10,285 research outputs found
M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planets
We perform three-dimensional numerical simulations of stellar winds of
early-M dwarf stars. Our simulations incorporate observationally reconstructed
large-scale surface magnetic maps, suggesting that the complexity of the
magnetic field can play an important role in the angular momentum evolution of
the star, possibly explaining the large distribution of periods in field dM
stars, as reported in recent works. In spite of the diversity of the magnetic
field topologies among the stars in our sample, we find that stellar wind
flowing near the (rotational) equatorial plane carries most of the stellar
angular momentum, but there is no preferred colatitude contributing to mass
loss, as the mass flux is maximum at different colatitudes for different stars.
We find that more non-axisymmetric magnetic fields result in more asymmetric
mass fluxes and wind total pressures (defined as the sum of
thermal, magnetic and ram pressures). Because planetary magnetospheric sizes
are set by pressure equilibrium between the planet's magnetic field and , variations of up to a factor of in (as found in the
case of a planet orbiting at several stellar radii away from the star) lead to
variations in magnetospheric radii of about 20 percent along the planetary
orbital path. In analogy to the flux of cosmic rays that impact the Earth,
which is inversely modulated with the non-axisymmetric component of the total
open solar magnetic flux, we conclude that planets orbiting M dwarf stars like
DT~Vir, DS~Leo and GJ~182, which have significant non-axisymmetric field
components, should be the more efficiently shielded from galactic cosmic rays,
even if the planets lack a protective thick atmosphere/large magnetosphere of
their own.Comment: 16 pages, 9 figures, to appear in MNRA
Preferred levels for background ducking to produce esthetically pleasing audio for TV with clear speech
In audio production, background ducking facilitates speech intelligibility while allowing the background to fulfill its purpose, e.g., to create ambience, set the mood, or convey semantic cues. Technical details for recommended ducking practices are not currently documented in the literature. Hence, we first analyzed common practices found in TV documentaries. Second, a listening test investigated the preferences of 22 normal-hearing participants on the Loud- ness Difference (LD) between commentary and background during ducking. Highly personal preferences were observed, highlighting the importance of object-based personalization. Sta- tistically significant difference was found between non-expert and expert listeners. On average, non-experts preferred LDs that were 4 LU higher than the ones preferred by experts. A sta- tistically significant difference was also found between Commentary over Music (CoM) and Commentary over Ambience (CoA). Based on the test results, we recommend at least 10 LU difference for CoM and at least 15 LU for CoA. Moreover, a computational method based on the Binaural Distortion-Weighted Glimpse Proportion (BiDWGP) was found to match the median preferred LD for each item with good accuracy (mean absolute error = 1.97 LU ± 2.50)
Decoherence-protected memory for a single-photon qubit
The long-lived, efficient storage and retrieval of a qubit encoded on a
photon is an important ingredient for future quantum networks. Although systems
with intrinsically long coherence times have been demonstrated, the combination
with an efficient light-matter interface remains an outstanding challenge. In
fact, the coherence times of memories for photonic qubits are currently limited
to a few milliseconds. Here we report on a qubit memory based on a single atom
coupled to a high-finesse optical resonator. By mapping and remapping the qubit
between a basis used for light-matter interfacing and a basis which is less
susceptible to decoherence, a coherence time exceeding 100 ms has been measured
with a time-independant storage-and-retrieval efficiency of 22%. This
demonstrates the first photonic qubit memory with a coherence time that exceeds
the lower bound needed for teleporting qubits in a global quantum internet.Comment: 3 pages, 4 figure
A homomorphism between link and XXZ modules over the periodic Temperley-Lieb algebra
We study finite loop models on a lattice wrapped around a cylinder. A section
of the cylinder has N sites. We use a family of link modules over the periodic
Temperley-Lieb algebra EPTL_N(\beta, \alpha) introduced by Martin and Saleur,
and Graham and Lehrer. These are labeled by the numbers of sites N and of
defects d, and extend the standard modules of the original Temperley-Lieb
algebra. Beside the defining parameters \beta=u^2+u^{-2} with u=e^{i\lambda/2}
(weight of contractible loops) and \alpha (weight of non-contractible loops),
this family also depends on a twist parameter v that keeps track of how the
defects wind around the cylinder. The transfer matrix T_N(\lambda, \nu) depends
on the anisotropy \nu and the spectral parameter \lambda that fixes the model.
(The thermodynamic limit of T_N is believed to describe a conformal field
theory of central charge c=1-6\lambda^2/(\pi(\lambda-\pi)).)
The family of periodic XXZ Hamiltonians is extended to depend on this new
parameter v and the relationship between this family and the loop models is
established. The Gram determinant for the natural bilinear form on these link
modules is shown to factorize in terms of an intertwiner i_N^d between these
link representations and the eigenspaces of S^z of the XXZ models. This map is
shown to be an isomorphism for generic values of u and v and the critical
curves in the plane of these parameters for which i_N^d fails to be an
isomorphism are given.Comment: Replacement of "The Gram matrix as a connection between periodic loop
models and XXZ Hamiltonians", 31 page
Detecting Text Reuse in Cryptocurrency Whitepapers
Thousands of new cryptocurrencies have been introduced in recent years. Most are introduced with a so-called "whitepaper" containing a mix of technical documentation, legal boilerplate and marketing material. Notably, many proposed currencies reuse text from previous established cryptocurrencies. We analyze the whitepapers from 1 260 actively traded cryptocurrencies and 2 039 ICOs. We develop two measures of similarity. Moderately similar papers reuse text in a portion of the paper, often the legal disclaimers. By contrast, some highly similar whitepapers appear to copy most of the text. 4% of coin and 19% of ICO whitepapers are highly similar to those of traded coins. The fraction rises to 64% for coins and 67% for ICOs when we consider moderate text reuse
Refined conformal spectra in the dimer model
Working with Lieb's transfer matrix for the dimer model, we point out that
the full set of dimer configurations may be partitioned into disjoint subsets
(sectors) closed under the action of the transfer matrix. These sectors are
labelled by an integer or half-integer quantum number we call the variation
index. In the continuum scaling limit, each sector gives rise to a
representation of the Virasoro algebra. We determine the corresponding
conformal partition functions and their finitizations, and observe an
intriguing link to the Ramond and Neveu-Schwarz sectors of the critical dense
polymer model as described by a conformal field theory with central charge
c=-2.Comment: 44 page
Cool Stars and Space Weather
Stellar flares, winds and coronal mass ejections form the space weather. They
are signatures of the magnetic activity of cool stars and, since activity
varies with age, mass and rotation, the space weather that extra-solar planets
experience can be very different from the one encountered by the solar system
planets. How do stellar activity and magnetism influence the space weather of
exoplanets orbiting main-sequence stars? How do the environments surrounding
exoplanets differ from those around the planets in our own solar system? How
can the detailed knowledge acquired by the solar system community be applied in
exoplanetary systems? How does space weather affect habitability? These were
questions that were addressed in the splinter session "Cool stars and Space
Weather", that took place on 9 Jun 2014, during the Cool Stars 18 meeting. In
this paper, we present a summary of the contributions made to this session.Comment: Proceedings of the 18th Cambridge Workshop on Cool Stars, Stellar
Systems, and the Sun, Eds G. van Belle & H. Harris, 13 pages, 1 figur
Quadrupolar Order in Isotropic Heisenberg Models with Biquadratic Interaction
Through Quantum Monte Carlo simulation, we study the biquadratic-interaction
model with the SU(2) symmetry in two and three dimensions. The zero-temperature
phase diagrams for the two cases are identical and exhibit an intermediate
phase characterized by finite quadrupole moment, in agreement with mean-field
type arguments and the semi-classical theory. In three dimensions, we
demonstrate that the model in the quadrupolar regime has a phase transition at
a finite temperature. In contrast to predictions by mean-field theories, the
phase transition to the quadrupolar phase turns out to be of the second order.
We also examine the critical behavior in the two marginal cases with the SU(3)
symmetry.Comment: 4 pages 5 figure
Complex magnetic topology and strong differential rotation on the low-mass T Tauri star V2247 Oph
From observations collected with the ESPaDOnS spectropolarimeter at the
Canada-France-Hawaii Telescope, we report the detection of Zeeman signatures on
the low-mass classical TTauri star (cTTS) V2247Oph. Profile distortions and
circular polarisation signatures detected in photospheric lines can be
interpreted as caused by cool spots and magnetic regions at the surface of the
star. The large-scale field is of moderate strength and highly complex;
moreover, both the spot distribution and the magnetic field show significant
variability on a timescale of only one week, as a likely result of strong
differential rotation. Both properties make V2247Oph very different from the
(more massive) prototypical cTTS BPTau; we speculate that this difference
reflects the lower mass of V2247Oph.
During our observations, V2247Oph was in a low-accretion state, with emission
lines showing only weak levels of circular polarisation; we nevertheless find
that excess emission apparently concentrates in a mid-latitude region of strong
radial field, suggesting that it is the footpoint of an accretion funnel.
The weaker and more complex field that we report on V2247Oph may share
similarities with those of very-low-mass late-M dwarfs and potentially explain
why low-mass cTTSs rotate on average faster than intermediate mass ones. These
surprising results need confirmation from new independent data sets on V2247Oph
and other similar low-mass cTTSs.Comment: MNRAS (in press) - 12 pages, 9 figure
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