33,247 research outputs found
The optimally-sampled galaxy-wide stellar initial mass function - Observational tests and the publicly available GalIMF code
Here we present a full description of the integrated galaxy-wide initial mass
function (IGIMF) theory in terms of the optimal sampling and compare it with
available observations. Optimal sampling is the method we use to discretize the
IMF into stellar masses deterministically. Evidence has been indicating that
nature may be closer to deterministic sampling as observations suggest a
smaller scatter of various relevant observables than random sampling would
give, which may result from a high level of self-regulation during the star
formation process. The variation of the IGIMFs under various assumptions are
documented. The results of the IGIMF theory are consistent with the empirical
relation between the total mass of a star cluster and the mass of its most
massive star, and the empirical relation between a galaxy's star formation rate
(SFR) and the mass of its most massive cluster. Particularly, we note a natural
agreement with the empirical relation between the IMF's power-law index and a
galaxy's SFR. The IGIMF also results in a relation between the galaxy's SFR and
the mass of its most massive star such that, if there were no binaries,
galaxies with SFR M/yr should host no Type II supernova
events. In addition, a specific list of initial stellar masses can be useful in
numerical simulations of stellar systems. For the first time, we show
optimally-sampled galaxy-wide IMFs (OSGIMF) which mimics the IGIMF with an
additional serrated feature. Finally, A Python module, GalIMF, is provided
allowing the calculation of the IGIMF and OSGIMF in dependence on the
galaxy-wide SFR and metallicity.Comment: 15 pages, 15 figures, A&A, in press; paper remains unchanged
(version1 equals version2); the GalIMF module is downloadable at githu
A two component jet model for the X-ray afterglow flat segment in short GRB 051221A
In the double neutron star merger or neutron star-black hole merger model for
short GRBs, the outflow launched might be mildly magnetized and neutron rich.
The magnetized neutron-rich outflow will be accelerated by the magnetic and
thermal pressure and may form a two component jet finally, as suggested by
Vlahakis, Peng & K\"{o}nigl (2003). We show in this work that such a two
component jet model could well reproduce the multi-wavelength afterglow
lightcurves, in particular the X-ray flat segment, of short GRB 051221A. In
this model, the central engine need not to be active much longer than the
prompt ray emission.Comment: 11 pages, 2 figure; Accepted for publication by ApJ
Controlled quantum teleportation and secure direct communication
We present a controlled quantum teleportation protocol. In the protocol,
quantum information of an unknown state of a 2-level particle is faithfully
transmitted from a sender (Alice) to a remote receiver (Bob) via an initially
shared triplet of entangled particles under the control of the supervisor
Charlie. The distributed entangled particles shared by Alice, Bob and Charlie
function as a quantum information channel for faithful transmission. We also
propose a controlled and secure direct communication scheme by means of this
teleportation. After insuring the security of the quantum channel, Alice
encodes the secret message directly on a sequence of particle states and
transmits them to Bob supervised by Charlie using this controlled quantum
teleportation. Bob can read out the encoded message directly by the measurement
on his qubit. In this scheme, the controlled quantum teleportation transmits
Alice's message without revealing any information to a potential eavesdropper.
Because there is not a transmission of the qubit carrying the secret message
between Alice and Bob in the public channel, it is completely secure for
controlled and direct secret communication if perfect quantum channel is used.
The feature of this scheme is that the communication between two sides depends
on the agreement of the third side.Comment: 4 page
Direct laser acceleration of electrons assisted by strong laser-driven azimuthal plasma magnetic fields.
A high-intensity laser beam propagating through a dense plasma drives a strong current that robustly sustains a strong quasistatic azimuthal magnetic field. The laser field efficiently accelerates electrons in such a field that confines the transverse motion and deflects the electrons in the forward direction. Its advantage is a threshold rather than resonant behavior, accelerating electrons to high energies for sufficiently strong laser-driven currents. We study the electron dynamics via a test-electron model, specifically deriving the corresponding critical current density. We confirm the model's predictions by numerical simulations, indicating energy gains two orders of magnitude higher than achievable without the magnetic field
The impact of the metallicity and star formation rate on the time-dependent galaxy-wide stellar initial mass function
The stellar initial mass function (IMF) is commonly assumed to be an
invariant probability density distribution function of initial stellar masses
being represented by the canonical IMF. As a consequence the galaxy-wide IMF
(gwIMF), defined as the sum of the IMFs of all star forming regions, should
also be invariant. Recent observational and theoretical results challenge the
hypothesis that the gwIMF is invariant. In order to study the possible reasons
for this variation we use the IMF determined in resolved star clusters and
apply the IGIMF-theory to calculate a grid of gwIMF models for metallicities,
-3<[Fe/H]<1, and galaxy-wide star formation rates,
<SFR<. For a galaxy with metallicy
[Fe/H]/yr, which is a common condition in the early
Universe, we find that the gwIMF is top-heavy (more massive stars), when
compared to the canonical IMF. For a SFR the gwIMF
becomes top-light regardless of the metallicity. For metallicities
the gwIMF can become bottom-heavy regardless of the SFR.
The IGIMF models predict that massive elliptical galaxies should have formed
with a gwIMF that is top-heavy within the first few hundred Myr of the galaxy's
life and that it evolves into a bottom-heavy gwIMF in the metal-enriched
galactic center. We study the SFRH relation, its dependency on
metallicity and the SFR, the correction factors to the Kennicutt SFRH relation, and provide new fitting functions Late-type dwarf
galaxies show significantly higher SFRs with respect to Kennicutt SFRs, while
star forming massive galaxies have significantly lower SFRs than hitherto
thought. This has implications for the gas-consumption time scales and for the
main sequence of galaxies. The Leo P and ultra-faint dwarf galaxies are
discussed explicitly. [abridged]Comment: Astronomy and Astrophysics (A&A) in press. 15 pages, 8 figure
Deep Multi-instance Networks with Sparse Label Assignment for Whole Mammogram Classification
Mammogram classification is directly related to computer-aided diagnosis of
breast cancer. Traditional methods rely on regions of interest (ROIs) which
require great efforts to annotate. Inspired by the success of using deep
convolutional features for natural image analysis and multi-instance learning
(MIL) for labeling a set of instances/patches, we propose end-to-end trained
deep multi-instance networks for mass classification based on whole mammogram
without the aforementioned ROIs. We explore three different schemes to
construct deep multi-instance networks for whole mammogram classification.
Experimental results on the INbreast dataset demonstrate the robustness of
proposed networks compared to previous work using segmentation and detection
annotations.Comment: MICCAI 2017 Camera Read
A Review of Three-Family Grand Unified String Models
We review the construction and classification of three-family grand unified
models within the framework of asymmetric orbifolds in perturbative heterotic
superstring. We give a detailed survey of all such models which is organized to
aid analysis of their phenomenological properties. We compute tree-level
superpotentials for these models. These superpotentials are used to analyze the
issues of proton stability (doublet-triplet splitting and R-parity violating
terms) and Yukawa mass matrices. To have agreement with phenomenological data
all these models seem to require certain degree of fine-tuning. We also analyze
the possible patterns of supersymmetry breaking in these models.
We find that the supersymmetry breaking scale comes out either too high to
explain the electroweak hierarchy problem, or below the electroweak scale
unless some degree of fine-tuning is involved. Thus, none of the models at hand
seem to be phenomenologically flawless.Comment: 49 pages, Revtex 3.0; one ps figure included. To appear in the Review
section of Int.J.Mod.Phy
Developing fluid flow and heat transfer in a channel partially filled with porous medium
A three-dimensional computational model is developed to analyze fluid flow in a channel partially filled with porous medium. In order to understand the developing fluid flow and heat transfer mechanisms inside the channel partially filled with porous medium, the conventional Navier–Stokes equations for gas channel, and volume-averaged Navier–Stokes equations for porous medium layer are adopted individually in this study. Conservation of mass, momentum and energy equations are solved numerically in a coupled gas and porous media domain along a channel using the vorticity–velocity method with power law scheme. Detailed development of axial velocity, secondary flow and temperature field at various axial positions in the entrance region are presented. The friction factor and Nusselt number are presented as a function of axial position, and the effects of the size of porous media inside the channel partially filled with porous medium are also analyzed in the present study
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