50,390 research outputs found
Against the Wind: Radio Light Curves of Type Ia Supernovae Interacting with Low-Density Circumstellar Shells
For decades, a wide variety of observations spanning the radio through
optical and on to the x-ray have attempted to uncover signs of type Ia
supernovae (SNe Ia) interacting with a circumstellar medium (CSM). The goal of
these studies is to constrain the nature of the hypothesized SN Ia mass-donor
companion. A continuous CSM is typically assumed when interpreting observations
of interaction. However, while such models have been successfully applied to
core-collapse SNe, the assumption of continuity may not be accurate for SNe Ia,
as shells of CSM could be formed by pre-supernova eruptions (novae). In this
work, we model the interaction of SNe with a spherical, low density,
finite-extent CSM and create a suite of synthetic radio synchrotron light
curves. We find that CSM shells produce sharply peaked light curves, and
identify a fiducial set of models that all obey a common evolution and can be
used to generate radio light curves for interaction with an arbitrary shell.
The relations obeyed by the fiducial models can be used to deduce CSM
properties from radio observations; we demonstrate this by applying them to the
non-detections of SN 2011fe and SN 2014J. Finally, we explore a multiple shell
CSM configuration and describe its more complicated dynamics and resultant
radio light curves.Comment: 15 pages, 11 figures, ApJ accepte
On the absence of Shapiro-like steps in certain mesoscopic S-N-S junctions
In DC transport through mesoscopic S-N-S junctions, it is known that the
Josephson coupling decreases exponentially with increasing temperature, but the
phase dependence of the conductance persists to much higher temperatures and
decreases only as 1/T. It is pointed out here that, despite the fact that such
a phase-dependent conductance does bring about an AC current for a pure DC
voltage, it cannot, by itself, lead to the formation of Shapiro steps.Comment: 1 page, to be published in PRL (as Comment
Self-healing fuse
Fast-acting current limiting device provides current overload protection for vulnerable circuit elements and then re-establishes conduction path within milliseconds. Fuse can also perform as fast-acting switch to clear transient circuit overloads. Fuse takes advantage of large increase in electrical resistivity that occurs when liquid metal vaporizes
Instantaneous processing of "slow light": amplitude-duration control, storage, and splitting
Nonadiabatic change of the control field or of the low-frequency coherence
allows for an almost instantaneous change of the signal field propagating in a
thick resonant absorber where electromagnetically induced transparency is
realized. This finding is applied for the storage and retrieval of the signal,
for the creation of a signal copy and separation of this copy from the original
pulse without its destruction.Comment: 9 pages, 1 figure, submitted to PRL on 18, December, 200
Two Populations of Young Massive Star Clusters in Arp 220
We present new optical observations of young massive star clusters in Arp
220, the nearest ultraluminous infrared galaxy, taken in UBVI with the Hubble
Space Telescope ACS/HRC camera. We find a total of 206 probable clusters whose
spatial distribution is centrally concentrated toward the nucleus of Arp 220.
We use model star cluster tracks to determine ages, luminosities, and masses
for 14 clusters with complete UBVI indices or previously published
near-infrared data. We estimate rough masses for 24 additional clusters with I
< 24 mag from BVI indices alone. The clusters with useful ages fall into two
distinct groups: a ``young'' population (< 10 Myr) and an intermediate-age
population (~300 Myr). There are many clusters with masses clearly above 10^6
Msun and possibly even above 10^7 Msun in the most extreme instances. These
masses are high enough that the clusters being formed in the Arp 220 starburst
can be considered as genuine young globular clusters. In addition, this study
allows us to extend the observed correlation between global star formation rate
and maximum cluster luminosity by more than an order of magnitude in star
formation rate.Comment: 30 pages, 8 figures; figs 1-3 in color; accepted to Ap
Gravitationally enhanced depolarization of ultracold neutrons in magnetic-field gradients
Trapped ultracold neutrons (UCN) have for many years been the mainstay of experiments to search for the electric dipole moment (EDM) of the neutron, a critical parameter in constraining scenarios of new physics beyond the Standard Model. Because their energies are so low, UCN preferentially populate the lower region of their physical enclosure, and do not sample uniformly the ambient magnetic field throughout the storage volume. This leads to a substantial increase in the rate of depolarization, as well as to shifts in the measured frequency of the stored neutrons. Consequences for EDM measurements are discussed
Self-healing fuse development
The mercury-filled self-healing fuses developed for this program afford very good protection from circuit faults with rapid reclosure. Fuse performance and design parameters have been characterized. Life tests indicate a capability of 500 fuse operations. Fuse ratings are 150 v at 5, 15, 25 and 50 circuit A. A series of sample fuses using alumina and beryllia insulation have been furnished to NASA for circuit evaluation
Invasion threshold in heterogeneous metapopulation networks
We study the dynamics of epidemic and reaction-diffusion processes in
metapopulation models with heterogeneous connectivity pattern. In SIR-like
processes, along with the standard local epidemic threshold, the system
exhibits a global invasion threshold. We provide an explicit expression of the
threshold that sets a critical value of the diffusion/mobility rate below which
the epidemic is not able to spread to a macroscopic fraction of subpopulations.
The invasion threshold is found to be affected by the topological fluctuations
of the metapopulation network. The presented results provide a general
framework for the understanding of the effect of travel restrictions in
epidemic containment.Comment: 4 pages, 2 figure
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