Maximum stable central density of a starless core with (solid line) and without (dashed line) photoelectric heating at the core boundary. For the mass indicated on the abscissa, the log of the maximum density in the center of the core is indicated on the ordinate. The figure shows that the inclusion of the photoelectric heating at the core edge marginally improves the stability of the more massive cores. The cores here are modeled with the only internal energy being thermal. The addition of some nonthermal energy in the form of microturbulence or magnetic energy would raise the curves to higher density.