Possible Young stellar objects without detectable CO emission

Young stellar objects (YSOs) usually appear in molecular clouds as infrared objects associated with a molecular envelope. Wouterloot and Brand (1989, AAA 50.133.012) searched 1302 IRAS point sources with reliable fluxes at 25 , 60, and 100 mu m near to the galactic plane for (CO)-C-12 (J = 1-0) emiss ion; 1077 sources were detected. Among their far-infrared sources without d etectable CO emission, we found that at least 18 objects are invisible at o ptical and near-infrared wavelengths. The infrared spectral indices between 2.2 mu m and 25 mu m correspond to those of class I YSOs, and the IRAS col ors are similar to those of the usual YSOs. These peculiar far-infrared obj ects are highly concentrated around the galactic plane and the distances ar e estimated to be similar to 1 kpc. Although their distribution is away fro m molecular clouds, some of them seem to be associated with large dark clou ds or weak radio sources. These objects are possible YSOs with low CO abund ance in the envelopes.