The Upper Limit for CH4 in the Protostellar Disk toward HL Tauri

We used high-resolution infrared spectra of the heavily embedded T Tauri star HL Tau to search for evidence of absorption due to the  R 0,  R 1, and  R 2 gas-phase CH 4  ν 3  lines near 3.3 μm. From this, we report a 3 σ upper limit of 1.3 × 10 15  cm -2  for the CH 4  gas column density toward HL Tau. Our results are compared to those found for CO gas toward this source and to the recent model for chemistry in the inner (10 AU) disks around T Tauri stars by Markwick et al. We find that the upper limit of methane ice+gas column density toward HL Tau, when compared to CO, is somewhat lower than but consistent with that measured toward other interstellar sources (~1%) but that it is much lower than that predicted in the Markwick et al. model and much less than the CH 4 /CO ratio (10%-80%) found in cometary volatiles. This has important implications for the processing of interstellar material and its incorporation into planetary bodies.