We present far-infrared (50-200 {$μ$}m) 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... Show moreWe present far-infrared (50-200 {$μ$}m) 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, H$_{2}$O, CH$^{+}$. The most common feature is the [O i] 63 {$μ$}m 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 10$^{13}$ {lt} N$_{OH}$ {lt} 10$^{16}$ cm$^{-2}$, emitting radii 15 {lt} r {lt} 100 AU and excitation temperatures 100 {lt} T$_{ex}$ {lt} 400 K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n {ge} 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 H$_{2}$O 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/H$_{2}$O 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 {$μ$}m, [O i] 145 {$μ$}m, [C ii]) are analyzed with PDR models and require high gas density (n {gsim} 10$^{5}$ cm$^{-3}$) and high UV fluxes (G$_o$ ~{} 10$^{3}$ - 10$^{7}$), 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.orgShow less
We present 50-210 {$μ$}m spectral scans of 30 Class 0/I protostellar sources, obtained with Herschel-PACS, and 0.5-1000 {$μ$}m spectral energy distributions, as part of the Dust, Ice, and Gas in... Show moreWe present 50-210 {$μ$}m spectral scans of 30 Class 0/I protostellar sources, obtained with Herschel-PACS, and 0.5-1000 {$μ$}m spectral energy distributions, as part of the Dust, Ice, and Gas in Time Key Program. Some sources exhibit up to 75 H$_{2}$O lines ranging in excitation energy from 100 to 2000 K, 12 transitions of OH, and CO rotational lines ranging from J = 14 { arr} 13 up to J = 40 { arr} 39. [O I] is detected in all but one source in the entire sample; among the sources with detectable [O I] are two very low luminosity objects. The mean 63/145 {$μ$}m [O I] flux ratio is 17.2 {plusmn} 9.2. The [O I] 63 {$μ$}m line correlates with L $_{bol}$, but not with the time-averaged outflow rate derived from low-J CO maps. [C II] emission is in general not local to the source. The sample L $_{bol}$ increased by 1.25 (1.06) and T $_{bol}$ decreased to 0.96 (0.96) of mean (median) values with the inclusion of the Herschel data. Most CO rotational diagrams are characterized by two optically thin components (${$$ackslash$langle ${$ ${$N$}$$}$$ackslash$rangle$}$ = (0.70 +/- 1.12)${$${$$}$ $ackslash$times 10^{}${$49$}$$}$ total particles). ${$ ${$N$}$$}$_CO correlates strongly with L $_{bol}$, but neither T $_{rot}$ nor ${$ ${$N$}$$}$_CO(warm)/${$ ${$N$}$$}$_CO(hot) correlates with L $_{bol}$, suggesting that the total excited gas is related to the current source luminosity, but that the excitation is primarily determined by the physics of the interaction (e.g., UV-heating/shocks). Rotational temperatures for H$_{2}$O (${$$ackslash$langle ${$T_rot$}$$ackslash$rangle $}$ = 194 +/- 85 K) and OH (${$$ackslash$langle ${$T_rot$}$$ackslash$rangle $}$ =183 +/- 117 K) are generally lower than for CO, and much of the scatter in the observations about the best fit is attributed to differences in excitation conditions and optical depths among the detected lines. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Show less
CO is an important component of a protoplanetary disc as it is one of the most abundant gas phase species. Furthermore, observations of CO transitions can be used as a diagnostic of the gas,... Show moreCO is an important component of a protoplanetary disc as it is one of the most abundant gas phase species. Furthermore, observations of CO transitions can be used as a diagnostic of the gas, tracing conditions in both the inner and outer disc. We present Herschel/PACS spectroscopy of a sample of 22 Herbig Ae/Be (HAEBEs) and eight T Tauri stars (TTS), covering the pure rotational CO transitions from J = 14 { arr} 13 up to J = 49 { arr} 48. CO is detected in only five HAEBEs, namely AB Aur, HD 36112, HD 97048, HD 100546, and IRS 48, and in four TTS, namely AS 205, S CrA, RU Lup, and DG Tau. The highest transition detected is J = 36 { arr} 35 with E$_{up}$ of 3669 K, seen in HD 100546 and DG Tau. We construct rotational diagrams for the discs with at least three CO detections to derive T$_{rot}$ and find average temperatures of 270 K for the HAEBEs and 485 K for the TTS. The HD 100546 star requires an extra temperature component at T$_{rot}$ ~{} 900-1000 K, suggesting a range of temperatures in its disc atmosphere, which is consistent with thermo-chemical disc models. In HAEBEs, the objects with CO detections all have flared discs in which the gas and dust are thermally decoupled. We use a small model grid to analyse our observations and find that an increased amount of flaring means higher line flux, as it increases the mass in warm gas. CO is not detected in our flat discs as the emission is below the detection limit. We find that HAEBE sources with CO detections have high L$_{UV}$ and strong PAH emission, which is again connected to the heating of the gas. In TTS, the objects with CO detections are all sources with evidence of a disc wind or outflow. For both groups of objects, sources with CO detections generally have high UV luminosity (either stellar in HAEBEs or due to accretion in TTS), but this is not a sufficient condition for the detection of the far-IR CO lines. 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.orgShow less
Context. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a... Show moreContext. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a young stellar object. Spatially resolved information from its constituent gas and dust provides the necessary constraints to distinguish between different theories of accretion energy dissipation into the envelope. Aims. Our aims are to quantify the far-infrared line emission from low-mass protostars and the contribution of different atomic and molecular species to the gas cooling budget, to determine the spatial extent of the emission, and to investigate the underlying excitation conditions. Analysis of the line cooling will help us characterize the evolution of the relevant physical processes as the protostar ages. Methods. Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied in the context of the WISH key program. For most targets, the spectra include many wavelength intervals selected to cover specific CO, H$_{2}$O, OH, and atomic lines. For four targets the spectra span the entire 55-200 {$μ$}m region. The PACS field-of-view covers ~{}47'' with the resolution of 9.4''. Results. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 out of 18 objects (except TMC1A), including 5 Class I sources. The high-excitation H$_{2}$O 8$_{18}$-7$_{07}$ 63.3 {$μ$}m line (E$_u$/k$_B$ = 1071 K) is detected in 7 sources. CO transitions from J = 14-13 up to J = 49 - 48 are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ~{}350 K and ~{}700 K. H$_{2}$O has typical excitation temperatures of ~{}150 K. Emission from both Class 0 and I sources is usually spatially extended along the outflow direction but with a pattern that depends on the species and the transition. In the extended sources, emission is stronger off source and extended on {amp}{ge}10,000 AU scales; in the compact sample, more than half of the flux originates within 1000 AU of the protostar. The H$_{2}$O line fluxes correlate strongly with those of the high-J CO lines, both for the full array and for the central position, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH fluxes and not with [O I] fluxes. In contrast, [O I] and OH often peak together at the central position. Conclusions. The PACS data probe at least two physical components. The H$_{2}$O and CO emission very likely arises in non-dissociative (irradiated) shocks along the outflow walls with a range of pre-shock densities. Some OH is also associated with this component, most likely resulting from H$_{2}$O photodissociation. UV-heated gas contributes only a minor fraction to the CO emission observed by PACS, based on the strong correlation between the shock-dominated CO 24-23 line and the CO 14-13 line. [O I] and some of the OH emission probe dissociative shocks in the inner envelope. The total far-infrared cooling is dominated by H$_{2}$O and CO, with the fraction contributed by [O I] increasing for Class I sources. Consistent with previous studies, the ratio of total far-infrared line emission over bolometric luminosity decreases with the evolutionary state. Appendices A-J are available in electronic form at http://www.aanda.orgShow less
