1,996 research outputs found

### Seismic analysis of 70 Ophiuchi A: A new quantity proposed

The basic intent of this paper is to model 70 Ophiuchi A using the latest
asteroseismic observations as complementary constraints and to determine the
fundamental parameters of the star. Additionally, we propose a new quantity to
lift the degeneracy between the initial chemical composition and stellar age.
Using the Yale stellar evolution code (YREC7), we construct a series of stellar
evolutionary tracks for the mass range $M$ = 0.85 -- 0.93 $M_{\odot}$ with
different composition $Y_{i}$ (0.26 -- 0.30) and $Z_{i}$ (0.017 -- 0.023).
Along these tracks, we select a grid of stellar model candidates that fall
within the error box in the HR diagram to calculate the theoretical
frequencies, the large- and small- frequency separations using the Guenther's
stellar pulsation code. Following the asymptotic formula of stellar $p$-modes,
we define a quantity $r_{01}$ which is correlated with stellar age. Also, we
test it by theoretical adiabatic frequencies of many models. Many detailed
models of 70 Ophiuchi A have been listed in Table 3. By combining all
non-asteroseismic observations available for 70 Ophiuchi A with these
seismological data, we think that Model 60, Model 125 and Model 126, listed in
Table 3, are the optimum models presently. Meanwhile, we predict that the
radius of this star is about 0.860 -- 0.865 $R_{\odot}$ and the age is about
6.8 -- 7.0 Gyr with mass 0.89 -- 0.90 $M_{\odot}$. Additionally, we prove that
the new quantity $r_{01}$ can be a useful indicator of stellar age.Comment: 23 pages, 5 figures, accepted by New Astronom

### Asteroseismic study of solar-like stars: A method of estimating stellar age

Asteroseismology, as a tool to use the indirect information contained in
stellar oscillations to probe the stellar interiors, is an active field of
research presently. Stellar age, as a fundamental property of star apart from
its mass, is most difficult to estimate. In addition, the estimating of stellar
age can provide the chance to study the time evolution of astronomical
phenomena. In our poster, we summarize our previous work and further present a
method to determine age of low-mass main-sequence star.Comment: 2 pages, 1 figures, submitted to IAUS25

### Asteroseismic Stellar Modelling with AIMS

The goal of AIMS (Asteroseismic Inference on a Massive Scale) is to estimate
stellar parameters and credible intervals/error bars in a Bayesian manner from
a set of asteroseismic frequency data and so-called classical constraints. To
achieve reliable parameter estimates and computational efficiency, it searches
through a grid of pre-computed models using an MCMC algorithm -- interpolation
within the grid of models is performed by first tessellating the grid using a
Delaunay triangulation and then doing a linear barycentric interpolation on
matching simplexes. Inputs for the modelling consist of individual frequencies
from peak-bagging, which can be complemented with classical spectroscopic
constraints. AIMS is mostly written in Python with a modular structure to
facilitate contributions from the community. Only a few computationally
intensive parts have been rewritten in Fortran in order to speed up
calculations.Comment: 11 pages, 4 figures. Tutorial 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

### 'Rapid fire' spectroscopy of Kepler solar-like oscillators

The NASA Kepler mission has been continuously monitoring the same field of
the sky since the successful launch in March 2009, providing high-quality
stellar lightcurves that are excellent data for asteroseismology, far superior
to any other observations available at the present. In order to make a
meaningful analysis and interpretation of the asteroseismic data, accurate
fundamental parameters for the observed stars are needed. The currently
available parameters are quite uncertain as illustrated by e.g. Thygesen et al.
(A&A 543, A160, 2012), who found deviations as extreme as 2.0 dex in [Fe/H] and
log g, compared to catalogue values. Thus, additional follow-up observations
for these targets are needed in order to put firm limits on the parameter space
investigated by the asteroseismic modellers. Here, we propose a metod for
deriving accurate metallicities of main sequence and subgiant solar-like
oscillators from medium resolution spectra with a moderate S/N. The method
takes advantage of the additional constraints on the fundamental parameters,
available from asteroseismology and multi-color photometry. The approach
enables us to reduce the analysis overhead significantly when doing spectral
synthesis, which in turn will increases the efficiency of follow-up
observations.Comment: 3 pages, 2 figures. Proceedings from Asteroseismology of Stellar
Populations in the Milky Way 2013 to appear in 'Astrophysics and Space
Science Proceedings

### Extraction efficiency of drifting electrons in a two-phase xenon time projection chamber

We present a measurement of the extraction efficiency of quasi-free electrons
from the liquid into the gas phase in a two-phase xenon time-projection
chamber. The measurements span a range of electric fields from 2.4 to 7.1 kV/cm
in the liquid xenon, corresponding to 4.5 to 13.1 kV/cm in the gaseous xenon.
Extraction efficiency continues to increase at the highest extraction fields,
implying that additional charge signal may be attained in two-phase xenon
detectors through careful high-voltage engineering of the gate-anode region

### Calibration of a two-phase xenon time projection chamber with a $^{37}$Ar source

We calibrate a two-phase xenon detector at 0.27 keV in the charge channel and
at 2.8 keV in both the light and charge channels using a $^{37}$Ar source that
is directly released into the detector. We map the light and charge yields as a
function of electric drift field. For the 2.8 keV peak, we calculate the
Thomas-Imel box parameter for recombination and determine its dependence on
drift field. For the same peak, we achieve an energy resolution,
$E_{\sigma}/E_{mean}$, between 9.8% and 10.8% for 0.1 kV/cm to 2 kV/cm electric
drift fields.Comment: 12 pages, 7 figure

### A simple beam combination for stellar interferometry

In stellar interferometry, image quality improves significantly with the inclusion of more telescopes and the use of phase closure. We demonstrate, using first coherent and then partially coherent white light, a compact and efficient pair-wise combination of twelve or more beams. The input beams are lined up and spread through a cylindrical lens into a comb of parallel ellipses, which interferes with a perpendicular copy of itself to form a matrix of interferograms between all pairs. The diagonal elements show interference of each beam with itself, for in-tensity calibration. The measured white-light visibilities were high and stable

### Hypersonic Separated Flows About "Tick" Configurations With Sensitivity to Model Design

This paper presents computational results obtained by applying the direct simulation Monte Carlo (DSMC) method for hypersonic nonequilibrium flow about "tick-shaped" model configurations. These test models produces a complex flow where the nonequilibrium and rarefied aspects of the flow are initially enhanced as the flow passes over an expansion surface, and then the flow encounters a compression surface that can induce flow separation. The resulting flow is such that meaningful numerical simulations must have the capability to account for a significant range of rarefaction effects; hence the application of the DSMC method in the current study as the flow spans several flow regimes, including transitional, slip, and continuum. The current focus is to examine the sensitivity of both the model surface response (heating, friction and pressure) and flowfield structure to assumptions regarding surface boundary conditions and more extensively the impact of model design as influenced by leading edge configuration as well as the geometrical features of the expansion and compression surfaces. Numerical results indicate a strong sensitivity to both the extent of the leading edge sharpness and the magnitude of the leading edge bevel angle. Also, the length of the expansion surface for a fixed compression surface has a significant impact on the extent of separated flow

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### Analysis of 83mKr prompt scintillation signals in the PIXeY detector

Prompt scintillation signals from 83mKr calibration sources are a useful metric to calibrate the spatial variation of light collection efficiency and electric field magnitude of a two phase liquid-gas xenon time projection chamber. Because 83mKr decays in two steps, there are two prompt scintillation pulses for each calibration event, denoted S1a and S1b. We study the ratio of S1b to S1a signal sizes in the Particle Identification in Xenon at Yale (PIXeY) experiment and its dependence on the time separation between the two signals (Î” t), notably its increase at low Î” t. In PIXeY data, the Î” t dependence of S1b/S1a is observed to exhibit two exponential components: one with a time constant of 0.05 Â± 0.02 Î¼ s, which can be attributed to processing effects and pulse overlap and one with a time constant of 10.2 Â± 2.2 Î¼s that increases in amplitude with electric drift field, the origin of which is not yet understood

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