Aims.We present $3.65''$ $\times$ $3.34''$ angular-resolution IRAM Plateau de Bure Interferometer (PdBI) observations of the CS J = 2–1 line toward the Horsehead Photodissociation Region (PDR), ...complemented with IRAM-30m single-dish observations of several rotational lines of CS, C34S and HCS+. We analyse the CS and HCS+ photochemistry, excitation and radiative transfer to obtain their abundances and the physical conditions prevailing in the cloud edge. Since the CS abundance scales to that of sulfur, we determine the gas phase sulfur abundance in the PDR, an interesting intermediate medium between translucent clouds (where sulfur remains in the gas phase) and dark clouds (where large depletions have been invoked). Methods.A nonlocal non-LTE radiative transfer code including dust and cosmic background illumination adapted to the Horsehead geometry has been developed to carefuly analyse the CS, C34S, HCS+ and C18O rotational line emission. We use this model to consistently link the line observations with photochemical models to determine the CS/HCS+/S/S+ structure of the PDR. Results.Densities of $n({\rm H}_2)\simeq(0.5{-}1.0)$ $\times$ 105 cm-3 are required to reproduce the CS and C34S J = 2–1 and 3–2 line emission. CS J = 5–4 lines show narrower line widths than the CS low-J lines and require higher density gas components not resolved by the ~10'' IRAM-30m beam. These values are larger than previous estimates based in CO observations. We found $\chi({\rm CS})$ = (7 ± 3) $\times$ 10-9 and $\chi({\rm HCS}^{+})$ = (4 ± 2) $\times$ 10-11 as the averaged abundances in the PDR. According to photochemical models, the gas phase sulfur abundance required to reproduce these values is S/H = (3.5 ± 1.5) $\times$ 10-6, only a factor $\la$4 less abundant than the solar sulfur elemental abundance. Since only lower limits to the gas temperature are constrained, even lower sulfur depletion values are possible if the gas is significantly warmer. Conclusions.The combination of CS, C34S and HCS+ observations together with the inclusion of the most recent CS collisional and chemical rates in our models implies that sulfur depletion invoked to account for CS and HCS+ abundances is much smaller than in previous studies.
C+ is a key species in the interstellar medium, but its 158 μm fine structure line cannot be observed from ground-based telescopes. Current models of fluorine chemistry predict that CF+ is the ...second-most important fluorine reservoir in regions where C+ is abundant. We detected the J = 1-0 and J = 2-1 rotational lines of CF+ with high signal-to-noise ratio toward the photo-dissociation region and dense core positions in the Horsehead. Using a rotational diagram analysis, we derive a column density of N(CF+) = (1.5-2.0) × 1012 cm-2. Because of the simple fluorine chemistry, the CF+ column density is proportional to the fluorine abundance. We thus infer the fluorine gas-phase abundance to be F/H = (0.6-1.5) × 10-8. Photochemical models indicate that CF+ is found in the layers where C+ is abundant. The emission arises in the UV-illuminated skin of the nebula, tracing the outermost cloud layers. Indeed, CF+ and C+ are the only species observed to date in the Horsehead with a double-peaked line profile caused by kinematics. We therefore propose that CF+, which is detectable from the ground, can be used as a proxy of the C+ layers.
We report on the discovery of strong intensity variations in the high rotational lines of abundant molecular species towards the archetypical circumstellar envelope of IRC+10216. The observations ...have been carried out with the HIFI instrument on board
and with the IRAM 30-m telescope. They cover several observing periods spreading over 3 years. The line intensity variations for molecules produced in the external layers of the envelope most probably result from time variations in the infrared pumping rates. We analyze the main implications this discovery has on the interpretation of molecular line emission in the envelopes of Mira-type stars. Radiative transfer calculations have to take into account both the time variability of infrared pumping and the possible variation of the dust and gas temperatures with stellar phase in order to reproduce the observation of molecular lines at different epochs. The effect of gas temperature variations with stellar phase could be particularly important for lines produced in the innermost regions of the envelope. Each layer of the circumstellar envelope sees the stellar light radiation with a different lag time (phase). Our results show that this effect must be included in the models. The sub-mm and FIR lines of AGB stars cannot anymore be considered as safe intensity calibrators.
Context. The HIFI instrument on the Herschel Space Observatory performed over 9100 astronomical observations, almost 900 of which were calibration observations in the course of the nearly four-year ...Herschel mission. The data from each observation had to be converted from raw telemetry into calibrated products and were included in the Herschel Science Archive. Aims. The HIFI pipeline was designed to provide robust conversion from raw telemetry into calibrated data throughout all phases of the HIFI missions. Pre-launch laboratory testing was supported as were routine mission operations. Methods. A modular software design allowed components to be easily added, removed, amended and/or extended as the understanding of the HIFI data developed during and after mission operations. Results. The HIFI pipeline processed data from all HIFI observing modes within the Herschel automated processing environment as well as within an interactive environment. The same software can be used by the general astronomical community to reprocess any standard HIFI observation. The pipeline also recorded the consistency of processing results and provided automated quality reports. Many pipeline modules were in use since the HIFI pre-launch instrument level testing. Conclusions. Processing in steps facilitated data analysis to discover and address instrument artefacts and uncertainties. The availability of the same pipeline components from pre-launch throughout the mission made for well-understood, tested, and stable processing. A smooth transition from one phase to the next significantly enhanced processing reliability and robustness.
We report the detection of emission in the J = 1−0 rotational transition of hydrogen fluoride (HF), together with observations of the J = 1−0 to J = 3−2 rotational lines of H35Cl and H37Cl, towards ...the envelope of the carbon star IRC +10216. High-sensitivity, high-spectral resolution observations have been carried out with the HIFI instrument on board Herschel, allowing us to resolve the line profiles and providing insights into the spatial distribution of the emission. Our interpretation of the observations, with the use of radiative transfer calculations, indicates that both HF and HCl are formed in the inner regions of the envelope close to the AGB star. Thermochemical equilibrium calculations predict HF and HCl to be the major reservoirs of fluorine and chlorine in the atmospheres of AGB stars. The abundances relative to H2 derived for HF and HCl, 8 × 10-9 and 10-7 respectively, are substantially lower than those predicted by thermochemical equilibrium, indicating that F and Cl are likely affected by significant depletion onto dust grains, although some chlorine may be in the form of atomic Cl. The H35Cl/H37Cl abundance ratio is 3.3 ± 0.3. The low abundance derived for HF in IRC +10216 makes it likely that the fluorine abundance is not enhanced over the solar value by nucleosynthesis in the AGB star, although this conclusion may not be robust because the HF abundance we derive is a lower limit to the elemental abundance of F. These observations suggest that both HF and HCl should be detectable through low J rotational transitions in other evolved stars.
We report the detection of absorption lines by the reactive ions OH+, H2O+and H3O+ along the line of sight to the submillimeter continuum source G10.6–0.4 (W31C). We used the Herschel HIFI instrument ...in dual beam switch mode to observe the ground state rotational transitions of OH+ at 971 GHz, H2O+ at 1115 and 607 GHz, and H3O+ at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6–0.4 as well as in the molecular gas directly associated with that source. The OH+ spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O+ lines remains lower than 1 in the same velocity range, and the H3O+line shows only weak absorption. For LSR velocities between 7 and 50 km s-1 we estimate total column densities of N(OH+) ≥ 2.5 × 1014 cm-2, N(H2O+) ~6 × 1013 cm-2 and N(H3O+) ~4.0 × 1013 cm-2. These detections confirm the role of O+ and OH+ in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH+ by the H2O+ column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.
Strong winds and ultraviolet (UV) radiation from O-type stars disrupt and ionize their molecular core birthplaces, sweeping up material into parsec-size shells. Owing to dissociation by starlight, ...the thinnest shells are expected to host low molecular abundances and therefore little star formation. Here, we expand previous maps made with observations using the IRAM 30 m telescope (at 11″ ≃ 4500 AU resolution) and present square-degree 12CO and 13CO (J = 2–1) maps of the wind-driven “Veil bubble” that surrounds the Trapezium cluster and its natal Orion molecular core (OMC). Although widespread and extended CO emission is largely absent from the Veil, we show that several CO “globules” exist that are blueshifted in velocity with respect to OMC and are embedded in the C II 158 μm-bright shell that confines the bubble. This includes the first detection of quiescent CO at negative local standard of rest velocities in Orion. Given the harsh UV irradiation conditions in this translucent material, the detection of CO globules is surprising. These globules are small (Rg = 7100 AU), not massive (Mg = 0.3 M⊙), and are moderately dense: nH = 4 × 104 cm−3 (median values). They are confined by the external pressure of the shell, Pext∕k ≳ 107 cm−3 K, and are likely magnetically supported. They are either transient objects formed by instabilities or have detached from pre-existing molecular structures, sculpted by the passing shock associated with the expanding shell and by UV radiation from the Trapezium. Some represent the first stages in the formation of small pillars, others of isolated small globules. Although their masses (Mg <MJeans) do not suggest they will form stars, one globule matches the position of a known young stellar object. The lack of extended CO in the “Veil shell” demonstrates that feedback from massive stars expels, agitates, and reprocesses most of the disrupted molecular cloud gas, thereby limiting the star-formation rate in the region. The presence of molecular globules is a result of this feedback.
Aims. The late stages of stellar evolution are mainly governed by the mass of the stars. Low- and intermediate-mass stars lose copious amounts of mass during the asymptotic giant branch (AGB) which ...obscure the central star making it difficult to study the stellar spectra and determine the stellar mass. In this study, we present observational data that can be used to determine lower limits to the stellar mass. Methods. Spectra of nine heavily reddened AGB stars taken by the Herschel Space Observatory display numerous molecular emission lines. The strongest emission lines are due to H sub(2)O. We search for the presence of isotopologues of H sub(2)O in these objects. Results. We detected the super(16)O and super(17)O isotopologues of water in these stars, but lines due to H sub(2) super(18)O are absent. The lack of super(18)O is predicted by a scenario where the star has undergone hot-bottom burning which preferentially destroys super(18)O relative to super(16)O and super(17)O. From stellar evolution calculations, this process is thought to occur when the stellar mass is above 5 M sub(middot in circle) for solar metallicity. Hence, observations of different isotopologues of H sub(2)O can be used to help determine the lower limit to the initial stellar mass. Conclusions. From our observations, we deduce that these extreme OH/IR stars are intermediate-mass stars with masses of > or =5 M sub(middot in circle). Their high mass-loss rates of ~10 super(-4)M sub(middo t in circle) yr super(-1) may affect the enrichment of the interstellar medium and the overall chemical evolution of our Galaxy.
Context. A discrepancy exists between the abundance of ammonia (NH3) derived previously for the circumstellar envelope (CSE) of IRC+10216 from far-IR submillimeter rotational lines and that inferred ...from radio inversion or mid-infrared (MIR) absorption transitions. Aims. To address the discrepancy described above, new high-resolution far-infrared (FIR) observations of both ortho- and para-NH3 transitions toward IRC+10216 were obtained with Herschel, with the goal of determining the ammonia abundance and constraining the distribution of NH3 in the envelope of IRC+10216. Methods. We used the Heterodyne Instrument for the Far Infrared (HIFI) on board Herschel to observe all rotational transitions up to the J = 3 level (three ortho- and six para-NH3 lines). We conducted non-LTE multilevel radiative transfer modelling, including the effects of near-infrared (NIR) radiative pumping through vibrational transitions. The computed emission line profiles are compared with the new HIFI data, the radio inversion transitions, and the MIR absorption lines in the ν2 band taken from the literature. Results. We found that NIR pumping is of key importance for understanding the excitation of rotational levels of NH3. The derived NH3 abundances relative to molecular hydrogen were (2.8 ± 0.5) × 10-8 for ortho-NH3 and (3.2 -0.6+0.7) × 10 -8 for para-NH3, consistent with an ortho/para ratio of 1. These values are in a rough agreement with abundances derived from the inversion transitions, as well as with the total abundance of NH3 inferred from the MIR absorption lines. To explain the observed rotational transitions, ammonia must be formed near to the central star at a radius close to the end of the wind acceleration region, but no larger than about 20 stellar radii (1σ confidence level).
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