72,250 research outputs found
Mass retention efficiencies of He accretion onto carbon-oxygen white dwarfs and type Ia supernovae
Type Ia supernovae (SNe Ia) play a crucial role in studying cosmology and
galactic chemical evolution. They are thought to be thermonuclear explosions of
carbon-oxygen white dwarfs (CO WDs) when their masses reach the Chandrasekar
mass limit in binaries. Previous studies have suggested that He novae may be
progenitor candidates of SNe Ia. However, the mass retention efficiencies
during He nova outbursts are still uncertain. In this article, we aim to study
the mass retention efficiencies of He nova outbursts and to investigate whether
SNe Ia can be produced through He nova outbursts. Using the stellar evolution
code Modules for Experiments in Stellar Astrophysics, we simulated a series of
multicycle He-layer flashes, in which the initial WD masses range from 0.7 to
1.35 Msun with various accretion rates. We obtained the mass retention
efficiencies of He nova outbursts for various initial WD masses, which can be
used in the binary population synthesis studies. In our simulations, He nova
outbursts can increase the mass of the WD to the Chandrasekar mass limit and
the explosive carbon burning can be triggered in the center of the WD; this
suggests that He nova outbursts can produce SNe Ia. Meanwhile, the mass
retention efficiencies in the present work are lower than those of previous
studies, which leads to a lower birthrates of SNe Ia through the WD + He star
channel. Furthermore, we obtained the elemental abundances distribution at the
moment of explosive carbon burning, which can be used as the initial input
parameters in studying explosion models of SNe Ia.Comment: 8 pages, 12 figures, 2 tables, published in Astronomy & Astrophysics
(A&A 604, A31, 2017
Stokes Parameters as a Minkowskian Four-vector
It is noted that the Jones-matrix formalism for polarization optics is a
six-parameter two-by-two representation of the Lorentz group. It is shown that
the four independent Stokes parameters form a Minkowskian four-vector, just
like the energy-momentum four-vector in special relativity. The optical filters
are represented by four-by-four Lorentz-transformation matrices. This
four-by-four formalism can deal with partial coherence described by the Stokes
parameters. A four-by-four matrix formulation is given for decoherence effects
on the Stokes parameters, and a possible experiment is proposed. It is shown
also that this Lorentz-group formalism leads to optical filters with a symmetry
property corresponding to that of two-dimensional Euclidean transformations.Comment: RevTeX, 22 pages, no figures, submitted to Phys. Rev.
Transport in Graphene Tunnel Junctions
We present a technique to fabricate tunnel junctions between graphene and Al
and Cu, with a Si back gate, as well as a simple theory of tunneling between a
metal and graphene. We map the differential conductance of our junctions versus
probe and back gate voltage, and observe fluctuations in the conductance that
are directly related to the graphene density of states. The conventional
strong-suppression of the conductance at the graphene Dirac point can not be
clearly demonstrated, but a more robust signature of the Dirac point is found:
the inflection in the conductance map caused by the electrostatic gating of
graphene by the tunnel probe. We present numerical simulations of our
conductance maps, confirming the measurement results. In addition, Al causes
strong n-doping of graphene, Cu causes a moderate p-doping, and in high
resistance junctions, phonon resonances are observed, as in STM studies.Comment: 22 pages, 5 figure
User's guide for the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) CELL-ALL tape
The SMMR instrument onboard the Nimbus-7 satellite has been in operation since October 1978. It provided global coverage of passive microwave observations at 6.6, 10.7, 18, 21, and 37 GHz. The oberved brightness temperature can be used to retrieve geophysical parameters, principally sea surface temperature, atmospheric water vapor and liquid water content over oceans, sea ice concentration, and snow cover over land. The SMME CELL-ALL Tape contains earth-located calibrated brightness temperature data which have been appropriately binned into cells of various grid sizes, allowing intercomparisons of observations made at different frequencies (with corresponding different footprint sizes). This user's guide describes the operation of the instrument, the flow of the data processing the calibration procedure, and the characteristics of the calibrated brightness temperatures and how they are binned. Detailed tape specifications and lists of available data are also provided
Living on the edge : ground-state selection in quantum spin-ice pyrochlores
The search for new quantum phases, especially in frustrated magnets, is
central to modern condensed matter physics. One of the most promising places to
look is in rare-earth pyrochlore magnets with highly-anisotropic exchange
interactions, materials closely related to the spin ices Ho2Ti2O7 and Dy2Ti2O7.
Here we establish a general theory of magnetic order in these materials. We
find that many of their most interesting properties can be traced back to the
accidental degeneracies where phases with different symmetry meet. These
include the ordered ground state selection by fluctuations in Er2Ti2O7, the
dimensional-reduction observed in Yb2Ti2O7, and the absence of magnetic order
in Er2Sn2O7.Comment: A long-paper version of this preprint, "Living on the Edge", appears
as arXiv:1603.09466 [accepted for publication in Physical Review B]. The text
of v2 is otherwise unchanged from v1 (Submitted on 14 Nov 2013
Blending in Future Space-based Microlensing Surveys
We investigate the effect of blending in future gravitational microlensing
surveys by carrying out simulation of Galactic bulge microlensing events to be
detected from a proposed space-based lensing survey. From this simulation, we
find that the contribution of the flux from background stars to the total
blended flux will be equivalent to that from the lens itself despite the
greatly improved resolution from space observations, implying that
characterizing lenses from the analysis of the blended flux would not be easy.
As a method to isolate events for which most of the blended flux is
attributable to the lens, we propose to use astrometric information of source
star image centroid motion. For the sample of events obtained by imposing a
criterion that the centroid shift should be less than three times of the
astrometric uncertainty among the events for which blending is noticed with
blended light fractions , we estimate that the contamination of
the blended flux by background stars will be less than 20% for most () of the sample events. The expected rate of these events is
events/yr, which is large enough for the statistical analysis of the lens
populations.Comment: total 6 pages, including 5 figures, ApJ, in pres
Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) PARM tape user's guide
The Scanning Multichannel Microwave Radiometer (SMMR) instrument, onboard the Nimbus-7 spacecraft, collected data from Oct. 1978 until Jun. 1986. The data were processed to physical parameter level products. Geophysical parameters retrieved include the following: sea-surface temperatures, sea-surface windspeed, total column water vapor, and sea-ice parameters. These products are stored on PARM-LO, PARM-SS, and PARM-30 tapes. The geophysical parameter retrieval algorithms and the quality of these products are described for the period between Nov. 1978 and Oct 1985. Additionally, data formats and data availability are included
Thermodynamics of rotating Bose gases in a trap
Novel ground state properties of rotating Bose gases have been intensively
studied in the context of neutral cold atoms. We investigate the rotating Bose
gas in a trap from a thermodynamic perspective, taking the charged ideal Bose
gas in magnetic field (which is equivalent to a neutral gas in a synthetic
magnetic field) as an example. It is indicated that the Bose-Einstein
condensation temperature is irrelevant to the magnetic field, conflicting with
established intuition that the critical temperature decreases with the field
increasing. The specific heat and Landau diamagnetization also exhibit
intriguing behaviors. In contrast, we demonstrate that the condensation
temperature for neutral Bose gases in a rotating frame drops to zero in the
fast rotation limit, signaling a non-condensed quantum phase in the ground
state.Comment: 4 pages, 1 figur
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