557 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
VEGF guides angiogenic sprouting utilizing endothelial tip cell filopodia
Vascular endothelial growth factor (VEGF-A) is a major regulator of blood vessel formation and function. it controls several processes in endothelial cells, such as proliferation, survival, and migration, but it is not known how these are coordinately regulated to result in more complex morphogenetic events, such as tubular sprouting, fusion, and network formation. We show here that VEGF-A controls angiogenic sprouting in the early postnatal retina by guiding filopodial extension from specialized endothelial cells situated at the tips of the vascular sprouts. The tip cells respond to VEGF-A only by guided migration; the proliferative response to VEGF-A occurs in the sprout stalks. These two cellular responses are both mediated by agonistic activity of VEGF-A on VEGF receptor 2. Whereas tip cell migration depends on a gradient of VEGF-A, proliferation is regulated by its concentration. Thus, vessel patterning during retinal angiogenesis depends on the balance between two different qualities of the extracellular VEGF-A distribution, which regulate distinct cellular responses in defined populations of endothelial cells
An Introduction to Data Analysis in Asteroseismology
A practical guide is presented to some of the main data analysis concepts and
techniques employed contemporarily in the asteroseismic study of stars
exhibiting solar-like oscillations. The subjects of digital signal processing
and spectral analysis are introduced first. These concern the acquisition of
continuous physical signals to be subsequently digitally analyzed. A number of
specific concepts and techniques relevant to asteroseismology are then
presented as we follow the typical workflow of the data analysis process,
namely, the extraction of global asteroseismic parameters and individual mode
parameters (also known as peak-bagging) from the oscillation spectrum.Comment: Lecture presented at the IVth Azores International Advanced School in
Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars
and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta,
Azores Islands, Portugal in July 201
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
γ-Secretase modulators show selectivity for γ-secretase–mediated amyloid precursor protein intramembrane processing
The aggregation of β-amyloid peptide 42 results in the formation of toxic oligomers and plaques, which plays a pivotal role in Alzheimer's disease pathogenesis. Aβ42 is one of several Aβ peptides, all of Aβ30 to Aβ43 that are produced as a result of γ-secretase–mediated regulated intramembrane proteolysis of the amyloid precursor protein. γ-Secretase modulators (GSMs) represent a promising class of Aβ42-lowering anti-amyloidogenic compounds for the treatment of AD. Gamma-secretase modulators change the relative proportion of secreted Aβ peptides, while sparing the γ-secretase–mediated processing event resulting in the release of the cytoplasmic APP intracellular domain. In this study, we have characterized how GSMs affect the γ-secretase cleavage of three γ-secretase substrates, E-cadherin, ephrin type A receptor 4 (EphA4) and ephrin type B receptor 2 (EphB2), which all are implicated in important contexts of cell signalling. By using a reporter gene assay, we demonstrate that the γ-secretase–dependent generation of EphA4 and EphB2 intracellular domains is unaffected by GSMs. We also show that γ-secretase processing of EphA4 and EphB2 results in the release of several Aβ-like peptides, but that only the production of Aβ-like proteins from EphA4 is modulated by GSMs, but with an order of magnitude lower potency as compared to Aβ modulation. Collectively, these results suggest that GSMs are selective for γ-secretase–mediated Aβ production
Asteroseismic determination of obliquities of the exoplanet systems Kepler-50 and Kepler-65
Results on the obliquity of exoplanet host stars -- the angle between the
stellar spin axis and the planetary orbital axis -- provide important
diagnostic information for theories describing planetary formation. Here we
present the first application of asteroseismology to the problem of stellar
obliquity determination in systems with transiting planets and Sun-like host
stars. We consider two systems observed by the NASA Kepler Mission which have
multiple transiting small (super-Earth sized) planets: the previously reported
Kepler-50 and a new system, Kepler-65, whose planets we validate in this paper.
Both stars show rich spectra of solar-like oscillations. From the asteroseismic
analysis we find that each host has its rotation axis nearly perpendicular to
the line of sight with the sines of the angles constrained at the 1-sigma level
to lie above 0.97 and 0.91, respectively. We use statistical arguments to show
that coplanar orbits are favoured in both systems, and that the orientations of
the planetary orbits and the stellar rotation axis are correlated.Comment: Accepted for publication in ApJ; 46 pages, 11 figure
Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology
We present a study of 33 {\it Kepler} planet-candidate host stars for which
asteroseismic observations have sufficiently high signal-to-noise ratio to
allow extraction of individual pulsation frequencies. We implement a new
Bayesian scheme that is flexible in its input to process individual oscillation
frequencies, combinations of them, and average asteroseismic parameters, and
derive robust fundamental properties for these targets. Applying this scheme to
grids of evolutionary models yields stellar properties with median statistical
uncertainties of 1.2\% (radius), 1.7\% (density), 3.3\% (mass), 4.4\%
(distance), and 14\% (age), making this the exoplanet host-star sample with the
most precise and uniformly determined fundamental parameters to date. We assess
the systematics from changes in the solar abundances and mixing-length
parameter, showing that they are smaller than the statistical errors. We also
determine the stellar properties with three other fitting algorithms and
explore the systematics arising from using different evolution and pulsation
codes, resulting in 1\% in density and radius, and 2\% and 7\% in mass and age,
respectively. We confirm previous findings of the initial helium abundance
being a source of systematics comparable to our statistical uncertainties, and
discuss future prospects for constraining this parameter by combining
asteroseismology and data from space missions. Finally we compare our derived
properties with those obtained using the global average asteroseismic
observables along with effective temperature and metallicity, finding an
excellent level of agreement. Owing to selection effects, our results show that
the majority of the high signal-to-noise ratio asteroseismic {\it Kepler} host
stars are older than the Sun.Comment: 25 pages, 17 figures, MNRAS accepte
Asteroseismology of solar-type stars with K2
We present the first detections by the NASA K2 Mission of oscillations in
solar-type stars, using short-cadence data collected during K2 Campaign\,1
(C1). We understand the asteroseismic detection thresholds for C1-like levels
of photometric performance, and we can detect oscillations in subgiants having
dominant oscillation frequencies around . Changes to the
operation of the fine-guidance sensors are expected to give significant
improvements in the high-frequency performance from C3 onwards. A reduction in
the excess high-frequency noise by a factor of two-and-a-half in amplitude
would bring main-sequence stars with dominant oscillation frequencies as high
as into play as potential asteroseismic targets for
K2.Comment: Accepted for publication in PASP; 16 pages, 2 figure
- …