Evidence for Dust Stratification In Disks Around Young Stars
          T. Rettig (University of Notre Dame)
          Sean Brittain (NOAO)
          T. Simon (University of Hawaii)
          C. Kulesa (University of Arizona)
          E. Gibb (University of Notre Dame)

Abstract:           The physical structure of circumstellar disks plays a pivotal role in the sculpting of planetary systems.  If dust preferentially settles to the midplane from flared disks of young stellar systems, then we expect the gas-to-dust ratio along our line of sight to the central star to be higher for face-on systems than for highly inclined edge-on systems.  If the gas and grains remain mixed in the extended disk atmosphere, the gas/dust ratio in the disk should be generally independent of the disk inclination.  High-resolution absorption line spectroscopy in the near-infrared provides a unique blend of milliarcsecond-scale angular resolution, detailed kinematic information, and direct measurement of chemically important molecular species like CO.   Using absorption lines of CO from a sample of YSO disk systems with a diverse range of inclination angles and measurable dust extinction, the resulting gas/dust ratio provides a direct measure of the degree of dust settling a! nd thus, disk stratification.  The stratification of dust and gas in circumstellar disks bears directly on the process by which planetesimals form, a prerequisite for subsequent planet formation.  Identifying the processes that dominate the initial conditions for planet formation is critical to meaningful progress in this field.  Nevertheless, observational constraints on formation theories have remained elusive.  Infrared spectra of a statistical sample of disks at various inclinations will be used to measure the stratification of dust and gas to provide constraints for disk and planet formation processes.