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 μm near to the galactic plane for 12CO (J = 1–0) emission; 1077 sources were detected. Among their far-infrared sources without detectable CO emission, we found that at least 18 objects are invisible at optical and near-infrared wavelengths. The infrared spectral indices between 2.2 μm and 25 μm correspond to those of class I YSOs, and the IRAS colors are similar to those of the usual YSOs. These peculiar far-infrared objects are highly concentrated around the galactic plane and the distances are estimated to be ∼1 kpc. Although their distribution is away from molecular clouds, some of them seem to be associated with large dark clouds or weak radio sources. These objects are possible YSOs with low CO abundance in the envelopes.