2007 (IPP)

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    On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei
    (2007-10-27) Shaw, Gargi; Stancil, P. C.; Ferland, G. J.; et al.
    The spatial distribution of the cosmic-ray flux is important in understanding the Interstellar Medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive to the cosmic-ray ionization rate. Recently several groups have reported an enhanced cosmic-ray ionization rate (ζ=χCRζstandard) in diffuse clouds compared to the standard value, ζstandard (=2.5x10-17 s-1), measured toward dense molecular clouds. In an earlier work we reported an enhancement χCR =20 towards HD185418. McCall et al. have reported χCR =48 towards ζ Persei based on the observed abundance of H3+ while Le Petit et al. found χCR ≈ 10 to be consistent with their models for this same sight line. Here we revisit ζ Persei and perform a detailed calculation using a self-consistent treatment of the hydrogen chemistry, grain physics, energy and ionization balance, and excitation physics. We show that the value of χCR deduced from the H3 + column density, N(H3 +), in the diffuse region of the sightline depends strongly on the properties of the grains because they remove free electrons and change the hydrogen chemistry. The observations are largely consistent with χCR ≈ 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.
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    Hot Halos around High Redshift Protogalaxies: Observations of O VI and N V Absorption in Damped Lyman Alpha systems
    (2007-01-15) Fox, Andrew J.; Petitjean, Patrick; Ledoux, C´edric; et al.
    Aims. We present a study of the highly ionized gas (plasma) associated with damped Lyman-α (DLA) systems at z=2.1–3.1. Methods. We search for Ovi absorption and corresponding Si iv, Civ, and Nv in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering Ovi at S/N>10. We then use optical depth profile comparisons and ionization modelling to investigate the properties, phase structure, and origin of the plasma. Results. We report twelve DLAs (nine intervening and three at <5 000 kms −1 from the QSO redshift) with detections of Ovi absorption. There are no clear Ovi non-detections, so the incidence of Ovi in DLAs is between 34% (12/35) and 100%. Among these 12 DLAs, Civ and Si iv are seen whenever data is available, and Nv is detected in 3 cases. Analysis of the line widths together with photoionization modelling suggests that two phases of DLA plasma exist: a hot, collisionally ionized phase (seen in broad Ovi components), and a warm, photoionized phase (seen just in narrow Civ and Si iv components). The presence of inflows and/or outflows is indicated by individual Ovi and Civ components displaced from the neutral gas (either blueshifted or redshifted) by up to 400 kms −1 . We find tentative evidence (98% confidence) for correlations between the DLA metallicity (measured in the neutral gas) and high-ion column density, and between the DLA metallicity and high-ion line width, as would be expected if supernova-driven galactic outflows rather than accretion produced the high ions. Using conservative ionization corrections, we find lower limits to the total hydrogen column densities in the hot (Ovi-bearing) and warm (Civ-bearing) phases in the range logNHot H ii > 19.5 to > 21.1, and logNWarm H ii > 19.4 to > 20.9. On average, the hot and warm phases thus contain > ∼40% and > ∼20% of the baryonic mass of the neutral phase in DLAs, respectively. Conclusions. If the temperature in the Ovi phase is ≈ 106 K and so fOvi = Ovi/O ≪ 0.2, the plasma can make a significant contribution to the metal budget at high redshift. Additional searches for Ovi in Lyman Limit Systems (QSO absorbers with 17.0 < NHi < 20.3) will be necessary to determine the total quantity of baryons and metals hidden in hot halos at z ≈ 2.
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    Galactic dynamos supported by magnetic helicity fluxes
    (2007-03-08) Sur, Sharanya; Shukurov, A.; Subramanian, Kandaswamy
    We present a simple semi-analytical model of nonlinear, mean-field galactic dynamos and use it to study the effects of various magnetic helicity fluxes. The dynamo equations are reduced using the ‘no-z’ approximation to a nonlinear system of ordinary differential equations in time; we demonstrate that the model reproduces accurately earlier results, including those where nonlinear behaviour is driven by a magnetic helicity flux. We discuss the implications and interplay of two types of magnetic helicity flux, one produced by advection (e.g., due to the galactic fountain or wind) and the other, arising fromanisotropy of turbulence as suggested by Vishniac & Cho (2001). We argue that the latter is significant if the galactic differential rotation is strong enough: in ourmodel, forRω . −10 in terms of the corresponding turbulent magnetic Reynolds number. We confirm that the intensity of gas outflow from the galactic disc optimal for the dynamo action is close to that expected for normal spiral galaxies. The steady-state strength of the large-scale magnetic field supported by the helicity advection is still weaker than that corresponding to equipartition with the turbulent energy. However, the Vishniac-Cho helicity flux can boost magnetic field further to achieve energy equipartition with turbulence. For stronger outflows that may occur in starburst galaxies, the Vishniac-Cho flux can be essential for the dynamo action. However, this mechanism requires a large-scale magnetic field of at least≃ 1 Gto be launched, so that it has to be preceded by a conventional dynamo assisted by the advection of magnetic helicity by the fountain or wind.
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    Critical properties of spherically symmetric accretion in a fractal medium
    (2007-07-18) Roy, Nirupam; Ray, Arnab K.
    Spherically symmetric transonic accretion of a fractal medium has been studied in both the stationary and the dynamic regimes. The stationary transonic solution is greatly sensitive to infinitesimal deviations in the outer boundary condition, but the flow becomes transonic and stable, when its evolution is followed through time. The evolution towards transonicity is more pronounced for a fractal medium than what is it for a continuum. The dynamic approach also shows that there is a remarkable closeness between an equation of motion for a perturbation in the flow, and the metric of an analogue acoustic black hole. The stationary inflow solutions of a fractal medium are as much stable under the influence of linearised perturbations, as they are for the fluid continuum.
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    3D Automated Classification Scheme for the TAUVEX data pipeline
    (2007-11-28) Bora, Archana; Gupta, Ranjan; Singh, Harinder P.; et al.
    In order to develop a pipeline for automated classi cation of stars to be observed by the TAUVEX ultraviolet space Telescope, we employ an arti cial neural network (ANN) technique for classifying stars by using synthetic spectra in the UV region from 1250 A to 3220 A as the training set and International Ultraviolet Explorer (IUE) low resolution spectra as the test set. Both the data sets have been pre-processed to mimic the observations of the TAUVEX ultraviolet imager. We have successfully classi ed 229 stars from the IUE low resolution catalog to within 3-4 spectral sub-class using two di erent simulated training spectra, the TAUVEX spectra of 286 spectral types and UVBLUE spectra of 277 spectral types. Further, we have also been able to obtain the colour excess (i.e. E(B-V) in magnitude units) or the interstellar reddening for those IUE spectra which have known reddening to an accuracy of better than 0.1 magnitudes. It has been shown that even with the limitation of data from just photometric bands, ANNs have not only classi ed the stars, but also provided satisfactory estimates for interstellar extinction. The ANN based classi cation scheme has been successfully tested on the simulated TAUVEX data pipeline. It is expected that the same technique can be employed for data validation in the ultraviolet from the virtual observatories. Finally, the interstellar extinction estimated by applying the ANNs on the TAUVEX data base would provide an extensive extinction map for our galaxy and which could in turn be modeled for the dust distribution in the galaxy.