Also I am currently engaged in writing out a hundred times, “symmetrical”. I knew there was something wrong with my first and second attempts at spelling it, but I forgot to look it up.

Ivan.

If it was simmetrical, it would not have attracted as much attention, but being lop-sided, we have to account for it. So if we have parts of two arms on one side, from our perspective, then we can expect more mass and/or greater extension in the view. By the way, one could see the same effect looking at a propellor or even a fan – three-bladed types of course.

Have we had enough time to get a fix on any differential movement of stars or mass of gas in those spirals?

Ivan.

Of course, the particles do interact. They collide, they have gravitational interactions, the bulk mass of the disk pulls them around, and probably fifteen other effects I haven’t thought of. Any such interaction is going to change the orbits of the objects involved. It probably won’t circularize them directly, but at the least the eccentricity and particularly the orientation of the ellipse they follow will change. If you imagine it as gradually randomizing the orientations, then after some amount of time you’ll have a lot of particles moving in ellipses, but the eliipses will be pointing every which way. If they get to the point that they’re evenly distributed, so that the typical size in every direction is the same, then you have a circular disk, even though all the orbits might be elliptical!

The orbits probably won’t stay elliptical, since elliptical orbits in an arrangement like that will be constantly crossing each other, causing lots of interactions; the system will tend to circularize (since circular orbits don’t interact so much, it’s a more stable configuration) and we’ll be back to a normal disk situation.

The interactions that happen in a circular disk situation are what allow material to migrate inward and ultimately fall into the central object. When the interactions fall off, because there just aren’t that many objects left, you’re left with a very long-lived disk or ring, like Saturn’s; it may be so sparse it’s a planetary system.

What I find really interesting about this one is that it must have gotten a kick – a particular, well-ordered kick – pretty recently, so it hasn’t had time to circularize itself. I wonder how long that process takes? I don’t even have an order-of-magnitude idea of the timescale. The shorter it is, the harder it is to believe some rare event caused this (because there’s less time for it to have happened in). So I bet the disk modelling people are having a field day with this one (and maybe a few shouted arguments with their cluster dynamics colleagues). Anyone have a number for the circularization time?

I think that question would be better answered by the BA, though, as I am a toxicologist, not an astronomer.

I wonder how the protoplanetary disk’s eccentricity effects planetary accretion? It might kick it into overdrive… Anybody you know running simulations?
Richard B. Drumm
Vice President, CAS

Oh, not that dust?