Surface density of gas in a viscously spreading disk. Crosses are as in Fig. 1. The lines represent the surface densities at various dimensionless times T, as in eq. (9). Assuming ﹩\alpha =3\times 10^{-3}﹩, these correspond to physical times (top to bottom) 0, 0.1, 0.3 (in bold), 1, 2 (dotted), 3, and 10 Myr. While an instantaneous match to the surface density profile can be found, the gas density rapidly decreases afterward.