19,643 research outputs found
The Sun, stellar-population models, and the age estimation of high-redshift galaxies
Given sufficiently deep optical spectroscopy, the age estimation of
high-redshif t () galaxies has been claimed to be a relatively robust
process (e.g. Dunlop et al. 1996) due to the fact that, for ages Gyr, the
near-ultraviolet light of a stellar population is expected to be dominated by
`well-understood' main-sequence (MS) stars. Recently, however, the reliability
of this process has been called into question by Yi et al (2000), who claim to
have developed models in which the spectrum produced by the main sequence
reddens much more rapidly than in the models of Jimenez et al (2000a), leading
to much younger age estimates for the reddest known high-redshift ellipticals.
In support of their revised age estimates, Yi et al cite the fact that their
models can reproduce the spectrum of the Sun at an age of 5 Gyr, whereas the
solar spectrum is not reproduced by the Jimenez et al models until
Gyr. Here we confirm this discrepancy, but point out that this is in fact a
{\it strength} of the Jimenez et al models and indicative of some flaw in the
models of Yi et al (which, in effect, imply that the Sun will turn into a red
giant any minute now). We have also explored the models of Worthey (1994)
(which are known to differ greatly from those of Jimenez et al in the treatment
of post-MS evolution) and find that the main-sequence component of Worthey's
models also cannot reproduce the solar spectrum until an age of 9-10 Gyr. We
conclude that either the models of Yi et al are not as main-sequence dominated
at 4-5 Gyr as claimed, or that the stellar evolutionary timescale in these
models is in error by a factor possibly as high as two. (abridged)Comment: Submitted to MNRAS, final versio
Sensitivity analysis of the solar rotation to helioseismic data from GONG, GOLF and MDI observations
Accurate determination of the rotation rate in the radiative zone of the sun
from helioseismic observations requires rotational frequency splittings of
exceptional quality as well as reliable inversion techniques. We present here
inferences based on mode parameters calculated from 2088-days long MDI, GONG
and GOLF time series that were fitted to estimate very low frequency rotational
splittings (nu < 1.7 mHz). These low frequency modes provide data of
exceptional quality, since the width of the mode peaks is much smaller than the
rotational splitting and hence it is much easier to separate the rotational
splittings from the effects caused by the finite lifetime and the stochastic
excitation of the modes. We also have implemented a new inversion methodology
that allows us to infer the rotation rate of the radiative interior from mode
sets that span l=1 to 25. Our results are compatible with the sun rotating like
a rigid solid in most of the radiative zone and slowing down in the core (R_sun
< 0.2). A resolution analysis of the inversion was carried out for the solar
rotation inverse problem. This analysis effectively establishes a direct
relationship between the mode set included in the inversion and the sensitivity
and information content of the resulting inferences. We show that such an
approach allows us to determine the effect of adding low frequency and low
degree p-modes, high frequency and low degree p-modes, as well as some g-modes
on the derived rotation rate in the solar radiative zone, and in particular the
solar core. We conclude that the level of uncertainties that is needed to infer
the dynamical conditions in the core when only p-modes are included is unlikely
to be reached in the near future, and hence sustained efforts are needed
towards the detection and characterization of g-modes.Comment: Accepted for publication in Astrophysical journal. 15 pages, 19
figure
Analysis of the acoustic cut-off frequency and HIPs in six Kepler stars with stochastically excited pulsations
Gravito-acoustic modes in the Sun and other stars propagate in resonant
cavities with a frequency below a given limit known as the cut-off frequency.
At higher frequencies, waves are no longer trapped in the stellar interior and
become traveller waves. In this article we study six pulsating solar-like stars
at different evolutionary stages observed by the NASA Kepler mission. These
high signal-to-noise targets show a peak structure that extends at very high
frequencies and are good candidates for studying the transition region between
the modes and the interference peaks or pseudo-modes. Following the same
methodology successfully applied on Sun-as-a-star measurements, we uncover the
existence of pseudo-modes in these stars with one or two dominant interference
patterns depending on the evolutionary stage of the star. We also infer their
cut-off frequency as the midpoint between the last eigenmode and the first peak
of the interference patterns. By using ray theory we show that, while the
period of one of the interference pattern is very close to half the large
separation the other, one depends on the time phase of mixed waves, thus
carrying additional information on the stellar structure and evolution.Comment: Accepted for publication in A&A. 14 pages, 28 figure
Variations of the solar granulation motions with height using the GOLF/SoHO experiment
Below 1 mHz, the power spectrum of helioseismic velocity measurements is
dominated by the spectrum of convective motions (granulation and
supergranulation) making it difficult to detect the low-order acoustic modes
and the gravity modes. We want to better understand the behavior of solar
granulation as a function of the observing height in the solar atmosphere and
with magnetic activity during solar cycle 23. We analyze the Power Spectral
Density (PSD) of eleven years of GOLF/SOHO velocity-time series using a
Harvey-type model to characterize the properties of the convective motions in
the solar oscillation power spectrum. We study then the evolution of the
granulation with the altitude in the solar atmosphere and with the solar
activity. First, we show that the traditional use of a lorentzian profile to
fit the envelope of the p modes is not well suitable for GOLF data. Indeed, to
properly model the solar spectrum, we need a second lorentzian profile. Second,
we show that the granulation clearly evolves with the height in the photosphere
but does not present any significant variation with the activity cycle.Comment: Paper accepted in A&A. 7 pages, 4 figures, 2 table
F stars, metallicity, and the ages of red galaxies at z > 1
We explore whether the rest-frame near-UV spectral region, observable in
high-redshift galaxies via optical spectroscopy, contains sufficient
information to allow the degeneracy between age and metallicity to be lifted.
We do this by testing the ability of evolutionary synthesis models to reclaim
the correct metallicity when fitted to the near-UV spectra of F stars of known
(sub-solar and super-solar) metallicity. F stars are of particular interest
because the rest-frame near-UV spectra of the oldest known elliptical galaxies
at z > 1 appear to be dominated by F stars near to the main-sequence turnoff.
We find that, in the case of the F stars, where the HST ultraviolet spectra
have high signal:noise, model-fitting with metallicity allowed to vary as a
free parameter is rather successful at deriving the correct metallicity. As a
result, the estimated turnoff ages of these stars yielded by the model fitting
are well constrained. Encouraged by this we have fitted these same variable-
metallicity models to the deep, optical spectra of the z \simeq 1.5 mJy radio
galaxies 53W091 and 53W069 obtained with the Keck telescope. While the
age-metallicity degeneracy is not so easily lifted for these galaxies, we find
that even when metallicity is allowed as a free parameter, the best estimates
of their ages are still \geq 3 Gyr, with ages younger than 2 Gyr now strongly
excluded. Furthermore, we find that a search of the entire parameter space of
metallicity and star formation history using MOPED (Heavens et al., 2000) leads
to the same conclusion. Our results therefore continue to argue strongly
against an Einstein-de Sitter universe, and favour a lambda-dominated universe
in which star formation in at least these particular elliptical galaxies was
completed somewhere in the redshift range z = 3 - 5.Comment: 10 pages, LaTeX, uses MNRAS style file, incorporates 14 postscript
figures, submitted to MNRAS. Changes include: inclusion of single stellar
atmosphere model fits; more rigorous calculation of confidence regions; some
re-structurin
GONG p-mode frequency changes with solar activity
We present a correlation analysis of GONG p-mode frequencies with nine solar
activity indices for the period 1995 August to 1997 August. This study includes
spherical harmonic degree in the range 2 to 150 and the frequency range of
1500-3500 \mu Hz. Using three statistical tests, the measured mean frequency
shifts show strong to good correlation with activity indices. A decrease of
0.06 \mu Hz in frequency, during the descending phase of solar cycle 22 and an
increase of 0.04 \mu Hz in the ascending phase of solar cycle 23 is observed.
These results provide the first evidence for change in p-mode frequencies
around the declining phase of solar cycle 22 and beginning of new cycle 23.
This analysis further confirms that the temporal behaviour of the solar
frequency shifts closely follow the phase of the solar activity cycle.Comment: 14 pages, 3 figures. To appear in Ap.
The Cosmic Neutrino Background and the Age of the Universe
We discuss the cosmological degeneracy between the age of the Universe, the
Hubble parameter and the effective number of relativistic particles N_eff. We
show that independent determinations of the Hubble parameter H(z) as those
recently provided by Simon,Verde, Jimenez (2006), combined with other
cosmological data sets can provide the most stringent constraint on N_eff,
yielding N_eff=3.7 (-1.2) (+1.1) at 95% confidence level. A neutrino background
is detected with high significance: N_eff >1.8 at better than 99% confidence
level. Constraints on the age of the universe in the framework of an extra
background of relativistic particles are improved by a factor 3.Comment: JCAP, in pres
Community Fitness Center-Based Physical Activity Interventions: A Brief Review
Sedentary lifestyle is associated with cardiovascular and metabolic diseases.
A compelling body of evidence demonstrates the amelioration and prevention of such conditions with increased levels of physical activity (PA). Despite this evidence, many public health initiatives aimed at increasing PA have failed to demonstrate clinically relevant effects on public health. It has been hypothesized that the highly controlled environments in which PA and health research is conducted limits its replicability in real-world community settings. This review aimed to evaluate the effectiveness of community fitness center-based interventions on inactivity-related diseases in adults. Data from 11 investigations highlighted 3 factors: 1) a lack of community-based PA studies, 2) a lack of clinically relevant data, and 3) further reliance on self-report and rudimentary measurements. It is concluded that the current laboratory-based evidence for PA and health is to be replicated yet in real-world settings and that rigorous and clinically relevant naturalistic research is required
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