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Na I D lines in the spectrum of the young binary KH 15D have been analyzed indetail. We find an excess absorption component that may be attributed toforeground interstellar absorption, and to gas possibly associated with thesolids in the circumbinary disk. The derived column density is log N_NaI = 12.5cm^-2, centered on a radial velocity that is consistent with the systemicvelocity. Subtracting the likely contribution of the ISM leaves log N_NaI ~12.3 cm^-2. There is no detectable change in the gas column density across the"knife edge" formed by the opaque grain disk, indicating that the gas andsolids have very different scale heights, with the solids being highly settled.Our data support a picture of this circumbinary disk as being composed of avery thin particulate grain layer composed of millimeter-sized or largerobjects that are settled within whatever remaining gas may be present. Thisphase of disk evolution has been hypothesized to exist as a prelude to theformation of planetesimals through gravitational fragmentation, and is expectedto be short-lived if much gas were still present in such a disk. Our analysisalso reveals the presence of excess Na I emission relative to the comparisonspectrum at the radial velocity of the currently visible star that plausiblyarises within the magnetosphere of this still-accreting young star.

Gas Absorption in the KH 15D System: Further Evidence for Dust Settling in the Circumbinary Disk