1,387 research outputs found
Screening of heterogeneous surfaces: charge renormalization of Janus particles
Nonlinear ionic screening theory for heterogeneously charged spheres is
developed in terms of a mode-decomposition of the surface charge. A far-field
analysis of the resulting electrostatic potential leads to a natural
generalization of charge renormalization from purely monopolar to dipolar,
quadropolar, etc., including mode-couplings. Our novel scheme is generally
applicable to large classes of surface heterogeneities, and is explicitly
applied here to Janus spheres with differently charged upper and lower
hemispheres, revealing strong renormalization effects for all multipoles.Comment: 2 figure
A Semantic Basis for Parallel Algorithm Design
As computing demands increase, emphasis is being placed on parallel architectures- To efficiently use parallel machines, software must be designed to take advantage of these machines. This research concentrates on an abstraction of algorithm design to permit the expression of parallel programs. The abstraction emphasizes thought about algorithms at a high level as opposed to algorithm implementation at a statement level. A model based on data flow allows algorithm expression using flow diagrams. The model specifies operating system requirements that support parallel programming at a module level. Paths are used to carry data between modules. Data enter modules through ports. Module activation is triggered by the satisfaction of data availability conditions. Continual module presence within the system, dynamic activation criteria, and a high level of programming distinguishes this model from other parallel programming systems
Optical polarisation variability of radio loud narrow line Seyfert 1 galaxies. Search for long rotations of the polarisation plane
Narrow line Seyfert 1 galaxies (NLSy1s) constitute the AGN subclass
associated with systematically smaller black hole masses. A few radio loud ones
have been detected in MeV -- GeV energy bands by Fermi and evidence for the
presence of blazar-like jets has been accumulated. In this study we wish to
quantify the temporal behaviour of the optical polarisation, fraction and
angle, for a selected sample of radio loud NLSy1s. We also search for rotations
of the polarisation plane similar to those commonly observed in blazars. We
have conducted R-band optical polarisation monitoring of a sample of 10 RL
NLSy1s 5 of which have been previously detected by Fermi. The dataset includes
observations with the RoboPol, KANATA, Perkins and Steward polarimeters. In the
cases where evidences for long rotations of the polarisation plane are found,
we carry out numerical simulations to assess the probability that they are
caused by intrinsically evolving EVPAs instead of observational noise. Even our
moderately sampled sources show indications of variability, both in
polarisation fraction and angle. For the four best sampled objects in our
sample we find multiple periods of significant polarisation angle variability.
In the two best sampled cases, namely J1505+0326 and J0324+3410, we find
indications for three long rotations. We show that although noise can induce
the observed behaviour, it is much more likely that the apparent rotation is
caused by intrinsic evolution of the EVPA. To our knowledge this is the very
first detection of such events in this class of sources. In the case of the
largest dataset (J0324+3410) we find that the EVPA concentrates around a
direction which is at 49.3\degr to the 15-GHz radio jet implying a projected
magnetic field at an angle of 40.7\degr to that axis.Comment: Accepted for publication in section 2. Astrophysical processes of
Astronomy and Astrophysic
Resolving the pulsations of subdwarf B stars: HS 0039+4302, HS 0444+0458, and an examination of the group properties of resolved pulsators
We continue our program of single-site observations of pulsating subdwarf B
(sdB) stars and present the results of extensive time series photometry of HS
0039+4302 and HS 0444+0458. Both were observed at MDM Observatory during the
fall of 2005. We extend the number of known frequencies for HS 0039+4302 from 4
to 14 and discover one additional frequency for HS 0444+0458, bringing the
total to three. We perform standard tests to search for multiplet structure,
measure amplitude variations, and examine the frequency density to constrain
the mode degree .
Including the two stars in this paper, 23 pulsating sdB stars have received
follow-up observations designed to decipher their pulsation spectra. It is
worth an examination of what has been detected. We compare and contrast the
frequency content in terms of richness and range and the amplitudes with
regards to variability and diversity. We use this information to examine
observational correlations with the proposed pulsation mechanism as
well as alternative theories.Comment: 32 pages, 18 figures, 7 tables. Accepted for publication in MNRA
The Age of the Oldest Stars in the Local Galactic Disk From Hipparcos Parallaxes of G and K Subgiants
We review the history of the discovery of field subgiant stars and their
importance in the age dating of the Galactic disk. We use the cataloged data
from the Hipparcos satellite in this latter capacity. Based on Hipparcos
parallaxes accurate to 10% or better, the absolute magnitude of the lower
envelope of the nearly horizontal subgiant sequence for field stars in the H-R
diagram for B-V colors between 0.85 and 1.05 is measured to be M_V = 4.03 +/-
0.06. The age of the field stars in the solar neighborhood is found to be 7.9
+/- 0.7 Gyr by fitting the theoretical isochrones for [Fe/H] = +0.37 to the
lower envelope of the Hipparcos subgiants. The same grid of isochrones yields
ages, in turn, of 4.0 +/- 0.2 Gyr, 6.2 +/- 0.5 Gyr, and 7.5 to 10 Gyr for the
old Galactic clusters M67, NGC188, and NGC6791. The ages of both the Galactic
disk in the solar neighborhood and of NGC6791 are, nevertheless, likely between
3 and 5 Gyr younger than the oldest halo globular clusters, which have ages of
13.5 Gyr. The most significant results are (1) the supermetallicity of the
oldest local disk stars, and (2) the large age difference between the most
metal-poor component of the halo and the thick and thin disk in the solar
neighborhood. These facts are undoubtedly related and pose again the problem of
the proper scenario for the timing of events in the formation of the halo and
the Galactic disk in the solar neighborhood. [Abstract Abridged]Comment: 44 pages, 12 Figures; accepted for publication in PASP; high
resolution versions of Figures 1, 2, 6 and 9 available at
http://bubba.ucdavis.edu/~lubin/Sandage
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