25,891 research outputs found
A Damping of the de Haas-van Alphen Oscillations in the superconducting state
Deploying a recently developed semiclassical theory of quasiparticles in the
superconducting state we study the de Haas-van Alphen effect. We find that the
oscillations have the same frequency as in the normal state but their amplitude
is reduced. We find an analytic formulae for this damping which is due to
tunnelling between semiclassical quasiparticle orbits comprising both
particle-like and hole-like segments. The quantitative predictions of the
theory are consistent with the available data.Comment: 7 pages, 5 figure
Water does partially dissociate on the perfect TiO2(110) surface : a quantitative structure determination
There has been a long-standing controversy as to whether water can dissociate on perfect areas of a TiO2(110) surface; most early theoretical work indicated this dissociation was facile, while experiments indicated little or no dissociation. More recently the consensus of most theoretical calculations is that no dissociation occurs. New results presented here, based on analysis of scanned-energy mode photoelectron diffraction data from the OH component of O 1s photoemission, show the coexistence of molecular water and OH species in both atop (OHt) and bridging (OHbr) sites. OHbr can arise from reaction with oxygen vacancy defect sites (Ovac), but OHt have only been predicted to arise from dissociation on the perfect areas of the surface. The relative concentrations of OHt and OHbr sites arising from these two dissociation mechanisms are found to be fully consistent with the initial concentration Ovac sites, while the associated Ti-O bondlengths of the OHt and OHbr species are found to be 1.85±0.08Å and 1.94±0.07 Å, respectively
Hubble Space Telescope Imaging of the Globular Cluster System around NGC 5846
Bimodal globular cluster metallicity distributions have now been seen in a
handful of large ellipticals. Here we report the discovery of a bimodal
distribution in the dominant group elliptical NGC 5846, using the Hubble Space
Telescope's Wide Field and Planetary Camera 2 (WFPC2). The two peaks are
located at V-I = 0.96 and 1.17, which roughly correspond to metallicities of
[Fe/H] = -1.2 and -0.2 respectively. The luminosity functions of the blue and
red subpopulations appear to be the same, indicating that luminosity does not
correlate with metallicity within an individual galaxy's globular cluster
system. Our WFPC2 data cover three pointings allowing us to examine the spatial
distribution of globular clusters out to 30 kpc (or 2.5 galaxy effective
radii). We find a power law surface density with a very flat slope, and a
tendency for globular clusters to align close to the galaxy minor axis. An
extrapolation of the surface density profile, out to 50 kpc, gives a specific
frequency S_N = 4.3 +/- 1.1. Thus NGC 5846 has a much lower specific frequency
than other dominant ellipticals in clusters but is similar to those in groups.
The central galaxy regions reveal some filamentary dust features, presumably
from a past merger or accretion of a gas-rich galaxy. This dust reaches to the
very nucleus and so provides an obvious source of fuel for the radio core. We
have searched for proto-globular clusters that may have resulted from the
merger/accretion and find none. Finally, we briefly discuss the implications of
our results for globular cluster formation mechanisms.Comment: 22 pages, Latex. To be published in the Astronomical Journal. Full
paper available at http://www.ucolick.org/~forbes/home.htm
Cold gas and star formation in a merging galaxy sequence
We explore the evolution of the cold gas and star-formation activity during
galaxy interactions, using a merging galaxy sequence comprising both pre- and
post-mergers. Data for this study come from the literature but supplemented by
new radio observations presented here. Firstly, we confirm that the
star-formation efficiency (SFE) increases close to nuclear coalescence. At
post-merger stages there is evidence that the SFE declines to values typical of
ellipticals. This trend can be attributed to M(H_2) depletion due to
interaction induced star-formation. However, there is significant scatter,
likely to arise from differences in the interaction details of individual
systems. Secondly, we find that the central molecular hydrogen surface density,
increases close to the final stages of the merging of the two nuclei. Such a
trend is also predicted by numerical simulations. Furthermore, there is
evidence for a decreasing fraction of cold gas mass from early interacting
systems to merger remnants, attributed to gas conversion into other forms. The
evolution of the total-radio to blue-band luminosity ratio, reflecting the
disk+nucleus star-formation activity, is also investigated. Although this ratio
is on average higher than that of isolated spirals, we find a marginal increase
along the merging sequence, attributed to the relative insensitivity of disk
star-formation to interactions. However, a similar result is also obtained for
the nuclear radio emission, although galaxy interactions are believed to
significantly affect the activity in the central galaxy regions. Finally, we
find that the FIR--radio flux ratio distribution of interacting galaxies is
consistent with star-formation being the main energising source.Comment: 18 pages, 17 figures, accepted for publication in MNRA
Noncompact sigma-models: Large N expansion and thermodynamic limit
Noncompact SO(1,N) sigma-models are studied in terms of their large N
expansion in a lattice formulation in dimensions d \geq 2. Explicit results for
the spin and current two-point functions as well as for the Binder cumulant are
presented to next to leading order on a finite lattice. The dynamically
generated gap is negative and serves as a coupling-dependent infrared regulator
which vanishes in the limit of infinite lattice size. The cancellation of
infrared divergences in invariant correlation functions in this limit is
nontrivial and is in d=2 demonstrated by explicit computation for the above
quantities. For the Binder cumulant the thermodynamic limit is finite and is
given by 2/(N+1) in the order considered. Monte Carlo simulations suggest that
the remainder is small or zero. The potential implications for ``criticality''
and ``triviality'' of the theories in the SO(1,N) invariant sector are
discussed.Comment: 46 pages, 2 figure
Radio Observations of Super Star Clusters in Dwarf Starburst Galaxies
We present new radio continuum observations of two dwarf starburst galaxies,
NGC3125 and NGC5408, with observations at 4.80GHz (6cm) and 8.64GHz (3cm),
taken with the Australia Telescope Compact Array (ATCA). Both galaxies show a
complex radio morphology with several emission regions, mostly coincident with
massive young star clusters. The radio spectral indices of these regions are
negative (with alpha ~ -0.5 - -0.7), indicating that the radio emission is
dominated by synchrotron emission associated with supernova activity from the
starburst. One emission region in NGC5408 has a flatter index (alpha ~ -0.1)
indicative of optically thin free-free emission, which could indicate it is a
younger cluster. Consequently, in these galaxies we do not see regions with the
characteristic positive spectral index indicative of optically obscured
star-formation regions, as seen in other dwarf starbursts such as Hen 2-10.Comment: Accepted for publication in MNRA
Keck Imaging of the Globular Cluster Systems in the Early--type Galaxies NGC 1052 and NGC 7332
The presence of two globular cluster subpopulations in early-type galaxies is
now the norm rather than the exception. Here we present two more examples for
which the host galaxy appears to have undergone a recent merger. Using
multi-colour Keck imaging of NGC 1052 and NGC 7332 we find evidence for a
bimodal globular cluster colour distribution in both galaxies, with roughly
equal numbers of blue and red globular clusters. The blue ones have similar
colours to those in the Milky Way halo and are thus probably very old and
metal-poor. If the red GC subpopulations are at least solar metallicity, then
stellar population models indicate young ages. We discuss the origin of
globular clusters within the framework of formation models. We conclude that
recent merger events in these two galaxies have had little effect on their
overall GC systems. We also derive globular cluster density profiles, global
specific frequencies and in the case of NGC 1052, radial colour gradients and
azimuthal distribution. In general these globular cluster properties are normal
for early-type galaxies.Comment: 11 pages, Latex, 15 figures, 2 tables, accepted by MNRA
Heating and cooling of coronal loops with turbulent suppression of parallel heat conduction
Using the "enthalpy-based thermal evolution of loops" (EBTEL) model, we
investigate the hydrodynamics of the plasma in a flaring coronal loop in which
heat conduction is limited by turbulent scattering of the electrons that
transport the thermal heat flux. The EBTEL equations are solved analytically in
each of the two (conduction-dominated and radiation-dominated) cooling phases.
Comparison of the results with typical observed cooling times in solar flares
shows that the turbulent mean free-path lies in a range
corresponding to a regime in which classical (collision-dominated) conduction
plays at most a limited role. We also consider the magnitude and duration of
the heat input that is necessary to account for the enhanced values of
temperature and density at the beginning of the cooling phase and for the
observed cooling times. We find through numerical modeling that in order to
produce a peak temperature ~K and a 200~s cooling time
consistent with observations, the flare heating profile must extend over a
significant period of time; in particular, its lingering role must be taken
into consideration in any description of the cooling phase. Comparison with
observationally-inferred values of post-flare loop temperatures, densities, and
cooling times thus leads to useful constraints on both the magnitude and
duration of the magnetic energy release in the loop, as well as on the value of
the turbulent mean free-path .Comment: 16 pages, 4 figures, to be published in The Astrophysical Journa
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