968 research outputs found
Why should we correct reported pulsation frequencies for stellar line-of-sight Doppler velocity shifts?
In the age of Kepler and Corot, extended observations have provided estimates
of stellar pulsation frequencies that have achieved new levels of precision,
regularly exceeding fractional levels of a few parts in . These high
levels of precision now in principle exceed the point where one can ignore the
Doppler shift of pulsation frequencies caused by the motion of a star relative
to the observer. We present a correction for these Doppler shifts and use
previously published pulsation frequencies to demonstrate the significance of
the effect. We suggest that reported pulsation frequencies should be routinely
corrected for stellar line-of-sight velocity Doppler shifts, or if a
line-of-sight velocity estimate is not available, the frame of reference in
which the frequencies are reported should be clearly stated.Comment: 5 pages, 1 figure, accepted for publication in MNRAS Letter
Super-Nyquist asteroseismology of solar-like oscillators with Kepler and K2 - expanding the asteroseismic cohort at the base of the red-giant branch
We consider the prospects for detecting solar-like oscillations in the
"super-Nyquist" regime of long-cadence (LC) Kepler photometry, i.e., above the
associated Nyquist frequency of approximately 283 {\mu}Hz. Targets of interest
are cool, evolved subgiants and stars lying at the base of the red-giant
branch. These stars would ordinarily be studied using the short-cadence (SC)
data, since the associated SC Nyquist frequency lies well above the frequencies
of the detectable oscillations. However, the number of available SC target
slots is quite limited. This imposes a severe restriction on the size of the
ensemble available for SC asteroseismic study.We find that archival Kepler LC
data from the nominal Mission may be utilized for asteroseismic studies of
targets whose dominant oscillation frequencies lie as high as approximately 500
{\mu}Hz, i.e., about 1.75- times the LC Nyquist frequency. The frequency
detection threshold for the shorter-duration science campaigns of the
re-purposed Kepler Mission, K2, is lower. The maximum threshold will probably
lie somewhere between approximately 400 and 450 {\mu}Hz. The potential to
exploit the archival Kepler and K2 LC data in this manner opens the door to
increasing significantly the number of subgiant and low-luminosity red-giant
targets amenable to asteroseismic analysis, overcoming target limitations
imposed by the small number of SC slots.We estimate that around 400 such
targets are now available for study in the Kepler LC archive. That number could
potentially be a lot higher for K2, since there will be a new target list for
each of its campaigns.Comment: Accepted for publication in MNRAS; 11 pages, 7 figures; reference
list update
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
Optimal Growth Conditions for Selective Ge Islands Positioning on Pit-Patterned Si(001)
We investigate ordered nucleation of Ge islands on pit-patterned Si(001) using an original hybrid Kinetic Monte Carlo model. The method allows us to explore long time-scale evolution while using large simulation cells. We analyze the possibility to achieve selective nucleation and island homogeneity as a function of the various parameters (flux, temperature, pit period) able to influence the growth process. The presence of an optimal condition where the atomic diffusivity is sufficient to guarantee nucleation only within pits, but not so large to induce significant Ostwald ripening, is clearly demonstrated
A synthetic sample of short-cadence solar-like oscillators for TESS
NASA's Transiting Exoplanet Survey Satellite (TESS) has begun a two-year
survey of most of the sky, which will include lightcurves for thousands of
solar-like oscillators sampled at a cadence of two minutes. To prepare for this
steady stream of data, we present a mock catalogue of lightcurves, designed to
realistically mimic the properties of the TESS sample. In the process, we also
present the first public release of the asteroFLAG Artificial Dataset
Generator, which simulates lightcurves of solar-like oscillators based on input
mode properties. The targets are drawn from a simulation of the Milky Way's
populations and are selected in the same way as TESS's true Asteroseismic
Target List. The lightcurves are produced by combining stellar models,
pulsation calculations and semi-empirical models of solar-like oscillators. We
describe the details of the catalogue and provide several examples. We provide
pristine lightcurves to which noise can be added easily. This mock catalogue
will be valuable in testing asteroseismology pipelines for TESS and our methods
can be applied in preparation and planning for other observatories and
observing campaigns.Comment: 14 pages, 6 figures, accepted for publication in ApJS. Archives
containing the mock catalogue are available at
https://doi.org/10.5281/zenodo.1470155 and the pipeline to produce it at
https://github.com/warrickball/s4tess . The first public release of the
asteroFLAG Artificial Dataset Generator v3 (AADG3) is described at
https://warrickball.github.io/AADG3
Driving and damping mechanisms in hybrid pressure-gravity modes pulsators
We study the energetic aspects of hybrid pressure-gravity modes pulsations.
The case of hybrid beta Cephei-SPB pulsators is considered with special
attention. In addition to the already known sensitivity of the driving
mechanism to the heavy elements mixture (mainly the iron abundance), we show
that the characteristics of the propagation and evanescent regions play also a
major role, determining the extension of the stable gap in the frequency domain
between the unstable low order pressure and high order gravity modes. Finally,
we consider the case of hybrid delta Sct-gamma Dor pulsators.Comment: 7 pages, 9 figures, in the proceedings of the Helas II Conference:
"Helioseismology, Asteroseismology and MHD Connections", Goettingen, August
200
Galactic Archaeology with TESS: Prospects for Testing the Star Formation History in the Solar Neighbourhood
A period of quenching between the formation of the thick and thin disks of
the Milky Way has been recently proposed to explain the observed
age-[{\alpha}/Fe] distribution of stars in the solar neighbourhood. However,
robust constraints on stellar ages are currently available for only a limited
number of stars. The all-sky survey TESS (Transiting Exoplanet Survey
Satellite) will observe the brightest stars in the sky and thus can be used to
investigate the age distributions of stars in these components of the Galaxy
via asteroseismology, where previously this has been difficult using other
techniques. The aim of this preliminary study was to determine whether TESS
will be able to provide evidence for quenching periods during the star
formation history of the Milky Way. Using a population synthesis code, we
produced populations based on various stellar formation history models and
limited the analysis to red-giant-branch stars. We investigated the
mass-Galactic-disk-height distributions, where stellar mass was used as an age
proxy, to test for whether periods of quenching can be observed by TESS. We
found that even with the addition of 15% noise to the inferred masses, it will
be possible for TESS to find evidence for/against quenching periods suggested
in the literature (e.g. between 7 and 9 Gyr ago), therefore providing stringent
constraints on the formation and evolution of the Milky Way.Comment: 4 pages, 3 figures, proceedings of "Seismology of the Sun and the
Distant Stars 2016", Mario J. P. F. G. Monteiro, Margarida S. Cunha, Joao
Miguel T. Ferreira editor
CLES, Code Liegeois d'Evolution Stellaire
Cles is an evolution code recently developed to produce stellar models
meeting the specific requirements of studies in asteroseismology. It offers the
users a lot of choices in the input physics they want in their models and its
versatility allows them to tailor the code to their needs and implement easily
new features. We describe the features implemented in the current version of
the code and the techniques used to solve the equations of stellar structure
and evolution. A brief account is given of the use of the program and of a
solar calibration realized with it.Comment: Comments: 8 pages, Astrophys. Space Sci. CoRoT-ESTA Volume, in the
pres
CoRoT's view of newly discovered B-star pulsators: results for 358 candidate B pulsators from the initial run's exoplanet field data
We search for new variable B-type pulsators in the CoRoT data assembled
primarily for planet detection, as part of CoRoT's Additional Programme. We aim
to explore the properties of newly discovered B-type pulsators from the
uninterrupted CoRoT space-based photometry and to compare them with known
members of the Beta Cep and slowly pulsating B star (SPB) classes. We developed
automated data analysis tools that include algorithms for jump correction,
light-curve detrending, frequency detection, frequency combination search, and
for frequency and period spacing searches. Besides numerous new, classical,
slowly pulsating B stars, we find evidence for a new class of low-amplitude
B-type pulsators between the SPB and Delta Sct instability strips, with a very
broad range of frequencies and low amplitudes, as well as several slowly
pulsating B stars with residual excess power at frequencies typically a factor
three above their expected g-mode frequencies. The frequency data we obtained
for numerous new B-type pulsators represent an appropriate starting point for
further theoretical analyses of these stars, once their effective temperature,
gravity, rotation velocity, and abundances will be derived spectroscopically in
the framework of an ongoing FLAMES survey at the VLT.Comment: 22 pages, 30 figures, accepted for publication in A&
Development of energy system model of the Caspian region
The main objective of the project is to develop and to use the model of energy system
of Central Asia and Caspian region (CAC): Azerbaijan (AZJ), Kazakhstan (KZK), Turkmenistan (TKM)
and Uzbekistan (UZB) - TIMES-CAC-4R and to assess quantitatively the direct economic benefits of
cooperation in export of hydrocarbons among CAC countries
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