Physical State of Molecular Gas in High Galactic Latitude Translucent Clouds

The rotational transitions of carbon monoxide (CO) are the primary means of investigating the density and velocity structure of the molecular interstellar medium. Here we study the lowest four rotational transitions of CO towards high-latitude translucent molecular clouds (HLCs). We report new observations of the J = (4-3), (2-1), and (1-0) transitions of CO towards eight high-latitude clouds. The new observations are combined with data from the literature to show that the emission from all observed CO transitions is linearly correlated. This implies that the excitation conditions which lead to emission in these transitions are uniform throughout the clouds. Observed 13CO/12CO (1-0) integrated intensity ratios are generally much greater than the expected abundance ratio of the two species, indicating that the regions which emit 12CO (1-0) radiation are optically thick. We develop a statistical method to compare the observed line ratios with models of CO excitation and radiative transfer. This enables us to determine the most likely portion of the physical parameter space which is compatible with the observations. The model enables us to rule out CO gas temperatures greater than 30K since the most likely high-temperature configurations are 1 pc-sized structures aligned along the line of sight. The most probable solution is a high density and low temperature (HDLT) solution. The CO cell size is approximately 0.01 pc (2000 AU). These cells are thus tiny fragments within the 100 times larger CO-emitting extent of a typical high-latitude cloud. We discuss the physical implications of HDLT cells, and we suggest ways to test for their existence.Comment: 19 pages, 13 figures, 2 tables, emulateapj To be published in The Astrophysical Journa

HI Narrow Self-Absorption in Dark Clouds: Correlations with Molecular Gas and Implications for Cloud Evolution and Star Formation

We present the results of a comparative study of HI narrow self-absorption (HINSA), OH, 13CO, and C18O in five dark clouds. The HINSA follows the distribution of the emission of the carbon monoxide isotopologues, and has a characteristic size close to that of 13CO. This confirms that the HINSA is produced by cold HI which is well mixed with molecular gas in well-shielded regions. The ratio of the atomic hydrogen density to total proton density for these sources is 5 to 27 x 10^{-4}. Using cloud temperatures and the density of HI, we set an upper limit to the cosmic ray ionization rate of 10^{-16} s^{-1}. Comparison of observed and modeled fractional HI abundances indicates ages for these clouds to be 10^{6.5} to 10^{7} yr. The low values of the HI density we have determined make it certain that the time scale for evolution from an atomic to an almost entirely molecular phase, must be a minimum of several million years. This clearly sets a lower limit to the overall time scale for star formation and the lifetime of molecular clouds

Modeling of diffuse molecular gas applied to HD 102065 observations

Aims. We model a diffuse molecular cloud present along the line of sight to the star HD 102065. We compare our modeling with observations to test our understanding of physical conditions and chemistry in diffuse molecular clouds. Methods. We analyze an extensive set of spectroscopic observations which characterize the diffuse molecular cloud observed toward HD 102065. Absorption observations provide the extinction curve, H2, C I, CO, CH, and CH+ column densities and excitation. These data are complemented by observations of CII, CO and dust emission. Physical conditions are determined using the Meudon PDR model of UV illuminated gas. Results. We find that all observational results, except column densities of CH, CH+ and H2 in its excited (J > 2) levels, are consistent with a cloud model implying a Galactic radiation field (G~0.4 in Draine's unit), a density of 80 cm-3 and a temperature (60-80 K) set by the equilibrium between heating and cooling processes. To account for excited (J >2) H2 levels column densities, an additional component of warm (~ 250K) and dense (nH>10^4 cm-3) gas within 0.03 pc of the star would be required. This solution reproduces the observations only if the ortho-to-para H2 ratio at formation is 1. In view of the extreme physical conditions and the unsupported requirement on the ortho-to-para ratio, we conclude that H2 excitation is most likely to be accounted for by the presence of warm molecular gas within the diffuse cloud heated by the local dissipation of turbulent kinetic energy. This warm H2 is required to account for the CH+ column density. It could also contribute to the CH abundance and explain the inhomogeneity of the CO abundance indicated by the comparison of absorption and emission spectra.Comment: 10 pages, 17 figures. Accepted for publication in Astronomy and Astrophysics. Typos correcte

The anti-bacterial iron-restriction defence mechanisms of egg white; the potential role of three lipocalin-like proteins in resistance against Salmonella

Salmonella enterica serovar Enteritidis (SE) is the most frequently-detected Salmonella in foodborne outbreaks in the European Union. Among such outbreaks, egg and egg products were identified as the most common vehicles of infection. Possibly, the major antibacterial property of egg white is iron restriction, which results from the presence of the iron-binding protein, ovotransferrin. To circumvent iron restriction, SE synthesise catecholate siderophores (i.e. enterobactin and salmochelin) that can chelate iron from host iron-binding proteins. Here, we highlight the role of lipocalin-like proteins found in egg white that could enhance egg-white iron restriction through sequestration of certain siderophores, including enterobactin. Indeed, it is now apparent that the egg-white lipocalin, Ex-FABP, can inhibit bacterial growth via its siderophore-binding capacity in vitro. However, it remains unclear whether ex-FABP performs such a function in egg white or during bird infection. Regarding the two other lipocalins of egg white (Cal-γ and α-1-glycoprotein), there is currently no evidence to indicate that they sequester siderophores