To explain the Extreme Scattering Events, observed towards some radio quasars, requires an unseen population of self-gravitating gas clouds. We call these clouds "paleons", because they're probably ancient. The scintillation data suggest that individual paleons have masses in the planetary range (e.g. a few times the mass of the Earth), and radii that are a few AU (Astronomical Units: 1 AU is the mean distance between the Earth and the Sun). This interpretation of the Extreme Scattering Events implies that paleons contribute a substantial fraction of the mass of our Galaxy, and presumably other galaxies too. They are a type of "dark matter", and could perhaps be the dominant form of matter in the Universe. Because paleons are difficult to see, to understand them we have to turn to theory. Modelling their structure allows us to sketch out what their properties could be, because those properties must be consistent with the laws of physics.

Despite the large differences in physical characteristics, we model the structure of paleons in much the same way as one models the structure of stars. In both cases the main idea is that the outward pressure of the gas is balanced by the inward pull of gravity, and that limits the possible density and pressure variations within each object (i.e. paleon or star).

To proceed with the modelling we need to know how the pressure of the gas relates to its density and temperature. In part that relationship is determined by what the gas is made of. We expect it to be almost entirely hydrogen and helium, in the ratio 3:1 (by mass), because that seems to be the average composition of the things in the Universe that we can see. For paleons we also expect that the hydrogen atoms react with each other much more rapidly than they dissociate, so that the gas should be mainly molecular hydrogen (H2) and atomic helium.

If paleons were just a gas of H2 and helium they would be relatively simple to model. What makes them more interesting is that they also contain a sprinkling of solid H2. Hydrogen snowflakes can only appear in the outermost layers of a paleon, where the gas is coldest. But the solid is much denser than the gas so they drift downwards towards the centre. Although only a miniscule fraction of a paleon is in the form of snowflakes at any instant, snowfall is a continuous process so it gradually causes the outer layers to become hydrogen-poor and the central regions hydrogen-rich. That process will continue until the gas in the centre becomes slightly buoyant and starts to bubble up through the hydrogen-poor envelope - that causes mixing between the inner and outer regions, which stops any further change in the hydrogen/helium ratio.

Our models will provide us with some insights into the nature of paleons, and they will allow us to determine how paleons might appear to an observer. That is a critical issue for objects which have so far only been identified via radio-wave scintillation, and we will therefore explore all reasonable possibilities. For example: paleons are cold, but should nevertheless emit a little in the millimetre-wave region of the spectrum so we will calculate the expected properties of that emission. If we can predict the characteristics of the emission we're in a better position to search for it with a telescope. It's actually the snowflakes which emit the thermal millimetre-wave radiation, and they also introduce absorption and scattering of starlight. That's another way that we might be able to detect paleons - as one passes between us and a more-distant star - providing we know what to look for.

This work is in collaboration with Mark Wardle (Macquarie Uni).

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