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I'm trying to understand circumstellar disks. One term that keeps cropping up in this context is "surface density". The disks, though flattened, are still three-dimensional, so I'm confused about what "surface density" actually means. I can think of a couple of possibilities:

Try as I might, I can't seem to find a clear-cut definition for it.

- The hypothetical surface density of the projection of the disk onto its mean plane
- The surface density of its cross section taken at some distance R from the star

Cheers

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# Surface density in 3D?

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