlated with shock-tracing lines like sulfur ([S I]) and iron ([Fe I]). These findings suggest deuterium depletion onto dust grains, which is released in shocks, may explain the observed variations.

Discussion

The discrepancies between observed [D/H] values and theoretical predictions highlight the complexity of deuterium's behavior in the interstellar medium. The study found tentative evidence linking higher gas-phase iron abundances to enhanced [D/H], supporting the hypothesis of shock-induced grain destruction. Future modeling and deeper observations could clarify the interplay between deuterium depletion, shock environments, and Galactic chemical evolution.

Conclusions

This study provides new insights into deuterium distribution and its role in tracing Galactic evolution. The findings underscore the importance of advanced infrared observations for understanding chemical processes in the universe. By demonstrating the capabilities of JWST, the research lays groundwork for broader studies of isotope chemistry in star-forming regions.

Source: Francis