1,606 research outputs found
The progenitors of Type Ia supernovae with long delay times
The nature of the progenitors of Type Ia supernovae (SNe Ia) is still
unclear. In this paper, by considering the effect of the instability of
accretion disk on the evolution of white dwarf (WD) binaries, we performed
binary evolution calculations for about 2400 close WD binaries, in which a
carbon--oxygen WD accretes material from a main-sequence star or a slightly
evolved subgiant star (WD + MS channel), or a red-giant star (WD + RG channel)
to increase its mass to the Chandrasekhar (Ch) mass limit. According to these
calculations, we mapped out the initial parameters for SNe Ia in the orbital
period--secondary mass () plane for various WD
masses for these two channels, respectively. We confirm that WDs in the WD + MS
channel with a mass as low as can accrete efficiently and reach
the Ch limit, while the lowest WD mass for the WD + RG channel is . We have implemented these results in a binary population synthesis
study to obtain the SN Ia birthrates and the evolution of SN Ia birthrates with
time for both a constant star formation rate and a single starburst. We find
that the Galactic SN Ia birthrate from the WD + MS channel is according to our standard model, which is higher than
previous results. However, similar to previous studies, the birthrate from the
WD + RG channel is still low (). We also
find that about one third of SNe Ia from the WD + MS channel and all SNe Ia
from the WD + RG channel can contribute to the old populations (\ga1 Gyr) of
SN Ia progenitors.Comment: 11 pages, 9 figures, 1 table, accepted for publication in MNRA
Enhancing Hydrogen Generation Through Nanoconfinement of Sensitizers and Catalysts in a Homogeneous Supramolecular Organic Framework.
Enrichment of molecular photosensitizers and catalysts in a confined nanospace is conducive for photocatalytic reactions due to improved photoexcited electron transfer from photosensitizers to catalysts. Herein, the self-assembly of a highly stable 3D supramolecular organic framework from a rigid bipyridine-derived tetrahedral monomer and cucurbit[8]uril in water, and its efficient and simultaneous intake of both [Ru(bpy)3 ]2+ -based photosensitizers and various polyoxometalates, that can take place at very low loading, are reported. The enrichment substantially increases the apparent concentration of both photosensitizer and catalyst in the interior of the framework, which leads to a recyclable, homogeneous, visible light-driven photocatalytic system with 110-fold increase of the turnover number for the hydrogen evolution reaction
Precise Request Tracing and Performance Debugging for Multi-tier Services of Black Boxes
As more and more multi-tier services are developed from commercial components
or heterogeneous middleware without the source code available, both developers
and administrators need a precise request tracing tool to help understand and
debug performance problems of large concurrent services of black boxes.
Previous work fails to resolve this issue in several ways: they either accept
the imprecision of probabilistic correlation methods, or rely on knowledge of
protocols to isolate requests in pursuit of tracing accuracy. This paper
introduces a tool named PreciseTracer to help debug performance problems of
multi-tier services of black boxes. Our contributions are two-fold: first, we
propose a precise request tracing algorithm for multi-tier services of black
boxes, which only uses application-independent knowledge; secondly, we present
a component activity graph abstraction to represent causal paths of requests
and facilitate end-to-end performance debugging. The low overhead and tolerance
of noise make PreciseTracer a promising tracing tool for using on production
systems
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