1,709 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&
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
Experimental investigation on crack propagation paths in spur gears
Spur gears subjected to bending fatigue may nucleate cracks at the tooth root fillet.
In thin rim gears these cracks may propagate in a safe way (through the tooth) or in catastrophic
way (through the rim). Crack propagation direction is mainly influenced by both wheel geometry
parameters and crack initiation point, as already pointed out by theoretical and numerical results
available in literature. Aim of this work is to set up an experimental activity in order to verify
the onset of the bending crack and its propagation path in spur gears with different geometries.
In particular, a special device connected to a standard fatigue machine was realized to perform
bending tests for both standard and thin rim gears. During bending tests, an IR thermocamera
was utilized to monitor the surface thermal profile in the tooth root fillet zone
Characterizing high energy explosive eruptions at Stromboli volcano using multidisciplinary data: An example from the 9 January 2005 explosion
Stromboli is well known for its persistent, normal explosive activity, consisting of intermittent, mild to
moderate, Strombolian explosions that typically occur every 10–20 min. All tephras erupted during this
activity usually fall back into the crater terrace, and consist of volatile-poor scoriae fed by Highly Porphyritic
(HP) magma. More occasionally, large explosions or “paroxysms” eject a greater quantity of tephra, mainly
consisting of HP scoriae and pumice clasts of Low Porphyritic (LP) magma, but also including large lithic
blocks. In addition to this activity, between 2004 and 2006 high energy explosions, displaying an
intermediate eruptive style between that of normal and paroxysmal explosions in terms of column height,
duration and tephra dispersal, were observed to occur at a frequency of one to eight events per year. While
many volcanological, geochemical and geophysical studies have focused in the last few years on the two endmembers
of activity, i.e. normal or paroxysmal, a detailed investigation on these intermediate types of events
has not been carried out yet. Here we report of a study on the 9 January 2005 explosion, one of the high
energy explosions during which the main fountaining phase lasted nearly a minute causing ejection of coarse
bombs up to a height of 120 m, and of ash and lapilli to N200 m. An accompanying ash plume rose up to
500 m at the end of the explosion. We present a multidisciplinary approach that integrates the results from
analysis of live-camera images with compositional and textural characterization of the erupted products.
Major element composition of glassy groundmass and 3D views of textures in the erupted scoriae support
the hypothesis based on volcanological observations that this explosion falls between normal and
paroxysmal activity, for which we use the term “intermediate”. By comparing the video-camera images of
the 9 January 2005 explosion with volcanological features of other high energy explosions that occurred at
Stromboli between June 2004 and October 2006, we find that three additional events can be considered
intermediate explosions, suggesting that this type of activity may be fairly common on this volcano. The
results of this study, although preliminary given our limited dataset, clearly indicate that the methodology
used here can be successfully applied to better define the range of eruptive styles typifying the normal
explosive activity, potentially improving our capability of eruption forecasting and assessing volcanic hazard
at Stromboli
Spin alignment of stars in old open clusters
Stellar clusters form by gravitational collapse of turbulent molecular
clouds, with up to several thousand stars per cluster. They are thought to be
the birthplace of most stars and therefore play an important role in our
understanding of star formation, a fundamental problem in astrophysics. The
initial conditions of the molecular cloud establish its dynamical history until
the stellar cluster is born. However, the evolution of the cloud's angular
momentum during cluster formation is not well understood. Current observations
have suggested that turbulence scrambles the angular momentum of the
cluster-forming cloud, preventing spin alignment amongst stars within a
cluster. Here we use asteroseismology to measure the inclination angles of spin
axes in 48 stars from the two old open clusters NGC~6791 and NGC~6819. The
stars within each cluster show strong alignment. Three-dimensional
hydrodynamical simulations of proto-cluster formation show that at least 50 %
of the initial proto-cluster kinetic energy has to be rotational in order to
obtain strong stellar-spin alignment within a cluster. Our result indicates
that the global angular momentum of the cluster-forming clouds was efficiently
transferred to each star and that its imprint has survived after several
gigayears since the clusters formed.Comment: 14 pages, 3 figures, 1 table. Published in Nature Astronom
Testing the asymptotic relation for period spacings from mixed modes of red giants observed with the Kepler mission
Dipole mixed pulsation modes of consecutive radial order have been detected
for thousands of low-mass red-giant stars with the NASA space telescope Kepler.
Such modes have the potential to reveal information on the physics of the deep
stellar interior. Different methods have been proposed to derive an observed
value for the gravity-mode period spacing, the most prominent one relying on a
relation derived from asymptotic pulsation theory applied to the gravity-mode
character of the mixed modes. Our aim is to compare results based on this
asymptotic relation with those derived from an empirical approach for three
pulsating red-giant stars. We developed a data-driven method to perform
frequency extraction and mode identification. Next, we used the identified
dipole mixed modes to determine the gravity-mode period spacing by means of an
empirical method and by means of the asymptotic relation. In our methodology,
we consider the phase offset, , of the asymptotic
relation as a free parameter. Using the frequencies of the identified dipole
mixed modes for each star in the sample, we derived a value for the
gravity-mode period spacing using the two different methods. These differ by
less than 5%. The average precision we achieved for the period spacing derived
from the asymptotic relation is better than 1%, while that of our data-driven
approach is 3%. Good agreement is found between values for the period spacing
derived from the asymptotic relation and from the empirical method.
Full abstract in PDF file.Comment: 14 pages, 13 figures, accepted for publication in A&
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