3,225 research outputs found
New members of the TW Hydrae Association and two accreting M-dwarfs in Scorpius-Centaurus
We report the serendipitous discovery of several young mid-M stars found
during a search for new members of the 30-40 Myr-old Octans Association. Only
one of the stars may be considered a possible Octans(-Near) member. However,
two stars have proper motions, kinematic distances, radial velocities,
photometry and Li I 6708AA measurements consistent with membership in the 8-10
Myr-old TW Hydrae Association. Another may be an outlying member of TW Hydrae
but has a velocity similar to that predicted by membership in Octans. We also
identify two new lithium-rich members of the neighbouring Scorpius-Centaurus OB
Association (Sco-Cen). Both exhibit large 12 and 22 micron excesses and strong,
variable H-alpha emission which we attribute to accretion from circumstellar
discs. Such stars are thought to be incredibly rare at the ~16 Myr median age
of Sco-Cen and they join only one other confirmed M-type and three higher-mass
accretors outside of Upper Scorpius. The serendipitous discovery of two
accreting stars hosting large quantities of circumstellar material may be
indicative of a sizeable age spread in Sco-Cen, or further evidence that disc
dispersal and planet formation time-scales are longer around lower-mass stars.
To aid future studies of Sco-Cen we also provide a newly-compiled catalogue of
305 early-type Hipparcos members with spectroscopic radial velocities sourced
from the literature.Comment: 12 pages. Accepted for publication in MNRA
Generalized nonuniform dichotomies and local stable manifolds
We establish the existence of local stable manifolds for semiflows generated
by nonlinear perturbations of nonautonomous ordinary linear differential
equations in Banach spaces, assuming the existence of a general type of
nonuniform dichotomy for the evolution operator that contains the nonuniform
exponential and polynomial dichotomies as a very particular case. The family of
dichotomies considered allow situations for which the classical Lyapunov
exponents are zero. Additionally, we give new examples of application of our
stable manifold theorem and study the behavior of the dynamics under
perturbations.Comment: 18 pages. New version with minor corrections and an additional
theorem and an additional exampl
Non-Gaussian statistics, maxwellian derivation and stellar polytropes
In this letter we discuss the Non-gaussian statistics considering two
aspects. In the first, we show that the Maxwell's first derivation of the
stationary distribution function for a dilute gas can be extended in the
context of Kaniadakis statistics. The second one, by investigating the stellar
system, we study the Kaniadakis analytical relation between the entropic
parameter and stellar polytrope index . We compare also the
Kaniadakis relation with proposed in the Tsallis
framework.Comment: 10 pages, 1 figur
Generalized Chaplygin Gas Model: Dark Energy - Dark Matter Unification and CMBR Constraints
The generalized Chaplygin gas (GCG) model allows for an unified description
of the recent accelerated expansion of the Universe and the evolution of energy
density perturbations. This dark energy - dark matter unification is achieved
through an exotic background fluid whose equation of state is given by , where is a positive constant and .
Stringent constraints on the model parameters can be obtained from recent WMAP
and BOOMERanG bounds on the locations of the first few peaks and troughs of the
Cosmic Microwave Background Radiation (CMBR) power spectrum as well as SNe Ia
data.Comment: 9 pages, 2 figures; essay selected for an honorable mention by the
Gravity Research Foundation, 200
Adaptive Cluster Expansion for Inferring Boltzmann Machines with Noisy Data
We introduce a procedure to infer the interactions among a set of binary
variables, based on their sampled frequencies and pairwise correlations. The
algorithm builds the clusters of variables contributing most to the entropy of
the inferred Ising model, and rejects the small contributions due to the
sampling noise. Our procedure successfully recovers benchmark Ising models even
at criticality and in the low temperature phase, and is applied to
neurobiological data.Comment: Accepted for publication in Physical Review Letters (2011
Brane Cosmology from Heterotic String Theory
We consider brane cosmologies within the context of five-dimensional actions
with O(a') higher curvature corrections. The actions are compatible with bulk
string amplitude calculations from heterotic string theory. We find wrapped
solutions that satisfy the field equations in an approximate but acceptable
manner given their complexity, where the internal four-dimensional scale factor
is naturally inflating, having an exponential De-Sitter form. The temporal
dependence of the metric components is non-trivial so that this metric cannot
be factored as in a conformally flat case. The effective Planck mass is finite
and the brane solutions localize four-dimensional gravity, while the
four-dimensional gravitational constant varies with time. The Hubble constant
can be freely specified through the initial value of the scalar field, to
conform with recent data.Comment: 15 pages, 3 figures, LaTeX, Accepted for Publication in IJT
Links between the Indian, U.S. and Chinese Stock Markets
This study examines the bilateral relations between three pairs of stock markets, namely India-U.S., India-China and China-U.S. We use a Fractionally Integrated Vector Error Correction Model (FIVECM) to examine the cointegration mechanism between markets. By augmenting the FIVECM with a multivariate GARCH formulation, we study the first and second moment spillover effects simultaneously. Our empirical results show that all three pairs of stock markets are fractionally cointegrated. The U.S. stock market plays a dominant role in the relations with the other two markets, whereas there is an interactive relationship between the Indian and Chinese stock markets. In particular, the Indian stock market dominates the first moment feedback with the Chinese market, while the latter dominates the second moment feedback with the former.Stock market, Cointegration, Fractionally Integrated Vector Error Correction Model, Multivariate GARCH
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