CO line emission and absorption from the HL Tauri disk - Where is all the dust?

We present high-resolution infrared spectra of HL Tau, a heavily embedded young star. The spectra exhibit broad emission lines of (CO)-C-12 gas-phase molecules, as well as narrow absorption lines of (CO)-C-12, (CO)-C-13, and (CO)-O-18. The broad emission lines of vibrationally excited (CO)-C-12 are dominated by the hot (T ∼ 1500 K) inner disk. The narrow absorption lines of CO are found to originate from the circumstellar gas at a temperature of ∼ 100 K. The (CO)-C-12 column density for this cooler material [(7.5 ± 0.2); 10(18) cm(-2)] indicates a large column of absorbing gas along the line of sight. In dense interstellar clouds, this column density of CO gas is associated with A(V)∼ 52 mag. However, the extinction toward this source (A(V)∼ 23) suggests that there is less dust along the line of sight than inferred from the CO absorption data. We discuss three possibilities for the apparent paucity of dust along the line of sight through the flared disk: (1) the dust extinction has been underestimated because of differences in circumstellar grain properties, such as grain agglomeration; (2) the effect of scattering has been underestimated and the actual extinction is much higher; or (3) the line of sight through the disk is probing a gas-rich, dust-depleted region, possibly due to the stratification of gas and dust in a preplanetary disk. Through the analysis of hot rovibrational (CO)-C-12 line emission, we place strong constraints on grain growth and thermal infrared dust opacity, and separately constrain the enhancement of carbon-bearing species in the neighboring molecular envelope. The physical stratification of gas and dust in the HL Tau disk remains a viable explanation for the anomalous gas-to-dust ratio seen in this system. The measured radial velocity dispersion in the outer disk is consistent with the thermal line widths of the absorption lines, leaving only a small turbulent component to provide gas-dust mixing.