Chemistry in circumstellar disks: CS toward HL Tauri

Abstract

High-resolution millimeter-wave aperture synthesis images of the CS J = 2 → 1 and dust continuum emission toward the young star HL Tauri have been combined with single-dish spectra of the higher J CS transitions in order to probe the chemical and physical structure of circumstellar material in this source. We find that the extended molecular cloud surrounding HL Tau is similar to other Taurus dark cloud cores, having T_(kinetic) ≈ 10-20 K, n_(H2) ≈ 10^4-10^5 cm^(-3) , and x(CS) = N(CS)/N(H_2) ≈ (1-2) x 10^(-8). In contrast, the gas-phase CS abundance in the circumstellar disk is depleted by factors of at least 25-50, and perhaps considerably more. These results are consistent with substantial depletion onto grains, or a transition from kinetically controlled chemistry in the molecular cloud to thermodynamically controlled chemistry in the outer regions of the circumstellar disk. Dust continuum emission at 3.06 mm, although unresolved in a 3".0 beam, appears centered on the stellar position; combined with other millimeter-wave measurements its intensity indicates an emissivity index of β = 1.2 ± 0.3. This β may reflect grain growth via depletion and aggregation or compositional evolution, and suggests that the 3.06 mm dust opacity exceeds unity within 8-10 AU of HL Tauri. Even at millimeter and submillimeter wavelengths, observational studies of other high dipole moment molecules in circumstellar disks may also be hampered by the combination of grain mantle depletion and dust opacity structure in sources viewed nearly edge-on.