A Near-Infrared Multiplicity Survey of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We present new near-IR observations of 76 Class I/flat-spectrum objects inthe nearby (d < 320 pc) Perseus, Taurus, Chamaeleon I and II, rho Ophiuchi, andSerpens dark clouds. These observations are part of a larger systematicinfrared multiplicity survey of self-embedded objects in the nearest darkclouds. When combined with the results of our previously publishednear-infrared multiplicity survey, we find a restricted companion star fractionof 14/79 (18% +/- 4%) of the sources surveyed to be binary or higher ordermultiple systems over a separation range of ~300 - 2000 AU with a magnitudedifference $\Delta$ K <= 4, and with no correction for background contaminationor completeness. This is consistent with the fraction of binary/multiplesystems found among older pre-main-sequence T Tauri stars in each of theTaurus, rho Ophiuchi, and Chamaeleon star-forming regions over a similarseparation range, as well as the combined companion star fraction for theseregions. However, the companion star fraction for solar-type, and lower mass Mdwarf, main-sequence stars in the solar neighborhood in this separation range(11% +/- 3%) is approximately one-half that of our sample. Together withmultiplicity statistics derived for previously published samples of Class 0 andClass I sources, our study suggests that a significant number ofbinary/multiple objects may remain to be discovered at smaller separationsamong our Class I/flat-spectrum sample and/or most of the evolution ofbinary/multiple systems occurs during the Class 0 phase of early stellarevolution.Comment: 23 pages, 8 figures, to appear in the March 2004 A