11,929 research outputs found
OB stars at the lowest Local Group metallicity: GTC-OSIRIS observations of Sextans A
Our aim is to find and classify OB stars in Sextans A, to later determine
accurate stellar parameters of these blue massive stars in this low metallicity
region .
Using UBV photometry, the reddening-free index Q and GALEX imaging, we built
a list of blue massive star candidates in Sextans A. We obtained low resolution
(R 1000) GTC-OSIRIS spectra for a fraction of them and carried out
spectral classification. For the confirmed O-stars we derive preliminary
stellar parameters.
The target selection criteria and observations were successful and have
produced the first spectroscopic atlas of OB-type stars in Sextans A. From the
whole sample of 18 observed stars, 12 were classified as early OB-types,
including 5 O-stars. The radial velocities of all target stars are in agreement
with their Sextans A membership, although three of them show significant
deviations. We determined the stellar parameters of the O-type stars using the
stellar atmosphere code FASTWIND, and revisited the sub-SMC temperature scale.
Two of the O-stars are consistent with relatively strong winds and enhanced
helium abundances, although results are not conclusive. We discuss the position
of the OB stars in the HRD. Initial stellar masses run from slightly below 20
up to 40 solar masses.
The target selection method worked well for Sextans A, confirming the
procedure developed in Garcia \& Herrero (2013). The stellar temperatures are
consistent with findings in other galaxies. Some of the targets deserve
follow-up spectroscopy because of indications of a runaway nature, an enhanced
helium abundance or a relatively strong wind. We observe a correlation between
HI and OB associations similar to the irregular galaxy IC1613, confirming the
previous result that the most recent star formation of Sextans A is currently
on-going near the rim of the H\,{\sc I} cavity
Centralized vs Decentralized Markets in the Laboratory: The Role of Connectivity
This paper compares the performance of centralized and decentralized markets experimentally. We constrain trading exchanges to happen on an exogenously predetermined network, representing the trading relationships in markets with differing levels of connectivity. Our experimental results show that, despite having lower trading volumes, decentralized markets are generally not less efficient. Although information can propagate quicker through highly connected markets, we show that higher connectivity also induces informed traders to trade faster and exploit further their information advantages before the information becomes fully incorporated into prices. This not only reduces market efficiency, but it increases wealth inequality. We show that, in more connected markets, informed traders trade not only relatively quicker, but also more, in the right direction, despite not doing it at better prices
White dwarfs as test objects of Lorentz violations
In the present work the thermodynamical properties of bosonic and fermionic
gases are analyzed under the condition that a modified dispersion relation is
present. This last condition implies a breakdown of Lorentz symmetry. The
implications upon the condensation temperature will be studied, as well, as
upon other thermodynamical variables such as specific heat, entropy, etc.
Moreover, it will be argued that those cases entailing a violation of time
reversal symmetry of the motion equations could lead to problems with the
concept of entropy. Concerning the fermionic case it will be shown that Fermi
temperature suffers a modification due to the breakdown of Lorentz symmetry.
The results will be applied to white dwarfs and the consequences upon the
Chandrasekhar mass--radius relation will be shown. The possibility of resorting
to white dwarfs for the testing of modified dispersion relations is also
addressed. It will be shown that the comparison of the current observations
against the predictions of our model allows us to discard some values of one of
the parameters appearing in the modifications of the dispersion relation.Comment: Accepted in Classical and Quantum Gravitatio
Naturally graded Zinbiel algebras with nilindex n - 3
We present the classification of a subclass of n-dimensional naturally graded Zinbiel
algebras. This subclass has the nilindex n â 3 and the characteristic sequence (n â 3, 2, 1). In fact,
this result completes the classification of naturally graded Zinbiel algebras of nilindex n â 3
Wave Packets Propagation in Quantum Gravity
Wave packet broadening in usual quantum mechanics is a consequence of
dispersion behavior of the medium which the wave propagates in it. In this
paper, we consider the problem of wave packet broadening in the framework of
Generalized Uncertainty Principle(GUP) of quantum gravity. New dispersion
relations are derived in the context of GUP and it has been shown that there
exists a gravitational induced dispersion which leads to more broadening of the
wave packets. As a result of these dispersion relations, a generalized
Klein-Gordon equation is obtained and its interpretation is given.Comment: 9 pages, no figur
Time delay of light signals in an energy-dependent spacetime metric
In this note we review the problem of time delay of photons propagating in a
spacetime with a metric that explicitly depends on the energy of the particles
(Gravity-Rainbow approach). We show that corrections due to this approach --
which is closely related to DSR proposal -- produce for small redshifts
() smaller time delays than in the generic Lorentz Invariance Violating
case.Comment: 5 pages. This version contains two new references with respect to the
published versio
Leibniz Algebras Whose Semisimple Part is Related to sl2
In this paper we identify the structure of complex finite-dimensional Leibniz algebras
with associated Lie algebras sl1
2âsl2
2â· · ·âsls
2âR, where R is a solvable radical. The classifications
of such Leibniz algebras in the cases dimR = 2, 3 and dimI 6= 3 have been obtained. Moreover, we
classify Leibniz algebras with L/I âŒ= sl1
2âsl2
2 and some conditions on ideal I = id < [x, x] | x â L
WKB formalism and a lower limit for the energy eigenstates of bound states for some potentials
In the present work the conditions appearing in the WKB approximation
formalism of quantum mechanics are analyzed. It is shown that, in general, a
careful definition of an approximation method requires the introduction of two
length parameters, one of them always considered in the text books on quantum
mechanics, whereas the second one is usually neglected. Afterwards we define a
particular family of potentials and prove, resorting to the aforementioned
length parameters, that we may find an energy which is a lower bound to the
ground energy of the system. The idea is applied to the case of a harmonic
oscillator and also to a particle freely falling in a homogeneous gravitational
field, and in both cases the consistency of our method is corroborated. This
approach, together with the Rayleigh--Ritz formalism, allows us to define an
energy interval in which the ground energy of any potential, belonging to our
family, must lie.Comment: Accepted in Modern Physics Letters
Effects of water movement and aeration system on the survival and growth of hatchery bred sugpo (Penaeus monodon Fabricius) in earthen nursery ponds
An experiment was conducted to determine the effects of water movement and airlift aeration on the survival and growth of P. monodon fry reared from P4P5 to P32P33 in earthen brackishwater ponds. The high survival rates obtained justify the need for aeration when using the earliest stages of fry (P4P5) at higher stocking densities. For older stages regardless of source and at lower stocking densities, nursery operations based on traditional methods could also achieve better survival rates
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