Effect of Ice Charging on the Astrochemistry of Interstellar Sulfur Bearing Species on Amorphous Solid Water

summary

Video file (mp4)

The gist

The provided data consists of computational results detailing benchmarks for interstellar sulfur-bearing species, specifically focusing on SO 2 and related species (BE0, BE-1, and their difference

In short

The episode discusses a paper analyzing how an excess negative charge on interstellar ice affects sulfur-bearing species. The authors propose that treating ice as a charged, active surface—rather than a neutral one—significantly increases the binding energy of molecules like ext SO 2. This suggests that much of the missing sulfur may be trapped in these solid phases, requiring major revisions to current chemical models.

Key concepts

Ice Charging
Interstellar ice grains can gain an excess negative charge from cosmic rays and energetic particles. The authors argue that this charge makes the ice a 'charged, active surface,' which fundamentally changes how molecules interact with it and increases binding strength.
Sulfur Depletion Problem
This decades-old puzzle concerns the apparent lack of observable sulfur in gas clouds. Researchers test the hypothesis that instead of being missing, sulfur is locked away or trapped within solid ice mantles on dust grains.
Amorphous Solid Water
This refers to interstellar ice that lacks a perfect crystal structure. It is described as 'messier,' featuring a huge variety of different binding sites across its surface, which affects the energy required to remove molecules from it.

Terminology used across episodes

This episode discusses

The paper

Effect of Ice Charging on the Astrochemistry of Interstellar Sulfur Bearing Species on Amorphous Solid Water · Read on arXiv

University of Antwerp · University of Montana

We aim to derive statistically robust and physically interpretable BE distributions for atomic S and the sulfur-bearing molecules H 2 S, SO 2, and OCS on neutral and negatively charged ASW, and assess how excess negative charge alters their retention and potential role in the sulfur reservoir of cold dense molecular clouds. Basis set superposition error (BSSE) and zero-point energy (ZPE) corrected BEs of S, SO 2, OCS and H 2 S, are calculated by density functional theory (DFT), using the ORCA software. Molecule-specific DFT levels of theory were first selected from benchmark calculations on neutral and charged small water complexes, against coupled-cluster reference energies. The selected protocols were then applied to study adsorption on neutral and charged ASW clusters. A range of adsorption sites were sampled on five independent amorphous ice clusters, yielding BE distributions that account for the site heterogeneity of ASW. Neutral ASW yields broad, site-dependent BE distributions consistent with previous water-ice estimates. On charged ASW, three general cases are identified: the BE will either always increase due to electron transfer (S-atom, SO 2), the BE increases slightly without any electron transfer (H 2 S) or the BE remains the same unless an electron transfer occurs under specific conditions (OCS). These findings are inherently linked to the molecular properties of these molecules.

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Next we'll be talking about the paper "Effect of Ice Charging on the Astrochemistry of Interstellar Sulfur Bearing Species on Amorphous Solid Water".

Jocelyn: The paper was written by T. Vorsselmans, I. Grubova, K. Verhagen, T. Guldentops, R. Buimer et al. from University of Antwerp and University of Montana.

Vera: Stay tuned as we take you through the paper and discuss its implications.

Title: Vera: We're looking at 'Effect of Ice Charging on the Astrochemistry of Interstellar Sulfur–Bearing Species on Amorphous Solid Water' by T. Vorsselmans and a team from the University of Antwerp.

Jocelyn: That title is quite a mouthful, Vera, but I'm guessing the 'ice charging' part is the real hook here?

Vera: It definitely is, because they're moving past the idea of ice being just a neutral, passive layer.

Jocelyn: Most people think of ice in space as just a frozen coating on a rock.

Vera: But these authors, Vorsselmans and his colleagues, are treating it as a charged, active surface.

Jocelyn: Is that because of cosmic rays or something?

Vera: Exactly, things like cosmic rays and energetic particles can leave these grains with an excess negative charge.

Jocelyn: Does this relate to that weird problem where we can't find enough sulfur in the gas clouds?

Vera: It's the sulfur depletion problem, which has been bothering us for decades.

Subrahmanyan: The missing sulfur is a massive headache for us because it's one of the essential elements for life, yet it's just... gone from our observations.

Jocelyn: So, are these authors suggesting the sulfur is just hiding on these charged ice mantles?

Subrahmanyan: That's the hypothesis they're testing, and it could fundamentally change how we track the chemical evolution of a molecular cloud.

Vera: They're specifically looking at how this charge might act as a trap for sulfur-bearing molecules.

Jocelyn: And they're using 'amorphous solid water,' which I assume isn't just a perfect ice cube?

Vera: No, it's much messier than that, with a huge variety of different binding sites across the surface.

Jocelyn: How does that messiness affect things like binding energy?

Vera: Binding energy is basically how much effort it takes to pull a molecule off the ice and back into the gas.

Jocelyn: So if the energy is high, the molecule stays put?

Vera: Exactly, and that's what's at stake here.

Jocelyn: I want to see if the actual data supports that idea of it being a trap.

Summary: Jocelyn: So, Vera, what did the team actually find when they ran these simulations on the different sulfur species?

Vera: The results were quite dramatic, especially for the sulfur atom and SO2.

Jocelyn: How dramatic are we talking?

Vera: For the sulfur atom, the binding energy jumped from about one thousand five hundred seventy-one Kelvin on neutral ice to over forty-three thousand Kelvin on the charged surface.

Jocelyn: That's a massive leap; it's like going from a light dusting to being glued down.

Vera: And SO2 saw a similar jump, going from around three thousand one hundred forty-five Kelvin to nearly thirty-five thousand Kelvin.

Subrahmanyan: The physical reason is that the SO2 molecule actually accepts an extra electron, becoming a negatively charged anion.

Jocelyn: What about H2S, since that's usually the big candidate for the sulfur reservoir?

Vera: H2S only saw a slight increase, from two thousand two hundred seventy-six Kelvin to about two thousand seven hundred seventeen Kelvin.

Subrahmanyan: The H2S anion is just too unstable to be a major player in this specific charged interaction.

Jocelyn: And OCS? I remember seeing that mentioned in the abstract as being a bit of an outlier.

Vera: OCS was mostly indifferent to the charge, staying around one thousand five hundred Kelvin, unless it lands right next to the electron.

Subrahmanyan: It's a very proximity-dependent situation for OCS, because it has to bend its structure to accept that electron.

Jocelyn: So the charge doesn't affect every molecule in the same way?

Vera: Not at all; it's completely dependent on the electronic properties of the molecule itself.

Jocelyn: They even showed spin density plots where you can see the extra electron sitting in a pocket between the H-atoms for H2S.

Vera: Yeah, it's a very specific way of visualizing where that charge actually lives in the complex.

Jocelyn: I'm curious how they actually managed to calculate these specific energy jumps with such precision.

Improvements: Vera: They used density functional theory, or DFT, specifically with the ORCA software to model these amorphous solid water clusters.

Jocelyn: Modeling amorphous ice sounds like a nightmare because it's so messy and irregular, doesn't it?

Vera: It is, so they used five different independent ice clusters to make sure their binding energy distributions were statistically robust.

Jocelyn: Did they have to double-check that their DFT math was actually accurate for these sulfur species?

Vera: They did a massive benchmark against much more expensive coupled-cluster calculations to pick the best settings for each molecule.

Subrahmanyan: That level of rigor is necessary because even a small error in binding energy can lead to massive mistakes in gas-grain models.

Jocelyn: So, if we've been using neutral ice in our models all this time, how much have we been missing?

Vera: We might have been missing the entire mechanism that locks sulfur into the solid phase.

Subrahmanyan: The paper basically describes a one-way valve where sulfur hits a charged grain and gets stuck almost immediately.

Jocelyn: That would certainly explain why our surveys aren't picking up the gas-phase abundance we expect.

Vera: They even modeled it as a random hit-and-stick process, which is how molecules actually land on grains in space.

Subrahmanyan: It's that site heterogeneity and the availability of dangling hydrogen atoms that makes the whole process so complex.

Jocelyn: It sounds like we need to overhaul our current chemical templates.

Vera: We definitely do, and this paper is a huge step in that direction.

Conclusion: Vera: We've covered a lot of ground with 'Effect of Ice Charging on the Astrochemistry of Interstellar Sulfur–Bearing Species on Amorphous Solid Water.'

Jocelyn: It really feels like a missing piece of the puzzle for the sulfur depletion problem.

Vera: It certainly does, and it's a reminder that even the 'empty' spaces between stars are chemically complex.

Subrahmanyan: If this holds up, we have to rethink the entire lifecycle of sulfur from diffuse clouds to protoplanetary disks.

Jocelyn: I'm looking forward to seeing the next generation of models that actually include these charged surfaces.

Subrahmanyan: The theoretical implications for how we interpret future JWST data are going to be massive.

Vera: Well, that's all the time we have for this one, but we'll be back with more soon.

Jocelyn: Goodbye for now!

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