Infrared observations of young stars - V. The faint members of the Orion population.

New infrared observations are presented of I ZO faint stars of the Orion Population in the - to I 8- region. Brightness in the infrared and a flat energy distribution are found to be common characteristics. Nebulous and non-nebulous T Tauri variables in the Taurus dark clouds are found to be distinct in a number of infrared properties: it is suggested that the nebulous objects are more massive and/or somewhat younger than the other stars. The hypothesis that circumstellar dust grains thermalize stellar ultraviolet and visible radiation to produce the infrared by thermal emission 15 strengthened by the new data. A remarkably uniform picture of early stellar evolution is emerging from the infrared observations of stars in several major complexes of young objects.