1,014 research outputs found
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
Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission
Asteroseismic constraints on K giants make it possible to infer radii, masses
and ages of tens of thousands of field stars. Tests against independent
estimates of these properties are however scarce, especially in the metal-poor
regime. Here, we report the detection of solar-like oscillations in 8 stars
belonging to the red-giant branch and red-horizontal branch of the globular
cluster M4. The detections were made in photometric observations from the K2
Mission during its Campaign 2. Making use of independent constraints on the
distance, we estimate masses of the 8 stars by utilising different combinations
of seismic and non-seismic inputs. When introducing a correction to the Delta
nu scaling relation as suggested by stellar models, for RGB stars we find
excellent agreement with the expected masses from isochrone fitting, and with a
distance modulus derived using independent methods. The offset with respect to
independent masses is lower, or comparable with, the uncertainties on the
average RGB mass (4-10%, depending on the combination of constraints used). Our
results lend confidence to asteroseismic masses in the metal poor regime. We
note that a larger sample will be needed to allow more stringent tests to be
made of systematic uncertainties in all the observables (both seismic and
non-seismic), and to explore the properties of RHB stars, and of different
populations in the cluster.Comment: 6 pages, 3 figures, accepted for publication in MNRA
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
Solar-like oscillations in KIC11395018 and KIC11234888 from 8 months of Kepler data
We analyze the photometric short-cadence data obtained with the Kepler
Mission during the first eight months of observations of two solar-type stars
of spectral types G and F: KIC 11395018 and KIC 11234888 respectively, the
latter having a lower signal-to-noise ratio compared to the former. We estimate
global parameters of the acoustic (p) modes such as the average large and small
frequency separations, the frequency of the maximum of the p-mode envelope and
the average linewidth of the acoustic modes. We were able to identify and to
measure 22 p-mode frequencies for the first star and 16 for the second one even
though the signal-to-noise ratios of these stars are rather low. We also derive
some information about the stellar rotation periods from the analyses of the
low-frequency parts of the power spectral densities. A model-independent
estimation of the mean density, mass and radius are obtained using the scaling
laws. We emphasize the importance of continued observations for the stars with
low signal-to-noise ratio for an improved characterization of the oscillation
modes. Our results offer a preview of what will be possible for many stars with
the long data sets obtained during the remainder of the mission.Comment: 39 pages, 9 figures. Accepted for publication in Ap
Estimating the p-mode frequencies of the solar twin 18 Sco
Solar twins have been a focus of attention for more than a decade, because
their structure is extremely close to that of the Sun. Today, thanks to
high-precision spectrometers, it is possible to use asteroseismology to probe
their interiors. Our goal is to use time series obtained from the HARPS
spectrometer to extract the oscillation frequencies of 18 Sco, the brightest
solar twin. We used the tools of spectral analysis to estimate these
quantities. We estimate 52 frequencies using an MCMC algorithm. After
examination of their probability densities and comparison with results from
direct MAP optimization, we obtain a minimal set of 21 reliable modes. The
identification of each pulsation mode is straightforwardly accomplished by
comparing to the well-established solar pulsation modes. We also derived some
basic seismic indicators using these values. These results offer a good basis
to start a detailed seismic analysis of 18 Sco using stellar models.Comment: 12 pages, 6 figures, to be published in A&
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
Calibrations of Atmospheric Parameters Obtained from the First Year of SDSS-III APOGEE Observations
The SDSS-III Apache Point Observatory Galactic Evolution Experiment (APOGEE)
is a three year survey that is collecting 100,000 high-resolution spectra in
the near-IR across multiple Galactic populations. To derive stellar parameters
and chemical compositions from this massive data set, the APOGEE Stellar
Parameters and Chemical Abundances Pipeline (ASPCAP) has been developed. Here,
we describe empirical calibrations of stellar parameters presented in the first
SDSS-III APOGEE data release (DR10). These calibrations were enabled by
observations of 559 stars in 20 globular and open clusters. The cluster
observations were supplemented by observations of stars in NASA's Kepler field
that have well determined surface gravities from asteroseismic analysis. We
discuss the accuracy and precision of the derived stellar parameters,
considering especially effective temperature, surface gravity, and metallicity;
we also briefly discuss the derived results for the abundances of the
alpha-elements, carbon, and nitrogen. Overall, we find that ASPCAP achieves
reasonably accurate results for temperature and metallicity, but suffers from
systematic errors in surface gravity. We derive calibration relations that
bring the raw ASPCAP results into better agreement with independently
determined stellar parameters. The internal scatter of ASPCAP parameters within
clusters suggests that, metallicities are measured with a precision better than
0.1 dex, effective temperatures better than 150 K, and surface gravities better
than 0.2 dex. The understanding provided by the clusters and Kepler giants on
the current accuracy and precision will be invaluable for future improvements
of the pipeline.Comment: 40 pages, 15 figures, 4 tables, accepted to A
Stellar ages and convective cores in field main-sequence stars: first asteroseismic application to two Kepler targets
Using asteroseismic data and stellar evolution models we make the first
detection of a convective core in a Kepler field main-sequence star, putting a
stringent constraint on the total size of the mixed zone and showing that extra
mixing beyond the formal convective boundary exists. In a slightly less massive
target the presence of a convective core cannot be conclusively discarded, and
thus its remaining main-sequence life time is uncertain. Our results reveal
that best-fit models found solely by matching individual frequencies of
oscillations corrected for surface effects do not always properly reproduce
frequency combinations. Moreover, slightly different criteria to define what
the best-fit model is can lead to solutions with similar global properties but
very different interior structures. We argue that the use of frequency ratios
is a more reliable way to obtain accurate stellar parameters, and show that our
analysis in field main-sequence stars can yield an overall precision of 1.5%,
4%, and 10% in radius, mass and age, respectively. We compare our results with
those obtained from global oscillation properties, and discuss the possible
sources of uncertainties in asteroseismic stellar modeling where further
studies are still needed.Comment: 46 pages, 10 figures, ApJ accepte
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