434 research outputs found
The scalar complex potential and the Aharonov-Bohm effect
The Aharonov-Bohm effect is traditionally attributed to the effect of the
electromagnetic 4-potential , even in regions where both the electric field
and the magnetic field are zero. The AB effect
reveals that multiple-valued functions play a crucial role in the description
of an electromagnetic field. We argue that the quantity measured by AB
experiments is a difference in values of a multiple-valued complex function,
which we call a complex potential or {pre-potential. We show that any
electromagnetic field can be described by this pre-potential, and give an
explicit expression for the electromagnetic field tensor through this
potential. The pre-potential is a modification of the two scalar potential
functions.Comment: 10 pages 2 figure
Unravelling the chemical inhomogeneity of PNe with VLT FLAMES integral-field unit spectroscopy
Recent weak emission-line long-slit surveys and modelling studies of PNe have
convincingly argued in favour of the existence of an unknown component in the
planetary nebula plasma consisting of cold, hydrogen-deficient gas, as an
explanation for the long-standing recombination-line versus forbidden-line
temperature and abundance discrepancy problems. Here we describe the rationale
and initial results from a detailed spectroscopic study of three Galactic PNe
undertaken with the VLT FLAMES integral-field unit spectrograph, which advances
our knowledge about the small-scale physical properties, chemical abundances
and velocity structure of these objects across a two-dimensional field of view,
and opens up for exploration an uncharted territory in the study and modelling
of PNe and photoionized nebulae in general.Comment: 4 pages; 3 figures; invited paper to appear in proceedings of IAU
Symp. No. 234, 2006, Planetary Nebulae in our Galaxy and Beyond (held in
Hawaii, April 2006
Getting Jobs, Keeping Jobs, and Earning a Living Wage: Can Welfare Reform Work?
Most discussions of welfare and work have focused on how demographic characteristics, schooling, training, and work experience limit welfare mothers’ employment and wages, but they have largely ignored factors such as inappropriate workplace behaviors, expectations of discrimination and harassment, depression, alcoholism, and domestic violence, all of which may affect welfare mothers and make employment difficult. In this paper we review the prevalence of these individual-level barriers and argue that they, in combination with an economy which does not pay low-skill workers well, are likely to impede employment and self-sufficiency for a large proportion of welfare mothers. At the end of the review, we summarize the current state of knowledge about barriers to the employment of welfare recipients and suggest several ways in which welfare-to-work programs might address these barriers.
A deep survey of heavy element lines in planetary nebulae -- II. Recombination line abundances and evidence for ultra-cold plasma
[Abridged] Deep optical observations of the spectra of 12 Galactic planetary
nebulae (PNe) and 3 Magellanic Cloud PNe were presented in Paper I by Tsamis et
al. (2003b), who carried out an abundance analysis using the collisionally
excited forbidden lines. Here, the relative intensities of faint optical
recombination lines (ORLs) from ions of carbon, nitrogen and oxygen are
analysed in order to derive the abundances of these ions relative to hydrogen.
We define an abundance discrepancy factor (ADF) as the ratio of the abundance
derived for a heavy element ion from its recombination lines to that derived
for the same ion from its ultraviolet, optical or infrared collisionally
excited lines (CELs). All of the PNe in our sample are found to have ADF's that
exceed unity. There is no dependence of the magnitude of the ADF upon the
excitation energy of the UV, optical or IR CEL transition used, indicating that
classical nebular temperature fluctuations--i.e. in a chemically homogeneous
medium--are not the cause of the observed abundance discrepancies. Instead, we
conclude that the main cause of the discrepancy is enhanced ORL emission from
cold ionized gas located in hydrogen-deficient clumps inside the main body of
the nebulae. We have developed a new electron temperature diagnostic, based
upon the relative intensities of the OII 4f-3d 4089A and 3p-3s 4649A
recombination transitions. For six out of eight PNe for which both transitions
are detected, we derive O2+ ORL electron temperatures of <300 K, very much less
than the O2+ forbidden-line and Balmer jump temperatures derived for the same
nebulae. These results provide direct observational evidence for the presence
of H-deficient, cold plasma regions within the nebulae, consistent with gas
cooled largely by infrared fine structure and recombination transitions.Comment: 27 pages, 7 figures, submitted to the MNRA
A deep survey of heavy element lines in planetary nebulae - I. Observations and forbidden-line densities, temperatures and abundances
We present spectrophotometry of 12 Galactic and 3 Magellanic Cloud planetary
nebulae (PNe). Nine of the Galactic PNe were observed by scanning the slit
across the PN. We use the fluxes of collisionally excited lines (CELs) to
derive electron densities (D's) and temperatures (T's), and ionic abundances.
We find that the D's derived from optical CEL ratios are systematically higher
than those derived from the ratios of the IR fine-structure (FS) lines of
[OIII], indicating the presence of significant density variations within the
PNe. We also compare T's obtained from the ratio of optical nebular to auroral
[OIII] lines with those obtained from the ratio of [OIII] optical to IR FS
lines. We find that when the latter are derived using D's based on the [OIII]
52um/88um ratio, they yield values that are significantly higher than the
optical [OIII] T's. Contrasting this, [OIII] optical/IR T's derived using the
higher D's obtained from [ClIII] 5517A/5537A ratios show much closer agreement
with optical [OIII] T's, implying that the observed [OIII] optical/IR ratios
are significantly weighted by D's in excess of the critical densities of both
[OIII] FS lines. Consistent with this, ionic abundances derived from [OIII] and
[NIII] FS lines using D's from optical CELs show much better agreement with
abundances derived for the same ions from optical and UV CELs than do
abundances derived from the FS lines using the lower D's obtained from the
52um/88um ratios. The behaviour of T's obtained making use of the T-insensitive
IR FS lines provides no support for significant T-fluctuations within the PNe
that could be responsible for derived Balmer jump T's being lower than those
obtained from the much more T-sensitive [OIII] optical lines.Comment: 36 pages; 3 figures; to be published in MNRA
Spectral statistics of random geometric graphs
We use random matrix theory to study the spectrum of random geometric graphs,
a fundamental model of spatial networks. Considering ensembles of random
geometric graphs we look at short range correlations in the level spacings of
the spectrum via the nearest neighbour and next nearest neighbour spacing
distribution and long range correlations via the spectral rigidity Delta_3
statistic. These correlations in the level spacings give information about
localisation of eigenvectors, level of community structure and the level of
randomness within the networks. We find a parameter dependent transition
between Poisson and Gaussian orthogonal ensemble statistics. That is the
spectral statistics of spatial random geometric graphs fits the universality of
random matrix theory found in other models such as Erdos-Renyi, Barabasi-Albert
and Watts-Strogatz random graph.Comment: 19 pages, 6 figures. Substantially updated from previous versio
Dust Formation and He II 4686 emission in the Dense Shell of the Peculiar Type Ib Supernova 2006jc
We present evidence for the formation of dust grains in an unusual Type Ib SN
based on late-time spectra of SN 2006jc. The progenitor suffered an LBV-like
outburst just 2 yr earlier, and we propose that the dust formation is a
consequence of the SN blast wave overtaking that LBV-like shell. The key
evidence for dust formation is (a) the appearance of a red/near-IR continuum
source fit by 1600 K graphite grains, and (b) fading of the redshifted sides of
He I emission lines, yielding progressively more asymmetric blueshifted lines
as dust obscures receding material. This provides the strongest case yet for
dust formation in any SN Ib/c. Both developments occurred between 51 and 75 d
after peak, while other SNe observed to form dust did so after a few hundred
days. Geometric considerations indicate that dust formed in the dense swept-up
shell between the forward and reverse shocks, and not in the freely expanding
SN ejecta. Rapid cooling leading to dust formation may have been aided by
extremely high shell densities, as indicated by He I line ratios. The brief
epoch of dust formation is accompanied by He II 4686 emission and enhanced
X-ray emission. These clues suggest that the unusual dust formation in this
object was not due to properties of the SN itself, but instead -- like most
peculiarities of SN 2006jc -- was a consequence of the dense environment
created by an LBV-like eruption 2 yr before the SN.Comment: ApJ, accepted. added some discussion and 2 figures, better title,
conclusions same as previous version. 12 pages, 4 color fig
Implementation of Photonic True Time Delay Using High-Order-Mode Dispersion Compensating Fibers
Dynamics and Excitation of Radio Galaxy Emission-Line Regions - I. PKS 2356-61
Results are presented from a programme of detailed longslit spectroscopic
observations of the extended emission-line region (EELR) associated with the
powerful radio galaxy PKS 2356-61. The observations have been used to construct
spectroscopic datacubes, which yield detailed information on the spatial
variations of emission-line ratios across the EELR, together with its kinematic
structure. We present an extensive comparison between the data and results
obtained from the MAPPINGS II shock ionization code, and show that the physical
properties of the line-emitting gas, including its ionization, excitation,
dynamics and overall energy budget, are entirely consistent with a scenario
involving auto-ionizing shocks as the dominant ionization mechanism. This has
the advantage of accounting for the observed EELR properties by means of a
single physical process, thereby requiring less free parameters than the
alternative scheme involving photoionization by radiation from the active
nucleus. Finally, possible mechanisms of shock formation are considered in the
context of the dynamics and origin of the gas, specifically scenarios involving
infall or accretion of gas during an interaction between the host radio galaxy
and a companion galaxy.Comment: 35 pages, LaTeX, uses aas2pp4.sty file, includes 9 PostScript
figures. Two additional colour plates are available from the authors upon
request. Accepted for publication in the Astrophysical Journa
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