2,765 research outputs found
Precificação de opções financeiras: um estudo sobre os modelos de Black Scholes e Garch
Precificação de opções financeiras: um estudo sobre os modelos de Black Scholes e Garc
An upper limit to the secular variation of the gravitational constant from white dwarf stars
A variation of the gravitational constant over cosmological ages modifies the
main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art
stellar evolutionary code we compute the effects of a secularly varying G on
the main sequence ages and, employing white dwarf cooling ages computed taking
into account the effects of a running G, we place constraints on the rate of
variation of Newton's constant. This is done using the white dwarf luminosity
function and the distance of the well studied open Galactic cluster NGC 6791.
We derive an upper bound G'/G ~ -1.8 10^{-12} 1/yr. This upper limit for the
secular variation of the gravitational constant compares favorably with those
obtained using other stellar evolutionary properties, and can be easily
improved if deep images of the cluster allow to obtain an improved white dwarf
luminosity function.Comment: 15 pages, 4 figures, accepted for publication in JCA
An independent constraint on the secular rate of variation of the gravitational constant from pulsating white dwarfs
A secular variation of the gravitational constant modifies the structure and
evolutionary time scales of white dwarfs. Using an state-of-the-art stellar
evolutionary code and an up-to-date pulsational code we compute the effects of
a secularly varying on the pulsational properties of variable white dwarfs.
Comparing the the theoretical results obtained taking into account the effects
of a running with the observed periods and measured rates of change of the
periods of two well studied pulsating white dwarfs, G117--B15A and R548, we
place constraints on the rate of variation of Newton's constant. We derive an
upper bound yr using the variable
white dwarf G117--B15A, and yr using
R548. Although these upper limits are currently less restrictive than those
obtained using other techniques, they can be improved in a future measuring the
rate of change of the period of massive white dwarfs.Comment: 13 pages, 4 tables, 3 figures. To be published in the Journal of
Cosmology and Astroparticle Physic
Axions and White Dwarfs
White dwarfs are almost completely degenerate objects that cannot obtain
energy from the thermonuclear sources and their evolution is just a
gravothermal process of cooling. The simplicity of these objects, the fact that
the physical inputs necessary to understand them are well identified, although
not always well understood, and the impressive observational background about
white dwarfs make them the most well studied Galactic population. These
characteristics allow to use them as laboratories to test new ideas of physics.
In this contribution we discuss the robustness of the method and its
application to the axion case.Comment: 4 pages, 1 figure, to appear in the Proceedings for the 6th Patras
meeting on Axions, WIMPs and WISP
Monte Carlo simulations of the halo white dwarf population
The interpretation of microlensing results towards the Large Magellanic Cloud
(LMC) still remains controversial. Whereas white dwarfs have been proposed to
explain these results and, hence, to contribute significantly to the mass
budget of our Galaxy, there are as well several constraints on the role played
by white dwarfs. In this paper we analyze self-consistently and simultaneously
four different results, namely, the local halo white dwarf luminosity function,
the microlensing results reported by the MACHO team towards the LMC, the
results of Hubble Deep Field (HDF) and the results of the EROS experiment, for
several initial mass functions and halo ages. We find that the proposed
log-normal initial mass functions do not contribute to solve the problem posed
by the observed microlensing events and, moreover, they overproduce white
dwarfs when compared to the results of the HDF and of the EROS survey. We also
find that the contribution of hydrogen-rich white dwarfs to the dynamical mass
of the halo of the Galaxy cannot be more than .Comment: 17 pages, 10 figures; accepted for publication in Astronomy and
Astrophysic
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