Keck Diffraction-limited Imaging of the Young Quadruple Star System HD 98800

This paper presents diffraction-limited 1-18 ?m images of the young quadruple star system HD 98800 obtained with the W. M. Keck 10 m telescopes using speckle and adaptive optics imaging at near-infrared wavelengths and direct imaging at mid-infrared wavelengths. The two components of the visual binary, A and B, both themselves spectroscopic binaries, were separable at all wavelengths, allowing us to determine their stellar and circumstellar properties. Combining these observations with spectroscopic data from the literature, we derive an age of ~107yr, masses of 0.93 and 0.64 M?, and an inclination angle of 58° for the spectroscopic components of HD 98800 B and an age of ~107yr and a mass of 1.1 M?for HD 98800 Aa. Our data confirm that the large mid-infrared excess is entirely associated with HD 98800 B. This excess exhibits a blackbody temperature of 150 K and a strong 10 ?m silicate emission feature. The theoretical equilibrium radius of large, perfectly absorbing, 150 K grains around HD 98800 B is 2.4 AU, suggesting a circumspectroscopic binary distribution. Our observations set important upper limits on the size of the inner dust radius of ~2 AU (from the mid-infrared data) and on the quantity of scattered light of less than 10% (from the H-band data). For an inner radius of 2 AU, the dust distribution must have a height of at least 1 AU to account for the fractional dust luminosity of ~20% LB. Based on the scattered-light limit, the dust grains responsible for the excess emission must have an albedo of less than 0.33. The presence of the prominent silicate emission feature at 10 ?m implies dust grain radii of 2 ?m. The total mass of the dust is greater than 0.002 M?. We conclude that the dust is located in a circumbinary disk around the HD 98800 B spectroscopic binary with an inner gap of ~2 AU and a height of 1 AU, and we speculate that the A-Bc orbital dynamics are responsible for the characteristics of the observed dust in the system.