15,935 research outputs found
Spin-Up/Spin-Down models for Type Ia Supernovae
In the single degenerate scenario for Type Ia supernova (SNeIa), a white
dwarf (WD) must gain a significant amount of matter from a companion star.
Because the accreted mass carries angular momentum, the WD is likely to achieve
fast spin periods, which can increase the critical mass, , needed for
explosion. When is higher than the maximum mass achieved by the WD,
the WD must spin down before it can explode. This introduces a delay between
the time at which the WD has completed its epoch of mass gain and the time of
the explosion. Matter ejected from the binary during mass transfer therefore
has a chance to become diffuse, and the explosion occurs in a medium with a
density similar to that of typical regions of the interstellar medium. Also,
either by the end of the WD's mass increase or else by the time of explosion,
the donor may exhaust its stellar envelope and become a WD. This alters,
generally diminishing, explosion signatures related to the donor star.
Nevertheless, the spin-up/spin-down model is highly predictive. Prior to
explosion, progenitors can be super- WDs in either wide binaries with
WD companions, or else in cataclysmic variables. These systems can be
discovered and studied through wide-field surveys. Post explosion, the
spin-up/spin-down model predicts a population of fast-moving WDs, low-mass
stars, and even brown dwarfs. In addition, the spin-up/spin-down model provides
a paradigm which may be able to explain both the similarities and the diversity
observed among SNeIa.Comment: Submitted to ApJ Letter
Intervalley Scattering and Localization Behaviors of Spin-Valley Coupled Dirac Fermions
We study the quantum diffusive transport of multivalley massive Dirac cones,
where time-reversal symmetry requires opposite spin orientations in
inequivalent valleys. We show that the intervalley scattering and intravalley
scattering can be distinguished from the quantum conductivity that corrects the
semiclassical Drude conductivity, due to their distinct symmetries and
localization trends. In immediate practice, it allows transport measurements to
estimate the intervalley scattering rate in hole-doped monolayers of group-VI
transition metal dichalcogenides (e.g., molybdenum dichalcogenides and tungsten
dichalcogenides), an ideal class of materials for valleytronics applications.
The results can be generalized to a large class of multivalley massive Dirac
systems with spin-valley coupling and time-reversal symmetry.Comment: 5 pages+4 pages of supplemental materials, 4 figure
Probing the momentum dependence of medium modifications of the nucleon-nucleon elastic cross sections
The momentum dependence of the medium modifications on nucleon-nucleon
elastic cross sections is discussed with microscopic transport theories and
numerically investigated with an updated UrQMD microscopic transport model. The
semi-peripheral Au+Au reaction at beam energy MeV is adopted as an
example. It is found that the uncertainties of the momentum dependence on
medium modifications of cross sections influence the yields of free nucleons
and their collective flows as functions of their transverse momentum and
rapidity. Among these observables, the elliptic flow is sensitively dependent
on detailed forms of the momentum dependence and more attention should be paid.
The elliptic flow is hardly influenced by the probable splitting effect of the
neutron-neutron and proton-proton cross sections so that one might pin down the
mass splitting effect of the mean-field level at high beam energies and high
nuclear densities by exploring the elliptic flow of nucleons or light clusters.Comment: 13 pages, 6 figures, 1 tabl
Reconstructing propagation networks with natural diversity and identifying hidden sources
Our ability to uncover complex network structure and dynamics from data is
fundamental to understanding and controlling collective dynamics in complex
systems. Despite recent progress in this area, reconstructing networks with
stochastic dynamical processes from limited time series remains to be an
outstanding problem. Here we develop a framework based on compressed sensing to
reconstruct complex networks on which stochastic spreading dynamics take place.
We apply the methodology to a large number of model and real networks, finding
that a full reconstruction of inhomogeneous interactions can be achieved from
small amounts of polarized (binary) data, a virtue of compressed sensing.
Further, we demonstrate that a hidden source that triggers the spreading
process but is externally inaccessible can be ascertained and located with high
confidence in the absence of direct routes of propagation from it. Our approach
thus establishes a paradigm for tracing and controlling epidemic invasion and
information diffusion in complex networked systems.Comment: 20 pages and 5 figures. For Supplementary information, please see
http://www.nature.com/ncomms/2014/140711/ncomms5323/full/ncomms5323.html#
Physical properties of CO-dark molecular gas traced by C
Neither HI nor CO emission can reveal a significant quantity of so-called
dark gas in the interstellar medium (ISM). It is considered that CO-dark
molecular gas (DMG), the molecular gas with no or weak CO emission, dominates
dark gas. We identified 36 DMG clouds with C emission (data from Galactic
Observations of Terahertz C+ (GOT C+) project) and HINSA features. Based on
uncertainty analysis, optical depth of HI of 1 is a reasonable
value for most clouds. With the assumption of , these clouds
were characterized by excitation temperatures in a range of 20 K to 92 K with a
median value of 55 K and volume densities in the range of
cm to cm with a median value of
cm. The fraction of DMG column density in the cloud ()
decreases with increasing excitation temperature following an empirical
relation +1.0. The relation
between and total hydrogen column density is given by
=. The values of in the
clouds of low extinction group ( mag) are consistent with the
results of the time-dependent, chemical evolutionary model at the age of ~ 10
Myr. Our empirical relation cannot be explained by the chemical evolutionary
model for clouds in the high extinction group ( mag). Compared to
clouds in the low extinction group ( mag), clouds in the high
extinction group ( mag) have comparable volume densities but
excitation temperatures that are 1.5 times lower. Moreover, CO abundances in
clouds of the high extinction group ( mag) are
times smaller than the canonical value in the Milky Way. #[Full version of
abstract is shown in the text.]#Comment: Accepted for publishing in Astronomy & Astrophysics. 13 pages, 8
figure
The Progenitors of Type Ia Supernovae: Are They Supersoft Sources?
In a canonical model, the progenitors of Type Ia supernovae (SNe Ia) are
accreting, nuclear-burning white dwarfs (NBWDs), which explode when the white
dwarf reaches the Chandrasekhar mass, M_C. Such massive NBWDs are hot (kT ~100
eV), luminous (L ~ 10^{38} erg/s), and are potentially observable as luminous
supersoft X-ray sources (SSSs). During the past several years, surveys for soft
X-ray sources in external galaxies have been conducted. This paper shows that
the results falsify the hypothesis that a large fraction of progenitors are
NBWDs which are presently observable as SSSs. The data also place limits on
sub-M_C models. While Type Ia supernova progenitors may pass through one or
more phases of SSS activity, these phases are far shorter than the time needed
to accrete most of the matter that brings them close to M_C.Comment: submitted to ApJ 18 November 2009; 17 pages, 2 figure
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