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Manly Astrophysics is a scientific institute dedicated to research in astronomy.

We focus on understanding astronomical data in terms of the underlying physics.

Our research is described in the "Projects" section, at a level suitable for non-specialists.


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A long-running research theme here at Manly Astro is the physics of the very cold regions of the interstellar medium. Material that is very cold and dense emits little radiation and is thus a type of dark matter. Because such material is difficult to see it's important to pursue theoretical work that can provide us with some insights - both for a better understanding of the possibilities and also to stimulate novel observational approaches. Our latest study highlights a new feature associated with solid molecular hydrogen: a plethora of optical absorption lines that arise when ions are trapped inside the solid.

Previous work has revealed properties of the sort of clouds wherein solid H2 snowflakes might form; some features are quite surprising - e.g. the cloud surfaces can be colder than the 3 degree background radiation that permeates all of space (see these project pages). Lately we've been wondering how H2 snowflakes might appear if they're removed from their native clouds (e.g. by the action of radiation pressure) and subjected to the harsh conditions of interstellar space. In that environment we already know that there will be ionisation of the molecules in the solid - in consequence of ultraviolet radiation from stars, for example. Interestingly, the resulting molecular ions can be quite different from the ones that arise in the gas phase of molecular hydrogen (see these project pages). We also know that the electric fields from the ions can suppress sublimation from the surface of the solid, and as a result H2 snowflakes may be able to survive for a long time in interstellar space (see these project pages).

An electric field suppresses the sublimation of H2 because it makes each molecule more tightly bound to the solid. In other words: the molecules are in a state of lower energy because they're sitting in an electric field. Each molecule also tends to align with the field, increasingly so for stronger fields - as shown below.

Alignment of H2 in electric fields of various strengths In the absence of an electric field the separation vector between the two atomic nuclei in H2 is equally likely to point in any direction, so the corresponding probability distribution is spherical. But in the presence of an electric field the molecule achieves a lower energy state when the nuclear separation vector is aligned with the electric field, so that is the preferred orientation - as shown here for various field strengths. The electric field is oriented vertically in each of the cases shown.

All of the molecules in the solid are normally in their lowest energy state, but it's possible to kick any one of them up to a higher energy state for a short time. And indeed that happens routinely when light passes through the solid because some of that light is absorbed by the molecules.

Rotation and vibration of an H2 molecule in an electric field

If they're given some energy, H2 molecules are able to rotate and vibrate, even when they're situated in a solid matrix, as demonstrated in the animation to the left. In strong electric fields the character of the rotation changes because the energy of the molecule depends on its orientation relative to the field direction. And the character of the vibration changes because the field modifies the internuclear potential energy.

Now the really interesting thing is that an electric field modifies each of the rotation-vibration states of the H2 molecule differently, so that light is absorbed at wavelengths that are quite different from the pattern that is usually expected from H2. So different, in fact, that if you were simply shown the absorptions themselves you wouldn't guess they were due to molecular hydrogen - you'd be left puzzled as to the cause.

It might be possible to find the predicted absorptions. Spectroscopy of stars is routinely undertaken by astronomers, and if there are some H2 snowflakes between us and the star that we're observing then we should see less starlight at wavelengths where it's absorbed by the molecules.

Stellar spectra do indeed reveal a large number of interstellar absorption lines that don't correspond to any known spectral pattern of substances that have been studied in the laboratory. These lines are known as "Diffuse Interstellar Bands", or DIBs for short, and were first noticed a century ago by Mary Lea Heger when she was a graduate student at Berkeley. Heger recorded two of the lines that we now call DIBs - two of the strongest, of course - and ascertained that they might be interstellar in nature. Modern studies, using large telescopes and more sensitive spectrographs, have catalogued hundreds of DIBs; their origin remains mysterious.

Star spectra with/without DIBs How DIBs might appear to the eye if you could view a hot, bright star through a glass prism behind a large telescope. The lower spectrum shows the DIBs as dark, vertical lines; the upper spectrum shows a comparison star without any DIBs.

So are the DIBs actually just due to H2 molecules in an electric field? It's possible, but our theoretical models of the absorptions are not yet good enough to decide one way or the other. Ideally we'd like to be able to compare wavelengths and line strengths from model predictions (or laboratory measurements) with those of the DIBs, but more work is needed before those comparisons can be made. Stay tuned!

If you're interested in all the technical details you can find the full paper here (at number 32); it was published today in The Astrophysical Journal.


MW 8th June 2022