2,327 research outputs found
Asteroseismic stellar activity relations
In asteroseismology an important diagnostic of the evolutionary status of a
star is the small frequency separation which is sensitive to the gradient of
the mean molecular weight in the stellar interior. It is thus interesting to
discuss the classical age-activity relations in terms of this quantity.
Moreover, as the photospheric magnetic field tends to suppress the amplitudes
of acoustic oscillations, it is important to quantify the importance of this
effect by considering various activity indicators. We propose a new class of
age-activity relations that connects the Mt. Wilson index and the average
scatter in the light curve with the small frequency separation and the
amplitude of the p-mode oscillations. We used a Bayesian inference to compute
the posterior probability of various empirical laws for a sample of 19
solar-like active stars observed by the Kepler telescope. We demonstrate the
presence of a clear correlation between the Mt. Wilson index and the
relative age of the stars as indicated by the small frequency separation, as
well as an anti-correlation between the index and the oscillation
amplitudes. We argue that the average activity level of the stars shows a
stronger correlation with the small frequency separation than with the absolute
age that is often considered in the literature. The phenomenological laws
discovered in this paper have the potential to become new important diagnostics
to link stellar evolution theory with the dynamics of global magnetic fields.
In particular we argue that the relation between the Mt. Wilson index and
the oscillation amplitudes is in good agreement with the findings of direct
numerical simulations of magneto-convection.Comment: 5 pages, 4 figures, 2 tables. Accepted for publication in A&
Dynamical System Analysis of Cosmologies with Running Cosmological Constant from Quantum Einstein Gravity
We discuss a mechanism that induces a time-dependent vacuum energy on
cosmological scales. It is based on the instability induced renormalization
triggered by the low energy quantum fluctuations in a Universe with a positive
cosmological constant. We employ the dynamical systems approach to study the
qualitative behavior of Friedmann-Robertson-Walker cosmologies where the
cosmological constant is dynamically evolving according with this
nonperturbative scaling at low energies. It will be shown that it is possible
to realize a "two regimes" dark energy phases, where an unstable early phase of
power-law evolution of the scale factor is followed by an accelerated expansion
era at late times.Comment: 26 pages, 4 figures. To appear in New Journal of Physic
Quantum Gravity effects near the null black hole singularity
The structure of the Cauchy Horizon singularity of a black hole formed in a
generic collapse is studied by means of a renormalization group equation for
quantum gravity. It is shown that during the early evolution of the Cauchy
Horizon the increase of the mass function is damped when quantum fluctuations
of the metric are taken into account.Comment: 15 Pages, one figure. Minor changes in the presentation, to appear on
Phys.Rev.
An effective action for asymptotically safe gravity
Asymptotically safe theories of gravitation have received great attention in
recent times. In this framework an effective action embodying the basic
features of the renormalized flow around the non-gaussian fixed point is
derived and its implications for the early universe are discussed. In
particular, a "landscape" of a countably infinite number of cosmological
inflationary solutions characterized by an unstable de Sitter phase lasting for
a large enough number of e-folds is found.Comment: 5 pages, to appear as a Rapid Communication in Physical Review
Magnetic activity, differential rotation and dynamo action in the pulsating F9IV star KIC 5955122
We present photometric spot modeling of the nearly four-year long light-curve
of the Kepler target KIC 5955122 in terms of persisting dark circular surface
features. With a Bayesian technique, we produced a plausible surface map that
shows dozens of small spots. After some artifacts are removed, the residuals
are at \,mmag. The shortest rotational period found is days. The equator-to-pole extrapolated differential rotation is rad/d. The spots are roughly half as bright as the unperturbed stellar
photosphere. Spot latitudes are restricted to the zone latitude.
There is no indication for any near-pole spots. In addition, the p-mode
pulsations enabled us to determine the evolutionary status of the star, the
extension of the convective zone, and its radius and mass. We discuss the
possibility that the clear signature of active regions in the light curve of
the F9IV star KIC 5955122 is produced by a flux-transport dynamo action at the
base of the convection zone. In particular, we argue that this star has evolved
from an active to a quiet status during the Q0--Q16 period of observation, and
we predict, according to our dynamo model, that the characteristic activity
cycle is of the order of the solar one.Comment: 9 pages, 12 figures, to be published on A&
Spacetime Structure of an Evaporating Black Hole in Quantum Gravity
The impact of the leading quantum gravity effects on the dynamics of the
Hawking evaporation process of a black hole is investigated. Its spacetime
structure is described by a renormalization group improved Vaidya metric. Its
event horizon, apparent horizon, and timelike limit surface are obtained taking
the scale dependence of Newton's constant into account. The emergence of a
quantum ergosphere is discussed. The final state of the evaporation process is
a cold, Planck size remnant.Comment: 23 pages, BibTeX, revtex4, 7 figure
Theory, Epistemology and Critical Rural Sociology
Article originally published in International Journal of Sociology of Agriculture and FoodThis paper would like to provide an alternative to the Marx-Weber dichotomy which has recently emerged in rural sociological studies. It consists of the re-proposition of critical sociology as a mode of scientific investigation which, while remaining within the Marxian tradition, addresses many of the central concerns of Weberian scholarship. Though a merger between Marx and Weber is not proposed, it is assumed that a lack of knowledge of critical sociology has hampered further development of the theoretical debate in rural sociology. More importantly, this lack of knowledge has prevented the diffusion of the basic tenets of critical sociology among sociologists concerned with the study of agriculture and food, limiting their ability to inform empirical investigations and to instruct students.Sociolog
Running Gauge Coupling in Asymptotically Safe Quantum Gravity
We investigate the non-perturbative renormalization group behavior of the
gauge coupling constant using a truncated form of the functional flow equation
for the effective average action of the Yang-Mills-gravity system. We find a
non-zero quantum gravity correction to the standard Yang-Mills beta function
which has the same sign as the gauge boson contribution. Our results fit into
the picture according to which Quantum Einstein Gravity (QEG) is asymptotically
safe, with a vanishing gauge coupling constant at the non-trivial fixed point.Comment: 27 page
The Accelerated expansion of the Universe as a crossover phenomenon
We show that the accelerated expansion of the Universe can be viewed as a
crossover phenomenon where the Newton constant and the Cosmological constant
are actually scaling operators, dynamically evolving in the attraction basin of
a non-Gaussian infrared fixed point, whose existence has been recently
discussed. By linearization of the renormalized flow it is possible to evaluate
the critical exponents, and it turns out that the approach to the fixed point
is ruled by a marginal and a relevant direction. A smooth transition between
the standard Friedmann--Lemaitre--Robertson--Walker (FLRW) cosmology and the
observed accelerated expansion is then obtained, so that at late times.Comment: 12 pages, latex, use bibtex. In the final version, the presentation
has been improved, and new references have been adde
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