Title: Dynamically dominant magnetic fields in the diffuse interstellar medium Authors: Fletcher, A.; Korpi, M.; Shukurov, A. Publication: eprint arXiv:0901.1975 Publication Date: 01/2009 Origin: ARXIV Keywords: Astrophysics - Galactic Astrophysics Comment: 2 pages, 1 figure. In proceedings of IAU Symposium 259: "Cosmic magnetic fields: from planets to stars and galaxies", eds. K.G. Strassmeier, A.G. Kosovichev & J.E. Beckman in press Bibliographic Code: 2009arXiv0901.1975F Abstract Observations show that magnetic fields in the interstellar medium (ISM) often do not respond to increases in gas density as would be naively expected for a frozen-in field. This may suggest that the magnetic field in the diffuse gas becomes detached from dense clouds as they form. We have investigated this possibility using theoretical estimates, a simple magneto-hydrodynamic model of a flow without mass conservation and numerical simulations of a thermally unstable flow. Our results show that significant magnetic flux can be shed from dense clouds as they form in the diffuse ISM, leaving behind a magnetically dominated diffuse gas. Title: The Outer Disk of the Milky Way Seen in 21-cm Absorption Authors: Dickey, John M.; Strasser, Simon; Gaensler, B. M.; Haverkorn, Marijke; Kavars, Dain; McClure-Griffiths, N. M.; Stil, Jeroen; Taylor, A. R. Publication: eprint arXiv:0901.0968 Publication Date: 01/2009 Origin: ARXIV Keywords: Astrophysics - Galactic Astrophysics Comment: Accepted for Ap J and scheduled for 2009 March 1 volume 693. LaTeX 27 pages with 29 figures Bibliographic Code: 2009arXiv0901.0968D Abstract Three recent surveys of 21-cm line emission in the Galactic plane, combining single dish and interferometer observations to achieve resolution of 1 arcmin to 2 arcmin, 1 km/s, and good brightness sensitivity, have provided some 650 absorption spectra with corresponding emission spectra for study of the distribution of warm and cool phase H I in the interstellar medium. These emission-absorption spectrum pairs are used to study the temperature of the interstellar neutral hydrogen in the outer disk of the Milky Way, outside the solar circle, to a radius of 25 kpc. The cool neutral medium is distributed in radius and height above the plane with very similar parameters to the warm neutral medium. In particular, the ratio of the emission to the absorption, which gives the mean spin temperature of the gas, stays nearly constant with radius to 25 kpc radius. This suggests that the mixture of cool and warm phases is a robust quantity, and that the changes in the interstellar environment do not force the H I into a regime where there is only one temperature allowed. The mixture of atomic gas phases in the outer disk is roughly 15% to 20% cool (40 K to 60 K), the rest warm, corresponding to mean spin temperature 250 to 400 K. The Galactic warp appears clearly in the absorption data, and other features on the familiar longitude-velocity diagram have analogs in absorption with even higher contrast than for 21-cm emission. In the third and fourth Galactic quadrants the plane is quite flat, in absorption as in emission, in contrast to the strong warp in the first and second quadrants. The scale height of the cool gas is similar to that of the warm gas, and both increase with Galactic radius in the outer disk. Title: Formation of Interstellar Clouds: Parker Instability with Phase Transitions Authors: Mouschovias, Telemachos Ch.; Kunz, Matthew W.; Christie, Duncan A. Publication: eprint arXiv:0901.0914 Publication Date: 01/2009 Origin: ARXIV Keywords: Astrophysics - Galactic Astrophysics Comment: 11 pages, 7 figures, accepted for publication in MNRAS; high-resolution color figures may be found at https://netfiles.uiuc.edu/mkunz/MKC08/ Bibliographic Code: 2009arXiv0901.0914M Abstract We follow numerically the nonlinear evolution of the Parker instability in the presence of phase transitions from a warm to a cold HI interstellar medium in two spatial dimensions. The nonlinear evolution of the system favors modes that allow the magnetic field lines to cross the galactic plane. Cold HI clouds form with typical masses ~= 10^5 M_sun, mean densities ~= 20 cm^-3, mean magnetic field strengths ~= 4.3 muG (rms field strengths ~= 6.4 muG), mass-to-flux ratios ~= 0.1 - 0.3 relative to critical, temperatures ~= 50 K, (two-dimensional) turbulent velocity dispersions ~= 1.6 km s^-1, and separations ~= 500 pc, in agreement with observations. The maximum density and magnetic field strength are ~= 10^3 cm^-3 and ~= 20 muG, respectively. Approximately 60% of all HI mass is in the warm neutral medium. The cold neutral medium is arranged into sheet-like structures both perpendicular and parallel to the galactic plane, but it is also found almost everywhere in the galactic plane, with the density being highest in valleys of the magnetic field lines. `Cloudlets' also form whose physical properties are in quantitative agreement with those observed for such objects by Heiles (1967). The nonlinear phase of the evolution takes ~< 30 Myr, so that, if the instability is triggered by a nonlinear perturbation such as a spiral density shock wave, interstellar clouds can form within a time suggested by observations. Title: Modeling the Disk (three-phase) Interstellar Medium Authors: Hensler, Gerhard Publication: eprint arXiv:0810.3347 Publication Date: 10/2008 Origin: ARXIV Keywords: Astrophysics Comment: 14 pages, to be published in Proceed. IAU Symp. No. 254, "The Galaxy Disk in Cosmological Context", J. Andersen, J. Bland-Hawthorn, & B. Nordstroem (eds.), 2009, invited review Bibliographic Code: 2008arXiv0810.3347H Abstract The evolution of galactic disks from their early stages is dominated by gasdynamical effects such as gas infall, galactic fountains, and galactic outflows, and further more. The influence of these processes is only understandable in the framework of diverse gas phases differing in their thermal energies, dynamics, and element abundances. To trace the temporal and chemical evolution of galactic disks, it is therefore essential to model the interstellar gasdynamics combined with stellar dynamics, the interactions between gas phases, and star-gas mass and energy exchanges as detailed as possible. This article reviews the potential of state-of-the-art numerical schemes like Smooth-Particle and grid-based hydrodynamics as well as the inherent processes as of star-formation criteria and feedback, energy deposit and metal enrichment by stars and on the influence of gas-phase interactions on the galactic gas dynamics and chemistry. Title: High angular resolution millimeter observations of circumstellar disks Authors: Testi, Leonard; Leurini, Silvia Affiliation: ESO, Karl-Scharzschild-Strasse 2, D-85748 Garching-bei-München, Germany. Corresponding author. Publication: New Astronomy Reviews, Volume 52, Issue 2-5, p. 105-116. Publication Date: 06/2008 Origin: ELSEVIER Abstract Copyright: Elsevier B.V. DOI: 10.1016/j.newar.2008.04.010 Bibliographic Code: 2008NewAR..52..105T Abstract In this lecture, we review the properties of protoplanetary disks as derived from high angular resolution observations at millimeter wavelengths. We discuss how the combination of several different high angular resolution techniques allow us to probe different regions of the disk around young stellar objects and to derive the properties of the dust when combined with sophisticated disk models. The picture that emerges is that the dust in circumstellar disks surrounding pre-main sequence stars is in many cases significantly evolved compared to the dust in molecular clouds and the interstellar medium. It is however still difficult to derive a consistent picture and timeline for dust evolution in disks as the observations are still limited to small samples of objects. We also review the evidence for and properties of disks around high-mass young stellar objects and the implications on their formation mechanisms. The study of massive YSOs is complicated by their short lifetimes and larger average distances. In most cases high angular resolution data at millimeter wavelengths are the only method to probe the structure of disks in these objects. We provide a summary of the characteristics of available high angular resolution millimeter and submillimeter observatories. We also describe the characteristics of the ALMA observatory being constructed in the Chilean Andes. ALMA is going to be the world leading observatory at millimeter wavelengths in the coming decades, the project is now in its main construction phase with early science activities envisaged for 2010 and full science operations for 2012. Title: Turbulence measurements from HI absorption spectra Authors: Roy, Nirupam; Peedikakkandy, Leshma; Chengalur, Jayaram N. Affiliation: AA(NCRA-TIFR, Post Bag 3, Ganeshkhind, Pune 411 007, India), AB(Department of Physics, Cochin University of Science and Technology, Kochi 682 022, India), AC(NCRA-TIFR, Post Bag 3, Ganeshkhind, Pune 411 007, India) Publication: Monthly Notices of the Royal Astronomical Society: Letters, Volume 387, Issue 1, pp. L18-L22. Publication Date: 06/2008 Origin: MNRAS MNRAS Keywords: turbulence , ISM: atoms , ISM: general , ISM: kinematics and dynamics , ISM: structure , radio lines: ISM Abstract Copyright: (c) Journal compilation © 2008 RAS DOI: 10.1111/j.1745-3933.2008.00473.x Bibliographic Code: 2008MNRAS.387L..18R Abstract We use the millennium Arecibo 21-cm absorption-line survey measurements to examine the issue of the non-thermal contribution to the observed Galactic HI linewidths. If we assume a simple, constant pressure model for the HI in the Galaxy, we find that the non-thermal contribution to the linewidth, vnt scales as v2nt ~ lα, for vnt larger than ~0.7 km s-1. Here, l is a derived length-scale and α ~ 0.7 +/- 0.1. This is consistent with what one would expect from a turbulent medium with a Kolmogorov scaling. Such a scaling is also predicted by theoretical models and numerical simulations of turbulence in a magnetized medium. For non-thermal linewidths narrower than ~0.7 km s-1, this scaling breaks down, and we find that the likely reason is ambiguities arising from Gaussian decomposition of intrinsically narrow, blended lines. We use the above estimate of the non-thermal contribution to the linewidth to determine corrected HI kinetic temperature. The new limits that we obtain imply that a significantly smaller (~40 per cent as opposed to 60 per cent) fraction of the atomic interstellar medium in our Galaxy is in the warm neutral medium phase. Title: AGB stars as an origin of dust and gas in the interstellar medium of galaxies Authors: Matsuura, M.; Zijlstra, A. A.; Wood, P. R.; Sloan, G. C.; Groenewegen, M. A. T.; Lagadec, E.; van Loon, J. Th.; Whitelock, P. A.; Bernard-Salas, J.; Menzies, J. W.; Cioni, M.-R. L.; Feast, M. W.; Harris, G. J. Affiliation: AA(), AB(), AC(), AD(), AE(), AF(), AG(), AH(), AI(), AJ(), AK(), AL(), AM() Publication: ORIGIN OF MATTER AND EVOLUTION OF GALAXIES: The 10th International Symposium on Origin of Matter and Evolution of Galaxies: From the Dawn of Universe to the Formation of Solar System. AIP Conference Proceedings, Volume 1016, pp. 383-388 (2008). Publication Date: 05/2008 Origin: AIP Keywords: giant stars, interstellar matter, infrared spectra, Small Magellanic Cloud Abstract Copyright: (c) 2008: American Institute of Physics DOI: 10.1063/1.2943602 Bibliographic Code: 2008AIPC.1016..383M Abstract We have obtained infrared spectra of carbon stars in four nearby galaxies-the Large and Small Magellanic Clouds, and the Fornax dwarf spheroidal galaxy. Our primary aim is to investigate mass-loss rate and gas compositions of these stars as a function of metallicity, by comparing AGB stars in several galaxies with different metallicities. These stars were observed using the Infrared Spectrometer (IRS) onboard the Spitzer Space Telescope which covers 5-35 μm region, and the Infrared Spectrometer And Array Camera (ISAAC) on the Very Large Telescope which covers the 2.9-4.1 μm region. HCN, CH and C2H2 molecular bands, as well as SiC and MgS dust features are identified in the spectra. We find no evidence that mass-loss rates depend on metallicity. Carbon stars are strongly affected by carbon production during the AGB phase; primarily mass-loss of carbon-rich stars are driven by amorphous carbon dust grains, that explains the little metallicity dependence of mass-loss rate for carbon-rich stars. We found that C2H2 bands are prominent features at 3-15 μm among extragalactic carbon stars, which is not always the case for Galactic carbon stars. We argue that carbon produced in AGB stars dominate the gas and dust chemistry in these stars in low metallicities. Title: Exotic Clouds in the Local Interstellar Medium Authors: Stanimirović, Snežana Affiliation: AA(Department of Astronomy, University of Wisconsin) Publication: Space Science Reviews, Online First Publication Date: 04/2008 Origin: SPRINGER Keywords: Interstellar medium: Physical properties, Interstellar medium: Solar neighborhood Abstract Copyright: (c) 2008: Springer Science+Business Media B.V. DOI: 10.1007/s11214-008-9363-y Bibliographic Code: 2008SSRv..tmp..134S Abstract The neutral interstellar medium (ISM) inside the Local Bubble (LB) has been known to have properties typical of the warm neutral medium (WNM). However, several recent neutral hydrogen (HI) absorption experiments show evidence for the existence of at least several cold diffuse clouds inside or at the boundary of the LB, with properties highly unusual relative to the traditional cold neutral medium. These cold clouds have a low HI column density, and AU-scale sizes. As the kinematics of cold and warm gas inside the LB are similar, this suggests a possibility of all these different flavors of the local ISM belonging to the same interstellar flow. The co-existence of warm and cold phases inside the LB is exciting as it can be used to probe the thermal pressure inside the LB. In addition to cold clouds, several discrete screens of ionized scattering material are clearly located inside the LB. The cold exotic clouds inside the LB are most likely long-lived, and we expect many more clouds with similar properties to be discovered in the future with more sensitive radio observations. While physical mechanisms responsible for the production of such clouds are still poorly understood, dynamical triggering of phase conversion and/or interstellar turbulence are likely to play an important role. Title: Interaction of massive stars with their surroundings Authors: Hensler, Gerhard Affiliation: AA(Institute of Astronomy, University of Vienna, Tuerkenschanzstr. 17, 1180 Vienna, Austria hensler@astro.univie.ac.at) Publication: The Art of Modeling Stars in the 21st Century, Proceedings of the International Astronomical Union, IAU Symposium, Volume 252, p. 309-315 Publication Date: 04/2008 Origin: CUP Keywords: Stars: supergiants, (ISM:)Hii regions, ISM: kinematics and hydrodynamics, galaxies: evolution Abstract Copyright: (c) 2008: Copyright © International Astronomical Union 2008 DOI: 10.1017/S1743921308023090 Bibliographic Code: 2008IAUS..252..309H Abstract Due to their short lifetimes but their enormous energy release in all stages of their lives massive stars are the major engines for the comic matter circuit. They affect not only their close environment but are also responsible to drive mass flows on galactic scales. Recent 2D models of radiation-driven and wind-blown Hii regions are summarized which explore the impact of massive stars to the interstellar medium but find surprisingly small energy transfer efficiencies while an observable Carbon self-enrichment in the Wolf-Rayet phase is detected in the warm ionized gas. Finally, the focus is set on state-of-the-art modelling of Hii regions and its present weaknesses with respect to uncertainties and simplifications but on a perspective of the requested art of their modelling in the 21st century. Title: Fragmentation and Turbulence in the Interstellar Medium Authors: Audit, E.; Hennebelle, P. Publication: Numerical Modeling of Space Plasma Flows: Astronum 2007 ASP Conference Series, Vol. 385, proceedings of the conference held 10-15 June, 2007, at Hotel Concorde Montparnasse, Paris, France. Edited by Nikolai V. Pogorelov, Edouard Audit, and Gary P. Zank. San Francisco: Astronomical Society of the Pacific, 2008., p.73 Publication Date: 04/2008 Origin: ASP Bibliographic Code: 2008ASPC..385...73A Abstract The neutral atomic gas (HI) is ubiquitous in the galaxies and is of great importance for the star formation process since molecular clouds form by contraction of HI. We present in this short paper results obtained using high resolution simulations of the turbulent 2-phase atomic gas. We discuss the peculiar properties of the turbulence in this diphasic media and illustrate the strong imprint of thermal instability. We also present the statistical properties of the cold structures formed and propose a theoretical explanation for their mass distribution. Finally, we show that these simulations reproduce various observational features reasonably well. Title: The role of stellar mass and mass functions in the ISM dust feedback Authors: Falceta-Gonçalves, D. Affiliation: AA(Núcleo de Astrofísica Teórica, CETEC, Universidade Cruzeiro do Sul, Rua Galvão Bueno 868, CEP 01506-000 São Paulo, Brazil diego.goncalves@unicsul.br) Publication: Astronomy and Astrophysics, Volume 478, Issue 1, January IV 2008, pp.151-154 Publication Date: 01/2008 Origin: EDP Keywords: ISM: dust, extinction, stars: luminosity function, mass function, stars: mass-loss, stars: winds, outflows, evolution DOI: 10.1051/0004-6361:20077844 Bibliographic Code: 2008A&A...478..151F Abstract Context: The dust component of the interstellar medium (ISM) has been extensively studied in past decades. Late-type stars have been assumed as the main source of dust to the ISM, but recent observations show that supernova remnants may play a role in the ISM dust feedback. Aims: In this work, we study the importance of low and high mass stars, along with their evolutionary phase, in the ISM dust feedback process. We also determine the changes in the obtained results considering different mass distribution functions and star formation history. Methods: We describe a semi-empirical calculation of the relative importance of each star at each evolutionary phase in the dust ejection to the ISM. We compared the obtained results for two stellar mass distribution functions, the classic Salpeter initial mass function, and the current mass function. We used the evolutionary track models for each stellar mass and the empirical mass-loss rates and dust-to-gas ratio. Results: We show that the relative contribution of each stellar mass depends on the used distribution. Ejecta from massive stars represent the most important objects for the ISM dust replenishment using the Salpeter IMF. On the other hand, for the present mass function, low and intermediate mass stars are dominant. Conclusions: We confirm that late-type giant and supergiant stars dominate the ISM dust feedback in our Galaxy, but this may not the case for galaxies experiencing high star formation rates or at high redshifts. In those cases, SNe are dominant in the dust feedback process. Title: Stellar Feedback Through Cosmic Time: Starbursts & Superwinds Authors: Dopita, Michael A. Affiliation: AA(Research School of Astronomy & Astrophysics, The Australian National University Cotter Road, Weston Creek, ACT 2611, Australia Michael.Dopita@anu.edu.au) Publication: Massive Stars as Cosmic Engines, Proceedings of the International Astronomical Union, IAU Symposium, Volume 250, p. 367-378 Publication Date: 00/2008 Origin: CUP Keywords: galaxies: evolution, galaxies: high-redshift, intergalactic medium, ISM: evolution, ISM: jets and outflows, stars: winds, outflows Abstract Copyright: (c) 2008: Copyright © International Astronomical Union 2008 DOI: 10.1017/S1743921308020711 Bibliographic Code: 2008IAUS..250..367D Abstract Throughout cosmic time, the feedback of massive star winds and supernova explosions has been instrumental in determining the phase structure of the interstellar medium, controlling important aspects of both the formation and evolution of galaxies, producing galactic winds and enriching the intergalactic medium with heavy elements. In this paper, I review progress made in our theoretical understanding of how these feedback processes have operated throughout cosmic time from the epoch of the first stars through to the present day. Title: Carbonaceous dust grains in luminous infrared galaxies. Spitzer/IRS reveals a-C:H as an abundant and ubiquitous ISM component Authors: Dartois, E.; Muñoz-Caro, G. M. Affiliation: AA(Institut d'Astrophysique Spatiale (IAS), Université Paris-Sud 11, CNRS (UMR 8617), Bâtiment 121, 91405 Orsay, France emmanuel.dartois@ias.u-psud.fr), AB(Centro de Astrobiología, INTA-CSIC, Carretera de Ajalvir, km. 4, Torrejón de Ardoz, 28850 Madrid, Spain) Publication: Astronomy and Astrophysics, Volume 476, Issue 3, December IV 2007, pp.1235-1242 Publication Date: 12/2007 Origin: EDP Keywords: ISM: dust, extinction, galaxies: ISM, methods: laboratory DOI: 10.1051/0004-6361:20077798 Bibliographic Code: 2007A&A...476.1235D Abstract Aims.The available ground- and space-based spectroscopic capabilities of observatories now allow us to extend Galactic interstellar medium composition studies to extragalactic cases. Absorptions in the mid-infrared shows evidence for silicate and carbonaceous grains in other galaxies. Methods: A set of extragalactic spectra of luminous infrared galaxies (LIRGs) has been extracted from the Spitzer database and compared to the spectra of laboratory-produced interstellar carbon dust analogues. Results: These highly obscured lines-of-sight display the characteristic absorptions at ~6.85 and 7.25 μm of the CH3/CH2 deformation modes of hydrogenated amorphous carbon (a-C:H) grains. They are compared to laboratory-produced a-C:H and imply carbon atom column densities in the solid phase exceeding ~1018 cm-2. Conclusions: These observations further demonstrate the ubiquitousness of a-C:H in the diffuse interstellar medium (DISM) of galaxies, for a long time almost only observed in the Milky-Way ISM lines-of-sights. Whereas PAH emission lines trace the re-processing of energetic young stellar radiation, the observed a-C:H features underline the existence of large masses of amorphous carbon dust in (extra-)galactic dust budgets. The difficulty in observing such an interstellar component in the mid-infrared is linked to its low absorption contrast (A_V/τ(6.85)≈ 625 ± 40) for the strongest band, which therefore requires high column densities to detect a-C:H grains. Such carbon grains might be present but spectroscopically hidden in many other galactic environments. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, and of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Title: Diffuse interstellar medium and the formation of molecular clouds Authors: Hennebelle, P.; Mac Low, M. -M.; Vazquez-Semadeni, E. Publication: eprint arXiv:0711.2417 Publication Date: 11/2007 Origin: ARXIV Keywords: Astrophysics Comment: Proceeding of conference "Structure formation in the universe", held in Chamonix 2007. To be published in Structure formation in Astrophysics, Ed. G. Chabrier edited by Cambridge University Press, 2008 Bibliographic Code: 2007arXiv0711.2417H Abstract (Abridged) The formation of molecular clouds (MCs) from the diffuse interstellar gas appears to be a necessary step for star formation, as young stars invariably occur within them. However, the mechanisms controlling the formation of MCs remain controversial. In this contribution, we focus on their formation in compressive flows driven by interstellar turbulence and large-scale gravitational instability. Turbulent compression driven by supernovae appears insufficient to explain the bulk of cloud and star formation. Rather, gravity must be important at all scales, driving the compressive flows that form both clouds and cores. Cooling and thermal instability allow the formation of dense gas out of moderate, transonic compressions in the warm diffuse gas, and drive turbulence into the dense clouds. MCs may be produced by an overshoot beyond the thermal-pressure equilibrium between the cold and warm phases of atomic gas, caused by some combination of the ram pressure of compression and the self-gravity of the compressed gas. In this case, properties of the clouds such as their mass, mass-to-magnetic flux ratio, and total kinetic and gravitational energies are in general time-variable quantities. MCs may never enter a quasi-equilibrium or virial equilibrium state but rather continuously collapse to stars. Title: Molecular gas in high-velocity clouds: revisited scenario Authors: Dessauges-Zavadsky, M.; Combes, F.; Pfenniger, D. Affiliation: AA(Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland miroslava.dessauges@obs.unige.ch), AB(Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France), AC(Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland) Publication: Astronomy and Astrophysics, Volume 473, Issue 3, October III 2007, pp.863-870 Publication Date: 10/2007 Origin: EDP Keywords: Galaxy: halo, ISM: clouds, ISM: molecules, radio lines: ISM DOI: 10.1051/0004-6361:20077277 Bibliographic Code: 2007A&A...473..863D Abstract We report a new search for 12CO(1-0) emission in high-velocity clouds (HVCs) performed with the IRAM 30 m millimeter-wave telescope. This search was motivated by the recent detection of cold dust emission (T˜ 10.7 K) in the HVCs of Complex C, implying a total gas column density 5 times larger than the column density measured in H I and suggesting undetected gas, presumably in molecular form. Despite a spatial resolution which is three times better and sensitivity twice as good compared to previous studies, no CO emission is detected in the HVCs of Complex C down to a best 5σ limit of 0.16 K km s-1 at a 22'' resolution. The non-detection of both the 12CO(1-0) emission and of the diffuse H2 absorption with the Far Ultraviolet Spectroscopic Explorer does not provide any evidence in favor of large amounts of molecular gas in these HVCs and hence in favor of the infrared findings. We discuss different configurations which, however, allow us to reconcile the negative CO result with the presence of molecular gas and cold dust emission. H2 column densities higher than our detection limit, N(H_2) = 3× 1019 cm-2, are expected to be confined in very small and dense clumps with 20 times smaller sizes than the 0.5 pc clumps resolved in our observations according to the results obtained in cirrus clouds, and might thus still be highly diluted. As a consequence, the inter-clump gas at the 1 pc scale, as resolved in our data, has a volume density lower than 20 cm-3 and already appears as too diffuse to excite the CO molecules. The observed physical conditions in the HVCs of Complex C also play an important role against CO emission detection. The sub-solar metallicity of 0.1-0.3 dex affects the H2 formation rate onto dust grains, and it has been shown that the CO-to-H2 conversion factor in low metallicity media is 60 times higher than at the solar metallicity, leading for a given H2 column density to a 60 times weaker integrated CO intensity. And the very low dust temperature estimated in these HVCs implies the possible presence of gas cold enough (<20 K) to cause CO condensation onto dust grains under interstellar medium pressure conditions and thus CO depletion in gas-phase observations. Based on observations collected with the IRAM 30 m telescope at Pico Veleta, Spain, in June 20-23, 2006. Title: Small Ionized and Neutral Structures: A Theoretical Review Authors: Spangler, S. R.; Vázquez-Semadeni, E. Publication: SINS - Small Ionized and Neutral Structures in the Diffuse Interstellar Medium ASP Conference Series, Vol. 365, Proceedings of the conference held 21-24 May, 2006 at the National Radio Astronomy Observatory in Socorro, New Mexico, USA. Edited by M. Haverkorn and W. M. Goss. San Francisco: Astronomical Society of the Pacific, 2007., p.347 Publication Date: 07/2007 Origin: ASP Bibliographic Code: 2007ASPC..365..347S Abstract The workshop on Small Ionized and Neutral Structures in the Interstellar Medium featured many contributions on the theory of the objects which are responsible for ``Tiny Scale Atomic Structures'' (TSAS) and ``Extreme Scattering Events'' (ESE). The main demand on theory is accounting for objects that have the high densities and small sizes apparently required by the observations, but also persist over a sufficiently long time to be observable. One extensively-discussed mechanism is compressions by transonic turbulence in the warm interstellar medium, followed by thermal instabilities leading to an even more compressed state. In addressing the requirements for overpressured but persistent objects, workshop participants also discussed fundamental topics in the physics of the interstellar medium, such as the timescale for evaporation of cool dense clouds, the relevance of thermodynamically-defined phases of the ISM, the effect of magnetic fields, statistical effects, and the length and time scales introduced by interstellar processes. Title: Physical conditions in the neutral interstellar medium at z = 2.43 toward Q 2348-011 Authors: Noterdaeme, P.; Petitjean, P.; Srianand, R.; Ledoux, C.; Le Petit, F. Affiliation: AA(European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile pnoterda@eso.org; LERMA, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France), AC(IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India anand@iucaa.ernet.in), AD(European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile cledoux@eso.org), AE(LUTH, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France franck.lepetit@obspm.fr) Publication: Astronomy and Astrophysics, Volume 469, Issue 2, July II 2007, pp.425-436 Publication Date: 07/2007 Origin: EDP Keywords: galaxies: ISM, quasars: absorption lines, quasars:, individuals: Q 2348-011 DOI: 10.1051/0004-6361:20066897 Bibliographic Code: 2007A&A...469..425N Abstract Aims.We aim at deriving the physical conditions in the neutral gas associated with damped Lyman-α systems using observation and analysis of H2 and C i absorptions. Methods: We obtained a high-resolution VLT-UVES spectrum of the quasar Q 2348-011 over a wavelength range that covers most of the prominent metal and molecular absorption lines from the log N(H i) = 20.50 ± 0.10 damped Lyman-α system at z_abs=2.4263. We detected H2 in this system and measured column densities of H2, C i, C i^*, C i**, Si ii, P ii, S ii, Fe ii, and Ni ii. From the column density ratios and, in particular, the relative populations of H2 rotational and C i fine-structure levels, we derived the physical conditions in the gas (relative abundances, dust-depletion, particle density, kinetic temperature, and ionising flux) and discuss physical conditions in the neutral phase. Results: Molecular hydrogen was detected in seven components in the first four rotational levels (J = 0-3) of the vibrational ground state. Absorption lines of H2 J=4 (resp. J = 5) rotational levels are detected in six (resp. two) of these components. This leads to a total molecular fraction of log f ≃ -1.69+0.37-0.58. Fourteen components are needed to reproduce the metal-line profiles. The overall metallicity is found to be -0.80, -0.62, -1.17 ± 0.10 for, respectively, [Si/H], [S/H] and [Fe/H]. We confirm the earlier findings that there is a correlation between log N(Fe ii)/N(S ii) and log N(Si ii)/N(S ii) from different components indicative of a dust-depletion pattern. Surprisingly, however, the depletion of metals onto dust in the H2 components is not large in this system: [Fe/S] = -0.8 to -0.1. The gas in H2-bearing components is found to be cold but still hotter than similar gas in our Galaxy (T > 130 K, instead of typically 80 K) and dense (n ˜ 100-200 cm-3). There is an anti-correlation (R=-0.97) between the logarithm of the photo-absorption rate, log β_0, and log N(H2)/N(C i) derived for each H2 component. We show that this is mostly due to shielding effects and imply that the photo-absorption rate β0 is a good indicator of the physical conditions in the gas. We find that the gas is immersed in an intense UV field, about one order of magnitude higher than in the solar vicinity. These results suggest that the gas in H2-bearing DLAs is clumpy, and star-formation occurs in the associated object. Based on observations carried out at the European Southern Observatory (ESO) under prog. ID No. 072.A-0346 with the UVES spectrograph installed at the Very Large Telescope (VLT) Unit 2, Kueyen, on Cerro Paranal, Chile.