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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 project at Manly Astrophysics, in collaboration with Mark Wardle (Macquarie Uni), has been investigating possible structures for interstellar clouds that are cold and dense. Such clouds may be of great importance because they are expected to be difficult to detect: there might be a lot of mass in this form that we have not yet discovered. Insights or constraints obtained from models are therefore valuable. It turns out that the theoretical situation is very interesting, with novel aspects of cloud structure emerging as a result of the condensation and precipitation of molecular hydrogen. Condensation of hydrogen occurs because of the exceptionally cold and dense conditions under consideration. For most of the parameter space the condensed form is solid- rather than liquid-H2, so that inside each cloud there are hydrogen snowflakes that are drifting toward the centre. These clouds are therefore termed "hydrogen snow clouds" in our paper, published this week in the Astrophysical Journal. Here and elsewhere on this site we use instead the term "paleons", motivated by the idea that such objects may have formed in the early universe and would thus be ancient.

Mass-Radius

 

Clouds that have low temperatures will always tend to have large radii, for a given mass, because the mass-to-radius ratio of a cloud in equilibrium is closely related to its internal temperature. The presence of hydrogen snowflakes makes our model paleons larger still, puffing them out to radii much greater than they would have in the absence of any snow. So although our model structures all have masses in the planetary range, their radii are much greater than that of any planet, as shown in the diagram at left. The green area shows roughly the region occupied by our model paleons, whereas the planets of the Solar System are shown as red dots, and the blue line shows the locus of main-sequence stars. Note that both axes of this plot are logarithmic and each spans a range of 1010.

 

 

 

An interesting aspect of the models is that the surface temperatures approach absolute zero. That is important because it means that there is negligible thermal evaporation from the surface of each paleon, whereas previously it had been assumed that significant mass would be lost over the age of the Universe. It is also surprising, because space is filled with a thermal background radiation at approximately 3 degrees above absolute zero; the background radiation heats the gas in each paleon and it had been thought to set a lower limit on the temperature everywhere inside. Our models show us that the surprisingly low surface temperatures are in turn due to another surprising feature of the paleons: heat is convected by fluid motions from the cold surface to the warmer interior, so heat supplied by the background radiation is moved from the surface to the interior, whence it is radiated away. This novel behaviour is a direct consequence of an internal composition gradient - hydrogen-rich in the centre, hydrogen-poor near the surface - which arises because of internal snowfall.

Other aspects of these models are described on the paleons page. Full details are given in the paper, which can be found here.

MW 15th August 2019