21,168 research outputs found
Accurate fundamental parameters for Lower Main Sequence Stars
We derive an empirical effective temperature and bolometric luminosity
calibration for G and K dwarfs, by applying our own implementation of the
InfraRed Flux Method to multi-band photometry. Our study is based on 104 stars
for which we have excellent BVRIJHK photometry, excellent parallaxes and good
metallicities. Colours computed from the most recent synthetic libraries
(ATLAS9 and MARCS) are found to be in good agreement with the empirical colours
in the optical bands, but some discrepancies still remain in the infrared.
Synthetic and empirical bolometric corrections also show fair agreement. A
careful comparison to temperatures, luminosities and angular diameters obtained
with other methods in literature shows that systematic effects still exist in
the calibrations at the level of a few percent. Our InfraRed Flux Method
temperature scale is 100K hotter than recent analogous determinations in the
literature, but is in agreement with spectroscopically calibrated temperature
scales and fits well the colours of the Sun. Our angular diameters are
typically 3% smaller when compared to other (indirect) determinations of
angular diameter for such stars, but are consistent with the limb-darkening
corrected predictions of the latest 3D model atmospheres and also with the
results of asteroseismology. Very tight empirical relations are derived for
bolometric luminosity, effective temperature and angular diameter from
photometric indices. We find that much of the discrepancy with other
temperature scales and the uncertainties in the infrared synthetic colours
arise from the uncertainties in the use of Vega as the flux calibrator. Angular
diameter measurements for a well chosen set of G and K dwarfs would go a long
way to addressing this problem.Comment: 34 pages, 20 figures. Accepted by MNRAS. Landscape table available
online at http://users.utu.fi/luccas/IRFM
Dark energy: the absolute electric potential of the universe
Is there an absolute cosmic electric potential?. The recent discovery of the
accelerated expansion of the universe could be indicating that this is
certainly the case. In this essay we show that the consistency of the covariant
and gauge invariant theory of electromagnetism is truly questionable when
considered on cosmological scales. Out of the four components of the
electromagnetic field, Maxwell's theory only contains two physical degrees of
freedom. However, in the presence of gravity, one of the "unphysical" states
cannot be consistently eliminated, thus becoming real. This third polarization
state is completely decoupled from charged matter, but can be excited
gravitationally thus breaking gauge invariance. On large scales the new state
can be seen as a homogeneous cosmic electric potential, whose energy density
behaves as a cosmological constant.Comment: 9 pages, 2 figures. Essay selected for "Honorable Mention" in the
2009 Awards for Essays on Gravitation (Gravity Research Foundation
Cosmological magnetic fields from inflation in extended electromagnetism
In this work we consider an extended electromagnetic theory in which the
scalar state which is usually eliminated by means of the Lorenz condition is
allowed to propagate. This state has been shown to generate a small
cosmological constant in the context of standard inflationary cosmology. Here
we show that the usual Lorenz gauge-breaking term now plays the role of an
effective electromagnetic current. Such a current is generated during inflation
from quantum fluctuations and gives rise to a stochastic effective charge
density distribution. Due to the high electric conductivity of the cosmic
plasma after inflation, the electric charge density generates currents which
give rise to both vorticity and magnetic fields on sub-Hubble scales. Present
upper limits on vorticity coming from temperature anisotropies of the CMB are
translated into lower limits on the present value of cosmic magnetic fields. We
find that, for a nearly scale invariant vorticity spectrum, magnetic fields
G are typically generated with coherence lengths
ranging from sub-galactic scales up to the present Hubble radius. Those fields
could act as seeds for a galactic dynamo or even account for observations just
by collapse and differential rotation of the protogalactic cloud.Comment: 5 pages, 2 figures. Final version to appear in Phys. Rev.
Electromagnetic nature of dark energy
Out of the four components of the electromagnetic field, Maxwell's theory
only contains two physical degrees of freedom. However, in an expanding
universe, consistently eliminating one of the "unphysical" states in the
covariant (Gupta-Bleuler) formalism turns out to be difficult to realize. In
this work we explore the possibility of quantization without subsidiary
conditions. This implies that the theory would contain a third physical state.
The presence of such a new (temporal) electromagnetic mode on cosmological
scales is shown to generate an effective cosmological constant which can
account for the accelerated expansion of the universe. This new polarization
state is completely decoupled from charged matter, but can be excited
gravitationally. In fact, primordial electromagnetic quantum fluctuations
produced during electroweak scale inflation could naturally explain the
presence of this mode and also the measured value of the cosmological constant.
The theory is compatible with all the local gravity tests, it is free from
classical or quantum instabilities and reduces to standard QED in the flat
space-time limit. Thus we see that, not only the true nature of dark energy can
be established without resorting to new physics, but also the value of the
cosmological constant finds a natural explanation in the context of standard
inflationary cosmology. Possible signals, discriminating this model from LCDM,
are also discussed.Comment: 10 pages, 2 figures. Contribution to the Proceedings of Invisible
Universe International Conference, UNESCO, Paris, June 29-July 3, 200
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