692 research outputs found
The relation between and for solar-like oscillations
Establishing relations between global stellar parameters and asteroseismic
quantities can help improve our understanding of stellar astrophysics and
facilitate the interpretation of observations. We present an observed relation
between the large frequency separation, , and the frequency of
maximum power, . We find that is proportional to
, allowing prediction of to about 15 per cent
given . Our result is further supported by established scaling
relations for and and by extended stellar model
calculations, which confirm that can be estimated using this
relation for basically any star showing solar-like oscillations in the
investigated range (0.5<M/Msol<4.0).Comment: 5 pages, 8 figures, Letter accepted by MNRA
Amplitude variability in satellite photometry of the non-radially pulsating O9.5V star zeta Oph
We report a time-series analysis of satellite photometry of the non-radially
pulsating Oe star zeta Oph, principally using data from SMEI obtained
2003--2008, but augmented with MOST and WIRE results. Amplitudes of the
strongest photometric signals, at 5.18, 2.96, and 2.67/d, each vary
independently over the 6-year monitoring period (from ca. 30 to <2 mmag at
5.18/d), on timescales of hundreds of days. Signals at 7.19/d and 5.18/d have
persisted (or recurred) for around two decades. Supplementary spectroscopic
observations show an H-alpha emission episode in 2006; this coincided with
small increases in amplitudes of the three strongest photometric signals.Comment: MNRAS, in pres
NGC 6819: testing the asteroseismic mass scale, mass loss, and evidence for products of non-standard evolution
We present an extensive peakbagging effort on Kepler data of 50 red
giant stars in the open star cluster NGC 6819. By employing sophisticated
pre-processing of the time series and Markov Chain Monte Carlo techniques we
extracted individual frequencies, heights and linewidths for hundreds of
oscillation modes.
We show that the "average" asteroseismic parameter , derived
from these, can be used to distinguish the stellar evolutionary state between
the red giant branch (RGB) stars and red clump (RC) stars.
Masses and radii are estimated using asteroseismic scaling relations, both
empirically corrected to obtain self-consistency as well as agreement with
independent measures of distance, and using updated theoretical corrections.
Remarkable agreement is found, allowing the evolutionary state of the giants to
be determined exclusively from the empirical correction to the scaling
relations. We find a mean mass of the RGB stars and RC stars in NGC 6819 to be
and ,
respectively. The difference is
almost insensitive to systematics, suggesting very little RGB mass loss, if
any.
Stars that are outliers relative to the ensemble reveal overmassive members
that likely evolved via mass-transfer in a blue straggler phase. We suggest
that KIC 4937011, a low-mass Li-rich giant, is a cluster member in the RC phase
that experienced very high mass-loss during its evolution. Such over- and
undermassive stars need to be considered when studying field giants, since the
true age of such stars cannot be known and there is currently no way to
distinguish them from normal stars.Comment: 21 pages, 11 figure
Asteroseismic surface gravity for evolved stars
Context: Asteroseismic surface gravity values can be of importance in
determining spectroscopic stellar parameters. The independent log(g) value from
asteroseismology can be used as a fixed value in the spectroscopic analysis to
reduce uncertainties due to the fact that log(g) and effective temperature can
not be determined independently from spectra. Since 2012, a combined analysis
of seismically and spectroscopically derived stellar properties is ongoing for
a large survey with SDSS/APOGEE and Kepler. Therefore, knowledge of any
potential biases and uncertainties in asteroseismic log(g) values is now
becoming important. Aims: The seismic parameter needed to derive log(g) is the
frequency of maximum oscillation power (nu_max). Here, we investigate the
influence of nu_max derived with different methods on the derived log(g)
values. The large frequency separation between modes of the same degree and
consecutive radial orders (Dnu) is often used as an additional constraint for
the determination of log(g). Additionally, we checked the influence of small
corrections applied to Dnu on the derived values of log(g). Methods We use
methods extensively described in the literature to determine nu_max and Dnu
together with seismic scaling relations and grid-based modeling to derive
log(g). Results: We find that different approaches to derive oscillation
parameters give results for log(g) with small, but different, biases for
red-clump and red-giant-branch stars. These biases are well within the quoted
uncertainties of ~0.01 dex (cgs). Corrections suggested in the literature to
the Dnu scaling relation have no significant effect on log(g). However somewhat
unexpectedly, method specific solar reference values induce biases of the order
of the uncertainties, which is not the case when canonical solar reference
values are used.Comment: 8 pages, 5 figures, accepted for publication by A&
Permeability evolution in sorbing media. Analogies between organic-rich shale and coal
Shale gas reservoirs like coalbed methane (CBM) reservoirs are promising targets for geological sequestration of carbon dioxide (CO2). However, the evolution of permeability in shale reservoirs on injection of CO2 is poorly understood unlike CBM reservoirs. In this study, we report measurements of permeability evolution in shales infiltrated separately by nonsorbing (He) and sorbing (CO2) gases under varying gas pressures and confining stresses. Experiments are completed on Pennsylvanian shales containing both natural and artificial fractures under nonpropped and propped conditions. We use the models for permeability evolution in coal (Journal of Petroleum Science and Engineering, Under Revision) to codify the permeability evolution observed in the shale samples. It is observed that for a naturally fractured shale, the He permeability increases by approximately 15% as effective stress is reduced by increasing the gas pressure from 1 MPa to 6 MPa at constant confining stress of 10 MPa. Conversely, the CO2 permeability reduces by a factor of two under similar conditions. A second core is split with a fine saw to create a smooth artificial fracture and the permeabilities are measured for both nonpropped and propped fractures. The He permeability of a propped artificial fracture is approximately 2- to 3fold that of the nonpropped fracture. The He permeability increases with gas pressure under constant confining stress for both nonpropped and propped cases. However, the CO2 permeability of the propped fracture decreases by between one-half to one-third as the gas pressure increases from 1 to 4 MPa at constant confining stress. Interestingly, the CO2 permeability of nonpropped fracture increases with gas pressure at constant confining stress. The permeability evolution of nonpropped and propped artificial fractures in shale is found to be similar to those observed in coals but the extent of permeability reduction by swelling is much lower in shale due to its lower organic content. Optical profilometry is used to quantify the surface roughness. The changes in surface roughness indicate significant influence of proppant indentation on fracture surface in the shale sample. The trends of permeability evolution on injection of CO2 in coals and shales are found analogous; therefore, the permeability evolution models previously developed for coals are adopted to explain the permeability evolution in shales
Characterisation of red-giant stars in the public Kepler data
The first public release of long-cadence stellar photometric data collected
by the NASA Kepler mission has now been made available. In this paper we
characterise the red-giant (G-K) stars in this large sample in terms of their
solar-like oscillations. We use published methods and well-known scaling
relations in the analysis. Just over 70% of the red giants in the sample show
detectable solar-like oscillations, and from these oscillations we are able to
estimate the fundamental properties of the stars. This asteroseismic analysis
reveals different populations: low-luminosity H-shell burning red-giant branch
stars, cool high-luminosity red giants on the red-giant branch and He-core
burning clump and secondary-clump giants.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Main Journa
Frequency, splitting, linewidth and amplitude estimates of low-l p modes of alpha Cen A: analysis of WIRE photometry
We present results of fitting the 50-day time series of photometry of alpha
Cen A taken by the WIRE satellite in 1999. Both power spectrum and
autocovariance function (ACF) fitting techniques were used in an attempt to
determine mode frequencies, rotational splittings, lifetimes and amplitudes of
low-l p-modes. In all, using both techniques, we managed to fit 18 modes (seven
l = 0, eight l = 1 and three l = 2) with frequencies determined to within 1 - 2
micro-Hz. These estimates are shown to be 0.6 +/- 0.3 micro-Hz lower, on
average, than the frequencies determined from two other more recent studies
(Bouchy & Carrier, 2002; Bedding et al. 2004) which used data gathered about 19
months after the WIRE observations. This could be indicative of an activity
cycle, although due to the large uncertainty, more data would be needed to
confirm this.
Over a range of 1700 to 2650 micro-Hz we were also able to use the ACF
fitting to determine an average lifetime of 3.9 +/- 1.4 days, and an average
rotational splitting of 0.54 +/- 0.22 micro-Hz, which is the first ever
reliable estimate of this parameter. In contrast to the ACF, the power spectrum
fitting was shown to return significantly biased results for these parameters.Comment: 11 pages, 10 figures (some figures are made up from multiple ps files
of which there are 17 in total
A large sample of calibration stars for Gaia: log g from Kepler and CoRoT
Asteroseismic data can be used to determine surface gravities with precisions
of < 0.05 dex by using the global seismic quantities Deltanu and nu_max along
with Teff and [Fe/H]. Surface gravity is also one of the four stellar
properties to be derived by automatic analyses for 1 billion stars from Gaia
data (workpackage GSP_Phot). We explore seismic data from MS F, G, K stars
(solar-like stars) observed by Kepler as a potential calibration source for
methods that Gaia will use for object characterisation (log g). We calculate
log g for bright nearby stars for which radii and masses are known, and using
their global seismic quantities in a grid-based method, we determine an
asteroseismic log g to within 0.01 dex of the direct calculation, thus
validating the accuracy of our method. We find that errors in Teff and mainly
[Fe/H] can cause systematic errors of 0.02 dex. We then apply our method to a
list of 40 stars to deliver precise values of surface gravity, i.e. sigma <
0.02 dex, and we find agreement with recent literature values. Finally, we
explore the precision we expect in a sample of 400+ Kepler stars which have
their global seismic quantities measured. We find a mean uncertainty
(precision) on the order of <0.02 dex in log g over the full explored range 3.8
< log g < 4.6, with the mean value varying only with stellar magnitude (0.01 -
0.02 dex). We study sources of systematic errors in log g and find possible
biases on the order of 0.04 dex, independent of log g and magnitude, which
accounts for errors in the Teff and [Fe/H] measurements, as well as from using
a different grid-based method. We conclude that Kepler stars provide a wealth
of reliable information that can help to calibrate methods that Gaia will use,
in particular, for source characterisation with GSP_Phot where excellent
precision (small uncertainties) and accuracy in log g is obtained from seismic
data.Comment: Accepted MNRAS, 15 pages (10 figures and 3 tables), v2=some rewording
of two sentence
Imaging elastodynamic and hydraulic properties of in situ fractured rock. An experimental investigation exploring effects of dynamic stressing and shearing
We describe laboratory experiments to elucidate the relationship between nonlinear elasticity and permeability evolution in fractured media subjected to local stress perturbations. This study is part of an effort to measure fluid pathways and fracture properties using active-source acoustic monitoring during fluid injection and shear of rough fractures. Experiments were conducted with L-shaped samples of Westerly granite fractured in situ under triaxial conditions with deionized water subsequently circulated through the resulting fractures. After in situ fracturing, we separately imposed oscillations of the applied normal stress and pore pressure with amplitudes ranging from 0.2 to 1 MPa and frequencies from 0.1 to 40 Hz. In response to normal stress and pore pressure oscillations, fractured Westerly granite samples exhibit characteristic transient softening, acoustic velocity fluctuations, and slow recovery, together with permeability enhancement or decay, informing us about the coupled nonlinear elastodynamic and poromechanical rock properties. Fracture interface properties (contact asperity stiffness, aperture) are then altered in situ by shearing, which generally decreases the measured elastic nonlinearity and permeability change for both normal stress and pore pressure oscillations
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