Search

Using the upGREAT instrument on SOFIA, we imaged the [C II] 158 µm fine structure line emission in bright-rimmed pillars located at the southern edge of the IC 1848 H II region, and carried out pointed observations of the [O I] 63 and 145 µm fine structure lines toward selected positions. The observations are used to characterize the morphology, velocity field, and the physical conditions in the G1–G3 filaments. The velocity-resolved [C II] spectra show evidence of a velocity shift at the head of the brightest G1 filament, possibly caused by radiation pressure from the impinging UV photons or the rocket effect of the evaporating gas. Archival Herschel PACS and SPIRE observations imply H2 column densities in the range 1021 –1022 cm−2 , corresponding to maximum visual extinction AV ≃ 10 mag, and average H2 volume density of ≃4500 cm−3 in the filaments. The [C II] emission traces ∼17% of the total H2 column density, as derived from dust SED fits. Photon-dominated region models are unable to explain the observed line intensities of the two [O I] fine structure lines in IC 1848, with the observed [O I] 145 µm line being too strong compared to the model predictions. The [O I] lines in IC 1848 are overall weak and the signal-to-noise ratio is limited. However, our observations suggest that the [O I] 63/145 µm intensity ratio is a sensitive probe of the physical conditions in photon-dominated regions such as IC 1848. These lines are thus excellent targets for future high-altitude balloon instruments, less affected by telluric absorption. [ABSTRACT FROM AUTHOR]

Context. Recent studies of the optical depth comparing [12C II] and [13C II] line profiles in Galactic star-forming regions have revealed strong self-absorption in [12C II] by low excitation foreground material. This implies a high column density for C+, corresponding to equivalent AV values of a few (up to about 10) mag. Aims. As the nature and origin of such a great column of cold C+ foreground gas are difficult to determine, it is essential to constrain the physical conditions of this material. Methods. We conducted high-resolution observations of [O I] 63 μm and [O I] 145 μm lines in M17 SW and Mon R2. The [O I] 145 μm transition traces warm PDR-material, while the [O I] 63 μm line traces the foreground material, as manifested by the absorption dips. Results. A comparison of both [O I] line profiles with [C II] isotopic lines confirm warm PDR-origin background emission and a significant column of cold foreground material, causing the self-absorption to be visible in the [12C II] and [O I] 63 μm profiles. In M17 SW, the C+ and O0 column densities are comparable for both layers. Mon R2 exhibits larger O0 columns compared to C+, indicating additional material where the carbon is neutral or in molecular form. Small-scale spatial variations in the foreground absorption profiles and the large column density (~1018 cm−2) of the foreground material suggest the emission is coming from high-density regions associated with the cloud complex – and not a uniform diffuse foreground cloud. Conclusions. The analysis confirms that the previously detected intense [C II] foreground absorption is attributable to a large column of low-excitation dense atomic material, where carbon is ionized and oxygen is in a neutral atomic form. [ABSTRACT FROM AUTHOR]

Context. Mid-infrared emission features are important probes of the properties of ionized gas and hot or warm molecular gas, which are difficult to probe at other wavelengths. The Orion Bar photodissociation region (PDR) is a bright, nearby, and frequently studied target containing large amounts of gas under these conditions. Under the "PDRs4All" Early Release Science Program for JWST, a part of the Orion Bar was observed with MIRI integral field unit (IFU) spectroscopy, and these high-sensitivity IR spectroscopic images of very high angular resolution (0.2″) provide a rich observational inventory of the mid-infrared (MIR) emission lines, while resolving the H II region, the ionization front, and multiple dissociation fronts. Aims. We list, identify, and measure the most prominent gas emission lines in the Orion Bar using the new MIRI IFU data. An initial analysis summarizes the physical conditions of the gas and demonstrates the potential of these new data and future IFU observations with JWST. Methods. The MIRI IFU mosaic spatially resolves the substructure of the PDR, its footprint cutting perpendicularly across the ionization front and three dissociation fronts. We performed an up-to-date data reduction, and extracted five spectra that represent the ionized, atomic, and molecular gas layers. We identified the observed lines through a comparison with theoretical line lists derived from atomic data and simulated PDR models. The identified species and transitions are summarized in the main table of this work, with measurements of the line intensities and central wavelengths. Results. We identified around 100 lines and report an additional 18 lines that remain unidentified. The majority consists of H I recombination lines arising from the ionized gas layer bordering the PDR. The H I line ratios are well matched by emissivity coefficients from H recombination theory, but deviate by up to 10% because of contamination by He I lines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni. We show how the Ne III/Ne II, S IV/S III, and Ar III/Ar II ratios trace the conditions in the ionized layer bordering the PDR, while Fe III/Fe II and Ni III/Ni II exhibit a different behavior, as there are significant contributions to Fe II and Ni II from the neutral PDR gas. We observe the pure-rotational H2 lines in the vibrational ground state from 0–0 S(1) to 0–0 S (8), and in the first vibrationally excited state from 1–1 S (5) to 1–1 S(9). We derive H2 excitation diagrams, and for the three observed dissociation fronts, the rotational excitation can be approximated with one thermal (~700 K) component representative of an average gas temperature, and one nonthermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model of the Orion Bar PDR, and find that the predicted excitation matches the data qualitatively, while adjustments to the parameters of the PDR model are required to reproduce the intensity of the 0–0 S (6) to S (8) lines. [ABSTRACT FROM AUTHOR]

We present new neutral atomic carbon [C i] (3P1! 3P0) mapping observations within the inner ~7 kpc and ~4 kpc of the disks of NGC3627 and NGC4321 at a spatial resolution of 190 pc and 270 pc, respectively, using the Atacama Large Millimeter/Submillimeter Array (ALMA) Atacama Compact Array (ACA). We combine these with the CO(21) data from PHANGS-ALMA, and literature [C i] and CO data for two other starburst and/or active galactic nucleus (AGN) galaxies (NGC 1808, NGC7469) with the aim of studying: (a) the spatial distributions of C i and CO emission; (b) the observed line ratio RCi=CO = I[Ci](10)=ICO(21) as a function of various galactic properties; and (c) the abundance ratio of [C i/CO]. We find excellent spatial correspondence between C i and CO emission and nearly uniform RCi=CO ~ 0:1 across the majority of the star-forming disks of NGC3627 and NGC4321. However, RCi=CO strongly varies from ~0:05 at the center of NGC4321 to >0:20:5 in NGC1808's starbursting center and NGC7469's center with an X-ray-luminous AGN. Meanwhile, RCi=CO does not obviously vary with hUi, which is in line with predictions from photodissociation-dominated region (PDR) models. We also find a mildly decreasing RCi=CO value with an increasing metallicity over 0:70:85 Z, which is consistent with the literature. Assuming various typical interstellar medium (ISM) conditions representing giant molecular clouds, active star-forming regions, and strong starbursting environments, we calculated the (non)local-thermodynamic-equilibrium radiative transfer and estimated the [C i/CO] abundance ratio to be ~0:1 across the disks of NGC3627 and NGC4321, similar to previous large-scale findings in Galactic studies. However, this abundance ratio likely experiences a substantial increase, up to ~1 and &15 in NGC1808's starburst and NGC7469's strong AGN environments, respectively. This result is in line with the expectations for cosmic-ray dominated region (CRDR) and X-ray dominated region (XDR) chemistry. Finally, we do not find robust evidence for a generally CO-dark-and-C i-bright gas in the disk areas we probed. [ABSTRACT FROM AUTHOR]

Context. The discovery of a broad, ∼1.5° long filamentary [OIII] 5007 emission ∼1.2° south-east of the M 31 nucleus has recently been reported. More than 100 hours of exposures of a wide field (3.48° × 2.32°) have allowed this pioneering detection based on 30 Å narrow-band filters and several small refractors equipped with large cameras. Aims. We report a first velocity measurement in this extensive [OIII] emission line region. Methods. We used the low-resolution spectrograph MISTRAL (R ∼ 750), a facility of the Haute-Provence Observatory 193 cm telescope. The velocity measurement is based on the Hα, [NII], [SII] and [OIII] lines. Results. The best solution to fit the spectrum indicates that the Hα and [OIII] emissions are at the same heliocentric line-of-sight velocity of −96±4 km s−1. This was measured within an area of ∼250 arcsec2 selected on a bright knot along the long filament of ∼1.5°, together with a [OIII]5007 surface brightness of 4.2±2.1×10−17 erg s−1 cm−2 arcsec−2. This agrees moderately well with the previous measurement. We also estimated the Hα/[NII] line ratio as ∼1.1. Conclusions. The radial velocities at which the Hα and [OIII] lines were detected seem to show that these hydrogen and oxygen atoms belong to the same layer, but we cannot exclude that another weaker [OIII] line, belonging to another structure, that is, at another velocity, is below our detection threshold. Different scenarios have been considered to explain this filamentary structure. The extra-galactic origin was excluded in favour of Galactic origins. We tentatively assume that this filament is a piece of a supernova remnant located at a distance of ∼0.7 kpc from the Sun, of which we only see a small fraction of the shells with a radius of ∼35 pc. The progenitor may be along the line of sight of the galaxy M 31, but this observation might also just be part of a large-scale filamentary structure that should be investigated further. [ABSTRACT FROM AUTHOR]

The 21 cm emission from atomic hydrogen (H I) is one of the most important tracers of the structure and dynamics of the interstellar medium. Thanks to Galactic rotation, the line is Doppler shifted and, assuming a model for the velocity field, data from gas line surveys can be deprojected along the line of sight. However, given our vantage point in the Galaxy, such a reconstruction suffers from a number of ambiguities. Here, we argue that those can be cured by exploiting the spatial coherence of the gas density that is implied by the physical processes shaping it. We have adopted a Bayesian inference framework that allows reconstructing the three-dimensional map of H I and quantifying its uncertainty. We employ data from the HI4PI compilation to produce three-dimensional maps of Galactic H I. The reconstructed density shows structure on a variety of scales. In particular, some spurs and spiral arms can be identified with ease. We discuss the morphology of the surface mass density and the radial and vertical profiles. [ABSTRACT FROM AUTHOR]

Context. Shocks are ubiquitous in the interstellar and intergalactic media, where their chemical and radiative signatures reveal the physical conditions in which they arise. Detailed astrochemical models of shocks at all velocities are necessary to understand the physics of many environments including protostellar outflows, supernova remnants, and galactic outflows. Aims. We present an accurate treatment of the self-generated ultraviolet (UV) radiation in models of intermediate velocity (VS = 25–60 km s−1), stationary, weakly magnetised, J-type, molecular shocks. We show how these UV photons modify the structure and chemical properties of shocks and quantify how the initial mechanical energy is reprocessed into line emission. Methods. We develop an iterative scheme to calculate the self-consistent UV radiation field produced by molecular shocks. The shock solutions computed with the Paris–Durham shock code are post-processed using a multi-level accelerated Λ-iteration radiative transfer algorithm to compute Lyman α, Lyman β, and two-photon continuum emission. The subsequent impacts of these photons on the ionisation and dissociation of key atomic and molecular species as well as on the heating by the photoelectric effect are calculated by taking the wavelength dependent interaction cross-sections and the fluid velocity profile into account. This leads to an accurate description of the propagation of photons and the thermochemical properties of the gas in both the postshock region and in the material ahead of the shock called the radiative precursor. With this new treatment, we analyse a grid of shock models with velocities in the range VS = 25–60 km s−1, propagating in dense (nH ≥ 104 cm−3) and shielded gas. Results. Self-absorption traps Lyα photons in a small region in the shock, though a large fraction of this emission escapes by scattering into the line wings. We find a critical velocity VS ~ 30 km s−1 above which shocks generate Lyα emission with a photon flux exceeding the flux of the standard interstellar radiation field. The escaping photons generate a warm slab of gas (T ~ 100 K) ahead of the shock front as well as pre-ionising C and S. Intermediate velocity molecular shocks are traced by bright emission of many atomic fine structure (e.g. O and S) and metastable (e.g. O and C) lines, substantive molecular emission (e.g. H2, OH, and CO), enhanced column densities of several species including CH+ and HCO+, as well as a severe destruction of H2O. As much as 13–21% of the initial kinetic energy of the shock escapes in Lyα and Lyβ photons if the dust opacity in the radiative precursor allows it. Conclusions. A rich molecular emission is produced by interstellar shocks regardless of the input mechanical energy. Atomic and molecular lines reprocess the quasi totality of the kinetic energy, allowing for the connection of observable emission to the driving source for that emission. [ABSTRACT FROM AUTHOR]

We present a study of the filamentary structure in the emission from the neutral atomic hydrogen (HI) at 21 cm across velocity channels in the 40′′ and 1.5-km s−1 resolution position-position-velocity cube, resulting from the combination of the single-dish and interferometric observations in The HI/OH/recombination-line survey of the inner Milky Way. Using the Hessian matrix method in combination with tools from circular statistics, we find that the majority of the filamentary structures in the HI emission are aligned with the Galactic plane. Part of this trend can be assigned to long filamentary structures that are coherent across several velocity channels. However, we also find ranges of Galactic longitude and radial velocity where the HI filamentary structures are preferentially oriented perpendicular to the Galactic plane. These are located (i) around the tangent point of the Scutum spiral arm and the terminal velocities of the Molecular Ring, around l ≈ 28° and vLSR ≈ 100 km s−1, (ii) toward l ≈ 45° and vLSR ≈ 50 km s−1, (iii) around the Riegel-Crutcher cloud, and (iv) toward the positive and negative terminal velocities. A comparison with numerical simulations indicates that the prevalence of horizontal filamentary structures is most likely the result of large-scale Galactic dynamics and that vertical structures identified in (i) and (ii) may arise from the combined effect of supernova (SN) feedback and strong magnetic fields. The vertical filamentary structures in (iv) can be related to the presence of clouds from extra-planar HI gas falling back into the Galactic plane after being expelled by SNe. Our results indicate that a systematic characterization of the emission morphology toward the Galactic plane provides an unexplored link between the observations and the dynamical behavior of the interstellar medium, from the effect of large-scale Galactic dynamics to the Galactic fountains driven by SNe. [ABSTRACT FROM AUTHOR]

Context. Molecular clouds, which harbor the birthplaces of stars, form out of the atomic phase of the interstellar medium (ISM). To understand this transition process, it is crucial to investigate the spatial and kinematic relationships between atomic and molecular gas. Aims. We aim to characterize the atomic and molecular phases of the ISM and set their physical properties into the context of cloud formation processes. Methods. We studied the cold neutral medium (CNM) by means of H I self-absorption (HISA) toward the giant molecular filament GMF20.0-17.9 (distance = 3.5 kpc, length ~170 pc) and compared our results with molecular gas traced by 13CO emission. We fitted baselines of HISA features to H I emission spectra using first and second order polynomial functions. Results. The CNM identified by this method spatially correlates with the morphology of the molecular gas toward the western region. However, no spatial correlation between HISA and 13CO is evident toward the eastern part of the filament. The distribution of HISA peak velocities and line widths agrees well with 13CO within the whole filament. The column densities of the CNM probed by HISA are on the order of 1020 cm−2 while those of molecular hydrogen traced by 13CO are an order of magnitude higher. The column density probability density functions (N-PDFs) of HISA (CNM) and H I emission (tracing both the CNM and the warm neutral medium, WNM) have a log-normal shape for all parts of the filament, indicative of turbulent motions as the main driver for these structures. The H2 N-PDFs show a broad log-normal distribution with a power-law tail suggesting the onset of gravitational contraction. The saturation of H I column density is observed at ~25 M⊙ pc−2. Conclusions. We conjecture that different evolutionary stages are evident within the filament. In the eastern region, we witness the onset of molecular cloud formation out of the atomic gas reservoir while the western part is more evolved, as it reveals pronounced H2 column density peaks and signs of active star formation. [ABSTRACT FROM AUTHOR]

In this article, we simulated the collisions of an OH• projectile impacting on a methanol cluster formed by ten units of methanol to mimic an ice mantle (CH3OH)10. The chemical processes occurring after the impact were studied through Born-Oppenheimer (ab-initio) molecular dynamics. We focus on collisions with initial kinetic impact energy of 10–22 eV, where the richest chemistry happens. We report the formation mechanisms of stable complex organic molecules (COMs) such as methoxymethanol CH3OCH2OH, formic acid HCOOH, formyl radical HCO, formaldehyde H2CO and its elusive HCOH isomer. We show that CH2(OH)2, •CH2OH or +CH2OH are key intermediates to generate H2CO and other COMs. We compare the outcomes using OH• with those using OH− projectiles. These processes are likely relevant to the production of COMs in astrophysical environments. We discuss its formation mechanism and the astrophysical implications of these chemical pathways in star-forming regions. [ABSTRACT FROM AUTHOR]

In this article, we simulated the collisions of an OH• projectile impacting on a methanol cluster formed by ten units of methanol to mimic an ice mantle (CH3OH)10. The chemical processes occurring after the impact were studied through Born-Oppenheimer (ab-initio) molecular dynamics. We focus on collisions with initial kinetic impact energy of 10–22 eV, where the richest chemistry happens. We report the formation mechanisms of stable complex organic molecules (COMs) such as methoxymethanol CH3OCH2OH, formic acid HCOOH, formyl radical HCO, formaldehyde H2CO and its elusive HCOH isomer. We show that CH2(OH)2, •CH2OH or +CH2OH are key intermediates to generate H2CO and other COMs. We compare the outcomes using OH• with those using OH− projectiles. These processes are likely relevant to the production of COMs in astrophysical environments. We discuss its formation mechanism and the astrophysical implications of these chemical pathways in star-forming regions. [ABSTRACT FROM AUTHOR]

Context. Complex organic molecules (COMs) have been identified toward high- and low-mass protostars as well as molecular clouds, suggesting that these interstellar species originate from the early stage(s) of starformation. The reaction pathways resulting in COMs described by the formula C2HnO, such as acetaldehyde (CH3CHO), vinyl alcohol (CH2CHOH), ketene (CH2CO), and ethanol (CH3CH2OH), are still under debate. Several of these species have been detected in both translucent and dense clouds, where chemical processes are dominated by (ground-state) atom and radical surface reactions. Therefore, efficient formation pathways are needed to account for their appearance well before the so-called catastrophic CO freeze-out stage starts. Aims. In this work, we investigate the laboratory possible solid-state reactions that involve simple hydrocarbons and OH-radicals along with H2O ice under translucent cloud conditions (1 ≤ AV ≤ 5 and nH ~ 103 cm-3). We focus on the interactions of C2H2 with H-atoms and OH-radicals, which are produced along the H2O formation sequence on grain surfaces at 10 K. Methods. Ultra-high vacuum experiments were performed to study the surface chemistry observed during C2H2 + O2 + H codeposition, where O2 was used for the in situ generation of OH-radicals. These C2H2 experiments were extended by a set of similar experiments involving acetaldehyde (CH3CHO) - an abundant product of C2H2 + O2 + H codeposition. Reflection absorption infrared spectroscopy was applied to in situ monitor the initial and newly formed species. After that, a temperature-programmed desorption experiment combined with a quadrupole mass spectrometer was used as a complementary analytical tool. The IR and QMS spectral assignments were further confirmed in isotope labeling experiments using 18O2. Results. The investigated 10 K surface chemistry of C2H2 with H-atoms and OH-radicals not only results in semi and fully saturated hydrocarbons, such as ethylene (C2H4) and ethane (C2H6), but it also leads to the formation of COMs, such as vinyl alcohol, acetaldehyde, ketene, ethanol, and possibly acetic acid. It is concluded that OH-radical addition reactions to C2H2, acting as a molecular backbone, followed by isomerization (i.e., keto-enol tautomerization) via an intermolecular pathway and successive hydrogenation provides so far an experimentally unreported solid-state route for the formation of these species without the need of energetic input. The kinetics of acetaldehyde reacting with impacting H-atoms leading to ketene and ethanol is found to have a preference for the saturated product. The astronomical relevance of the reaction network introduced here is discussed. [ABSTRACT FROM AUTHOR]

Context. More gas is inferred to be present in molecular cloud complexes than can be accounted for by H I and CO emission, a phenomenon known as dark neutral medium (DNM) or CO-dark gas for the molecular part. Aims. We aim to investigate whether molecular gas can be detected in Chamaeleon where gas column densities in the DNM were inferred and CO emission was not detected. Methods. We took λ3 mm absorption profiles of HCO+ and other molecules toward 13 background quasars across the Chamaeleon complex, only one of which had detectable CO emission. We derived the H2 column density assuming N(HCO+)/N(H2) = 3 × 10−9 as before. Results. With the possible exception of one weak continuum target, HCO+ absorption was detected in all directions, C2H in eight directions and HCN in four directions. The sightlines divide into two groups according to their DNM content, with one group of eight directions having N(DNM) ≳ 2 × 1020 cm−2 and another group of five directions having N(DNM) < 0.5 × 1020 cm−2. The groups have comparable mean N(H I) associated with Chamaeleon 6−7 × 1020 cm−2 and total hydrogen column density per unit reddening 6−7 × 1021 cm−2 mag−1. They differ, however, in having quite different mean reddening 0.33 vs. 0.18 mag, mean N(DNM) 3.3 vs. 0.14 × 1020 cm−2 and mean molecular column density 2N(H2) = 5.6 vs. 0.8 × 1020 cm−2. The gas at more positive velocities is enriched in molecules and DNM. Conclusions. Overall the quantity of H2 inferred from HCO+ can fully account for the previously inferred DNM along the sightlines studied here. H2 is concentrated in the high-DNM group, where the molecular fraction is 46% vs. 13% otherwise and 38% overall. Thus, neutral gas in the outskirts of the complex is mostly atomic but the DNM is mostly molecular. Saturation of the H I emission line profile may occur along three of the four sightlines having the largest DNM column densities, but there is no substantial reservoir of "dark" atomic or molecular gas that remains undetected as part of the inventory of dark neutral medium. [ABSTRACT FROM AUTHOR]

Context. Dense highly ionized boundary layers (IBLs) outside of the neutral Photon Dominated Regions (PDRs) have recently been detected via the 122 and 205 μm transitions of ionized nitrogen. These layers have higher densities than in the Warm Ionized Medium (WIM) but less than typically found in H II regions. Observations of [C II] emission, which is produced in both the PDR and IBL, do not fully define the characteristics of these sources. Observations of additional probes which just trace the PDRs, such as the fine structure lines of atomic oxygen, are needed derive their properties and distinguish among different models for [C II] and [N II] emissison. Aims. We derive the properties of the PDRs adjacent to dense highly ionized boundary layers of molecular clouds. Methods. We combine high-spectral resolution observations of the 63 μm [O I] fine structure line taken with the upGREAT HFA-band instrument on SOFIA with [C II] observations to constrain the physical conditions in the PDRs. The observations consist of samples along four lines of sight (LOS) towards the inner Galaxy containing several dense molecular clouds. We interpret the conditions in the PDRs using radiative transfer models for [C II] and [O I]. Results. We have a 3.5-σ detection of [O I] toward one source but only upper limits towards the others. We use the [O I] to [C II] ratio, or their upper limits, and the column density of C+ to estimate the thermal pressure, Pth, in these PDRs. In two LOS the thermal pressure is likely in the range 2–5 × 105 in units of K cm−3, with kinetic temperatures of order 75–100 K and H2 densities, n(H2) ~ 2–4 × 103 cm−3. For the other two sources, where the upper limits on [O I] to [C II] are larger, Pth ≲105 (K cm−3). We have also used PDR models that predict the [O I] to [C II] ratio, along with our observations of this ratio, to limit the intensity of the Far UV radiation field. Conclusions. The [C II] and [N II] emission with either weak, or without any, evidence of [O I] indicates that the source of dense highly ionized gas traced by [N II] most likely arises from the ionized boundary layers of clouds rather than from H II regions. [ABSTRACT FROM AUTHOR]

The central area (40″  × 40″) of the bipolar nebula S106 was mapped in the [O I] line at 63.2 μm (4.74 THz) with high angular (6″) and spectral (0.24 MHz) resolution, using the GREAT heterodyne receiver on board SOFIA. The spatial and spectral emission distribution of [O I] is compared to emission in the CO 16 →15, [C II] 158 μm, and CO 11 →10 lines, mm-molecular lines, and continuum. The [O I] emission is composed of several velocity components in the range from –30 to 25 km s−1. The high-velocity blue- and red-shifted emission (v = −30 to –9 km s−1 and 8 to 25 km s−1) can be explained as arising from accelerated photodissociated gas associated with a dark lane close to the massive binary system S106 IR, and from shocks caused by the stellar wind and/or a disk–envelope interaction. At velocities from –9 to –4 km s−1 and from 0.5 to 8 km s−1 line wings are observed in most of the lines that we attribute to cooling in photodissociation regions (PDRs) created by the ionizing radiation impinging on the cavity walls. The velocity range from –4 to 0.5 km s−1 is dominated by emission from the clumpy molecular cloud, and the [O I], [C II], and high-J CO lines are excited in PDRs on clump surfaces that are illuminated by the central stars. Modelling the line emission in the different velocity ranges with the KOSMA-τ code constrains a radiation field χ of a few times 104 and densities n of a few times 104 cm−3. Considering self-absorption of the [O I] line results in higher densities (up to 106 cm−3) only for the gas component seen at high blue- and red velocities. We thus confirm the scenario found in other studies that the emission of these lines can be explained by a two-phase PDR, but attribute the high-density gas to the high-velocity component only. The dark lane has a mass of ~275 M⊙ and shows a velocity difference of ~1.4 km s−1 along its projected length of ~1 pc, determined from H13CO+ 1 →0 mapping. Its nature depends on the geometry and can be interpreted as a massive accretion flow (infall rate of ~2.5 × 10−4M⊙ yr−1), or the remains of it, linked to S106 IR/FIR. The most likely explanation is that the binary system is at a stage of its evolution where gas accretion is counteracted by the stellar winds and radiation, leading to the very complex observed spatial and kinematic emission distribution of the various tracers. [ABSTRACT FROM AUTHOR]

We present ALMA detections of the [CI] 1-0, CO J = 3-2, and CO J = 4-3 emission lines, as well as the ALMA band 4 continuum for a compact star-forming galaxy (cSFG) at z = 2.225, 3D-HST GS30274. As is typical for cSFGs, this galaxy has a stellar mass of 1.89 ± 0.47 × 1011M⊙, with a star formation rate (SFR) of 214 ± 44 M⊙ yr-1 putting it on the star-forming "main-sequence", but with an H-band effective radius of 2.5 kpc, making it much smaller than the bulk of "main-sequence" star-forming galaxies. The intensity ratio of the line detections yield an ISM density (~6 × 104 cm-3) and a UV-radiation field (~2 × 104G0), similar to the values in local starburst and ultra-luminous infrared galaxy environments. A starburst phase is consistent with the short depletion times (tH2,dep ≤ 140 Myr) we find in 3D-HST GS30274 using three different proxies for the H2 mass ([CI], CO, dust mass). This depletion time is significantly shorter than in more extended SFGs with similar stellar masses and SFRs. Moreover, the gas fraction of 3D-HST GS30274 is smaller than typically found in extended galaxies. We measure the CO and [CI] kinematics and find a FWHM line width of ~750 ± 41 km s-1. The CO and [CI] FWHM are consistent with a previously measured Hα FWHM for this source. The line widths are consistent with gravitational motions, suggesting we are seeing a compact molecular gas reservoir. A previous merger event, as suggested by the asymmetric light profile, may be responsible for the compact distribution of gas and has triggered a central starburst event. This event gives rise to the starburst-like ISM properties and short depletion times in 3D-HST GS30274. The centrally located and efficient star formation is quickly building up a dense core of stars, responsible for the compact distribution of stellar light in 3D-HST GS30274. [ABSTRACT FROM AUTHOR]

Aims. We aim to understand the unexpected presence of mm-wave molecular absorption at -200 km s-1 < v < -140 km s-1 in a direction that is well away from regions of the Galactic bulge where CO emission at such velocities is prominent. Methods. We compared 89 GHz Cycle 2 ALMA absorption spectra of HCO+, HCN, and HNC toward the extragalactic continuum source B1741-312 at l = -2.14°, b = -1.00° with existing CO, H I, and dust emission and absorption measurements. We placed the atomic and molecular gas in the bulge and disk using circular and non-circular galactic kinematics, deriving N(H I) from a combination of 21 cm emission and absorption and we derive N(H2) from scaling of the HCO+ absorption. We then inverted the variation of near-IR reddening E(J - K) with distance modulus and scale E(J - K) to a total gas column density N(H) that may be compared to N(H I) and N(H2). Results. At galactocentric radii Rgal > 1.5 kpc, conventional measures such as the standard CO-H2 conversion factor and locally observed N(HCO+)/N(H2) ratio separately imply that H I and H2 contribute about equally to N(H), and the gas-derived N(H) values are in broad agreement with those derived from E(J - K). Within the Galactic bulge at Rgal < 1.5 kpc, H I contributes less than 10% of the material inferred from E(J - K), so that the molecular absorption detected here is needed to understand the extinction. [ABSTRACT FROM AUTHOR]

To study the atomic, molecular, and ionized emission of giant molecular clouds (GMCs) in the Milky Way, we initiated a large program with the Karl G. Jansky Very Large Array (VLA): "THOR: The Hi, OH, Recombination line survey of the Milky Way". We map the 21 cm Hi line, 4 OH lines, up to 19 Hα recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way (l = 15°-67°, [ABSTRACT FROM AUTHOR]

Context. The formation of carbon dioxide ice in quiescent regions of molecular clouds has not yet been fully understood, even though CO2 is one the most abundant species in interstellar ices. Aims. CO2 formation was studied via oxidation of formaldehyde molecules on cold surfaces under conditions close to those encountered in quiescent molecular clouds to evaluate the efficiency and the activation barrier of the H2CO + O reaction. Methods. Formaldehyde ices were exposed to O atoms using a differentially pumped beam line. The H2CO + O reaction experiments were carried out on two different surfaces of astrophysical interest (amorphous water ice and oxidised graphite) held at 10 or 55 K. The products were probed via infrared and mass spectroscopy by using RAIRS and temperature-programmed desorption techniques. Results. In this paper we show that the H2CO + O reaction can efficiently form carbon dioxide in the solid phase. The activation barrier for the reaction, based on a model fit to the experimental data, was estimated to be 335 ± 55 K. Conclusions. The H2CO+O reaction on cold surfaces can be added to the set of pathways that lead to carbon dioxide in the interstellar ices. Astrophysically, the abundance of CO2 in quiescent molecular clouds may potentially be explained by three reactions occurring on cosmic grains: CO + OH,CO + O, and H2CO + O. [ABSTRACT FROM AUTHOR]

Context. The [C II] 158 μm line is one of the dominant coolants of the ISM, and an important probe with which to study the star formation process. Recent Herschel/HIFI and SOFIA/GREAT observations showed that assuming the total velocity-integrated intensity of this line is directly associated with the star-forming material is inadequate. Aims. We probe the column densities and masses traced by the ionized and neutral atomic carbon with spectrally resolved maps, and compare them to the diffuse and dense molecular gas traced by [C I] and low-J CO lines toward the star-forming region M17 SW. Methods. We mapped a 4.1 pc × 4.7 pc region in the [C I] 609 μm line using the APEX telescope, as well as the CO isotopologues with the IRAM 30 m telescope. Because of the velocity-resolved spectra, we analyze the data based on velocity channel maps that are 1 km s-1 wide. We correlate their spatial distribution with that of the [C II] map obtained with SOFIA/GREAT. Optically thin approximations were used to estimate the column densities of [C I] and [C II] in each velocity channel. Results. The distribution of the emission from the isotopologues 13CO, C17O, and C18O resembles more closely that of the [C I] emission than that of the 12CO emission. The spatial distribution of the [C I] and all CO isotopologues emission was found to be associated with that of [C II] in about 20%-80% of the mapped region, with the high correlation found in the central (15-23 km s-1) velocity channels. Conclusions. The excitation temperature of [C I] ranges between 40 K and 100 K in the inner molecular region of M17 SW. Excitation temperatures up to 200 K are found along the ridge. Column densities in 1 km s-1 channels between ~1015 cm-2 and ~1017 cm-2 were found for [C I]. Just ~20 % of the velocity range (~40 km s-1) that the [C II] line spans is associated with the star-forming material traced by [C I] and CO. The total (integrated over the 0-40 km s-1 velocity range) gas mass estimated from the [C II] emission gives a lower limit of ~4.4 × 103 M⊙. A very large fraction of at least 64% of this mass is not associated with the star-forming material in M17 SW. We also found that about 36%, 17%, and 47% of the [C II] emission is associated with the H II, H I, and H2 regimes, respectively. Comparisons with the H41α line shows an ionization region mixed with the neutral and part of the molecular gas, in agreement with the clumped structure and dynamical processes at play in M17 SW. These results are also relevant to extra-galactic studies in which [C II] is often used as a tracer of star-forming material. [ABSTRACT FROM AUTHOR]

Context. Numerical simulations have explored the possibility of forming molecular clouds through either a quasi-static, selfgravitating mechanism or the collision of gas streams or lower density clouds. They also quantitatively predict the distribution of matter at the transition from atomic to molecular gases. Aims. We aim to observationally test these models by studying the environment of W43, a molecular cloud complex recently identified near the tip of the Galactic long bar. Methods. Using Galaxy-wide Hi and 12CO 1-0 surveys, we searched for gas flowing toward the W43 molecular cloud complex. We also estimated the Hi and H2 mass surface densities to constrain the transition from atomic to molecular gas around and within W43. Results. We found three cloud ensembles within the position-velocity diagrams of 12CO and Hi gases. They are separated by ~20 km s-1 along the line of sight and extend into the 13CO velocity structure of W43. Since their velocity gradients are consistent with free fall, they could be nearby clouds attracted by and streaming toward the W43 ~107 M☉ potential well. We show that the Hi surface density, ΣHI = 45-85 M☉ pc-2, does not reach any threshold level but increases when entering the 130 pc-wide molecular complex previously defined. This suggests that an equilibrium between H2 formation and photodissociation has not yet been reached. The H2-to-H I ratio measured over the W43 region and its surroundings, RH2 ~ 3.5±32 , is high, indicating that most of the gas is already in molecular form in W43 and in structures several hundred parsecs downstream along the Scutum-Centaurus arm. Conclusions. The W43 molecular cloud complex may have formed and, in fact may still be accreting mass from the agglomeration of clouds. Already in the molecular-dominated regime, most of these clouds are streaming from the Scutum-Centaurus arm. This clearly disagrees with quasi-static and steady-state models of molecular cloud formation. [ABSTRACT FROM AUTHOR]

We present high resolution [N II] 205 μm (3P1 – 3 P0) spectra obtained with Herschel-HIFI towards a small sample of far-infrared bright star forming regions in the Galactic plane: W31C (G10.6–0.4), W49N (G43.2–0.1), W51 (G49.5–0.4), and G34.3+0.1. All sources display an emission line profile associated directly with the H II regions themselves. For the first time we also detect absorption of the [N II] 205 μm line by extended low-density foreground material towards W31C and W49N over a wide range of velocities. We attribute this absorption to the warm ionised medium (WIM) and find N(N+) ≈ 1.5 × 1017 cm-2 towards both sources. This is in agreement with recent Herschel-HIFI observations of [C II] 158 μm, also observed in absorption in the same sight-lines, if ≈7–10% of all C+ ions exist in the WIM on average. Using an abundance ratio of [N]/[H] = 6.76 × 10-5 in the gas phase we find that the mean electron and proton volume densities are ~0.1–0.3 cm-3 assuming a WIM volume filling fraction of 0.1–0.4 with a corresponding line-of-sight filling fraction of 0.46–0.74. A low density and a high WIM filling fraction are also supported by RADEX modelling of the [N II] 205 μm absorption and emission together with visible emission lines attributed mainly to the WIM. The detection of the 205 μm line in absorption emphasises the importance of a high spectral resolution, and also offers a new tool for investigation of the WIM. [ABSTRACT FROM AUTHOR]

Context. The distribution of various interstellar gas components and the pressure in the interstellar medium (ISM) is a result of the interplay of different dynamical mechanisms and energy sources on the gas in the Milky Way. The scale heights of the different gas tracers, such as Hi and CO, are a measure of these processes. The scale height of [C ii] emission in the Galactic plane is important for understanding those ISM components not traced by CO or Hi. Aims. We determine the average distribution of [Cii] perpendicular to the plane in the inner Galactic disk and compare it to the distributions of other key gas tracers, such as CO and Hi. Methods. We calculated the vertical, z, distribution of [C ii] in the inner Galactic disk by adopting a model for the emission that combines the latitudinal, b, spectrally unresolved BICE survey, with the spectrally resolved Herschel Galactic plane survey of [C ii] at b = 0°. Our model assumed a Gaussian emissivity distribution vertical to the plane, and related the distribution in z to that of the latitude b using the spectrally resolved [C ii] Herschel survey as the boundary solution for the emissivity at b = 0°. Results. We find that the distribution of [Cii] perpendicular to the plane has a full-width half-maximum of 172 pc, larger than that of CO, which averages ∼110 pc in the inner Galaxy, but smaller than that of Hi, ∼230 pc, and is offset by -28 pc. Conclusions. We explain the difference in distributions of [Cii], CO, and Hi as due to [Cii] tracing a mix of ISM components. Models of hydrostatic equilibrium of clouds in the disk predict different scale heights, for the same interstellar pressure. The diffuse molecular clouds with [Cii] but no CO emission likely have a scale height intermediate between the low density atomic hydrogen Hi clouds and the dense CO molecular clouds. [ABSTRACT FROM AUTHOR]

Context. Since 1996, during periods of low solar activity, the HSTOF instrument on board the SOHO satellite has been measuring weak fluxes of He atoms of 28-58 keV/n (helium energetic neutral atoms, He ENAs). The probable source region is the inner heliosheath. Aims. We aim to understand the emission mechanism of He ENAs based on knowledge of the heliosheath spatial extent and plasma content resulting from Voyager 1 and 2 measurements in the period after termination shock crossings. Methods. He ENAs are generated by charge-exchange neutralization of energetic helium ions on interstellar neutral H and He. Energy spectra of helium ions in the heliosheath are calculated by following the evolution of their velocity distribution functions when carried by and undergoing binary interactions with plasma constituents of a background flow whose particle populations are modeled to approximately render post-termination-shock Voyager data. Results. The observed HSTOF He ENAs form a higher energy part of general heliospheric He ENA fluxes and can be explained by the proposed mechanism to within 2σ error. The main factor determining the level of emission (and its uncertainty) is the energy spectrum of He+ pickup ions in post-termination-shock plasmas. [ABSTRACT FROM AUTHOR]

Context. We report on the detection of an atomic jet associated with the protostellar source SMM1 (FIRS1) in Serpens. The morphological and physical characteristics of the atomic jet suggest that there is a more evolved protostellar companion to the Class 0 source SMM1. Aims. We unravel the molecular and atomic emission around the protostellar source Serpens-SMM1, identify the emission line origin, and assess the evolutionary stage of the driving sources. Methods. The surroundings of SMM1 were mapped with the Spitzer Infrared Spectrograph (IRS) in slit-scan mode. The complex outflow morphology of the molecular (H2) and atomic ([FeII], [NeII], [SiII], [SI]) emission from Spitzer is examined along with deconvolved Spitzer IRAC and MIPS images and high-velocity CO J = 3-2 outflow maps. The physical conditions of the atomic jet are assessed assuming LTE, non-LTE conditions, and shock models. Results. The atomic jet is firmly detected in five different [FeII] and [NeII] lines, with possible contributions from [SI] and [SiII]. It is traced very close to SMM1 and peaks at ~5" from the source at a position angle of ~125°. H2 emission becomes prominent at distances >5" from SMM1 and extends at a position angle of 160°. The morphological differences suggest that the atomic emission arises from a companion source, lying in the foreground of the envelope surrounding the embedded protostar SMM1. The molecular and atomic emissions disentangle the large-scale CO emission into two distinct bipolar outflows, giving further support to a protobinary source. The LTE and non-LTE analysis at the peaks of the [FeII] jet show that emission arises from warm and dense gas (T ~ 1000 K, ne ~ 105-106 cm-3). These conditions are suggestive of dissociative J-type shocks, and this is further supported by strong water-maser emission observed on the axis of the atomic jet. The mass flux of the jet derived independently for the [FeII] and [NeII] emission is ~107 M☉ yr-1, pointing to a more evolved Class I/II protostar as the driving source. Comparisons of the large-scale outflow and atomic jet momentum fluxes show that the latter has adequate thrust to support the CO outflow. Conclusions. The atomic jet detected by Spitzer for the first time gives the opportunity to disentangle the complex outflow morphology around SMM1 into two precessing outflows. The morphological and physical properties of the outflows reveal that SMM1 is a protobinary source. The momentum flux of the atomic jet indicates that the companion to the deeply embedded Class 0 protostar SMM1 is a more evolved Class I/II source. [ABSTRACT FROM AUTHOR]

Context. A new hypothesis is offered to explain the so-called ribbon feature appearing in the all-sky flux maps of energetic neutral atoms presently observed with the IBEX spacecraft, namely that the ribbon is a consequence of inhomogeneities in the local interstellar medium. Aims. The study aims at a detailed presentation of this hypothesis and its implications for the interpretation of ENA measurements with IBEX. Methods. Theoretical considerations regarding three different topics, namely IBEX measurements related to high-energy neutral atoms, Lyman-α observations (made with the Voyager 1 spacecraft) related to low-energy neutral atoms, and astronomical data related to the structure of the interstellar medium, are critically discussed in order to corroborate the hypothesis. Results. It is found that inhomogeneities in the local interstellar medium can explain not only the IBEX ribbon and outer heliospheric Lyman-α observations, but can also account for the interstellar Lyman-α absorption that could only with difficulty be fully attributed to the hydrogen wall in the outer heliosheath if the heliospheric bow shock would indeed be absent. Conclusions. The IBEX observations of the ribbon provide a unique opportunity to learn more about the nature of the interstellar medium surrounding the heliosphere. [ABSTRACT FROM AUTHOR]

Context. The formation of carbon dioxide in quiescent regions of molecular clouds has not yet been fully understood, even though CO2 is one of the most abundant species in interstellar ices. Aims. CO2 formation is studied via oxidation of CO molecules on cold surfaces under conditions close to those encountered in quiescent molecular clouds. Methods. Carbon monoxide and oxygen atoms are codeposited using two differentially pumped beam lines on two different surfaces (amorphous water ice or oxydized graphite) held at given temperatures between 10 and 60 K. The products are probed via mass spectroscopy by using the temperature-programmed desorption technique. Results. We show that the reaction CO+O can form carbon dioxide in solid phase with an efficiency that depends on the temperature of the surface. The activation barrier for the reaction, based on modelling results, is estimated to be in the range of 780-475 K/kb. Our model also allows us to distinguish the mechanisms (Eley Rideal or Langmuir-Hinshelwood) at play in different temperature regimes. Our results suggest that competition between CO2 formation via CO+O and other surface reactions of O is a key factor in the yields of CO2 obtained experimentally. Conclusions. CO2 can be formed by the CO+O reaction on cold surfaces via processes that mimic carbon dioxide formation in the interstellar medium. Astrophysically, the presence of CO2 in quiescent molecular clouds could be explained by the reaction CO+O occurring on interstellar dust grains. [ABSTRACT FROM AUTHOR]

Context. Direct sampling of neutral interstellar (NIS) atoms by the Interstellar Boundary Explorer (IBEX) can potentially provide a complementary method for studying element abundances in the Local Interstellar Cloud (LIC) and processes in the heliosphere interface. Aims. We set the stage for abundance-aimed in-depth analysis of measurements of NIS He, Ne, and O by IBEX and determine systematic differences between abundances derived from various calculation methods and their uncertainties. Methods. Using a model of ionization rates of the NIS species in the heliosphere, based on independent measurements of the solar wind and solar EUV radiation, we developed a time-dependent method of calculating the survival probabilities of NIS atoms from the termination shock (TS) of the solar wind to IBEX.With them, we calculated densities of these species along the Earth's orbit and simulated the fluxes of NIS species as observed by IBEX.We studied pairwise ratios of survival probabilities, densities, and fluxes of NIS species at IBEX to calculate correction factors for inferring the abundances at TS. Results. The analytic method of calculating the survival probabilities gives acceptable results only for He and Ne during low solar activity. For the remaining portions of the solar cycle, and at all times for O, a fully time-dependent model should be used. Electronimpact ionization is surprisingly important for NIS O. Interpreting the IBEX observations using the time-dependent model yields the LIC Ne/O abundance of 0.16 ± 40%. The uncertainty is mostly due to uncertainties in the ionization rates and in the NIS gas flow vector. Conclusions. The Ne/He, O/He, and Ne/O ratios for survival probabilities, local densities, and fluxes scaled to TS systematically differ and thus an analysis based only on survival probabilities or densities is not recommended, except the Ne/O abundance for observations at low solar activity. [ABSTRACT FROM AUTHOR]

Context. The abundance of deuterium in the interstellar gas in front of the Sun gives insight into the processes of filtration of neutral interstellar species through the heliospheric interface and potentially into the chemical evolution of the Galactic gas. Aims. We investigate the possibility of detection of neutral interstellar deuterium at 1 AU from the Sun by direct sampling by the Interstellar Boundary Explorer (IBEX). Methods. Using both previous and the most recent determinations of the flow parameters of neutral gas in the local interstellar cloud (LIC) and an observation-based model of solar radiation pressure and ionization in the heliosphere, we simulated the flux of neutral interstellar D at IBEX for the actual measurement conditions. We assessed the number of interstellar D atom counts expected during the first three years of IBEX operation.We also simulated the observations expected during an epoch of high solar activity. In addition, we calculated the expected counts of D atoms from the thin terrestrial water layer covering the IBEX-Lo conversion surface, sputtered by neutral interstellar He atoms. Results. Most D counts registered by IBEX-Lo are expected to come from the water layer, exceeding the interstellar signal by 2 orders of magnitude. However, the sputtering should stop once the Earth leaves the portion of orbit traversed by interstellar He atoms. We identify seasons during the year when mostly the genuine interstellar D atoms are expected in the signal. During the first 3 years of IBEX operations about 2 detectable interstellar D atoms are expected. This number is comparable to the expected number of sputtered D atoms registered during the same time intervals. Conclusions. The most favorable conditions for the detection occur during low solar activity, in an interval including March and April each year. The detection chances could be improved by extending the instrument duty cycle, say, by making observations in the special deuterium mode of IBEX-Lo. [ABSTRACT FROM AUTHOR]

Context. Studying the HI line profiles in the Leiden-Argentina-Bonn (LAB) all-sky database, we have found a population of very cold H?i clouds. So far, the role of these clouds in the interstellar medium (ISM) has remained unclear. Aims. In this paper, we attempt to confirm the existence of the narrow-line HI emission (NHIE) clouds by using the data from the Parkes Galactic all-sky survey (GASS) and try to find their place among other coldest constituents of the ISM. Methods. We repeat the search of NHIE with the GASS data and derive or compile some preliminary estimates for the distribution, temperatures, distances, linear sizes, column and number densities, masses and the composition of NHIE clouds, and compare these data with corresponding estimates for HI self-absorption (HISA) features, the Planck cold clumps (CC) and infrared dark clouds (IRDC). Results. We demonstrate that from LAB and GASS we can separate comparable NHIE complexes and the properties of the obtained NHIE clouds are very similar to those of HISA features, but both of these types of clouds are somewhat warmer, more extended and of lower densities than the cores in the Planck CC and IRDC. Conclusions. We conclude that NHIE may be of the same type of clouds as HISA, but in different observing conditions, as the Planck CC and IRDC are most likely similar ISM structures in different observing conditions and probably in slightly different evolutionary stages. Both NHIE and HISA may be an intermediate phase between the diffuse cold neutral medium and star forming molecular clumps represented by the Planck CC and IRDC. [ABSTRACT FROM AUTHOR]

Aims. A model of heliosheath density and energy spectra of α-particles and He+ ions carried by the solar wind is developed. Neutralization of heliosheath He ions, mainly by charge exchange (CX) with neutral interstellar H and He atoms, gives rise to ∼0.2-∼100 keV fluxes of energetic neutral He atoms (He ENA). Such fluxes, if observed, would give information about plasmas in the heliosheath and heliospheric tail. Methods. Helium ions crossing the termination shock (TS) constitute suprathermal (test) particles convected by (locally also diffusing through) hydrodynamically calculated background plasma flows (three versions of flows are employed). The He ions proceed from the TS towards heliopause (HP) and finally to the heliospheric tail (HT). Calculations of the evolution of α- and He+ particle densities and energy spectra include binary interactions with background plasma and interstellar atoms (radiative and dielectronic recombinations, single and double CX, stripping, photoionization and impact ionizations), adiabatic heating (cooling) resulting from flow compression (rarefaction), and Coulomb scattering on background plasma. Results. Neutralization of suprathermal He ions leads to the emergence of He ENA fluxes with energy spectra modified by the Compton-Getting effect at emission and ENA loss during flight to the Sun. Energy-integrated He ENA intensities are in the range ∼0.05-∼50 cm-2 s-1 sr-1 depending on spectra at the TS (assumed kappa-distributions), background plasma model, and look direction. The tail/apex intensity ratio varies between ∼1.8 and ∼800 depending on model assumptions. Energy spectra are broad with maxima in the ∼0.2-∼3 keV range depending on the look direction and model. Conclusions. Expected heliosheath He ENA fluxes may be measurable based on the capabilities of the IBEX spacecraft. Data could offer insight into the heliosheath structure and improve understanding of the post-TS solar wind plasmas. HT direction and extent could be assessed. [ABSTRACT FROM AUTHOR]

We used the dual-band receiver GREAT on board the SOFIA airborne telescope to perform observations of the [C II] 158 pm fine- structure line at the postitions of two giant molecular clouds (GMC) in the center of IC 342 (GMCs C and E) and compared the spectra with corresponding ground-based data for low- and mid-J CO and [C I]. We performed model calculations assuming a clumpy photo- dissociation region (PDR) environment using the KOSMA-τ- PDR model code to derive physical parameters of the local medium. The [C II] 158 μm emission resembles the spectral signature of ground-based atomic and molecular lines, which indicates a common origin. The emission from GMC E can be decomposed into a cool, molecular component with weak far-ultraviolet (FUV) fields and low, mean densities of 10³ cm-3 and a strongly excited starburst/PDR region with higher densities of 104 cm-3 and FUV intensities of 250-300 Draine fields. The emission from GMC C is consistent with gas densities of 5000 cm-3, FIJV intensities of a few Draine fields and total gas masses of 20 x 10-6 Mʘ. The high spectral resolution of the GREAT receiver allowed us to decompose the [C II] emission of the GMC E into a strongly excited gas component resembling a PDR/starburst environment and a quieter, less excited gas component and to analyze the different components within a single beam individually. [ABSTRACT FROM AUTHOR]

Context. We analyze the NGC 2024 Ho region and molecular cloud interface using [12CII] and [13 CII] observations. Alma. We attempt to gain insight into the physical structure of the interface layer between the molecular cloud and the HII region. Methods. Observations of [`12CII] and [13Co] emission at 158 μm with high spatial and spectral resolution allow us to study the detailed structure of the ionization front and estimate the column densities and temperatures of the ionized carbon layer in the photon- dominated region. Results. The [12CII] emission closely follows the distribution of the 8 μm continuum. Across most of the source, the spectral lines have two velocity peaks similar to lines of rare CO isotopes. The [13;CII] emission is detected near the edge-on ionization front. It has only a single velocity component, which implies that the [°Cti] line shape is caused by self-absorption. An anomalous hyperfine line-intensity ratio observed in [13 CII] cannot yet be explained. Conclusions. Our analysis of the two isotopes results in a total column density of N(H) 1 .6 x 1023 cm-2 in the gas emitting the [CII] line. A large fraction of this gas has to beat a temperature of several hundred K. The self-absorption is caused by a cooler (T ≤ 100 K) foreground component containing a column density of N(H) ≈ 1022 cm-2. [ABSTRACT FROM AUTHOR]

Aims. With new THz maps that cover an area of ∼3.3 x 2.1 pc² we probe the spatial distribution and association of She ionized, neutral and molecular gas components in the M17 SW nebula. Methods. We used the dual hand receiver GREAT on board the SOFIA airborne telescope to obtain a 5'.7 x 3'.7 map of the 12CO J = 13-12 transition and the [C II] 158 μm fine-structure line in M17 SW and compare the spectroscopically resolved maps with corresponding ground-baaed data for low- and mid-J CO and [C 1] emission. Results. For the first time SOFIA/GREAT allow us to compare velocity-reaolved [C II] emission maps with molecular tracers. We see a large pail of the [C II] emission, both spatially and in velocity, that is completely non-associated with the other tracers of photon- dominated regions (PDR). Only particular narrow channel maps of the velocity-resolved [C II] spectra show a correlation between the different gas components, which is not seen at all in the integrated intensity maps. These a how different morphology in all lines but give hardly any information on the origin of the emission. The [C II] 158 μm emission extends for more than 2 pc into the M17 SW molecular cloud and its line profile covers a broader velocity range than the 12CO J = 13-12 and [CI] emissions, which we interpret as several clumps and layers of ionized carbon gas within the telescope beam. The high-J 12CO emission emerges from a dense region between the ionized and neutral carbon emissions, indicating the presence of high-density clumps that allow the fast formation of hot CO in the irradiated complex structure of M17 SW. The [CII] observations in the southern PDR cannot be explained with stratified or clumpy PDR models. [ABSTRACT FROM AUTHOR]

A correction to the article "New effective recombination coefficients for nebular N II lines," that was published in a previous issue of the jorunal is presented.

A correction to the article "UV optical measurements of the Nozomi spacecraft interpreted with a two-component LIC-flow model" that was published in the 2008 issue of the journal is presented.