DIGIT survey of far-infrared lines from protoplanetary disks. I. [O i], [C ii], OH, H₂O, and CH⁺

Abstract

We present far-infrared (50-200 μm) spectroscopic observations of young pre-main-sequence stars taken with Herschel/PACS as part of the DIGIT key project. The sample includes 16 Herbig AeBe and 4 T Tauri sources observed in SED mode covering the entire spectral range. An additional 6 Herbig AeBe and 4 T Tauri systems have been observed in SED mode with a limited spectral coverage. Multiple atomic fine structure and molecular lines are detected at the source position: [O i], [C ii], CO, OH, H₂O, CH⁺. The most common feature is the [O i] 63 μm line detected in almost all of the sources, followed by OH. In contrast with CO, OH is detected toward both Herbig AeBe groups (flared and non-flared sources). An isothermal LTE slab model fit to the OH lines indicates column densities of 10¹³ < N_OH < 10¹⁶ cm^-2, emitting radii 15 < r < 100 AU and excitation temperatures 100 < T_ex < 400 K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n ≥ 10¹⁰ cm^-3) are needed to reproduce the observations. The OH emission thus comes from a warm layer in the disk at intermediate stellar distances. Warm H₂O emission is detected through multiple lines toward the T Tauri systems AS 205, DG Tau, S CrA and RNO 90 and three Herbig AeBe systems HD 104237, HD 142527, HD 163296 (through line stacking). Overall, Herbig AeBe sources have higher OH/H₂O abundance ratios across the disk than do T Tauri disks, from near- to far-infrared wavelengths. Far-infrared CH⁺ emission is detected toward HD 100546 and HD 97048. The slab model suggests moderate excitation (T_ex ~ 100 K) and compact (r ~ 60 AU) emission in the case of HD 100546. Off-source [O i] emission is detected toward DG Tau, whose origin is likely the outflow associated with this source. The [C ii] emission is spatially extended in all sources where the line is detected. This suggests that not all [C ii] emission is associated with the disk and that there is a substantial contribution from diffuse material around the young stars. The flux ratios of the atomic fine structure lines ([O i] 63 μm, [O i] 145 μm, [C ii]) are analyzed with PDR models and require high gas density (n ≳ 10⁵ cm^-3) and high UV fluxes (G_o ~ 10³ - 10⁷), consistent with a disk origin for the oxygen lines for most of the sources.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.org