EVIDENCE FOR CLEARED REGIONS IN THE DISKS AROUND PRE-MAIN-SEQUENCE SPECTROSCOPIC BINARIES

Combining new infrared photometry with data in the literature, we have compiled spectral energy distributions (SEDs) from optical to millime ter wavelengths for four low-mass pre-main-sequence spectroscopic bina ries (a<1 AU) showing infrared excesses. Three of these binaries have small or no excess emission at lambda = 1-5 mu m but large excess emis sion at longer wavelengths, suggesting cleared regions within circumbi nary disks. The inferred sizes of these gaps or holes are consistent w ith theoretical predictions for dynamical clearing by stellar companio ns. In contrast, the binary AK Sco has a power-law SED with no evidenc e of reduced near-infrared emission. Nonetheless, a strong 9.8 mu m si licate emission feature suggests the presence of optically thin dust, and the near-infrared emission can be produced by very small amounts ( similar to 10(-9) M.) of material within a gap or hole. Thus a dynamic ally cleared region may exist in AK Sco but is not required by the dat a. That three of these binaries (and several other short-period pre-ma in-sequence binaries) have near-infrared excesses indicative of hot ma terial near the stars is significant given that circumstellar disk rad ii are limited to less than 0.06 AU (or several stellar radii). The sp atial distribution of the near-infrared-emitting material may not be d isk-like. Continued replenishment is necessary, perhaps from circumbin ary disks. Circumbinary disk masses around short-period pre-main-seque nce binaries range from 0.06 M. to less than 10(-4) M.. The larger dis k masses are sufficient to substantially influence the binaries' orbit al eccentricities during their pre-main-sequence lifetimes. (C) 1997 A merican Astronomical Society.