To appear in the July 1997 issue of the Astronomical Journal



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



Eric L. N. Jensen and Robert D. Mathieu - UW-Madison


ABSTRACT

Combining new infrared photometry with data in the literature, we have
compiled spectral energy distributions (SEDs) from optical to
millimeter wavelengths for four low-mass pre--main-sequence
spectroscopic binaries (a < 1 AU) showing infrared excesses.  Three of
these binaries have small or no excess emission at wavelengths of 1--5
microns but large excess emission at longer wavelengths, suggesting
cleared regions within circumbinary disks.  The inferred sizes of
these gaps or holes are consistent with theoretical predictions for
dynamical clearing by stellar companions.  In contrast, the binary AK
Sco has a power-law SED with no evidence of reduced near-infrared
emission.  Nonetheless, a strong 9.8 micron silicate emission feature
suggests the presence of optically thin dust, and the near-infrared
emission can be produced by very small amounts (roughly 10^{-9} M_sun)
of material within a gap or hole.  Thus a dynamically cleared region
may exist in AK Sco but is not required by the data.

That three of these binaries (and several other short-period
pre--main-sequence binaries) have near-infrared excesses indicative of
hot material near the stars is significant given that circumstellar
disk radii are limited to less than 0.06 AU (or several stellar
radii).  The spatial distribution of the near-infrared-emitting
material may not be disk-like.  Continued replenishment is necessary,
perhaps from circumbinary disks.

Circumbinary disk masses around short-period pre--main-sequence
binaries range from 0.06 M_sun to less than 10^{-4} M_sun. The larger
disk masses are sufficient to substantially influence the binaries'
orbital eccentricities during their pre--main-sequence lifetimes.