Mid-Infrared Observations of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We have obtained new mid-infrared observations of 65 Class I/flat-spectrum (FS) objects in the Perseus, Taurus, Chamaeleon I and II, r Ophiuchi, and Serpens dark clouds. These objects represent a subset of the young stellar objects (YSOs) from our previous near-infrared multiplicity surveys. We detected 45 out of 48 (94%) of the single sources, 16 out of 16 (100%) of the primary components, and 12 out of 16 (75%) of the secondary/triple components of the binary/multiple objects surveyed. One target, IRS 34, a 031 separation FS binary, remains unresolved at near-infrared wavelengths. The composite spectral energy distributions for all of our sample YSOs are either Class I or FS, and in 15 out of 16 multiple systems at least one of the individual components displays a Class I or FS spectral index. However, the occurrence of mixed pairings, such as FS with Class I, FS with Class II, and, in one case, an FS with a Class III (Cha I T33B), is suprisingly frequent. Such behavior is not consistent with that of multiple systems among T Tauri stars (TTSs), where the companion of a classical TTS also tends to be a classical TTS, although other mixed pairings have been previously observed among Class II YSOs. Based on an analysis of the spectral indices of the individual binary components, there appears to be a higher proportion of mixed Class I/FS systems (65%-80%) than that of mixed classical and weak-lined TTSs (25%-40%), demonstrating that the envelopes of Class I/FS systems are rapidly evolving during this evolutionary phase. In general, the individual binary/multiple components suffer very similar extinctions, Av, suggesting that most of the line-of-sight material is either in the foreground of the molecular cloud or circumbinary. We report the discovery of a steep spectral index secondary companion to ISO-Cha I 97, detected for the first time via our mid-infrared observations. In our previous near-infrared imaging survey of binary/multiple Class I and FS YSOs, ISO-Cha I 97 appeared to be single. With a spectral index of a >= +3.9, the secondary component of this system is a member of a rare class of very steep spectral index YSOs, those with a > +3. Only three such objects have previously been reported, all of which are either Class 0 or Class I.