5,399 research outputs found
Superfluid instability of r-modes in "differentially rotating" neutron stars
Superfluid hydrodynamics affects the spin-evolution of mature neutron stars,
and may be key to explaining timing irregularities such as pulsar glitches.
However, most models for this phenomenon exclude the global instability
required to trigger the event. In this paper we discuss a mechanism that may
fill this gap. We establish that small scale inertial r-modes become unstable
in a superfluid neutron star that exhibits a rotational lag, expected to build
up due to vortex pinning as the star spins down. Somewhat counterintuitively,
this instability arises due to the (under normal circumstances dissipative)
vortex-mediated mutual friction. We explore the nature of the superfluid
instability for a simple incompressible model, allowing for entrainment
coupling between the two fluid components. Our results recover a previously
discussed dynamical instability in systems where the two components are
strongly coupled. In addition, we demonstrate for the first time that the
system is secularly unstable (with a growth time that scales with the mutual
friction) throughout much of parameter space. Interestingly, large scale
r-modes are also affected by this new aspect of the instability. We analyse the
damping effect of shear viscosity, which should be particularly efficient at
small scales, arguing that it will not be sufficient to completely suppress the
instability in astrophysical systems.Comment: RevTex, 11 figure
AXA General Insurance Ltd v Lord Advocate : analysis
Discusses the Supreme Court decision in AXA General Insurance Ltd, Petitioners on whether Scottish legislation which reversed a House of Lords ruling concerning the actionability of certain asbestos-related conditions breached insurers' rights
A maximum-likelihood method for improving faint source flux and color estimates
Flux estimates for faint sources or transients are systematically biased high
because there are far more truly faint sources than bright. Corrections which
account for this effect are presented as a function of signal-to-noise ratio
and the (true) slope of the faint-source number-flux relation. The corrections
depend on the source being originally identified in the image in which it is
being photometered. If a source has been identified in other data, the
corrections are different; a prescription for calculating the corrections is
presented. Implications of these corrections for analyses of surveys are
discussed; the most important is that sources identified at signal-to-noise
ratios of four or less are practically useless.Comment: 9 pp., accepted for publication in PAS
A survey of the properties of early-type galaxies
A compilation of the properties of elliptical and early disk galaxies was completed. In addition to material from the literature, such as Infrared Astronomy Satellite (IRAS) fluxes, the compilation includes recent measurements of HI and CO, as well as a review of the x ray properties by Forman and Jones. The data are used to evaluate the gas content of early systems and to search for correlations with x ray emission. The interstellar medium in early-type galaxies is generally dominated by hot interstellar gas (T approx. 10 to the 7th power K; c.f. the review by Fabbiano 1989 and references therein). In addition, a significant fraction of these galaxies show infrared emission (Knapp, et al., 1989), optical emission lines, and visible dust. Sensitive studies in HI and CO of a number of these galaxies have been completed recently, resulting in several detections, particularly of the later types. Researchers wish to understand the connection among these different forms of the interstellar medium, and to examine the theoretical picture of the fate of the hot gas. To do so, they compiled observations of several forms of interstellar matter for a well-defined sample of early-type galaxies. Here they present a statistical analysis of this data base and discuss the implications of the results
Measurements of positive ions and air-earth current density at Maitri, Antarctica
Simultaneous measurements of the small-, intermediate- and large- positive
ions and air earth current density made at a coastal station, Maitri at
Antarctica during January to February 2005, are reported. Although, small and
large positive ion concentrations do not show any systematic diurnal
variations, variations in them are almost similar to each other. On the other
hand, variations in intermediate positive ion concentrations are independent of
variations in the small/large positive ions and exhibit a diurnal variation
which is similar to that in atmospheric temperature on fair weather days with a
maximum during the day and minimum during the night hours. No such diurnal
variation in intermediate positive ion concentration is observed on cloudy days
when variations in them are also similar to those insmall/large positive ion
concentrations. Magnitude of diurnal variation in intermediate positive ion
concentration on fair weather days increases with the lowering of atmospheric
temperature in this season. Scavenging of ions by snowfall and trapping of Alha
- rays from the ground radioactivity by a thin layer of snow on ground, is
demonstrated from observations. Variations in intermediate positive ion
concentration are explained on the basis of the formation of new particles by
the photolytic nucleation process.Comment: 38 pages, 11 figure and 2 tabl
Caltech Faint Field Galaxy Redshift Survey IX: Source detection and photometry in the Hubble Deep Field Region
Detection and photometry of sources in the U_n, G, R, and K_s bands in a 9x9
arcmin^2 region of the sky, centered on the Hubble Deep Field, are described.
The data permit construction of complete photometric catalogs to roughly
U_n=25, G=26, R=25.5 and K_s=20 mag, and significant photometric measurements
somewhat fainter. The galaxy number density is 1.3x10^5 deg^{-2} to R=25.0 mag.
Galaxy number counts have slopes dlog N/dm=0.42, 0.33, 0.27 and 0.31 in the
U_n, G, R and K_s bands, consistent with previous studies and the trend that
fainter galaxies are, on average, bluer. Galaxy catalogs selected in the R and
K_s bands are presented, containing 3607 and 488 sources, in field areas of
74.8 and 59.4 arcmin^2, to R=25.5 and and K_s=20 mag.Comment: Accepted for publication in ApJS; some tables and slightly nicer
figures available at http://www.sns.ias.edu/~hogg/deep
Study of electro-absorption effects in 1300nm In(Ga)As/GaAs quantum dot materials
No abstract available
Private Database Queries Using Quantum States with Limited Coherence Times
We describe a method for private database queries using exchange of quantum
states with bits encoded in mutually incompatible bases. For technology with
limited coherence time, the database vendor can announce the encoding after a
suitable delay to allow the user to privately learn one of two items in the
database without the ability to also definitely infer the second item. This
quantum approach also allows the user to choose to learn other functions of the
items, such as the exclusive-or of their bits, but not to gain more information
than equivalent to learning one item, on average. This method is especially
useful for items consisting of a few bits by avoiding the substantial overhead
of conventional cryptographic approaches.Comment: extended to generalized (POVM) measurement
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