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

Astronomy and Astrophysics 559 (2013): A77 reproduced with permission from Astronomy & AstrophysicsWe 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 < NOH < 1016 cm-2, emitting radii 15 < r < 100 AU and excitation temperatures 100 < Tex < 400 K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n ≥ 10 10 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 (Tex ~ 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 sourcesSupport for this work, part of the Herschel open time key project program, was provided by NASA through an award issued by the Jet Propulsion Laboratory, California Institute of Technology. We are grateful to the DIGIT team for stimulating discussions and scientific support. Astrochemistry in Leiden is supported by the Netherlands Research School for Astronomy (NOVA), by a Spinoza grant and grant 614.001.008 from the Netherlands Organisation for Scientific Research (NWO), and by the European Community’s Seventh Framework Programme FP7/2007−2013 under grant 238258 (LASSIE) and grant 291141 (CHEMPLAN). PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESAPRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain). The research of M.G. has been supported by the Austrian Research Promotion Agency (FFG) through the ASAP initiative of the Austrian Federal Ministry for Transport, Innovation and Technology (BMVIT