A Single Circumbinary Disk in the HD 98800 Quadruple System

We present sub-arcsecond thermal infrared imaging of HD 98800, a youngquadruple system composed of a pair of low-mass spectroscopic binariesseparated by 0.8'' (38 AU), each with a K-dwarf primary. Images at wavelengthsranging from 5 to 24.5 microns show unequivocally that the optically fainterbinary, HD 98800B, is the sole source of a comparatively large infrared excessupon which a silicate emission feature is superposed. The excess is detectedonly at wavelengths of 7.9 microns and longer, peaks at 25 microns, and has abest-fit black-body temperature of 150 K, indicating that most of the dust liesat distances greater than the orbital separation of the spectroscopic binary.We estimate the radial extent of the dust with a disk model that approximatesradiation from the spectroscopic binary as a single source of equivalentluminosity. Given the data, the most-likely values of disk properties in theranges considered are R_in = 5.0 +/- 2.5 AU, DeltaR = 13+/-8 AU, lambda_0 =2(+4/-1.5) microns, gamma = 0+/-2.5, and sigma_total = 16+/-3 AU^2, where R_inis the inner radius, DeltaR is the radial extent of the disk, lambda_0 is theeffective grain size, gamma is the radial power-law exponent of the opticaldepth, tau, and sigma_total is the total cross-section of the grains. The rangeof implied disk masses is 0.001--0.1 times that of the moon. These results showthat, for a wide range of possible disk properties, a circumbinary disk is farmore likely than a narrow ring.