1,042 research outputs found
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&
Magnetic activity, differential rotation and dynamo action in the pulsating F9IV star KIC 5955122
We present photometric spot modeling of the nearly four-year long light-curve
of the Kepler target KIC 5955122 in terms of persisting dark circular surface
features. With a Bayesian technique, we produced a plausible surface map that
shows dozens of small spots. After some artifacts are removed, the residuals
are at \,mmag. The shortest rotational period found is days. The equator-to-pole extrapolated differential rotation is rad/d. The spots are roughly half as bright as the unperturbed stellar
photosphere. Spot latitudes are restricted to the zone latitude.
There is no indication for any near-pole spots. In addition, the p-mode
pulsations enabled us to determine the evolutionary status of the star, the
extension of the convective zone, and its radius and mass. We discuss the
possibility that the clear signature of active regions in the light curve of
the F9IV star KIC 5955122 is produced by a flux-transport dynamo action at the
base of the convection zone. In particular, we argue that this star has evolved
from an active to a quiet status during the Q0--Q16 period of observation, and
we predict, according to our dynamo model, that the characteristic activity
cycle is of the order of the solar one.Comment: 9 pages, 12 figures, to be published on A&
The rotational shear layer inside the early red-giant star KIC 4448777
We present the asteroseismic study of the early red-giant star KIC 4448777,
complementing and integrating a previous work (Di Mauro et al. 2016), aimed at
characterizing the dynamics of its interior by analyzing the overall set of
data collected by the {\it Kepler} satellite during the four years of its first
nominal mission. We adopted the Bayesian inference code DIAMOND (Corsaro \& De
Ridder 2014) for the peak bagging analysis and asteroseismic splitting
inversion methods to derive the internal rotational profile of the star. The
detection of new splittings of mixed modes, more concentrated in the very inner
part of the helium core, allowed us to reconstruct the angular velocity profile
deeper into the interior of the star and to disentangle the details better than
in Paper I: the helium core rotates almost rigidly about 6 times faster than
the convective envelope, while part of the hydrogen shell seems to rotate at a
constant velocity about 1.15 times lower than the He core. In particular, we
studied the internal shear layer between the fast-rotating radiative interior
and the slow convective zone and we found that it lies partially inside the
hydrogen shell above and extends across the core-envelope
boundary. Finally, we theoretically explored the possibility for the future to
sound the convective envelope in the red-giant stars and we concluded that the
inversion of a set of splittings with only low-harmonic degree , even
supposing a very large number of modes, will not allow to resolve the
rotational profile of this region in detail.Comment: accepted for publication on Ap
FliPer: Checking the reliability of global seismic parameters from automatic pipelines
Our understanding of stars through asteroseismic data analysis is limited by
our ability to take advantage of the huge amount of observed stars provided by
space missions such as CoRoT, Kepler, K2, and soon TESS and PLATO. Global
seismic pipelines provide global stellar parameters such as mass and radius
using the mean seismic parameters, as well as the effective temperature. These
pipelines are commonly used automatically on thousands of stars observed by K2
for 3 months (and soon TESS for at least around 1 month). However, pipelines
are not immune from misidentifying noise peaks and stellar oscillations.
Therefore, new validation techniques are required to assess the quality of
these results. We present a new metric called FliPer (Flicker in Power), which
takes into account the average variability at all measured time scales. The
proper calibration of FliPer enables us to obtain good estimations of global
stellar parameters such as surface gravity that are robust against the
influence of noise peaks and hence are an excellent way to find faults in
asteroseismic pipelines.Comment: 4 pages, 3 figures, Proceedings for SF2A 2017 (Paris
Fast and Automated Peak Bagging with DIAMONDS (FAMED)
Stars of low and intermediate mass that exhibit oscillations may show tens of
detectable oscillation modes each. Oscillation modes are a powerful to
constrain the internal structure and rotational dynamics of the star, hence
tool allowing one to obtain an accurate stellar age. The tens of thousands of
solar-like oscillators that have been discovered thus far are representative of
the large diversity of fundamental stellar properties and evolutionary stages
available. Because of the wide range of oscillation features that can be
recognized in such stars, it is particularly challenging to properly
characterize the oscillation modes in detail, especially in light of large
stellar samples. Overcoming this issue requires an automated approach, which
has to be fast, reliable, and flexible at the same time. In addition, this
approach should not only be capable of extracting the oscillation mode
properties of frequency, linewidth, and amplitude from stars in different
evolutionary stages, but also able to assign a correct mode identification for
each of the modes extracted. Here we present the new freely available pipeline
FAMED (Fast and AutoMated pEak bagging with DIAMONDS), which is capable of
performing an automated and detailed asteroseismic analysis in stars ranging
from the main sequence up to the core-Helium-burning phase of stellar
evolution. This, therefore, includes subgiant stars, stars evolving along the
red giant branch (RGB), and stars likely evolving toward the early asymptotic
giant branch. In this paper, we additionally show how FAMED can detect rotation
from dipolar oscillation modes in main sequence, subgiant, low-luminosity RGB,
and core-Helium-burning stars. FAMED can be downloaded from its public GitHub
repository (https://github.com/EnricoCorsaro/FAMED).Comment: 46 pages, 19 figures, 4 tables. Accepted for publication in A&
The relevance of sperm morphology in male infertility
This brief report concerns the role of human sperm morphology assessment in different fields of male infertility: basic research, genetics, assisted reproduction technologies, oxidative stress. One of the best methods in studying sperm morphology is transmission electron microscopy (TEM) that enables defining the concept of sperm pathology and classifying alterations in non-systematic and systematic. Non-systematic sperm defects affect head and tail in variable ratio, whereas the rare systematic defects are characterized by a particular anomaly that marks most sperm of an ejaculate. TEM analysis and fluorescence in situ hybridization represent outstanding methods in the study of sperm morphology and cytogenetic in patients with altered karyotype characterizing their semen quality before intracytoplasmic sperm injection. In recent years, the genetic investigations on systematic sperm defects, made extraordinary progress identifying candidate genes whose mutations induce morphological sperm anomalies. The question if sperm morphology has an impact on assisted fertilization outcome is debated. Nowadays, oxidative stress represents one of the most important causes of altered sperm morphology and function and can be analyzed from two points of view: 1) spermatozoa with cytoplasmic residue produce reactive oxygen species, 2) the pathologies with inflammatory/oxidative stress background cause morphological alterations. Finally, sperm morphology is also considered an important endpoint in in vitro experiments where toxic substances, drugs, antioxidants are tested. We think that the field of sperm morphology is far from being exhausted and needs other research. This parameter can be still considered a valuable indicator of sperm dysfunction both in basic and clinical research
Human sperm as an in vitro model to assess the efficacy of antioxidant supplements during sperm handling: a narrative review
Spermatozoa are highly differentiated cells that produce reactive oxygen species (ROS) due to aerobic metabolism. Below a certain threshold, ROS are important in signal transduction pathways and cellular physiological processes, whereas ROS overproduction damages spermatozoa. Sperm manipulation and preparation protocols during assisted reproductive procedures-for example, cryopreservation-can result in excessive ROS production, exposing these cells to oxidative damage. Thus, antioxidants are a relevant topic in sperm quality. This narrative review focuses on human spermatozoa as an in vitro model to study which antioxidants can be used to supplement media. The review comprises a brief presentation of the human sperm structure, a general overview of the main items of reduction-oxidation homeostasis and the ambivalent relationship between spermatozoa and ROS. The main body of the paper deals with studies in which human sperm have been used as an in vitro model to test antioxidant compounds, including natural extracts. The presence and the synergic effects of different antioxidant molecules could potentially lead to more effective products in vitro and, in the future, in vivo
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