Friday, February 1, 2013

Mac Low, M.-M.; McCray, R.; Superbubbles in disk galaxies

Superbubbles in disk galaxies
by: Mac Low, M.-M.; McCray, R.
[ADS: 1988ApJ...324..776M, pdf, first author: personal site, second author]
This paper is part of a series of papers done by Mac Low and McCray on this subject. This paper contains the theory and analytic backing on the subject, another paper in 1989 contains more of the simulations on the same subject. This paper and others formed the basis of Mac Low's PhD dissertation, "Interactions of Massive Stars with the Interstellar Medium: Bow Shocks and Superbubbles". McCray was his advisor. Mac Low would later go on to advise my advisor Fabian Heitsch for his PhD.

This particular paper is heavy on the theory and equations. Basically they are looking at how supernovas (SNs) interact with the ISM. As supernovas explode they release significant energy into the ISM and create a bubble of hot gas. If the bubble is large enough it gets classified as a superbubble, which has the possibility of blowing out of the galactic disk, which then affects the halo and galactic accretion.

This paper represents a significant step forward in our understanding of the structure of the ISM. The models produced here are idealized and smooth, meaning everything looks nice, flat and symmetric. 25 years later the technology and experience available to researchers has improved and thus we have moved on to solving this exact same problem, except in 3D and with a much more complex setup.

There are two important conclusions that I wanted to mention. They provide a parameter that determines whether or not a superbubble will blow out of a stratified galactic disk. They give it as:
D = LSN ρ01/2 Pe-3/2 H-2
where LSN is the luminosity from the SNs, ρ0 is the density of the galactic disk (or ISM), Pe is the external pressure from the ISM, and H is the scale height of the disk. If D > 100 then there will be blowout even if the center of the superbubble begins to collapse. It will be interesting to find a corresponding parameter for a more complex set of simulations.

The second important point is that if there is a dense cloud in the ISM then when the edge of the superbubble over runs it will not "puncture" the bubble leading to a release of pressure. The bubble will instead travel around it and continue expanding. This is something that has become a very important consideration since it is the thing that allows molecular clouds to survive strong shocks like this. This was essentially a hint at the beginning of the study of the survivability of cold molecular clouds when they have been strongly shocked. The problem is that if molecular clouds are strongly shocked then they will heat up, expand and will not collapse gravitationally to form stars. So there has to be some way for them to survive long enough to form stars. Many people will look into this problem later, and research is still going on.

As a note, they used the 2D hydro code Zeus, which was very influential back in the day. The creators of Zeus rewrote the code for MHD and 3D and named it Athena (original site). Athena is the code that I use for my research.

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