1,821 research outputs found
Comment on the Nature of the and Mesons
Two charm-strange mesons, the and the , have
recently been observed by several experiments. There has been speculation in
the literature that the is the state and
the is the state. In this paper we explore
this and other explanations in the context of the relativized quark model and
the pseudoscalar emission decay model. We conclude that the is
most likely the state and the is most
likely the state with the resonances also
contributing to the observed signals and explaining the observed ratios of
branching ratios to and final states. We point out that measuring
the spin can support or eliminate this explanation and that
there are six excited states in this mass region; the , ,
, and two states. Observing some of the missing states
would help confirm the nature of the and the
states.Comment: 8 pages, 2 figure
Operator mixing in deformed D1D5 CFT and the OPE on the cover
We consider the D1D5 CFT near the orbifold point and develop methods for
computing the mixing of untwisted operators to first order by using the OPE on
the covering surface. We argue that the OPE on the cover encodes both the
structure constants for the orbifold CFT and the explicit form of the mixing
operators. We show this explicitly for some example operators. We start by
considering a family of operators dual to supergravity modes, and show that the
OPE implies that there is no shift in the anomalous dimension to first order,
as expected. We specialize to the operator dual to the dilaton, and show that
the leading order singularity in the OPE reproduces the correct structure
constant. Finally, we consider an unprotected operator of conformal dimension
(2,2), and show that the leading order singularity and one of the subleading
singularies both reproduce the correct structure constant. We check that the
operator produced at subleading order using the OPE method is correct by
calculating a number of three point functions using a Mathematica package we
developed. Further development of this OPE technique should lead to more
efficient calculations for the D1D5 CFT perturbed away from the orbifold point.Comment: 23 page
Conformal Invariance of (0,2) Sigma Models on Calabi-Yau Manifolds
Long ago, Nemeschansky and Sen demonstrated that the Ricci-flat metric on a
Calabi-Yau manifold could be corrected, order by order in perturbation theory,
to produce a conformally invariant (2,2) nonlinear sigma model. Here we extend
this result to (0,2) sigma models for stable holomorphic vector bundles over
Calabi-Yaus.Comment: 15 pages; references adde
3D mapping of young stars in the solar neighbourhood with Gaia DR2
We study the three dimensional arrangement of young stars in the solar
neighbourhood using the second release of the Gaia mission (Gaia DR2) and we
provide a new, original view of the spatial configuration of the star forming
regions within 500 pc from the Sun. By smoothing the star distribution through
a gaussian filter, we construct three dimensional density maps for early-type
stars (upper-main sequence, UMS) and pre-main sequence (PMS) sources. The PMS
and the UMS samples are selected through a combination of photometric and
astrometric criteria. A side product of the analysis is a three dimensional,
G-band extinction map, which we use to correct our colour-magnitude diagram for
extinction and reddening. Both density maps show three prominent structures,
Scorpius-Centaurus, Orion, and Vela. The PMS map shows a plethora of lower mass
star forming regions, such as Taurus, Perseus, Cepheus, Cassiopeia, and
Lacerta, which are less visible in the UMS map, due to the lack of large
numbers of bright, early-type stars. We report the finding of a candidate new
open cluster towards , which could be
related to the Orion star forming complex. We estimate ages for the PMS sample
and we study the distribution of PMS stars as a function of their age. We find
that younger stars cluster in dense, compact clumps, and are surrounded by
older sources, whose distribution is instead more diffuse. The youngest groups
that we find are mainly located in Scorpius-Centaurus, Orion, Vela, and Taurus.
Cepheus, Cassiopeia, and Lacerta are instead more evolved and less numerous.
Finally, we find that the three dimensional density maps show no evidence for
the existence of the ring-like structure which is usually referred to as the
Gould Belt.Comment: 17 pages, 17 figures, 6 appendixes; accepted for publication in A&A;
image quality decreased to comply with the arXiv.org rules on file siz
Exoplanet Transit Variability: Bow Shocks and Winds Around HD 189733b
By analogy with the solar system, it is believed that stellar winds will form
bow shocks around exoplanets. For hot Jupiters the bow shock will not form
directly between the planet and the star, causing an asymmetric distribution of
mass around the exoplanet and hence an asymmetric transit. As the planet orbits
thorough varying wind conditions, the strength and geometry of its bow shock
will change, thus producing transits of varying shape. We model this process
using magnetic maps of HD 189733 taken one year apart, coupled with a 3D
stellar wind model, to determine the local stellar wind conditions throughout
the orbital path of the planet. We predict the time-varying geometry and
density of the bow shock that forms around the magnetosphere of the planet and
simulate transit light curves. Depending on the nature of the stellar magnetic
field, and hence its wind, we find that both the transit duration and ingress
time can vary when compared to optical light curves. We conclude that
consecutive near-UV transit light curves may vary significantly and can
therefore provide an insight into the structure and evolution of the stellar
wind.Comment: 9 Pages, 7 figures. Accepted for publication in Monthly Notices of
The Royal Astronomical Societ
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
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