2MASS J−H versus H−K_s diagram showing most Be stars (black dots) distinctly separated from Herbig stars (squares), except a few Be stars with very large near-infrared excess (open circles). The sample of Be stars is taken from Zhang et al. (2005), and that of Herbig stars is from de Winter et al. (2001). The crosses indicate the levels of the free-free emission with varying electron number densities and emitting volumes. From bottom left, each cross represents an increase of an order, from 10⁰ to 10⁸ times in emissivity (see text) surrounding a B0 V star with a gas temperature of 20 000 K. The first three crosses (10⁰ to 10² times) and the last two crosses (10⁷ to 10⁸ times), are, respectively, almost overlapped. The gray and black curves are giant and dwarf loci (Bessell & Brett 1988) converted to the 2MASS system. The arrow shows the reddening vector (Rieke & Lebofsky 1985) for an average Galactic total-to-selective extinction (R = 3.1).