JOYS: The [D/H] abundance derived from protostellar outflows across the Galactic disk measured with JWST

Abstract

Context. The total deuterium abundance [D/H] in the universe is set by just two processes: the creation of deuterium in Big Bang nucleosynthesis at an abundance of [D/H] = 2.58 ± 0.13 × 10^‑5, and its destruction within stellar interiors (astration). Measurements of variations in the total [D/H] abundance can thus potentially provide a probe of Galactic chemical evolution. However, most observational measurements of [D/H] are only sensitive to the gas-phase deuterium, and the amount of deuterium sequestered in dust grains is debated. With the launch of the James Webb Space Telescope (JWST), it is now possible to measure the gas-phase [D/H] at unprecedented sensitivity and distances through observation of mid-IR lines of H₂ and HD. Comparisons of gas-phase [D/H] with the constraints on the total [D/H] from the primordial abundance and Galactic chemical evolution models can provide insight into the degree of Deuterium lock-up in grains and the star formation history of our Galaxy. Aims. We use data from the JWST Observations of Young protoStars (JOYS) program of 5 nearby and resolved low-mass protostellar outflows and 5 distant high-mass protostellar outflows taken with the JWST Mid Infrafred Instrument (MIRI) Medium Resolution Spectrometer (MRS) to measure gas-phase [D/H] via H₂ and HD lines, assuming the gas is fully molecular. Methods. We extract spectra from various locations in the outflows. Using a rotational diagram analysis covering lines of H₂ and HD with similar excitation energies, we derive the column density of HD and H₂ or their upper limits. We then calculate the gas-phase [D/H] from the column density results, and additionally apply a correction factor for the effect of chemical conversion of HD to atomic D and non-LTE excitation on the HD abundance in the shocks. To investigate the spatial distribution of the bulk gas and species refractory species associated with the dust grains, we also construct integrated line intensity maps of H₂, HD, [Fe II], [Fe I], and [S I] lines. Results. A comparison of gas-phase [D/H] between our low-mass sources shows variations of up to a factor of ~4, despite these sources likely having formed in nearly the same region of the Galactic disk that would be expected to have nearly constant total [D/H]. Most measurements of gas-phase [D/H] from our work or previous studies produce [D/H] ≲ 1.0 × 10^‑5, a factor of 2-4 lower than found from local UV absorption lines and as expected from Galactic chemical evolution models. In the integrated line intensity maps, the morphology of the HD R(6) line emission is strongly correlated with the H₂ S(7), [S I], and [Fe I] lines which mostly trace high velocity jet knots and bright bow-shocks. In our extracted spectra along the outflows, there is similarly a strong correlation between the H₂ and HD column density and the [S I] and [Fe I] line flux, however, no correlation is seen between [D/H] and the [S I] or [Fe I] line flux. Conclusions. The variations in [D/H] between our low-mass sources and the low [D/H] with respect to Galactic chemical evolution models suggest that our observations are not sensitive to the total [D/H]. Significant depletion of deuterium onto carbonaceous dust grains is a possible explanation, and tentative evidence of enhanced [D/H] toward positions with higher gas-phase Fe abundance is seen in the HH 211 outflow. Deeper observations of HD and H₂ across a wider range of shock conditions and modeling of the carbonaceous dust-grain destruction and shock conditions are warranted to test for the effects of depletion.