Reardon K.P.,Osservatorio Astrofisico di Arcetri |
Reardon K.P.,Queens University of Belfast |
Wang Y.-M.,U.S. Navy |
Muglach K.,NASA |
And 2 more authors.
Chromospheric fibrils are generally thought to trace out low-lying, mainly horizontal magnetic fields that fan out from flux concentrations in the photosphere. A high-resolution (∼01pixel-1) image, taken in the core of the Ca II854.2nm line and covering an unusually large area, shows the dark fibrils within an active region remnant as fine, looplike features that are aligned parallel to each other and have lengths comparable to a supergranular diameter. Comparison with simultaneous line-of-sight magnetograms confirms that the fibrils are centered above intranetwork areas (supergranular cell interiors), with one end rooted just inside the neighboring plage or strong unipolar network but the other endpoint less clearly defined. Focusing on a particular arcade-like structure lying entirely on one side of a filament channel (large-scale polarity inversion), we find that the total amount of positive-polarity flux underlying this "fibril arcade" is ∼50 times greater than the total amount of negative-polarity flux. Thus, if the fibrils represent closed loops, they must consist of very weak fields (in terms of total magnetic flux), which are interpenetrated by a more vertical field that contains most of the flux. This surprising result suggests that the fibrils in unipolar regions connect the network to the nearby intranetwork flux, while the bulk of the network flux links to remote regions of the opposite polarity, forming a second, higher canopy above the fibril canopy. The chromospheric field near the edge of the network thus has an interlaced structure resembling that in sunspot penumbrae. © 2011. The American Astronomical Society. All rights reserved. Source
Wienen M.,Max Planck Institute for Radio Astronomy |
Wyrowski F.,Max Planck Institute for Radio Astronomy |
Schuller F.,Max Planck Institute for Radio Astronomy |
Menten K.M.,Max Planck Institute for Radio Astronomy |
And 4 more authors.
Astronomy and Astrophysics
Context. The APEX Telescope Large Area Survey: the GALaxy (ATLASGAL) is an unbiased continuum survey of the inner Galactic disk at 870 μm. It covers ±60° in Galactic longitude and aims to find all massive clumps at various stages of high-mass star formation in the inner Galaxy, particularly the earliest evolutionary phases. Aims. We aim to determine properties such as the gas kinetic temperature and dynamics of new massive cold clumps found by ATLASGAL. Most importantly, we derived their kinematical distances from the measured line velocities. Methods. We observed the ammonia (J,K) = (1, 1) to (3, 3) inversion transitions toward 862 clumps of a flux-limited sample of submm clumps detected by ATLASGAL and extracted 13CO (1-0) spectra from the Galactic Ring Survey (GRS). We determined distances for a subsample located at the tangential points (71 sources) and for 277 clumps whose near/far distance ambiguity is resolved. Results. Most ATLASGAL clumps are cold with rotational temperatures from 10-30 K with a median of 17 K. They have a wide range of NH 3 linewidths (1-7 km s -1) with 1.9 km s -1 as median, which by far exceeds the thermal linewidth, as well as a broad distribution of high column densities from 10 14 to 10 16 cm -2 (median of 2 × 10 15 cm -2) with an NH 3 abundance in the range of 5 to 30 × 10 -8. ATLASGAL sources are massive, ≳ 100 M, and a fraction of clumps with a broad linewidth is in virial equilibrium. We found an enhancement of clumps at Galactocentric radii of 4.5 and 6 kpc. The comparison of the NH 3 lines as high-density probes with the GRS 13CO emission as low-density envelope tracer yields broader linewidths for 13CO than for NH 3. The small differences in derived clump velocities between NH 3 (representing dense core material) and 13CO (representing more diffuse molecular cloud gas) suggests that the cores are essentially at rest relative to the surrounding giant molecular cloud. Conclusions. The high detection rate (87%) confirms ammonia as an excellent probe of the molecular content of the massive, cold clumps revealed by ATLASGAL. A clear trend of increasing rotational temperatures and linewidths with evolutionary stage is seen for source samples ranging from 24 μm dark clumps to clumps with embedded HII regions. The survey provides the largest ammonia sample of high-mass star forming clumps and thus presents an important repository for the characterization of statistical properties of the clumps and the selection of subsamples for detailed, high-resolution follow-up studies. © 2012 ESO. Source
Chiuderi C.,University of Florence |
Pietrini P.,University of Florence |
Torricelli-Ciamponi G.,Osservatorio Astrofisico di Arcetri
Journal of Geophysical Research: Space Physics
We present a new approach to the study of two-fluid hydrodynamics of weakly collisional plasma systems, such as those frequently encountered in the astrophysical context. Our starting point is the novel procedure developed in past years by Chen, Rao, and Spiegel (CRS) for the fluid description of semicollisional neutral monoatomic gases. The new system of fluid equations that include viscous and conductive effects has been successfully tested. However, the extension of such a procedure to plasmas is not a straightforward one. We have to deal with (at least) two components, with vastly different values of the masses, and the nature of collisions between charged particles is substantially different from the neutral particle case. We perform a preliminary careful examination of the basic requirements of the CRS method and identify the conditions under which an extension of such method is indeed possible. We then derive the system of fluid equations appropriate to the description of a weakly collisional two-component plasma in the new scheme and discuss the differences with respect to the more familiar Navier-Stokes approach. This paper is therefore of a general theoretical nature. However, we also point out that solar wind is a good testing ground for the newly derived system of fluid equations. This application will be the subject of a subsequent paper. Copyright 2011 by the American Geophysical Union. Source
Jimenez-Serra I.,European Southern Observatory |
Jimenez-Serra I.,Harvard - Smithsonian Center for Astrophysics |
Caselli P.,University of Leeds |
Fontani F.,Osservatorio Astrofisico di Arcetri |
And 4 more authors.
Monthly Notices of the Royal Astronomical Society
Some theories of dense molecular cloud formation involve dynamical environments driven by converging atomic flows or collisions between preexisting molecular clouds.The determination of the dynamics and physical conditions of the gas in clouds at the early stages of their evolution is essential to establish the dynamical imprints of such collisions, and to infer the processes involved in their formation. We present multitransition 13CO and C18O maps towards the IRDC G035.39-00.33, believed to be at the earliest stages of evolution. The 13CO and C18O gas is distributed in three filaments (Filaments 1, 2 and 3), where the most massive cores are preferentially found at the intersecting regions between them. The filaments have a similar kinematic structurewith smooth velocity gradients of~0.4-0.8 kms-1 pc-1. Several scenarios are proposed to explain these gradients, including cloud rotation, gas accretion along the filaments, global gravitational collapse and unresolved sub-filament structures. These results are complemented by HCO+, HNC, H13CO+ and HN13C single-pointing data to search for gas infall signatures. The 13CO and C18O gas motions are supersonic across G035.39-00.33, with the emission showing broader linewidths towards the edges of the infrared dark cloud (IRDC). This could be due to energy dissipation at the densest regions in the cloud. The average H2 densities are ̃5000-7000 cm-3, with Filaments 2 and 3 being denser and more massive than Filament 1. The C18O data unveil three regions with high CO depletion factors (fD ̃ 5-12), similar to those found in massive starless cores. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source
Inoue S.,Kyoto University |
Salvaterra R.,National institute for astrophysics |
Choudhury T.R.,Harish Chandra Research Institute |
Ferrara A.,Normal School of Pisa |
And 2 more authors.
Monthly Notices of the Royal Astronomical Society
We discuss expectations for the absorption of high-energy gamma-rays by γγ pair production with intergalactic radiation fields (IRFs) at very high redshifts (z ∼ 5-20), and the prospects thereof for probing the cosmic reionization era. For the evolving IRF, a semi-analytical model incorporating both Population II and Population III stars is employed, which is consistent with a wide variety of existing high-. z observations including quasi-stellar object spectral measurements, Wilkinson Microwave Anisotropy Probe Thomson depth constraints, near-infrared source count limits, etc. We find that the ultraviolet IRF below the Lyman edge energy with intensities in the range of a few times 10-19 erg cm-2 s-1 Hz-1 sr-1 can cause appreciable attenuation above ∼12 GeV at z ∼ 5, down to ∼6-8 GeV at z ≳ 8-10. This may be observable in the spectra of blazars or gamma-ray bursts by the Fermi Gamma-ray Space Telescope or next-generation facilities such as the Cherenkov Telescope Array, Advanced Gamma-ray Imaging System or 5@5, providing invaluable insight into early star formation and cosmic reionization. © 2010 The Authors. Journal compilation © 2010 RAS. Source