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Monday, July 12, 2010 / Duncan Forgan

Thesis Anthology Part 3: Breaking Up a Disc Is Hard To Do


Here’s part 3 of my thesis anthology.  I’ve set the scene for my thesis, and I’ve told you a little about the methods I’ve developed to do my science.  I’m now going to tell you exactly what I did with those methods.

In part 2, I put a pretty picture of one of my simulated discs fragmenting into pieces:

That’s quite a nice picture, but we find that it’s very difficult in reality to make discs do this.  To find out why, we have to know what conditions we need to fulfill to make a disc break up.

Firstly, the disc needs to be gravitationally unstable.  This means the disc has to have a “strong” gravitational field (which means it needs to be massive), and it needs to be able to concentrate that mass well, which means the disc needs to be thin.  How thin the disc can get depends on two things:

i) How hot the disc is – if the disc is too hot, then the gas in the disc has to expand under its own pressure, and the gravity of the disc gets “diluted”.

ii) How fast the disc rotates.  Think about clay on a potter’s wheel.  If the wheel spins very fast, the clay will tend to fly away from the centre.  The same thing applies here – faster rotating discs spread out their gas further, making the disc thinner.

Let’s assume then that our gas is sufficiently cool and thin to be gravitationally unstable.  If that’s true, then it will start to develop spiral structure, like it does in the picture above.  But, this spiral structure is churning the disc, heating it up with shock waves (like the sonic boom from a fighter jet).  This means that the disc becomes hot, and stabilises.

The spiral structure can act like a thermostat: if the disc gets too cold, it heats up the disc with shocks – if it gets too hot, then the spiral structure disappears and the disc can cool by radiation.  Discs usually find a temperature that balances these effects, and the spiral structure remains in a quasi-steady state.

If we want to break up this disc, we need to break this thermostat.  We need to make the disc too cold or, give it too much rotation (i.e., too much angular momentum).  To make the disc too cold, it must find a way of quickly losing its energy (e.g. by radiation).  To make the disc spin too fast, we can add matter to the disc (say by accreting it from its surroundings).  Either of these things will result in the break-up of the disc like we see above (in fact, the reason this disc breaks up is because it starts out with too much rotation, so it is doomed to break up).

Unfortunately, the conditions required for disc break-up are just not found in nature.  The discs don’t cool quickly enough except in their outer regions (where there isn’t much mass).  Some scientists say (and I’m inclined to agree) the reason we don’t see fragmenting discs is because they break up very quickly (within a few thousand years, a blink of an eye for star systems).  We shouldn’t expect to be lucky enough to catch a system in the act (although we may have with HL Tau).

Anyway, if discs find it hard to break up, can we force them to do it? One question I asked was whether we could break up a disc by having a binary companion fly past.  If the gravitational pull of the binary dragged out enough matter from the disc into regions where it could cool quickly, perhaps we could break up the disc that way.  Sadly, I discovered that even when you use this trick, the discs in general don’t break up.

This doesn’t completely rule out this stimulated means of break-up – discs come in a lot of shapes and sizes, and there may be certain combinations of disc parameters that are susceptible to break-up if a companion comes by (with the right orbit – another set of parameters to tune as well).

Tune in next time for some more studies of stellar encounters, where I look at their potential role in outburst events.

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