4,070 research outputs found
Improved Current Densities in MgB2 By Liquid-Assisted Sintering
Polycrystalline MgB2 samples with GaN additions were prepared by reaction of
Mg, B, and GaN powders. The presence of Ga leads to a low melting eutectic
phase which allowed liquid phase sintering and produces plate-like grains. For
low-level GaN additions (5% at. % or less), the critical transition
temperature, Tc, remained unchanged and in 1T magnetic field, the critical
current density, Jc was enhanced by a factor of 2 and 10, for temperatures of
\~5K and 20K, respectively. The values obtained are approaching those of hot
isostatically pressed samples.Comment: 12 pages, 1 table, 4 figures, accepted in Applied Physics Letter
Structure formation in a colliding flow: The Herschel view of the Draco nebula
The Draco nebula is a high Galactic latitude interstellar cloud likely to
have been formed by the collision of a Galactic halo cloud entering the disk of
the Milky Way. Such conditions are ideal to study the formation of cold and
dense gas in colliding flows of warm gas. We present Herschel-SPIRE
observations that reveal the fragmented structure of the interface between the
infalling cloud and the Galactic layer. This front is characterized by a
Rayleigh-Taylor instability structure. From the determination of the typical
length of the periodic structure (2.2 pc) we estimated the gas kinematic
viscosity and the turbulence dissipation scale (0.1 pc) that is compatible with
that expected if ambipolar diffusion is the main mechanism of energy
dissipation in the WNM. The small-scale structures of the nebula are typical of
that seen in some molecular clouds. The gas density has a log-normal
distribution with an average value of cm. The size of the
structures is 0.1-0.2 pc but this estimate is limited by the resolution of the
observations. The mass ranges from 0.2 to 20 M and the distribution
of the more massive clumps follows a power law . We
identify a mass-size relation with the same exponent as that found in GMCs
() but only 15% of the mass of the cloud is in gravitationally
bound structures. We conclude that the increase of pressure in the collision is
strong enough to trigger the WNM-CNM transition caused by the interplay between
turbulence and thermal instability as self-gravity is not dominating the
dynamics.Comment: 16 pages, A&A, in pres
GHIGLS: HI mapping at intermediate Galactic latitude using the Green Bank Telescope
This paper introduces the data cubes from GHIGLS, deep Green Bank Telescope
surveys of the 21-cm line emission of HI in 37 targeted fields at intermediate
Galactic latitude. The GHIGLS fields together cover over 1000 square degrees at
9.55' spatial resolution. The HI spectra have an effective velocity resolution
about 1.0 km/s and cover at least -450 < v < +250 km/s. GHIGLS highlights that
even at intermediate Galactic latitude the interstellar medium is very complex.
Spatial structure of the HI is quantified through power spectra of maps of the
column density, NHI. For our featured representative field, centered on the
North Ecliptic Pole, the scaling exponents in power-law representations of the
power spectra of NHI maps for low, intermediate, and high velocity gas
components (LVC, IVC, and HVC) are -2.86 +/- 0.04, -2.69 +/- 0.04, and -2.59
+/- 0.07, respectively. After Gaussian decomposition of the line profiles, NHI
maps were also made corresponding to the narrow-line and broad-line components
in the LVC range; for the narrow-line map the exponent is -1.9 +/- 0.1,
reflecting more small scale structure in the cold neutral medium (CNM). There
is evidence that filamentary structure in the HI CNM is oriented parallel to
the Galactic magnetic field. The power spectrum analysis also offers insight
into the various contributions to uncertainty in the data. The effect of 21-cm
line opacity on the GHIGLS NHI maps is estimated.Comment: Accepted for publication in The Astrophysical Journal, 2015 July 16.
32 pages, 21 figures (Fig. 10 new). Minor revisions from review, particularly
Section 8 and Appendix C; results unchanged. Additional surveys added and
made available; new Appendix B. Added descriptions of available FITS files
and links to four illustrative movies on enhanced GHIGLS archive
(www.cita.utoronto.ca/GHIGLS/
Accurate Galactic 21-cm H I measurements with the NRAO Green Bank Telescope
Aims: We devise a data reduction and calibration system for producing
highly-accurate 21-cm H I spectra from the Green Bank Telescope (GBT) of the
NRAO.
Methods: A theoretical analysis of the all-sky response of the GBT at 21 cm
is made, augmented by extensive maps of the far sidelobes. Observations of
radio sources and the Moon are made to check the resulting aperture and main
beam efficiencies.
Results: The all-sky model made for the response of the GBT at 21 cm is used
to correct for "stray" 21-cm radiation reaching the receiver through the
sidelobes rather than the main beam. This reduces systematic errors in 21-cm
measurements by about an order of magnitude, allowing accurate 21-cm H I
spectra to be made at about 9' angular resolution with the GBT. At this
resolution the procedures discussed here allow for measurement of total
integrated Galactic H I line emission, W, with errors of 3 K km s^-1,
equivalent to errors in optically thin N_HI of 5 x 10^18 cm^-2.Comment: 49 pages, 25 figures; A&A, in pres
Extended HI Rotation Curve and Mass Distribution of M31
New HI observations of Messier 31 (M31) obtained with the Effelsberg and
Green Bank 100-m telescopes make it possible to measure the rotation curve of
that galaxy out to ~35 kpc. Between 20 and 35 kpc, the rotation curve is nearly
flat at a velocity of ~226 km/s. A model of the mass distribution shows that at
the last observed velocity point, the minimum dark-to-luminous mass ratio is
\~0.5 for a total mass of 3.4 10^11 Msol at R < 35 kpc. This can be compared to
the estimated MW mass of 4.9 10^11 Msol for R < 50 kpc.Comment: 4 pages, 2 figures, accepted for publication in ApJ Letter
The VLA Galactic Plane Survey
The VLA Galactic Plane Survey (VGPS) is a survey of HI and 21-cm continuum
emission in the Galactic plane between longitude 18 degrees 67 degr. with
latitude coverage from |b| < 1.3 degr. to |b| < 2.3 degr. The survey area was
observed with the Very Large Array (VLA) in 990 pointings. Short-spacing
information for the HI line emission was obtained by additional observations
with the Green Bank Telescope (GBT). HI spectral line images are presented with
a resolution of 1 arcmin x 1 arcmin x 1.56 km/s (FWHM) and rms noise of 2 K per
0.824 km/s channel. Continuum images made from channels without HI line
emission have 1 arcmin (FWHM) resolution. VGPS images are compared with images
from the Canadian Galactic Plane Survey (CGPS) and the Southern Galactic Plane
Survey (SGPS). In general, the agreement between these surveys is impressive,
considering the differences in instrumentation and image processing techniques
used for each survey. The differences between VGPS and CGPS images are small, <
6 K (rms) in channels where the mean HI brightness temperature in the field
exceeds 80 K. A similar degree of consistency is found between the VGPS and
SGPS. The agreement we find between arcminute resolution surveys of the
Galactic plane is a crucial step towards combining these surveys into a single
uniform dataset which covers 90% of the Galactic disk: the International
Galactic Plane Survey (IGPS). The VGPS data will be made available on the World
Wide Web through the Canadian Astronomy Data Centre (CADC).Comment: Accepted for publication in The Astronomical Journal. 41 pages, 13
figures. For information on data release, colour images etc. see
http://www.ras.ucalgary.ca/VGP
Carbon Recombination Lines from the Galactic Plane at 34.5 & 328 MHz
We present results of a search for carbon recombination lines in the Galaxy
at 34.5 MHz (C) made using the dipole array at Gauribidanur near
Bangalore. Observations made towards 32 directions, led to detections of lines
in absorption at nine positions. Followup observations at 328 MHz
(C) using the Ooty Radio Telescope detected these lines in emission.
A VLA D-array observation of one of the positions at 330 MHz yielded no
detection implying a lower limit of 10' for the angular size of the line
forming region.
The longitude-velocity distribution of the observed carbon lines indicate
that the line forming region are located mainly between 4 kpc and 7 kpc from
the Galactic centre. Combining our results with published carbon recombination
line data near 76 MHz (\nocite{erickson:95} Erickson \et 1995) we obtain
constraints on the physical parameters of the line forming regions. We find
that if the angular size of the line forming regions is , then
the range of parameters that fit the data are: \Te K, \ne \cm3 and pathlengths pc which may correspond to thin
photo-dissociated regions around molecular clouds. On the other hand, if the
line forming regions are in extent, then warmer gas (\Te K) with lower electron densities (\ne \cm3) extending
over several tens of parsecs along the line of sight and possibly associated
with atomic \HI gas can fit the data. Based on the range of derived parameters,
we suggest that the carbon line regions are most likely associated with
photo-dissociation regions.Comment: To appear in Journal of Astrophysics & Astronomy, March 200
Effective Vortex Pinning in MgB2 thin films
We discuss pinning properties of MgB2 thin films grown by pulsed-laser
deposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are
identified that contribute most effectively to the pinning of vortices in
randomly oriented films. The EB process produces low defected crystallites with
small grain size providing enhanced pinning at grain boundaries without
degradation of Tc. The PLD process produces films with structural disorder on a
scale less that the coherence length that further improves pinning, but also
depresses Tc
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