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.
Context. In bright photodissociation regions (PDR) associated with massive star formation, the presence of dense “clumps” that are immersed in a less dense interclump medium is often proposed to ...explain the difficulty of models to account for the observed gas emission in high-excitation lines. Aims. We aim to present a comprehensive view of the modelling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces. Methods. We observed the 12CO and 13CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard Herschel. We also considered line emission from key species in the gas cooling of PDRs (C+, O, and H2) and other tracers of PDR edges such as OH and CH+. All the intensities are collected from Herschel observations, the literature and the Spitzer archive and were analysed using the Meudon PDR code. Results. A grid of models was run to explore the parameter space of only two parameters: thermal gas pressure and a global scaling factor that corrects for approximations in the assumed geometry. We conclude that the emission in the high-J CO lines, which were observed up to Jup = 23 in the Orion Bar (Jup = 19 in NGC 7023), can only originate from small structures with typical thicknesses of a few 10−3 pc and at high thermal pressures (Pth ~ 108 K cm−3). Conclusions. Compiling data from the literature, we find that the gas thermal pressure increases with the intensity of the UV radiation field given by G0, following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help to rationalise the analysis of high-J CO emission in massive star formation and provides an observational constraint for models which study stellar feedback on molecular clouds.
Context. Alma Cycle 3 observations serendipitously showed strong absorption from diffuse molecular gas in the Galactic bulge at − 200 km s−1 < v < −140 km s−1 toward the compact extragalactic ...continuum source J1744-3116 at (l, b) = −2.13°, − 1.00°. Aims. We aimed to test whether molecular gas in the bulge could also be detected toward the three other, sufficiently strong mm-wave continuum sources seen toward the bulge at |b| < 3°. Methods. We took absorption profiles of HCO+ (1−0), HCN(1−0), C2H (1−0), CS(2−1) and H13CO+ (1−0) in ALMA Cycle 4 toward J1713-3418, J1717-3341, J1733-3722 and J1744-3116. Results. Strong molecular absorption from disk gas at |v|≲30 km s−1 was detected in all directions, and absorption from the 3 kpc arm was newly detected toward J1717 and J1744. However, only the sightline toward J1744 is dominated by molecular gas overall and no other sightlines showed molecular absorption from gas deep inside the bulge. No molecular absorption was detected toward J1717 where H I emission from the bulge was previously known. As observed in HCO+, HCN, C2H and CS, the bulge gas toward J1744 at v < −135 km s−1 has chemistry and kinematics like that seen near the Sun and in the Milky Way disk generally. We measured isotopologic ratios N(HCO+)/N(H13CO+) > 51 (3σ) for the bulge gas toward J1744 and 58 ± 9 and 64 ± 4 for the disk gas toward J1717 and J1744, respectively, all well above the value of 20−25 typical of the central molecular zone. Conclusions. The kinematics and chemistry of the bulge gas observed toward J1744 more nearly resemble that of gas in the Milky Way disk than in the central molecular zone.
Context. Barnard B1b has been revealed as one of the most interesting globules from the chemical and dynamical point of view. It presents a rich molecular chemistry characterized by large abundances ...of deuterated and complex molecules. Furthermore, this globule hosts an extremely young Class 0 object and one candidate for the first hydrostatic core (FHSC) proving the youth of this star-forming region. Aims. Our aim is to determine the cosmic ray ionization rate, ζH2, and the depletion factors in this extremely young star-forming region. These parameters determine the dynamical evolution of the core. Methods. We carried out a spectral survey toward Barnard 1b as part of the IRAM large program “IRAM Chemical survey of sun-like star-forming regions” (ASAI) using the IRAM 30-m telescope at Pico Veleta (Spain). This provided a very complete inventory of neutral and ionic C-, N-, and S- bearing species with, from our knowledge, the first secure detections of the deuterated ions DCS+ and DOCO+. We use a state-of-the-art pseudo-time-dependent gas-phase chemical model that includes the ortho and para forms of H2, H2+, D2+, H3+, H2D+, D2H+, D2, and D3+ to determine the local value of the cosmic ray ionization rate and the depletion factors. Results. Our model assumes n(H2) = 105 cm-3 and Tk = 12 K, as derived from our previous works. The observational data are well fitted with ζH2 between 3 × 10-17 s-1 and 10-16 s-1 and the elemental abundances O/H = 3 × 10-5, N/H = 6.4−8 × 10-5, C/H = 1.7 × 10-5, and S/H between 6.0 × 10-7 and 1.0 × 10-6. The large number of neutral/protonated species detected allows us to derive the elemental abundances and cosmic ray ionization rate simultaneously. Elemental depletions are estimated to be ~10 for C and O, ~1 for N, and ~25 for S. Conclusions. Barnard B1b presents similar depletions of C and O as those measured in prestellar cores. The depletion of sulfur is higher than that of C and O, but not as extreme as in cold cores. In fact, it is similar to the values found in some bipolar outflows, hot cores, and photon-dominated regions. Several scenarios are discussed to account for these peculiar abundances. We propose that it is the consequence of the initial conditions (important outflows and enhanced UV fields in the surroundings) and a rapid collapse (~0.1 Myr) that allows most S- and N-bearing species to remain in the gas phase to great optical depths. The interaction of the compact outflow associated with B1b-S with the surrounding material could enhance the abundances of S-bearing molecules, as well.
Aims. Ionized carbon is the main gas-phase reservoir of carbon in the neutral diffuse interstellar medium (ISM) and its 158 μm fine structure transition C ii is the most important cooling line of the ...diffuse ISM. We combine C ii absorption and emission spectroscopy to gain an improved understanding of physical conditions in the different phases of the ISM. Methods. We present high-resolution C ii spectra obtained with the Herschel/HIFI instrument towards bright dust continuum regions in the Galactic plane, probing simultaneously the diffuse gas along the line of sight and the background high-mass star forming regions. These data are complemented by single pointings in the 492 and 809 GHz fine structure lines of atomic carbon and by medium spectral resolution spectral maps of the fine structure lines of atomic oxygen at 63 and 145 μm with Herschel/PACS. Results. We show that the presence of foreground absorption may completely cancel the emission from the background source in medium spectral resolution PACS data and that high spectral resolution spectra are needed to interpret the C ii and O i emission and the C ii/FIR ratio. This phenomenon may explain part of the C ii/FIR deficit seen in external luminous infrared galaxies where the bright emission from the nuclear regions may be partially canceled by absorption from diffuse gas in the foreground. The C+ and C excitation in the diffuse gas is consistent with a median pressure of ~5900 K cm-3 for a mean kinetic temperature of ~100 K. A few higher pressure regions are detected along the lines of sight, as emission features in both fine structure lines of atomic carbon. The knowledge of the gas density allows us to determine the filling factor of the absorbing gas along the selected lines of sight. The derived median value of the filling factor is 2.4%, in good agreement with the properties of the Galactic cold neutral medium. The mean excitation temperature is used to derive the average cooling due to C+ in the Galactic plane : 9.5 × 10-26 erg-1H-1. Along the observed lines of sight, the gas phase carbon abundance does not exhibit a strong gradient as a function of Galacto-centric radius and has a weighted average of C/H = 1.5 ± 0.4 × 10-4.
Context. We previously detected 89.2 GHz J = 1−0 HCO+ absorption in 12 directions lacking detected CO emission in the outskirts of the Chamaeleon cloud complex and toward one sightline with ...integrated CO emission WCO = 2.4 K km s−1. Eight sightlines had a much larger mean column density of dark neutral medium (DNM) – gas not represented in HI or CO emission – and were found to have much higher mean molecular column density. The five other sightlines had little or no DNM and were found to have much smaller but still detectable N(HCO+). Aims. To determine the CO column density along previously observed Chamaeleon sightlines and to determine why CO emission was not detected in directions where molecular gas is present. Methods. We took 12CO J = 1−0 absorption profiles toward five sightlines having higher DNM and HCO+ column densities and one sightline with smaller N(DNM) and N(HCO+). We converted the integrated HCO+ optical depths to N(H2) in the weak-excitation limit using N(HCO+)/N(H2) = 3 × 10−9 and converted the integrated CO optical depths ϒCO to CO column density using the relationship N(CO) = 1.861 × 1015 cm−2 ϒCO1.131 found along comparable lines of sight that were previously studied in J = 1−0 and J = 2−1 CO absorption and emission. Results. CO absorption was detected along the five sightlines in the higher-DNM group, with CO column densities 4 × 1013 cm−2≲ N(CO) ≲1015 cm−2 that are generally below the detectability limit of CO emission surveys. Conclusions. In the outskirts of the Chamaeleon complex, the presence of molecular DNM resulted primarily from small CO column densities at the onset of CO formation around the HI/H2 transition in diffuse molecular gas. CO relative abundances N(CO)/H2 ≲2 × 10−6 in the outskirts of Chamaeleon are comparable to those seen in UV absorption toward early-type stars, including in Chamaeleon.
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.
La fièvre jaune est une zoonose due au virus amaril. Elle est transmise par un moustique présent dans les zones inter-tropicales d’Afrique sub-saharienne et d’Amérique du Sud. Elle entraîne chez ...l’homme une maladie fébrile pouvant évoluer vers la défaillance hépatique et rénale avec un syndrome hémorragique, parfois fatal. Le vaccin anti-amaril est un vaccin vivant atténué recommandé à toute personne de plus de 9 mois voyageant dans ces régions. Il comporte des effets indésirables bénins (syndrome viral post-vaccinal), des réactions allergiques et des effets indésirables graves viscérotropes (YEL-AVD) et neurotropes (YEL-AND). L’atteinte neurologique post-vaccinale auto-immune avec des signes neurologiques centraux (de type ADEM acute disseminated encephalo-myelitis) est définie par l’association : fièvre, céphalées, dysfonction neurologique, convulsions, pléiocytose et hyperprotéinorachie dans le liquide céphalorachidien (LCR), et démyélinisation multifocale à l’imagerie par résonance magnétique (IRM) cérébrale. La présence d’anticorps ou du virus dans le LCR, dans les 30jours suivant le vaccin est nécessaire au diagnostic, ainsi que l’élimination d’autres causes courantes d’encéphalopathies. Nous décrivons le cas d’un enfant de 4ans ayant présenté une encéphalopathie sévère à la suite d’un vaccin anti-amarile, d’évolution favorable. Le diagnostic a reposé sur la chronologie des signes cliniques et paracliniques et la présence d’anticorps anti-amarile dans le LCR. La prise en charge a consisté en un traitement symptomatique et l’injection d’immunoglobulines.
Yellow fever is a vector-borne disease transmitted by an endemic mosquito in sub-Saharan Africa and tropical South America. It causes fever and possibly liver and renal failure with hemorrhagic signs, which may be fatal. The yellow-fever vaccine is an attenuated vaccine that is recommended for all travelers over the age of 9 months in high-risk areas. Adverse effects have been reported: minor symptoms (such as viral syndrome), hypersensitivity reactions, and major symptoms such as viscerotropic disease (YEL-AVD) and neurotropic disease (YEL-AND). The yellow-fever vaccine-associated autoimmune disease with central nervous system involvement (such as acute disseminated encephalomyelitis) associates fever and headaches, neurologic dysfunction, seizures, cerebrospinal fluid (CSF) pleocytosis, and elevated protein, with neuroimaging consistent with multifocal areas of demyelization. The presence of antibodies or virus in CSF, within 1–30 days following vaccination, and the exclusion of other causes is necessary for diagnosis. We describe herein the case of a 4-year-old child who presented with severe encephalitis consecutive to a yellow-fever vaccine, with favorable progression. Diagnosis is based on the chronology of clinical and paraclinical signs and the presence of yellow-fever-specific antibodies in CSF. The treatment consists of symptomatic treatment and immunoglobulin injection.
Aims. We describe the assignment of a previously unidentified interstellar absorption line to ArH+ and discuss its relevance in the context of hydride absorption in diffuse gas with a low H2 ...fraction. The confidence of the assignment to ArH+ is discussed, and the column densities are determined toward several lines of sight. The results are then discussed in the framework of chemical models, with the aim of explaining the observed column densities. Methods. We fitted the spectral lines with multiple velocity components, and determined column densities from the line-to-continuum ratio. The column densities of ArH+ were compared to those of other species, tracing interstellar medium (ISM) components with different H2 abundances. We constructed chemical models that take UV radiation and cosmic ray ionization into account. Results. Thanks to the detection of two isotopologues, 36ArH+ and 38ArH+, we are confident about the carrier assignment to ArH+. NeH+ is not detected with a limit of NeH+/ArH+ ≤ 0.1. The derived column densities agree well with the predictions of chemical models. ArH+ is a unique tracer of gas with a fractional H2 abundance of 10-4 − 10-3 and shows little correlation to H2O+, which traces gas with a fractional H2 abundance of ≈0.1. Conclusions. A careful analysis of variations in the ArH+, OH+, H2O+, and HF column densities promises to be a faithful tracer of the distribution of the H2 fractional abundance by providing unique information on a poorly known phase in the cycle of interstellar matter and on its transition from atomic diffuse gas to dense molecular gas traced by CO emission. Abundances of these species put strong observational constraints upon magnetohydrodynamical (MHD)simulations of the interstellar medium, and potentially could evolve into a tool characterizing the ISM. Paradoxically, the ArH+ molecule is a better tracer of almost purely atomic hydrogen gas than Hi itself, since Hi can also be present in gas with a significant molecular content, but ArH+ singles out gas that is >99.9% atomic.
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