A single circumbinary disk in the HD 98800 quadruple system

We present subarcsecond thermal infrared imaging of HD 98800, a young quadr uple system composed of a pair of low-mass spectroscopic binaries separated by 0".8 (38 AU), each with a K-dwarf primary. Images at wavelengths rangin g from 5 to 24.5 mu m show unequivocally that the optically fainter binary, HD 98800B, is the sole source of a comparatively large infrared excess on which a silicate emission feature is superposed. The excess is detected onl y at wavelengths of 7.9 mu m and longer, peaks at 25 mu m, and has a best-f it blackbody temperature of 150 K, indicating that most of the dust lies at distances greater than the orbital separation of the spectroscopic binary. We estimate the radial extent of the dust with a disk model that approxima tes radiation from the spectroscopic binary as a single source of equivalen t luminosity. Given the data, the most likely values of disk properties in the ranges considered are R-in = 5.0 +/- 2.5 AU, Delta R = 13 +/- 8 AU, lam bda(0) = 2(-1.5)(+4) mu m, gamma = +/- 2.5 and sigma(total) = 16 +/- 3 AU(2 ), where R-in is the inner radius, Delta R is the radial extent of the disk , lambda(0) is the effective grain size, gamma is the radial power-law expo nent of the optical depth tau, and sigma(total) is the total cross section of the grains. The range of implied disk masses is 0.001-0.1 times that of the Moon. These results show that, for a wide range of possible disk proper ties, a circumbinary disk is far more likely than a narrow ring.