Since I haven’t had much to write about recently (or, haven’t felt much like writing recently) I thought I would highlight one of my recent papers. It was lead by James Cadman, a PhD student who is been working with me, and is essentially an extension of some work done by Cassandra Hall, who worked with me while a PhD student in Edinburgh (and who is about to start a tenure-track position at the University of Georgia).As I may have mentioned before, stars form from clouds of gas and dust that collapse under their own gravity. Conservation of angular momentum prevents most of the material from falling directly onto the young protostar in the centre; instead, most of the material forms a protostellar disc through which mass can then flow onto the central protostar.
When very young, these discs may be massive enough to be what we call self-gravitating, which could lead to them forming spiral density waves (left), much like what is seen in spiral galaxies (as an aside, these spiral waves may then play a role in driving mass onto the central protostar).
Recently, the latest astronomical instruments (ALMA) have become able to directly observe spiral density wavs in protostellar discs. These may be due to the disc self-gravity but they could also be due to some kind of interaction, with either an embedded planet, or a passing star.
In this latest work we wanted to better understand the properties of the systems that would most likely show evidence for these self-gravitating spiral density waves. The systems need to be young (less than 1 million years old) and they need to be reasonably strongly accreting. However, what we’ve shown in this recent paper is that it might be slightly easier to observe these spirals than previously thought, if some amount of grain grain has already occurred in the system.
The reason for this is related to some work I did about 15 years ago. It turns out that the interaction between dust grains and gas in a protostellar disc depends on the size of the dust grains. Very small grains are very strongly coupled to the gas, while very large grains are completely de-coupled. There’s an intermediate size (in the mm-cm range) where this interaction causes grains to drift towards pressure maxima. A consequence of this is that these grains will drift, and collect, in the spiral density waves.Hence, if grain growth has occurred in these discs, and you observe at wavelengths that are sensitive to the emissions from these grains, the collection of these grains will enhance emission in these spirals, making them easier to observe. So, even when these spirals are quite weak, we may still be able to observe them.
The figure on the right illustrates how, if you take this grain enhancement effect into account, the spirals become more evident in synthetic observations. What’s more, you can potentially use these observations to constrain the growth of solid particles in these discs. When these discs form, the grains are pre-dominantly micron-sized, and don’t emit much at wavelengths longer than this. If you start to see emission at longer wavelengths (in, for example, the sub-millimeter and millimeter) then you can infer that some amount of grain growth must have occurred.
So, what we’ve shown in this recent paper is that it may be easier to observe these self-gravitating spirals than we had previously thought, and we’ve illustrated the optimal observations for doing so. The reason this is important is that these spiral waves may play an important role in driving mass onto the central protostar. Directly observing them may then allow us to probe a crucial part of the star formation process.
In addition, these observations could also help us to better understand grain growth in these very young systems. Grain growth is, of course, a key part of the planet formation process and there are indications that it starts very early in the star formation process. It’s still not clear how micron-sized dust grains grow to form the kilometre-sized planetesimals that then combine to form terrestrial planets, or the cores of the gas giants. Being able to probe grain growth during the earliest stages of star formation may help us to resolve this mystery.
The observational impact of dust trapping in self gravitating discs – Cadman et al. 2020.
Spiral density waves – a post about one of the first observations of spiral density waves in a protostellar disc.
Observing the earliest stages of star and planet formation – another one of my posts about this topic.