42,478 research outputs found
Circumstellar effects on the Rb abundances in O-rich AGB stars
For the first time we explore the circumstellar effects on the Rb (and Zr)
abundance determination in O-rich asymptotic giant branch (AGB) stars by
considering the presence of a gaseous circumstellar envelope with a radial
wind. A modified version of the spectral synthesis code Turbospectrum was used
to deal with extended atmosphere models and velocity fields. The Rb and Zr
abundances were determined from the resonant 7800A Rb I line and the 6474A ZrO
bandhead, respectively, in five representative O-rich AGB stars with different
expansion velocity and metallicity. By using our new dynamical models, the Rb I
line profile (photospheric and circumstellar components) is very well
reproduced. Interestingly, the derived Rb abundances are much lower (by 1-2
dex) in those O-rich AGB stars showing the higher circumstellar expansion
velocities. The Zr abundances, however, remain close to the solar values. The
Rb abundances and Rb/Zr ratios derived here significantly alleviate the problem
of the present mismatch between the observations of intermediate-mass (4-8
solar masses) Rb-rich AGB stars and the AGB nucleosynthesis theoretical
predictions.Comment: Accepted for publication in Astronomy & Astrophysics Letters (7
pages, 5 figures, and 2 tables); final version (language corrected
On AdS to dS transitions in higher-curvature gravity
We study the possible existence of gravitational phase transitions from AdS
to dS geometries in the context of higher-curvature gravities. We use
Lanczos-Gauss-Bonnet (LGB) theory with a positive cosmological constant as a
toy model. This theory has two maximally symmetric vacua with positive (dS) and
negative (AdS) constant curvature. We show that a phase transition from the AdS
vacuum to a dS black hole geometry takes place when the temperature reaches a
critical value. The transition is produced by nucleation of bubbles of the new
phase that expand afterwards. We claim that this phenomenon is not particular
to the model under study, and shall also be part of generic gravitational
theories with higher-curvature terms.Comment: 12 pages, 3 figures; v2: comments and references adde
Controlling a triangular flexible formation of autonomous agents
In formation control, triangular formations consisting of three autonomous
agents serve as a class of benchmarks that can be used to test and compare the
performances of different controllers. We present an algorithm that combines
the advantages of both position- and distance-based gradient descent control
laws. For example, only two pairs of neighboring agents need to be controlled,
agents can work in their own local frame of coordinates and the orientation of
the formation with respect to a global frame of coordinates is not prescribed.
We first present a novel technique based on adding artificial biases to
neighboring agents' range sensors such that their eventual positions correspond
to a collinear configuration. Right after, a small modification in the bias
terms by introducing a prescribed rotation matrix will allow the control of the
bearing of the neighboring agents.Comment: 7 pages, accepted in the 20th World Congress of the International
Federation of Automatic Control (IFAC
Implications of the first detection of coherent elastic neutrino-nucleus scattering (CEvNS) with Liquid Argon
The CENNS-10 experiment of the COHERENT collaboration has recently reported
the first detection of coherent-elastic neutrino-nucleus scattering (CEvNS) in
liquid Argon with more than significance. In this work, we exploit
the new data in order to probe various interesting parameters which are of key
importance to CEvNS within and beyond the Standard Model. A dedicated
statistical analysis of these data shows that the current constraints are
significantly improved in most cases. We derive a first measurement of the
neutron rms charge radius of Argon, and also an improved determination of the
weak mixing angle in the low energy regime. We also update the constraints on
neutrino non-standard interactions, electromagnetic properties and light
mediators with respect to those derived from the first COHERENT-CsI data.Comment: discussion expanded including light mediators and nuclear
uncertainties, figures added, references added. V3: Fig. 7 corrected,
conclusions unchange
Matrix Product States: Symmetries and Two-Body Hamiltonians
We characterize the conditions under which a translationally invariant matrix
product state (MPS) is invariant under local transformations. This allows us to
relate the symmetry group of a given state to the symmetry group of a simple
tensor. We exploit this result in order to prove and extend a version of the
Lieb-Schultz-Mattis theorem, one of the basic results in many-body physics, in
the context of MPS. We illustrate the results with an exhaustive search of
SU(2)--invariant two-body Hamiltonians which have such MPS as exact ground
states or excitations.Comment: PDFLatex, 12 pages and 6 figure
Comparison between mirror Langmuir probe and gas puff imaging measurements of intermittent fluctuations in the Alcator C-Mod scrape-off layer
Statistical properties of the scrape-off layer (SOL) plasma fluctuations are
studied in ohmically heated plasmas in the Alcator C-Mod tokamak. For the first
time, plasma fluctuations as well as parameters that describe the fluctuations
are compared across measurements from a mirror Langmuir probe (MLP) and from
gas-puff imaging (GPI) that sample the same plasma discharge. This comparison
is complemented by an analysis of line emission time-series data, synthesized
from the MLP electron density and temperature measurements. The fluctuations
observed by the MLP and GPI typically display relative fluctuation amplitudes
of order unity together with positively skewed and flattened probability
density functions. Such data time series are well described by an established
stochastic framework which model the data as a superposition of uncorrelated,
two-sided exponential pulses. The most important parameter of the process is
the intermittency parameter, {\gamma} = {\tau}d / {\tau}w where {\tau}d denotes
the duration time of a single pulse and {\tau}w gives the average waiting time
between consecutive pulses. Here we show, using a new deconvolution method,
that these parameters can be consistently estimated from different statistics
of the data. We also show that the statistical properties of the data sampled
by the MLP and GPI diagnostic are very similar. Finally, a comparison of the
GPI signal to the synthetic line-emission time series suggests that the
measured emission intensity can not be explained solely by a simplified model
which neglects neutral particle dynamics
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