1,836 research outputs found
Kepler Mission Stellar and Instrument Noise Properties Revisited
An earlier study of the Kepler Mission noise properties on time scales of
primary relevance to detection of exoplanet transits found that higher than
expected noise followed to a large extent from the stars, rather than
instrument or data analysis performance. The earlier study over the first six
quarters of Kepler data is extended to the full four years ultimately
comprising the mission. Efforts to improve the pipeline data analysis have been
successful in reducing noise levels modestly as evidenced by smaller values
derived from the current data products. The new analyses of noise properties on
transit time scales show significant changes in the component attributed to
instrument and data analysis, with essentially no change in the inferred
stellar noise. We also extend the analyses to time scales of several days,
instead of several hours to better sample stellar noise that follows from
magnetic activity. On the longer time scale there is a shift in stellar noise
for solar-type stars to smaller values in comparison to solar values.Comment: 10 pages, 8 figures, accepted by A
Earth Occultation Imaging of the Low Energy Gamma-Ray Sky with GBM
The Earth Occultation Technique (EOT) has been applied to Fermi's Gamma-ray
Burst Monitor (GBM) to perform all-sky monitoring for a predetermined catalog
of hard X-ray/soft gamma-ray sources. In order to search for sources not in the
catalog, thus completing the catalog and reducing a source of systematic error
in EOT, an imaging method has been developed -- Imaging with a Differential
filter using the Earth Occultation Method (IDEOM). IDEOM is a tomographic
imaging method that takes advantage of the orbital precession of the Fermi
satellite. Using IDEOM, all-sky reconstructions have been generated for ~sim 4
years of GBM data in the 12-50 keV, 50-100 keV and 100-300 keV energy bands in
search of sources otherwise unmodeled by the GBM occultation analysis. IDEOM
analysis resulted in the detection of 57 sources in the 12-50 keV energy band,
23 sources in the 50-100 keV energy band, and 7 sources in the 100-300 keV
energy band. Seventeen sources were not present in the original GBM-EOT catalog
and have now been added. We also present the first joined averaged spectra for
four persistent sources detected by GBM using EOT and by the Large Area
Telescope (LAT) on Fermi: NGC 1275, 3C 273, Cen A, and the Crab
Modelling the Autocovariance of the Power Spectrum of a Solar-Type Oscillator
Asteroseismology is able to conduct studies on the interiors of solar-type
stars from the analysis of stellar acoustic spectra. However, such an analysis
process often has to rely upon subjective choices made throughout. A recurring
problem is to determine whether a signal in the acoustic spectrum originates
from a radial or a dipolar oscillation mode. In order to overcome this problem,
we present a procedure for modelling and fitting the autocovariance of the
power spectrum which can be used to obtain global seismic parameters of
solar-type stars, doing so in an automated fashion without the need to make
subjective choices. From the set of retrievable global seismic parameters we
emphasize the mean small frequency separation and, depending on the intrinsic
characteristics of the power spectrum, the mean rotational frequency splitting.
Since this procedure is automated, it can serve as a useful tool in the
analysis of the more than one thousand solar-type stars expected to be observed
as part of the Kepler Asteroseismic Investigation (KAI). We apply the
aforementioned procedure to simulations of the Sun. Assuming different apparent
magnitudes, we address the issues of how accurately and how precisely we can
retrieve the several global seismic parameters were the Sun to be observed as
part of the KAI.Comment: 10 pages, 8 figures, accepted for publication in MNRA
The amplitude of solar oscillations using stellar techniques
The amplitudes of solar-like oscillations depend on the excitation and
damping, both of which are controlled by convection. Comparing observations
with theory should therefore improve our understanding of the underlying
physics. However, theoretical models invariably compute oscillation amplitudes
relative to the Sun, and it is therefore vital to have a good calibration of
the solar amplitude using stellar techniques. We have used daytime spectra of
the Sun, obtained with HARPS and UCLES, to measure the solar oscillations and
made a detailed comparison with observations using the BiSON helioseismology
instrument. We find that the mean solar amplitude measured using stellar
techniques, averaged over one full solar cycle, is 18.7 +/- 0.7 cm/s for the
strongest radial modes (l=0) and 25.2 +/- 0.9 cm/s for l=1. In addition, we use
simulations to establish an equation that estimates the uncertainty of
amplitude measurements that are made of other stars, given that the mode
lifetime is known. Finally, we also give amplitudes of solar-like oscillations
for three stars that we measured from a series of short observations with HARPS
(gamma Ser, beta Aql and alpha For), together with revised amplitudes for five
other stars for which we have previously published results (alpha Cen A, alpha
Cen B, beta Hyi, nu Ind and delta Pav).Comment: 8 pages, accepted by ApJ. Minor wording changes and added a referenc
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
Determining global parameters of the oscillations of solar-like stars
Helioseismology has enabled us to better understand the solar interior, while
also allowing us to better constrain solar models. But now is a tremendous
epoch for asteroseismology as space missions dedicated to studying stellar
oscillations have been launched within the last years (MOST and CoRoT). CoRoT
has already proved valuable results for many types of stars, while Kepler,
which was launched in March 2009, will provide us with a huge number of seismic
data very soon. This is an opportunity to better constrain stellar models and
to finally understand stellar structure and evolution. The goal of this
research work is to estimate the global parameters of any solar-like
oscillating target in an automatic manner. We want to determine the global
parameters of the acoustic modes (large separation, range of excited pressure
modes, maximum amplitude, and its corresponding frequency), retrieve the
surface rotation period of the star and use these results to estimate the
global parameters of the star (radius and mass).To prepare the analysis of
hundreds of solar-like oscillating stars, we have developed a robust and
automatic pipeline. The pipeline consists of data analysis techniques, such as
Fast Fourier Transform, wavelets, autocorrelation, as well as the application
of minimisation algorithms for stellar-modelling. We apply our pipeline to some
simulated lightcurves from the asteroFLAG team and the Aarhus-asteroFLAG
simulator, and obtain results that are consistent with the input data to the
simulations. Our strategy gives correct results for stars with magnitudes below
11 with only a few 10% of bad determinations among the reliable results. We
then apply the pipeline to the Sun and three CoRoT targets.In particular we
determine the parameters of the Sun, HD49933, HD181906, and HD181420.Comment: 15 pages, 17 figures, accepted for publication in A&
Oscillation mode linewidths and heights of 23 main-sequence stars observed by Kepler
Solar-like oscillations have been observed by Kepler and CoRoT in many
solar-type stars, thereby providing a way to probe the stars using
asteroseismology. We provide the mode linewidths and mode heights of the
oscillations of various stars as a function of frequency and of effective
temperature. We used a time series of nearly two years of data for each star.
The 23 stars observed belong to the simple or F-like category. The power
spectra of the 23 main-sequence stars were analysed using both maximum
likelihood estimators and Bayesian estimators, providing individual mode
characteristics such as frequencies, linewidths, and mode heights. We study the
source of systematic errors in the mode linewidths and mode heights, and we
present a way to correct these errors with respect to a common reference fit.
Using the correction, we could explain all sources of systematic errors, which
could be reduced to less than 15% for mode linewidths and heights, and
less than 5% for amplitude, when compared to the reference fit. The effect
of a different estimated stellar background and a different estimated splitting
will provide frequency-dependent systematic errors that might affect the
comparison with theoretical mode linewidth and mode height, therefore affecting
the understanding of the physical nature of these parameters. All other sources
of relative systematic errors are less dependent upon frequency. We also
provide the dependence of the so-called linewidth dip, in the middle of the
observed frequency range, as a function of effective temperature. We show that
the depth of the dip decreases with increasing effective temperature. The
dependence of the dip on effective temperature may imply that the mixing length
parameter or the convective flux may increase with effective
temperature.Comment: Accepted by A&A, 38 pages, 35 figures, 26 table
Determinants of anti-vascular action by combretastatin A-4 phosphate: role of nitric oxide
The anti-vascular action of the tubulin binding agent combretastatin A-4 phosphate (CA-4-P) has been quantified in two types of murine tumour, the breast adenocarcinoma CaNT and the round cell sarcoma SaS. The functional vascular volume, assessed using a fluorescent carbocyanine dye, was significantly reduced at 18 h after CA-4-P treatment in both tumour types, although the degree of reduction was very different in the two tumours. The SaS tumour, which has a higher nitric oxide synthase (NOS) activity than the CaNT tumour, showed ~10-fold greater resistance to vascular damage by CA-4-P. This is consistent with our previous findings, which showed that NO exerts a protective action against this drug. Simultaneous administration of CA-4-P with a NOS inhibitor, Nω-nitro-L-arginine (L-NNA), resulted in enhanced vascular damage and cytotoxicity in both tumour types. Administration of diethylamine NO, an NO donor, conferred protection against the vascular damaging effects. Following treatment with CA-4-P, neutrophil infiltration into the tumours, measured by myeloperoxidase (MPO) activity, was significantly increased. Levels of MPO activity also correlated with the levels of vascular injury and cytotoxicity measured in both tumour types. Neutrophilic MPO generates free radicals and may therefore contribute to the vascular damage associated with CA-4-P treatment. MPO activity was significantly increased in the presence of L-NNA, suggesting that the protective effect of NO against CA-4-P-induced vascular injury may be, at least partially, mediated by limiting neutrophil infiltration. The data are consistent with the hypothesis that neutrophil action contributes to vascular injury by CA-4-P and that NO generation acts to protect the tumour vasculature against CA4-P-induced injury. The protective effect of NO is probably associated with an anti-neutrophil action. © 2000 Cancer Research Campaig
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