2013A&A...559A..77F

We present far-infrared (50-200µm) 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: [OI], [CII], CO, OH, H₂O, CH⁺. The most common feature is the [OI]63µm 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<10¹⁶cm^–2, emitting radii 15<r<100AU and excitation temperatures 100<T_ex<400K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n≥10¹⁰cm^–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∼100K) and compact (r∼60AU) emission in the case of HD 100546. Off-source [OI] emission is detected toward DG Tau, whose origin is likely the outflow associated with this source. The [CII] emission is spatially extended in all sources where the line is detected. This suggests that not all [CII] 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 ([OI]63µm, [OI]145µm, [CII]) are analyzed with PDR models and require high gas density (n>10⁵cm^–3) and high UV fluxes (G_o∼10³-10⁷), consistent with a disk origin for the oxygen lines for most of the sources.