The hypothesis that Herbig Ae/Be stars are young stars of intermediate mass (1.5 < M*/M. < 10) surrounded by circumstellar accretion disks is examined. Analysis of the spectral energy distributions for 47 cataloged Herbig Ae/Be stars leads to their classification into three groups. Group I includes 30 stars with large infrared (IR) excesses characterized by spectral slopes lambdaF(lambda) approximately lambda-4/3. Infrared spectral energy distributions (lambda greater-than-or-similar-to 2.2 mum) for these objects can be well fitted by assuming that excess emission above photospheric levels arises in a geometrically flat, optically thick circumstellar accretion disk. The inner regions of these accretion disks (from the stellar surface to a distance of several stellar radii) must be optically thin in order to account for distinctive inflections in their observed near-infrared (1.2 mum less-than-or-similar-to lambda less-than-or-similar-to 2.2 mum) spectral energy distributions. Group II includes 11 objects with flat or rising infrared spectra. These objects appear best interpreted as young, intermediate-mass stars or star/disk systems surrounded by gas and dust which is not confined to a disk. Indirect arguments suggest that most of these systems may be viewed through remnant infalling envelopes and, if so, might be regarded as the evolutionary precursors of the group I objects. Group III consists of six stars with small infrared excesses, whose infrared spectral energy distributions appear similar to those of classical Be stars in which modest excesses above photospheric levels seem to arise from free-free emission in a gaseous circumstellar disk or envelope. Nevertheless, their association with star-forming molecular clouds and their proximity to other young stars suggests that they are young, intermediate-mass stars which lack disks and which may be analogs of diskless T Tauri stars. Basic disk parameters for the group I objects are derived from extant optical and infrared photometry and from newly measured millimeter continuum flux densities: masses for the disks (along with any remnant envelope material contained within the millimeter-antenna beam) are in the range 0.01 < M(disk)/M. < 6; lower limits to outer disk radii are in the range 15 < R(disk)/AU < 175; inner optically thin disk regions have radii 3 < R(hole)/R* < 25; disk accretion luminosities are in the range 12 < L(acc)/L. < 1800, while the deduced disk mass accretion rates are in the range 6 x 10(-7) < M(acc) < 8 x 10(-5) M. yr-1. Derived Balmer line luminosities for the group I objects correlate well with our accretion luminosities and extend the relationships between these quantities derived for T Tauri stars. The group I Herbig Ae/Be stars thus appear to be the intermediate-mass analogs of T Tauri stars, whose unusual photometric and spectroscopic properties are believed to derive from a variety of physical processes (e.g., winds, boundary-layer emission) linked to the presence of a circumstellar accretion disk. If disk material arrives on the stellar surface at the derived rates (M(acc)) throughout the pre-main-sequence lifetime of an intermediate-mass star, then a significant fraction of the mass comprising the star must pass through a disk. Moreover, because the disk accretion luminosity can be comparable to or greater than the stellar luminosity, the energy input to the star by accreting disk material may be sufficient in many cases to alter the pre-main-sequence evolutionary paths followed by these intermediate-mass stars. Finally, it is noteworthy that among the Herbig Ae/Be stars, disk accretion rates and possibly envelope infall rates appear to be higher for stars of higher mass. This result may represent an important step toward understanding the protostellar core initial conditions which determine infall and accretion rates, and ultimately the stellar mass formed from collapsing core material.
