4,326 research outputs found
AME - Asteroseismology Made Easy. Estimating stellar properties by use of scaled models
We present a new method to obtain stellar properties for stars exhibiting
solar-like oscillations in an easy, fast, and transparent way. The method,
called Asteroseismology Made Easy (AME), can determine stellar masses,
mean-densities, radii, and surface gravities, as well as estimate ages. In this
writing we present AME as a visual and powerful tool which could be useful; in
particular in the light of the large number of exoplanets being found.
AME consists of a set of figures from which the stellar parameters are
deduced. These figures are made from a grid of stellar evolutionary models that
cover masses ranging from 0.7 Msun to 1.6 Msun in steps of 0.1 Msun and
metallicities in the interval -0.3 dex <= [Fe/H] <= +0.3 dex in increments of
0.1 dex. The stellar evolutionary models are computed using the Modules for
Experiments in Stellar Astrophysics (MESA) code with simple input physics.
We have compared the results from AME with results for three groups of stars;
stars with radii determined from interferometry (and measured parallaxes),
stars with radii determined from measurements of their parallaxes (and
calculated angular diameters), and stars with results based on the modelling of
their individual oscillation frequencies. We find that a comparison of the
radii from interferometry to those from AME yield a weighted mean of the
fractional differences of just 2%. This is also the level of deviation that we
find when we compare the parallax-based radii to the radii determined from AME.
The comparison between independently determined stellar parameters and those
found using AME show that our method can provide reliable stellar masses,
radii, and ages, with median uncertainties in the order of 4%, 2%, and 25%
respectively.Comment: 18 pages, 25 figures. To be published in Astronomy & Astrophysic
On the Conformal Geometry of Transverse Riemann-Lorentz Manifolds
Physical reasons suggested in \cite{Ha-Ha} for the \emph{Quantum Gravity
Problem} lead us to study \emph{type-changing metrics} on a manifold. The most
interesting cases are \emph{Transverse Riemann-Lorentz Manifolds}. Here we
study the conformal geometry of such manifolds
Barrio Adentro in Health: a Political and Social Project of the Bolivarian Republic of Venezuela
Objectives: To analyze and describe the implementation of Mission Barrio Adentro , inserted into the new National Public Health System in Venezuela
Constraining cosmological ultra-large scale structure using numerical relativity
Cosmic inflation, a period of accelerated expansion in the early universe,
can give rise to large amplitude ultra-large scale inhomogeneities on distance
scales comparable to or larger than the observable universe. The cosmic
microwave background (CMB) anisotropy on the largest angular scales is
sensitive to such inhomogeneities and can be used to constrain the presence of
ultra-large scale structure (ULSS). We numerically evolve nonlinear
inhomogeneities present at the beginning of inflation in full General
Relativity to assess the CMB quadrupole constraint on the amplitude of the
initial fluctuations and the size of the observable universe relative to a
length scale characterizing the ULSS. To obtain a statistically significant
number of simulations, we adopt a toy model in which inhomogeneities are
injected along a preferred direction. We compute the likelihood function for
the CMB quadrupole including both ULSS and the standard quantum fluctuations
produced during inflation. We compute the posterior given the observed CMB
quadrupole, finding that when including gravitational nonlinearities, ULSS
curvature perturbations of order unity are allowed by the data, even on length
scales not too much larger than the size of the observable universe. Our
results illustrate the utility and importance of numerical relativity for
constraining early universe cosmology.Comment: 14 pages, 6 figures v3: Clarifications added regarding the generality
of results - conclusions unchanged, version accepted for publication in PRD,
v2: updated with minor clarifications, submitte
On the asymptotic acoustic-mode phase in red-giant stars and its dependence on evolutionary state
Asteroseismic investigations based on the wealth of data now available,in
particular from the CoRoT and Kepler missions, require a good understanding of
the relation between the observed quantities and the properties of the
underlying stellar structure. Kallinger et al. 2012 found a relation between
their determination of the asymptotic phase of radial oscillations in evolved
stars and the evolutionary state, separating ascending-branch red giants from
helium-burning stars in the `red clump'. Here we provide a detailed analysis of
this relation, which is found to derive from differences between these two
classes of stars in the thermodynamic state of the convective envelope. There
is potential for distinguishing red giants and clump stars based on the phase
determined from observations that are too short to allow distinction based on
determination of the period spacing for mixed modes. The analysis of the phase
may also point to a better understanding of the potential for using the
helium-ionization-induced acoustic glitch to determine the helium abundance in
the envelopes of these stars.Comment: MNRAS, in the pres
Tests of the asymptotic large frequency separation of acoustic oscillations in solar-type and red giant stars
Asteroseismology, i.e. the study of the internal structures of stars via
their global oscillations, is a valuable tool to obtain stellar parameters such
as mass, radius, surface gravity and mean density. These parameters can be
obtained using certain scaling relations which are based on an asymptotic
approximation. Usually the observed oscillation parameters are assumed to
follow these scaling relations. Recently, it has been questioned whether this
is a valid approach, i.e., whether the order of the observed oscillation modes
are high enough to be approximated with an asymptotic theory. In this work we
use stellar models to investigate whether the differences between observable
oscillation parameters and their asymptotic estimates are indeed significant.
We compute the asymptotic values directly from the stellar models and derive
the observable values from adiabatic pulsation calculations of the same models.
We find that the extent to which the atmosphere is included in the models is a
key parameter. Considering a larger extension of the atmosphere beyond the
photosphere reduces the difference between the asymptotic and observable values
of the large frequency separation. Therefore, we conclude that the currently
suggested discrepancies in the scaling relations might have been overestimated.
Hence, based on the results presented here we believe that the suggestions of
Mosser et al. (2013) should not be followed without careful consideration.Comment: 6 pages, 4 figures, 1 table, accepted for publication by MNRAS as a
Letter to the Edito
Electron degeneracy and intrinsic magnetic properties of epitaxial Nb:SrTiO thin-films controlled by defects
We report thermoelectric power experiments in e-doped thin films of SrTiO
(STO) which demonstrate that the electronic band degeneracy can be lifted
through defect management during growth. We show that even small amounts of
cationic vacancies, combined with epitaxial stress, produce a homogeneous
tetragonal distortion of the films, resulting in a Kondo-like resistance upturn
at low temperature, large anisotropic magnetoresistance, and non-linear Hall
effect. Ab-initio calculations confirm a different occupation of each band
depending on the degree of tetragonal distortion. The phenomenology reported in
this paper for tetragonally distorted e-doped STO thin films, is similarto that
observed in LaAlO/STO interfaces and magnetic STO quantum wells.Comment: 5 pages, 5 figure
Pulsation Period Changes as a Tool to Identify Pre-Zero Age Horizontal Branch Stars
One of the most dramatic events in the life of a low-mass star is the He
flash, which takes place at the tip of the red giant branch (RGB) and is
followed by a series of secondary flashes before the star settles into the
zero-age horizontal branch (ZAHB). Yet, no stars have been positively
identified in this key evolutionary phase, mainly for two reasons: first, this
pre-ZAHB phase is very short compared to other major evolutionary phases in the
life of a star; and second, these pre-ZAHB stars are expected to overlap the
loci occupied by asymptotic giant branch (AGB), HB and RGB stars observed in
the color-magnitude diagram (CMD). We investigate the possibility of detecting
these stars through stellar pulsations, since some of them are expected to
rapidly cross the Cepheid/RR Lyrae instability strip in their route from the
RGB tip to the ZAHB, thus becoming pulsating stars along the way. As a
consequence of their very high evolutionary speed, some of these stars may
present anomalously large period change rates. We constructed an extensive grid
of stellar models and produced pre-ZAHB Monte Carlo simulations appropriate for
the case of the Galactic globular cluster M3 (NGC 5272), where a number of RR
Lyrae stars with high period change rates are found. Our results suggest that
some -- but certainly not all -- of the RR Lyrae stars in M3 with large period
change rates are in fact pre-ZAHB pulsators.Comment: Conference Proceedings HELAS Workshop on 'Synergies between solar and
stellar modelling', Rome, June 2009, Astrophys. Space Sci., in the pres
Testing Asteroseismic Radii of Dwarfs and Subgiants with Kepler and Gaia
We test asteroseismic radii of Kepler main-sequence and subgiant stars by
deriving their parallaxes which are compared with those of the first Gaia data
release. We compute radii based on the asteroseismic scaling relations as well
as by fitting observed oscillation frequencies to stellar models for a subset
of the sample, and test the impact of using effective temperatures from either
spectroscopy or the infrared flux method. An offset of 3%, showing no
dependency on any stellar parameters, is found between seismic parallaxes
derived from frequency modelling and those from Gaia. For parallaxes based on
radii from the scaling relations, a smaller offset is found on average;
however, the offset becomes temperature dependent which we interpret as
problems with the scaling relations at high stellar temperatures. Using the
hotter infrared flux method temperature scale, there is no indication that
radii from the scaling relations are inaccurate by more than about 5%. Taking
the radii and masses from the modelling of individual frequencies as reference
values, we seek to correct the scaling relations for the observed temperature
trend. This analysis indicates that the scaling relations systematically
overestimate radii and masses at high temperatures, and that they are accurate
to within 5% in radius and 13% in mass for main-sequence stars with
temperatures below 6400 K. However, further analysis is required to test the
validity of the corrections on a star-by-star basis and for more evolved stars.Comment: 12 pages, 9 figures. Accepted for publication in MNRA
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