A while ago I wrote a post about an ALMA observation of a very young stellar object (Elias 2-27) that shows spiral density waves in its circumstellar disc (see Figure on the right). A reason I wrote that post is because I had also written a Science Perspectives piece about it.
One of the interesting things about this ALMA observation is the possibility that the spiral density waves could be driven by a gravitational instability, which can occur if the disc mass is comparable to that of the central protostar (by comparable, I mean more than about one tenth the mass of the central protostar). The reason I’m writing this post is that we’ve just had a paper published (Hall et al. 2018) in which we tested this by running a suite of numerical simulations with a range of parameters similar to that estimated for Elias 2-27.In some cases, the disc was so unstable that it essentially broke up into fragments. In other cases, the instability was so weak as to essentially not even be present. However, in some cases, the instability did lead to the presence of obvious spiral density waves (Figure on the left).
One problem, though, is that simply looking at the output from some numerical simulation of a circumstellar disc does not really tell you what you would actually see if you observed such a system. Cass Hall, who did a PhD with me and is now a postdoc in Leicester, did some excellent work running the simulation output through a radiation transport code (which tells us what these discs will actually emit) and then taking the emissions maps and determining what the Atacama Large Millimeter/sub-millimeter Array (ALMA) would actually observe.The figure on the right shows the result from the analysis of one of our simulations and does indeed look quite similar to the actual observation of Elias 2-27 (first figure in this post). However, very few of our simulations produced results that seemed consistent with the Elias 2-27 observation. Essentially, the range of parameter space over which this seems possible appears quite narrow. This might suggest that the spiral waves observed in Elias 2-27 are due to something other than a gravitational instability, maybe a perturbation due to some kind of companion (as suggested in this paper).
There is, however, one other interesting possibility. We are pretty confident that the building blocks of planet formation must grow during the earliest stages of star formation. In the presence of spiral density waves, small particles will tend to collect near the density maxima, enhancing the emission from the spirals and potentially making them easier to observe. We did consider this in our paper (and I plan to do some more on this, if I ever get the time) and it is likely to somewhat increase the range of parameter space over which such spirals are observable.
Therefore, if we do start to see such structures in more and more discs around very young stars, and if they are (sometimes, at least) a consequence of a gravitational instability, then we could use these observations to better understand how the early stages of star formation influence the subsequent growth of planets. However, the range of parameter space in which this is possible is still reasonably narrow, so it may still be unlikely that we would regularly be able to observe such a phase. It’s an interesting possibility, nonetheless.