[Paper Review] Can Dust Explain Variations in the D/H Ratio?

B. T. Draine|arXiv (Cornell University)|Oct 12, 2004

Astrophysics and Star Formation Studies3 citations

TL;DR

The paper proposes that variations in the interstellar D/H ratio are primarily due to deuterium depletion onto dust grains, particularly polycyclic aromatic hydrocarbons (PAHs), rather than differences in stellar astration. Gas-grain reactions can strongly deuterate carbonaceous dust, and grain destruction releases D into the gas, predicting a positive correlation between D/H and gas-phase abundances of depleted elements like Fe, Si, and Ti—supported by observed correlations in recent data.

ABSTRACT

The D/H ratio in interstellar gas varies on scales of a few hundred pc in the local Milky Way, with D/H values ranging from ~7 ppm to ~22 ppm. The reduction in D/H relative to the primordial value of D/H (~ 26 ppm) is usually attributed to "astration" -- conversion of D into other elements by nuclear fusion in stars. However, it is shown here that astration has difficulty accounting for the observations because the expected associated variations in O/H are not seen. The lower D/H values are instead likely due to "depletion" of the D onto dust grains. Polycyclic aromatic hydrocarbons (PAHs) are a possible repository for the missing D, and it appears possible for gas-grain reactions to achieve extreme deuteration of carbonaceous grain material. Grain destruction will release D from the grains; the gas phase abundance of D should therefore be positively correlated with the gas phase abundances of other elements that exhibit strong depletions, such as Mg, Si, Ti, and Fe, which also will be returned to the gas by grain destruction.

Motivation & Objective

• Explain the observed 4-fold variation in interstellar D/H ratios (5–22 ppm) within a few hundred parsecs of the Sun.
• Challenge the conventional explanation based on variable stellar astration, which fails to account for the lack of correlated O/H variations.
• Propose that deuterium is sequestered in dust grains, particularly PAHs, as an alternative mechanism for D/H variation.
• Predict a positive correlation between gas-phase D/H and abundances of strongly depleted elements (e.g., Fe, Si, Ti) due to grain destruction.
• Provide a testable hypothesis via laboratory analysis of interstellar dust collected by the Stardust mission.

Proposed method

• Model gas-grain reactions involving D atoms and PAH cations to assess the efficiency of deuteration in cool H I clouds.
• Estimate time scales for D depletion from the gas phase via dust incorporation, finding depletion occurs on ~2 Myr timescales.
• Use chemical evolution models to compare expected O/H vs. D/H trends under astration scenarios, showing poor agreement with observations.
• Analyze observed D/H and O/H data from 14 sightlines to demonstrate the lack of anticorrelation expected under astration.
• Predict that interstellar dust grains—especially carbonaceous components—could have D/H ratios up to ~0.3, with PAHs potentially reaching D/H ≈ 0.36 in regions like M17.
• Propose laboratory analysis of Stardust mission samples to measure D/H in interstellar grains, testing the deuteration hypothesis.

Experimental results

Research questions

• RQ1 Why do observed D/H ratios vary by a factor of four across nearby sightlines without corresponding variations in O/H?
• RQ2 Can deuteration of interstellar dust grains, particularly PAHs, account for the observed D/H variations in the gas phase?
• RQ3 What is the expected correlation between gas-phase D/H and the abundances of refractory elements like Fe, Si, and Ti, given grain destruction as a source of D?
• RQ4 Can the deuteration of PAHs via gas-phase reactions achieve the necessary D/H ratios to explain the observed gas-phase D/H depletion?
• RQ5 Do the D/H ratios measured in interplanetary dust particles (IDPs) and Stardust samples support the hypothesis of highly deuterated interstellar dust?

Key findings

• The observed lack of anticorrelation between D/H and O/H in interstellar sightlines strongly contradicts the hypothesis that variations in D/H are due to differential astration.
• Deuteration of PAHs via gas-grain reactions can deplete D from the gas phase on timescales of ~2 Myr in cool H I clouds, sufficient to explain observed D/H variations.
• The predicted positive correlation between D/H and gas-phase abundances of depleted elements (e.g., Fe, Si, Ti) is supported by recent data showing a significant correlation between D/H and Fe/H.
• Polycyclic aromatic hydrocarbons (PAHs) are viable repositories for deuterium, with estimated D/H ratios in PAHs reaching up to 0.36 in regions like M17, consistent with required levels to explain gas-phase D/H depletion.
• Interstellar dust grains captured by the Stardust mission are expected to have D/H ratios of ~0.06 overall, with the aromatic component potentially reaching D/H ≈ 0.3, providing a direct test of the hypothesis.
• Laboratory measurements of D/H in Stardust samples could confirm whether interstellar dust is highly deuterated, offering a critical test of the proposed mechanism.