indicate sputtering timescale
We normally assume steady state but do not indicate sputtering timescales. We have this note from Peter:
Draine & Salpeter (1979) ApJ 231, 77 give a simple formula in Eq. 44.
tau = 2e4 yr / n_H * (a/0.01 micron)
We could add a test on this and, in the PRINT AGES report, say something safe like "a 0.01 micron grain would survive less than xxx years in this environment".
The above does not seem right - the Orion Nebula has n_H = 1e4 cm-3, so tau ~ 1 year, but there is hot dust??