25,184 research outputs found
Deep optical imaging of AGB circumstellar envelopes
We report results of a program to image the extended circumstellar envelopes
of asymptotic giant branch (AGB) stars in dust-scattered Galactic light. The
goal is to characterize the shapes of the envelopes to probe the mass-loss
geometry and the presence of hidden binary companions. The observations consist
of deep optical imaging of 22 AGB stars with high mass loss rates: 16 with the
ESO 3.5 m NTT telescope, and the remainder with other telescopes. The
circumstellar envelopes are detected in 15 objects, with mass loss rates > 2E-6
Msun/year. The surface brightness of the envelopes shows a strong decrease with
Galactic radius, which indicates a steep radial gradient in the interstellar
radiation field. The envelopes range from circular to elliptical in shape, and
we characterize them by the ellipticity (E = major/minor axis) of iso-intensity
contours. We find that about 50 percent of the envelopes are close to circular
with E
1.2. We interpret the shapes in terms of populations of single stars and
binaries whose envelopes are flattened by a companion. The distribution of E is
qualitatively consistent with expectations based on population synthesis models
of binary AGB stars. We also find that about 50 percent of the sample exhibit
small-scale, elongated features in the central regions. We interpret these as
the escape of light from the central star through polar holes, which are also
likely produced by companions. Our observations of envelope flattening and
polar holes point to a hidden population of companions within the circumstellar
envelopes of AGB stars. These companions are expected to play an important role
in the transition to post-AGB stars and the formation of planetary nebulae.Comment: 19 pages, 13 figures, color pictures in Appendix, accepted by A&
Reversibility Checking for Markov Chains
In this paper, we present reversibility preserving operations on Markov chain
transition matrices. Simple row and column operations allow us to create new
reversible transition matrices and yield an easy method for checking a Markov
chain for reversibility
Light pseudoscalar eta and H->eta eta decay in the simplest little Higgs mode
The SU(3) simplest little Higgs model in its original framework without the
so-called mu term inevitably involves a massless pseudoscalar boson eta, which
is problematic for b-physics and cosmological axion limit. With the mu term
introduced by hand, the eta boson acquires mass m_eta ~ mu, which can be
lighter than half the Higgs boson mass in a large portion of the parameter
space. In addition, the introduced mu term generates sizable coupling of
H-eta-eta. The Higgs boson can dominantly decay into a pair of eta's especially
when mH below the WW threshold. Another new decay channel of H->Z+eta can be
dominant or compatible with H -> WW for mH above the Z+eta threshold. We show
that the LEP bound on the Higgs boson mass is loosened to some extent due to
this new H->eta eta decay channel as well as the reduced coupling of H-Z-Z. The
Higgs boson mass bound falls to about 110 GeV for f=3-4 TeV. Since the eta
boson decays mainly into a bb pair, H-> eta eta -> 4b and H-> Z eta -> Z bb
open up other interesting search channels in the pursuit of the Higgs boson in
the future experiments. We discuss on these issues.Comment: major modification considering the simplest little Higgs model with
the mu ter
Thermal Diagnostics with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory: A Validated Method for Differential Emission Measure Inversions
We present a new method for performing differential emission measure (DEM)
inversions on narrow-band EUV images from the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO). The method yields positive
definite DEM solutions by solving a linear program. This method has been
validated against a diverse set of thermal models of varying complexity and
realism. These include (1) idealized gaussian DEM distributions, (2) 3D models
of NOAA Active Region 11158 comprising quasi-steady loop atmospheres in a
non-linear force-free field, and (3) thermodynamic models from a
fully-compressible, 3D MHD simulation of AR corona formation following magnetic
flux emergence. We then present results from the application of the method to
AIA observations of Active Region 11158, comparing the region's thermal
structure on two successive solar rotations. Additionally, we show how the DEM
inversion method can be adapted to simultaneously invert AIA and XRT data, and
how supplementing AIA data with the latter improves the inversion result. The
speed of the method allows for routine production of DEM maps, thus
facilitating science studies that require tracking of the thermal structure of
the solar corona in time and space.Comment: 21 pages, 18 figures, accepted for publication in Ap
Strange-Beauty Meson Production at Colliders
The production rates and transverse momentum distributions of the
strange-beauty mesons and at colliders are calculated
assuming fragmentation is the dominant process. Results are given for the
Tevatron in the large transverse momentum region, where fragmentation is
expected to be most important.Comment: Minor changes in the discussion section. Also available at
http://www.ph.utexas.edu/~cheung/paper.htm
Conformal Symmetries of Adiabatic Modes in Cosmology
We remark on the existence of non-linearly realized conformal symmetries for
scalar adiabatic perturbations in cosmology. These conformal symmetries are
present for any cosmological background, beyond any slow-roll or quasi-de
Sitter approximation. The dilatation transformation shifts the curvature
perturbation by a constant, and corresponds to the well-known symmetry under
spatial rescaling. We argue that the scalar sector is also invariant under
special conformal transformations, which shift the curvature perturbation by a
term linear in the spatial coordinates. We discuss whether these conformal
symmetries can be extended to include tensor perturbations. Tensor modes
introduce their own set of non-linearly realized symmetries. We identify an
infinite set of large gauge transformations which maintain the transverse,
traceless gauge condition, while shifting the tensor mode non-trivially.Comment: 16 page
Gamma-ray Constraints on Effective Interactions of the Dark Matter
Using an effective interaction approach to describe the interactions between
the dark matter particle and the light degrees of freedom of the standard
model, we calculate the gamma-ray flux due to the annihilation of the dark
matter into quarks, followed by fragmentation into neutral pions which
subsequently decay into photons. By comparison to the mid-latitude data
released from the Fermi-LAT experiment, we obtain useful constraints on the
size of the effective interactions and they are found to be comparable to those
deduced from collider, gamma-ray line and anti-matter search experiments.
However, the two operators induced by scalar and vector exchange among
fermionic dark matter and light quarks that contribute to spin-independent
cross sections are constrained more stringently by the recent XENON100 data.Comment: 22 pages, 4 figures; title fixed and a couple of references adde
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