572 research outputs found
Testing the asymptotic relation for period spacings from mixed modes of red giants observed with the Kepler mission
Dipole mixed pulsation modes of consecutive radial order have been detected
for thousands of low-mass red-giant stars with the NASA space telescope Kepler.
Such modes have the potential to reveal information on the physics of the deep
stellar interior. Different methods have been proposed to derive an observed
value for the gravity-mode period spacing, the most prominent one relying on a
relation derived from asymptotic pulsation theory applied to the gravity-mode
character of the mixed modes. Our aim is to compare results based on this
asymptotic relation with those derived from an empirical approach for three
pulsating red-giant stars. We developed a data-driven method to perform
frequency extraction and mode identification. Next, we used the identified
dipole mixed modes to determine the gravity-mode period spacing by means of an
empirical method and by means of the asymptotic relation. In our methodology,
we consider the phase offset, , of the asymptotic
relation as a free parameter. Using the frequencies of the identified dipole
mixed modes for each star in the sample, we derived a value for the
gravity-mode period spacing using the two different methods. These differ by
less than 5%. The average precision we achieved for the period spacing derived
from the asymptotic relation is better than 1%, while that of our data-driven
approach is 3%. Good agreement is found between values for the period spacing
derived from the asymptotic relation and from the empirical method.
Full abstract in PDF file.Comment: 14 pages, 13 figures, accepted for publication in A&
Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars
A knowledge of stellar ages is crucial for our understanding of many
astrophysical phenomena, and yet ages can be difficult to determine. As they
become older, stars lose mass and angular momentum, resulting in an observed
slowdown in surface rotation. The technique of 'gyrochronology' uses the
rotation period of a star to calculate its age. However, stars of known age
must be used for calibration, and, until recently, the approach was untested
for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for
stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for
old field stars whose ages have been determined with asteroseismology. The data
for the cluster agree with previous period-age relations, but these relations
fail to describe the asteroseismic sample. Here we report stellar evolutionary
modelling, and confirm the presence of unexpectedly rapid rotation in stars
that are more evolved than the Sun. We demonstrate that models that incorporate
dramatically weakened magnetic braking for old stars can---unlike existing
models---reproduce both the asteroseismic and the cluster data. Our findings
might suggest a fundamental change in the nature of ageing stellar dynamos,
with the Sun being close to the critical transition to much weaker magnetized
winds. This weakened braking limits the diagnostic power of gyrochronology for
those stars that are more than halfway through their main-sequence lifetimes.Comment: 25 pages, 3 figures in main paper, 6 extended data figures, 1 table.
Published in Nature, January 2016. Please see https://youtu.be/O6HzYgP5uyc
for a video description of the resul
Finding answers in lipid profile in COVID-19 patients
Introduction: A small percentage of patients will develop a severe form of COVID-19 caused by SARS-CoV-2 infection. Thus, it is important to predict the potential outcomes identifying early markers of poor prognosis. In this context, we evaluated the association of SARS-CoV-2 infection with lipid abnormalities and their role in prognosis. Methods: Single-center, retrospective, observational study of COVID-19 patients admitted from March to October 2020. Clinical and laboratory data, comorbidities, and treatments for COVID-19 were evaluated. Main outcomes including intensive care unit (ICU) admission and mortality were analyzed with a multivariable Cox proportional hazards regression model. Results: We selected 1489 from a total of 2038 consecutive patients with confirmed COVID-19, who had a complete lipid profile before ICU admission. During the follow-up performed in 1109 patients, we observed a decrease in T-c, HDL-c, and LDL-c in 28.6%, 42.9%, and 30.4% of patients, respectively, and an increase in TG in 76.8%. The decrease of both T-c and HDL- c was correlated with a decrease in albumin levels (r = 0.39 and r = 0.37, respectively). Kaplan–Meier survival curves found an increased ICU admission in patients with lower T-c (HR 0.55, CI 0.36–0.86), HDL-c (HR 0.61, CI 0.45–0.84), and LDL-c (HR 0.85, CI 0.74–0.97). Higher values of T-c (HR 0.45, CI 0.36–0.57), HDL-c (HR 0.66, CI 0.54–0.81), and LDL-c (HR 0.86, CI 0.78–0.94) showed a protective effect on mortality. Conclusions: Abnormalities in lipid profile are a frequent complication of SARS-CoV-2 infection and might be related to morbidity and mortalityThis work was supported by the following grants: Proyectos
de Investigación en Salud (FIS) PI16-02091 and PI19-00584 (funded
by Instituto de Salud Carlos III), TIRONET2-CM, B2017/BMD-3724
(funded by Comunidad de Madrid) and cofinanced by FEDER funds to
M.M
Propuesta para la corrección del factor de potencia en parques eólicos
En el presente trabajo, se propone un sistema para la corrección dinámica del factor de potencia de parques eólicos. El concepto propuesto consiste en la utilización de un capacitor variable por pasos binarios, el cual es conmutado sobre la red a través de llaves semiconductoras. Las llaves son controladas para conectar y desconectar las diferentes ramas del banco de capacitores sobre la red, en el momento del cruce por cero de la tensión en los bornes de la llave. El sistema propuesto tiene aplicaciones, tanto en sistemas con variaciones suaves de demanda de potencia reactiva, como tensión en casos de variaciones bruscas de la potencia reactiva debido a conexión y desconexión de los generadores eólicos integrantes de un parque.Tema: Instrumentación y Materiales usados en Energías Renovables.Asociación Argentina de Energía Sola
Limits on surface gravities of Kepler planet-candidate host stars from non-detection of solar-like oscillations
We present a novel method for estimating lower-limit surface gravities log g
of Kepler targets whose data do not allow the detection of solar-like
oscillations. The method is tested using an ensemble of solar-type stars
observed in the context of the Kepler Asteroseismic Science Consortium. We then
proceed to estimate lower-limit log g for a cohort of Kepler solar-type
planet-candidate host stars with no detected oscillations. Limits on
fundamental stellar properties, as provided by this work, are likely to be
useful in the characterization of the corresponding candidate planetary
systems. Furthermore, an important byproduct of the current work is the
confirmation that amplitudes of solar-like oscillations are suppressed in stars
with increased levels of surface magnetic activity.Comment: Accepted for publication in ApJ; 35 pages, 10 figures, 5 table
Propuesta para la corrección del factor de potencia en parques eólicos
En el presente trabajo, se propone un sistema para la corrección dinámica del factor de potencia de parques eólicos. El concepto propuesto consiste en la utilización de un capacitor variable por pasos binarios, el cual es conmutado sobre la red a través de llaves semiconductoras. Las llaves son controladas para conectar y desconectar las diferentes ramas del banco de capacitores sobre la red, en el momento del cruce por cero de la tensión en los bornes de la llave. El sistema propuesto tiene aplicaciones, tanto en sistemas con variaciones suaves de demanda de potencia reactiva, como tensión en casos de variaciones bruscas de la potencia reactiva debido a conexión y desconexión de los generadores eólicos integrantes de un parque.Tema: Instrumentación y Materiales usados en Energías Renovables.Asociación Argentina de Energía Sola
Propuesta para la corrección del factor de potencia en parques eólicos
En el presente trabajo, se propone un sistema para la corrección dinámica del factor de potencia de parques eólicos. El concepto propuesto consiste en la utilización de un capacitor variable por pasos binarios, el cual es conmutado sobre la red a través de llaves semiconductoras. Las llaves son controladas para conectar y desconectar las diferentes ramas del banco de capacitores sobre la red, en el momento del cruce por cero de la tensión en los bornes de la llave. El sistema propuesto tiene aplicaciones, tanto en sistemas con variaciones suaves de demanda de potencia reactiva, como tensión en casos de variaciones bruscas de la potencia reactiva debido a conexión y desconexión de los generadores eólicos integrantes de un parque.Tema: Instrumentación y Materiales usados en Energías Renovables.Asociación Argentina de Energía Sola
Testing the Asteroseismic Mass Scale Using Metal-Poor Stars Characterized with APOGEE and Kepler
Fundamental stellar properties, such as mass, radius, and age, can be
inferred using asteroseismology. Cool stars with convective envelopes have
turbulent motions that can stochastically drive and damp pulsations. The
properties of the oscillation frequency power spectrum can be tied to mass and
radius through solar-scaled asteroseismic relations. Stellar properties derived
using these scaling relations need verification over a range of metallicities.
Because the age and mass of halo stars are well-constrained by astrophysical
priors, they provide an independent, empirical check on asteroseismic mass
estimates in the low-metallicity regime. We identify nine metal-poor red giants
(including six stars that are kinematically associated with the halo) from a
sample observed by both the Kepler space telescope and the Sloan Digital Sky
Survey-III APOGEE spectroscopic survey. We compare masses inferred using
asteroseismology to those expected for halo and thick-disk stars. Although our
sample is small, standard scaling relations, combined with asteroseismic
parameters from the APOKASC Catalog, produce masses that are systematically
higher (=0.17+/-0.05 Msun) than astrophysical expectations. The
magnitude of the mass discrepancy is reduced by known theoretical corrections
to the measured large frequency separation scaling relationship. Using
alternative methods for measuring asteroseismic parameters induces systematic
shifts at the 0.04 Msun level. We also compare published asteroseismic analyses
with scaling relationship masses to examine the impact of using the frequency
of maximum power as a constraint. Upcoming APOKASC observations will provide a
larger sample of ~100 metal-poor stars, important for detailed asteroseismic
characterization of Galactic stellar populations.Comment: 4 figures; 1 table. Accepted to ApJ
Quantitative taphonomy of the Cacela site (late Tortonian; Algarve, S Portugal)
A quantitative taphonomic analysis were performed in the fossil assemblages of the late Tortonian of the
lower-mid part of the Cacela Formation (Cacela, Algarve, S Portugal). Taphonomic results, together with
sedimentological analysis and the faunal assemblages, allow to inferring palaeoenvironmental changes.
The upper Tortonian sedimentation started with fan-delta conglomerates that quickly changed to outerplatform
silty marls deposited in a setting with a relatively low sedimentation rate as indicated by the
concentration of glauconite. Higher up into the section, a progressive shallowing trend is inferred, and
sediments of inner platform settings were forme
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