19,271 research outputs found
Nucleosynthesis in Fast Expansions of High-Entropy, Proton Rich Matter
We demonstrate that nucleosynthesis in rapid, high-entropy expansions of
proton-rich matter from high temperature and density can result in a wider
variety of abundance patterns than heretofore appreciated. In particular, such
expansions can produce iron-group nuclides, p-process nuclei, or even heavy,
neutron-rich isotopes. Such diversity arises because the nucleosynthesis enters
a little explored regime in which the free nucleons are not in equilibrium with
the abundant alpha particles. This allows nuclei significantly heavier than
iron to form in t he presence of abundant free nucleons early in the expansion.
As the temperature drops, nucleons increasingly assemble into alpha particles
and heavier nuclei. If the assembly is efficient, the resulting depletion of
free neutrons allows disintegrat ion flows to drive nuclei back down to iron
and nickel. If this assembly is inefficient, then the large abundance of free
nucleons prevents the disintegration flows and leaves a distribution of heavy
nuclei after reaction freezeout. For cases in between, an intermediate
abundance distribution, enriched in p-process isotopes, is frozen out. These
last expansions may contribute to the solar system's supply of the p-process
nuclides if mildly proton-rich, high-entropy matter is ejected from
proto-neutron stars winds or other astrophysical sites. Also sign ificant is
the fact that, because the nucleosynthesis is primary, the signature of this
nucleosyn thesis may be evident in metal poor stars.Comment: 11 pages, 2 tables, 1 figure. Submitted to ApJ Letter
Uptake of nitrate and sulfate on dust aerosols during TRACE-P
Aerosol data collected near Asia on the DC-8 aircraft platform during TRACE-P has been examined for evidence of uptake of NO3− and SO4= on dust surfaces. Data is compared between a sector where dust was predominant and a sector where dust was less of an influence. Coincident with dust were higher mixing ratios of anthropogenic pollutants. HNO3, SO2, and CO were higher in the dust sector than the nondust sector by factors of 2.7, 6.2, and 1.5, respectively. The colocation of dust and pollution sources allowed for the uptake of NO3−and nss-SO4= on the coarse dust aerosols, increasing the mixing ratios of these particulates by factors of 5.7 and 2.6 on average. There was sufficient nss-SO4= to take up all of the NH4+present, with enough excess nss-SO4= to also react with dust CaCO3. This suggests that the enhanced NO3− was not in fine mode NH4NO3. Particulate NO3− (p-NO3−) constituted 54% of the total NO3− (t-NO3−) on average, reaching a maximum of 72% in the dust sector. In the nondust sector, p-NO3− contributed 37% to t-NO3−, likely due to the abundance of sea salts there. In two other sectors where the influence of dust and sea salt were minimal, p-NO3−accounted for \u3c15% of t-NO3−
Parity meter for charge qubits: an efficient quantum entangler
We propose a realization of a charge parity meter based on two double quantum
dots alongside a quantum point contact. Such a device is a specific example of
the general class of mesoscopic quadratic quantum measurement detectors
previously investigated by Mao et al. [Phys. Rev. Lett. 93, 056803 (2004)]. Our
setup accomplishes entangled state preparation by a current measurement alone,
and allows the qubits to be effectively decoupled by pinching off the parity
meter. Two applications of the parity meter are discussed: the measurement of
Bell's inequality in charge qubits and the realization of a controlled NOT
gate.Comment: 8 pages, 4 figures; v2: discussion of measurement time, references
adde
Dynamic Poisson Factorization
Models for recommender systems use latent factors to explain the preferences
and behaviors of users with respect to a set of items (e.g., movies, books,
academic papers). Typically, the latent factors are assumed to be static and,
given these factors, the observed preferences and behaviors of users are
assumed to be generated without order. These assumptions limit the explorative
and predictive capabilities of such models, since users' interests and item
popularity may evolve over time. To address this, we propose dPF, a dynamic
matrix factorization model based on the recent Poisson factorization model for
recommendations. dPF models the time evolving latent factors with a Kalman
filter and the actions with Poisson distributions. We derive a scalable
variational inference algorithm to infer the latent factors. Finally, we
demonstrate dPF on 10 years of user click data from arXiv.org, one of the
largest repository of scientific papers and a formidable source of information
about the behavior of scientists. Empirically we show performance improvement
over both static and, more recently proposed, dynamic recommendation models. We
also provide a thorough exploration of the inferred posteriors over the latent
variables.Comment: RecSys 201
Temporal variations in scattering and dispersion measure in the Crab Pulsar and their effect on timing precision
We have measured variations in scattering time scales in the Crab Pulsar over
a 30-year period, using observations made at 610 MHz with the 42-ft telescope
at Jodrell Bank Observatory. Over more recent years, where regular Lovell
Telescope observations at frequencies around 1400 MHz were available, we have
also determined the dispersion measure variations, after disentangling the
scattering delay from the dispersive delay. We demonstrate a relationship
between scattering and dispersion measure variations, with a correlation
coefficient of . The short time scales over which these quantities
vary, the size of the variations, and the close correlation between scattering
and dispersion measure all suggest that the effects are due to discrete
structures within the Crab Nebula, with size scales of AU
(corresponding to an angular size of mas at an assumed distance of 2200
pc). We mitigate the effects of scattering on the observed pulse shape by using
the measured scattering information to modify the template used for generating
the pulse arrival times, thus improving the precision to which the pulsar can
be timed. We test this on timing data taken during periods of high scattering,
and obtain a factor of two improvement in the root mean square of the timing
residuals.Comment: 10 pages, 7 figures. Accepted for publication in MNRA
Far Infrared Variability of Sagittarius A*: 25.5 Hours of Monitoring with
Variable emission from Sgr~A*, the luminous counterpart to the super-massive
black hole at the center of our Galaxy, arises from the innermost portions of
the accretion flow. Better characterization of the variability is important for
constraining models of the low-luminosity accretion mode powering Sgr~A*, and
could further our ability to use variable emission as a probe of the strong
gravitational potential in the vicinity of the
black hole. We use the \textit{Herschel}
Spectral and Photometric Imaging Receiver (SPIRE) to monitor Sgr~A* at
wavelengths that are difficult or impossible to observe from the ground. We
find highly significant variations at 0.25, 0.35, and 0.5 mm, with temporal
structure that is highly correlated across these wavelengths. While the
variations correspond to 1% changes in the total intensity in the
\textit{Herschel} beam containing Sgr~A*, comparison to independent,
simultaneous observations at 0.85 mm strongly supports the reality of the
variations. The lowest point in the light curves, 0.5 Jy below the
time-averaged flux density, places a lower bound on the emission of Sgr~A* at
0.25 mm, the first such constraint on the THz portion of the SED. The
variability on few hour timescales in the SPIRE light curves is similar to that
seen in historical 1.3 mm data, where the longest time series is available, but
the distribution of variations in the sub-mm do not show a tail of
large-amplitude variations seen at 1.3 mm. Simultaneous X-ray photometry from
XMM-Newton shows no significant variation within our observing period, which
may explain the lack of very large variations if X-ray and submillimeter flares
are correlated.Comment: Accepted for publication in Ap
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