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Monday, August 16, 2010 / Duncan Forgan

Thesis Anthology Part 4: Flick the Disc to Light it Up


Let’s soldier on with the thesis! Today I’m talking about encounters again.  Last time we showed that breaking up a disc this way was very difficult.  This time I’ll be talking about outburst phenomena.When stars are formed, they come ready made with a disc (just like we discussed previously).  The disc feeds the star over a period of about 100,000 years, growing it until it reaches its final mass.  Astronomers can estimate how quickly the star gets fed (what we call the accretion rate).  What they find is that the discs aren’t feeding the star quickly enough.  Unless those stars clean their plates quickly, they simply won’t form quickly enough for us to see the stars we see.

What we do see is that the discs go through phases of gorging.  The accretion rate goes up very quickly, then goes back to normal after a few years.  But why?  The current opinion suggests a mix of different instabilities in the disc joining together to force the star to chow down.  What I investigated in my thesis is the possibility of encounters resulting in an outburst.

And it turns out, that you can make a star gorge by passing a star nearby.  The passing star heats and twists the disc, spinning it up like a dynamo and boosting the accretion rate.  However, it doesn’t quite look like the gorging events astronomers see.  We looked into this as well – this particular type of event is pretty infrequent (you have to get stars to come very close to each other, about as close as Neptune is to our Sun, which happens very rarely except in really dense star clusters).  Also, because the stars end up embedded deep in the disc, they get heavily obscured (unless you observe at very long wavelengths, which reduces the obscuration).  I’ve talked about this quite a bit in previous posts, so I’ll leave it there.

Next time, we’ll be thinking about isolated discs – and examining the feeding process in much greater detail.

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