First-overtone CO variability in young stellar objects

We have monitored the 2.3 mu m Delta upsilon = 2 CO bands toward young stellar objects that are predominantly of low luminosity (L < 10,000 L-.) and show evidence of mass loss. The CO emission can be highly variable, on timescales as short as a few days. In DG Tau, the bands have been observed to disappear and reappear and may also have a periodic modulation. For the BN object, the equivalent width has varied by a factor of 3; and for V1331 Cyg, the equivalent width doubled in 9 days. Changes in equivalent width also occurred in SSV 13, AS 353A, and 1548C27, while WL 16 and S106 have not shown variations. Of the nine CO absorption sources reobserved, only the FU Ori-type star V1057 Cpg went through a significant change in equivalent width. It also presents an unusual and variable band profile.