Infrared properties of weak radio sources in the rho Ophiuchi molecular cloud

We report mid-infrared ISOCAM observations for a sample of radio continuum sources in the rho Ophiuchi molecular cloud core with unknown or poorly stu died infrared counterparts. These data are combined with existing infrared photometry, including recently published ISOCAM data, for previously studie d radio sources to investigate the evolutionary states of 35 radio-emitting young stars in the rho Oph cloud core. About 50% of the radio stars are fo und to have class I, flat, or class II spectral energy distributions with n ear-infrared excesses arising from circumstellar disks. Their radio emissio n is most likely thermal emission from gas ionized by stellar winds. The re maining radio emitters are young stars with class III spectral energy distr ibutions that lack infrared excesses and circumstellar disks. Their radio e mission likely is nonthermal emission from magnetic surface activity. The l ack of young stars with weak infrared excesses supports earlier suggestions that disk dissipation is rapid. Class III sources are twice as common as c lass II sources in this radio-selected sample in comparison with extinction -limited samples, which underscores the importance of radio surveys in obta ining a complete census of young stellar objects. The concentration of disk less class III objects in the high column-density molecular core, and their median age of 0.3 Myr derived from their positions in a Hertzsprung-Russel l diagram, indicate they are contemporaries of class II objects which inclu de the classical T Tauri stars. It appears that these class III objects hav e shorter disk survival times than class II objects in the cloud.