3,759 research outputs found
Fast and automated oscillation frequency extraction using Bayesian multi-modality
Since the advent of CoRoT, and NASA Kepler and K2, the number of low- and
intermediate-mass stars classified as pulsators has increased very rapidly with
time, now accounting for several targets. With the recent launch of NASA
TESS space mission, we have confirmed our entrance to the era of all-sky
observations of oscillating stars. TESS is currently releasing good quality
datasets that already allow for the characterization and identification of
individual oscillation modes even from single 27-days shots on some stars. When
ESA PLATO will become operative by the next decade, we will face the
observation of several more hundred thousands stars where identifying
individual oscillation modes will be possible. However, estimating the
individual frequency, amplitude, and lifetime of the oscillation modes is not
an easy task. This is because solar-like oscillations and especially their
evolved version, the red giant branch (RGB) oscillations, can vary
significantly from one star to another depending on its specific stage of the
evolution, mass, effective temperature, metallicity, as well as on its level of
rotation and magnetism. In this perspective I will present a novel, fast, and
powerful way to derive individual oscillation mode frequencies by building on
previous results obtained with \diamonds. I will show that the oscillation
frequencies obtained with this new approach can reach precisions of about 0.1 %
and accuracies of about 0.01 % when compared to published literature values for
the RGB star KIC~12008916.Comment: 10 pages, 2 figures, accepted for publication in Frontiers in
Astronomy and Space Sciences. Invited contribution for the research topic
"The Future of Asteroseismology
DIAMONDS: a new Bayesian Nested Sampling tool. Application to Peak Bagging of solar-like oscillations
To exploit the full potential of Kepler light curves, sophisticated and
robust analysis tools are now required more than ever. Characterizing single
stars with an unprecedented level of accuracy and subsequently analyzing
stellar populations in detail are fundamental to further constrain stellar
structure and evolutionary models. We developed a new code, termed Diamonds,
for Bayesian parameter estimation and model comparison by means of the nested
sampling Monte Carlo (NSMC) algorithm, an efficient and powerful method very
suitable for high-dimensional and multi-modal problems. A detailed description
of the features implemented in the code is given with a focus on the novelties
and differences with respect to other existing methods based on NSMC. Diamonds
is then tested on the bright F8 V star KIC~9139163, a challenging target for
peak-bagging analysis due to its large number of oscillation peaks observed,
which are coupled to the blending that occurs between peaks, and the
strong stellar background signal. We further strain the performance of the
approach by adopting a 1147.5 days-long Kepler light curve. The Diamonds code
is able to provide robust results for the peak-bagging analysis of KIC~9139163.
We test the detection of different astrophysical backgrounds in the star and
provide a criterion based on the Bayesian evidence for assessing the peak
significance of the detected oscillations in detail. We present results for 59
individual oscillation frequencies, amplitudes and linewidths and provide a
detailed comparison to the existing values in the literature. Lastly, we
successfully demonstrate an innovative approach to peak bagging that exploits
the capability of Diamonds to sample multi-modal distributions, which is of
great potential for possible future automatization of the analysis technique.Comment: 22 pages, 14 figures, 3 tables. Accepted for publication in A&
Peak Bagging of red giant stars observed by Kepler: first results with a new method based on Bayesian nested sampling
The peak bagging analysis, namely the fitting and identification of single
oscillation modes in stars' power spectra, coupled to the very high-quality
light curves of red giant stars observed by Kepler, can play a crucial role for
studying stellar oscillations of different flavor with an unprecedented level
of detail. A thorough study of stellar oscillations would thus allow for deeper
testing of stellar structure models and new insights in stellar evolution
theory. However, peak bagging inferences are in general very challenging
problems due to the large number of observed oscillation modes, hence of free
parameters that can be involved in the fitting models. Effciency and robustness
in performing the analysis is what may be needed to proceed further. For this
purpose, we developed a new code implementing the Nested Sampling Monte Carlo
(NSMC) algorithm, a powerful statistical method well suited for Bayesian
analyses of complex problems. In this talk we show the peak bagging of a sample
of high signal-to-noise red giant stars by exploiting recent Kepler datasets
and a new criterion for the detection of an oscillation mode based on the
computation of the Bayesian evidence. Preliminary results for frequencies and
lifetimes for single oscillation modes, together with acoustic glitches, are
therefore presented.Comment: Conference Proceeding - CoRoT3-KASC7 The Space Photometry Revolution,
Toulouse, France, July 6-11 2014, 4 pages, 3 figure
Asteroseismic stellar activity relations
In asteroseismology an important diagnostic of the evolutionary status of a
star is the small frequency separation which is sensitive to the gradient of
the mean molecular weight in the stellar interior. It is thus interesting to
discuss the classical age-activity relations in terms of this quantity.
Moreover, as the photospheric magnetic field tends to suppress the amplitudes
of acoustic oscillations, it is important to quantify the importance of this
effect by considering various activity indicators. We propose a new class of
age-activity relations that connects the Mt. Wilson index and the average
scatter in the light curve with the small frequency separation and the
amplitude of the p-mode oscillations. We used a Bayesian inference to compute
the posterior probability of various empirical laws for a sample of 19
solar-like active stars observed by the Kepler telescope. We demonstrate the
presence of a clear correlation between the Mt. Wilson index and the
relative age of the stars as indicated by the small frequency separation, as
well as an anti-correlation between the index and the oscillation
amplitudes. We argue that the average activity level of the stars shows a
stronger correlation with the small frequency separation than with the absolute
age that is often considered in the literature. The phenomenological laws
discovered in this paper have the potential to become new important diagnostics
to link stellar evolution theory with the dynamics of global magnetic fields.
In particular we argue that the relation between the Mt. Wilson index and
the oscillation amplitudes is in good agreement with the findings of direct
numerical simulations of magneto-convection.Comment: 5 pages, 4 figures, 2 tables. Accepted for publication in A&
High-precision acoustic helium signatures in 18 low-mass low-luminosity red giants. Analysis from more than four years of Kepler observations
High-precision frequencies of acoustic modes in red giant stars are now
available thanks to the long observing length and high-quality of the light
curves provided by the NASA Kepler mission, thus allowing to probe the interior
of evolved cool low-mass stars with unprecedented level of detail. We
characterize the acoustic signature of the helium second ionization zone in a
sample of 18 low-mass low-luminosity red giants by exploiting new mode
frequency measurements derived from more than four years of Kepler
observations. We analyze the second frequency differences of radial acoustic
modes in all the stars of the sample by using the Bayesian code Diamonds. We
find clear acoustic glitches due to the signature of helium second ionization
in all the stars of the sample. We measure the acoustic depth and the
characteristic width of the acoustic glitches with a precision level on average
around 2% and 8%, respectively. We find good agreement with
theoretical predictions and existing measurements from the literature. Lastly,
we derive the amplitude of the glitch signal at for the
second differences and for the frequencies with an average precision of
6%, obtaining values in the range 0.14-0.24 Hz, and 0.08-0.33
Hz, respectively, which can be used to investigate the helium abundance in
the stars.Comment: 12 pages, 19 figures, 3 tables. Accepted for publication in A&
Bayesian peak bagging analysis of 19 low-mass low-luminosity red giants observed with Kepler
The currently available Kepler light curves contain an outstanding amount of
information but a detailed analysis of the individual oscillation modes in the
observed power spectra, also known as peak bagging, is computationally
demanding and challenging to perform on a large number of targets. Our intent
is to perform for the first time a peak bagging analysis on a sample of 19
low-mass low-luminosity red giants observed by Kepler for more than four years.
This allows us to provide high-quality asteroseismic measurements that can be
exploited for an intensive testing of the physics used in stellar structure
models, stellar evolution and pulsation codes, as well as for refining existing
asteroseismic scaling relations in the red giant branch regime. For this
purpose, powerful and sophisticated analysis tools are needed. We exploit the
Bayesian code Diamonds, using an efficient nested sampling Monte Carlo
algorithm, to perform both a fast fitting of the individual oscillation modes
and a peak detection test based on the Bayesian evidence. We find good
agreement for the parameters estimated in the background fitting phase with
those given in the literature. We extract and characterize a total of 1618
oscillation modes, providing the largest set of detailed asteroseismic mode
measurements ever published. We report on the evidence of a change in regime
observed in the relation between linewidths and effective temperatures of the
stars occurring at the bottom of the RGB. We show the presence of a linewidth
depression or plateau around for all the red giants of the
sample. Lastly, we show a good agreement between our measurements of maximum
mode amplitudes and existing maximum amplitudes from global analyses provided
in the literature, useful as empirical tools to improve and simplify the future
peak bagging analysis on a larger sample of evolved stars.Comment: 78 pages, 46 figures, 22 tables. Accepted for publication in A&
Riscrittura e autocensura nei Dialoghi di Torquato Tasso
Il contributo ripercorre in particolare il dialogo del Gonzaga di Torquato Tasso nella luce di premesse metodologiche atte a illustrare la relazione tra la vicenda ‘pubblica’ del testo e la sua divulgazione manoscritta (che nel caso dei dialoghi tassiani avviene sovente in modo parallelo e tangenziale). Ne risultano, con nuova evidenza, gli scrupoli dell’autore su un argomento (quello dell’inquisizione spagnola in Napoli) particolarmente delicato, che portarono a rivedere non pochi luoghi del testo attraverso forme rilevanti di autocensura
Techniques and Patterns for Safe and Efficient Real-Time Middleware
Over 90 percent of all microprocessors are now used for real-time and embedded applications. The behavior of these applications is often constrained by the physical world. It is therefore important to devise higher-level languages and middleware that meet conventional functional requirements, as well as dependably and productively enforce real-time constraints. Real-Time Java is emerging as a safe, real-time environment. In this thesis we use it as our experimentation platform; however, our findings are easily adapted to other similar platforms. This thesis provides the following contributions to the study of safe and efficient real-time middleware. First, it identifies potential bottlenecks and problem with respect to guaranteeing real-time performance in middleware. Second, it presents a series of techniques and patterns that allow the design and implementation of safe, predictable, and highly efficient real-time middleware. Third, it provides a set of architectural and design patterns that application developers can use when designing real-time systems. Finally, it provides a methodology for evaluating the merits and benefits of real-time middleware. Empirical results are presented using that methodology for the techniques presented in this thesis. The methodology helps compare the performance and predictability of general, real-time middleware platforms
The status of music education in the public schools of Massachusetts as of the year 1948-1949
Thesis (Ed. M.)--Boston University, 195
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