1,837 research outputs found
Probing the core structure and evolution of red giants using gravity-dominated mixed modes observed with Kepler
We report for the first time a parametric fit to the pattern of the \ell = 1
mixed modes in red giants, which is a powerful tool to identify
gravity-dominated mixed modes. With these modes, which share the
characteristics of pressure and gravity modes, we are able to probe directly
the helium core and the surrounding shell where hydrogen is burning. We propose
two ways for describing the so-called mode bumping that affects the frequencies
of the mixed modes. Firstly, a phenomenological approach is used to describe
the main features of the mode bumping. Alternatively, a quasi-asymptotic
mixed-mode relation provides a powerful link between seismic observations and
the stellar interior structure. We used period \'echelle diagrams to emphasize
the detection of the gravity-dominated mixed modes. The asymptotic relation for
mixed modes is confirmed. It allows us to measure the gravity-mode period
spacings in more than two hundred red giant stars. The identification of the
gravity-dominated mixed modes allows us to complete the identification of all
major peaks in a red giant oscillation spectrum, with significant consequences
for the true identification of \ell = 3 modes, of \ell = 2 mixed modes, for the
mode widths and amplitudes, and for the \ell = 1 rotational splittings. The
accurate measurement of the gravity-mode period spacing provides an effective
probe of the inner, g-mode cavity. The derived value of the coupling
coefficient between the cavities is different for red giant branch and clump
stars. This provides a probe of the hydrogen-shell burning region that
surrounds the helium core. Core contraction as red giants ascend the red giant
branch can be explored using the variation of the gravity-mode spacing as a
function of the mean large separation.Comment: Accepted in A&
Seismic evidence for a weak radial differential rotation in intermediate-mass core helium burning stars
The detection of mixed modes that are split by rotation in Kepler red giants
has made it possible to probe the internal rotation profiles of these stars,
which brings new constraints on the transport of angular momentum in stars.
Mosser et al. (2012) have measured the rotation rates in the central regions of
intermediate-mass core helium burning stars (secondary clump stars). Our aim
was to exploit& the rotational splittings of mixed modes to estimate the amount
of radial differential rotation in the interior of secondary clump stars using
Kepler data, in order to place constraints on angular momentum transport in
intermediate-mass stars. We selected a subsample of Kepler secondary clump
stars with mixed modes that are clearly rotationally split. By applying a
thorough statistical analysis, we showed that the splittings of both
gravity-dominated modes (trapped in central regions) and p-dominated modes
(trapped in the envelope) can be measured. We then used these splittings to
estimate the amount of differential rotation by using inversion techniques and
by applying a simplified approach based on asymptotic theory (Goupil et al.
2013). We obtained evidence for a weak radial differential rotation for six of
the seven targets that were selected, with the central regions rotating
to times faster than the envelope. The last target was
found to be consistent with a solid-body rotation. This demonstrates that an
efficient redistribution of angular momentum occurs after the end of the main
sequence in the interior of intermediate-mass stars, either during the
short-lived subgiant phase, or once He-burning has started in the core. In
either case, this should bring constraints on the angular momentum transport
mechanisms that are at work.Comment: 16 pages, 8 figures, accepted in A&
Seismic diagnostics for transport of angular momentum in stars 2. Interpreting observed rotational splittings of slowly-rotating red giant stars
Asteroseismology with the space-borne missions CoRoT and Kepler provides a
powerful mean of testing the modeling of transport processes in stars.
Rotational splittings are currently measured for a large number of red giant
stars and can provide stringent constraints on the rotation profiles. The aim
of this paper is to obtain a theoretical framework for understanding the
properties of the observed rotational splittings of red giant stars with slowly
rotating cores. This allows us to establish appropriate seismic diagnostics for
rotation of these evolved stars. Rotational splittings for stochastically
excited dipolar modes are computed adopting a first-order perturbative approach
for two benchmark models assuming slowly rotating cores. For red
giant stars with slowly rotating cores, we show that the variation of the
rotational splittings of modes with frequency depends only on the
large frequency separation, the g-mode period spacing, and the ratio of the
average envelope to core rotation rates (). This leds us to propose a
way to infer directly from the observations. This method is
validated using the Kepler red giant star KIC 5356201. Finally, we provide a
theoretical support for the use of a Lorentzian profile to measure the observed
splittings for red giant stars.Comment: 15 pages, 15 figures, accepted for publication in A&
Magnetization dynamics in the single-molecule magnet Fe8 under pulsed microwave irradiation
We present measurements on the single molecule magnet Fe8 in the presence of
pulsed microwave radiation at 118 GHz. The spin dynamics is studied via time
resolved magnetization experiments using a Hall probe magnetometer. We
investigate the relaxation behavior of magnetization after the microwave pulse.
The analysis of the experimental data is performed in terms of different
contributions to the magnetization after-pulse relaxation. We find that the
phonon bottleneck with a characteristic relaxation time of 10 to 100 ms
strongly affects the magnetization dynamics. In addition, the spatial effect of
spin diffusion is evidenced by using samples of different sizes and different
ways of the sample's irradiation with microwaves.Comment: 14 pages, 12 figure
Neurological disorders in rural Africa - a systematic approach
Leishmaniasis in Northern and Western Africa: A Review Background/Objectives Empirical knowledge suggests that neurological disorders are common in sub-Saharan Africa. However, to date prevalence studies are scarce. The aims of our study were to assess the hospital-based prevalence of neurological disorders in a rural African setting and to describe the pattern of disease by using a systematic approach. Methods The study was conducted at the Haydom Lutheran Hospital in northern Tanzania, Manyara region. Over a period of eight months all patients admitted to hospital were seen prospectively in consecutive order by aneurologist (ASW). Results Out of 8676 admissions 740 patients (8.5%) were given a neurological diagnosis. The most frequent neurological disorders were seizures (26.6%) and infectious diseases (18.1%). The overall mortality of neurological disease was 21%. Cases were grouped according to diagnostic certainty. We suggest three major categories for neurological disorders (group 1 = no diagnostic uncertainties; group 2 = minor diagnostic uncertainties; group 3 = major diagnostic uncertainties) with implications regarding therapy and prognosis. Conclusions The above data emphasizes that neurological disease contributes substantially to morbidity and mortality in a rural African hospital. Based on the observed pattern of neurological disorders we suggest a systematic approac
Evidence for a sharp structure variation inside a red-giant star
The availability of precisely determined frequencies of radial and non-radial
oscillation modes in red giants is finally paving the way for detailed studies
of the internal structure of these stars. We look for the seismic signature of
regions of sharp structure variation in the internal structure of the CoRoT
target HR7349. We analyse the frequency dependence of the large frequency
separation and second frequency differences, as well as the behaviour of the
large frequency separation obtained with the envelope auto-correlation
function. We find evidence for a periodic component in the oscillation
frequencies, i.e. the seismic signature of a sharp structure variation in
HR7349. In a comparison with stellar models we interpret this feature as caused
by a local depression of the sound speed that occurs in the helium
second-ionization region. Using solely seismic constraints this allows us to
estimate the mass (M=1.2^{+0.6}_{-0.4} Msun) and radius (R=12.2^{+2.1}_{-1.8}
Rsun) of HR7349, which agrees with the location of the star in an HR diagram.Comment: 4 pages, 5 figures, accepted in A&A Letter
SYMPA, a dedicated instrument for Jovian Seismology. II. Real performance and first results
Context. Due to its great mass and its rapid formation, Jupiter has played a
crucial role in shaping the Solar System. The knowledge of its internal
structure would strongly constrain the solar system formation mechanism.
Seismology is the most efficient way to probe directly the internal structure
of giant planets. Aims. SYMPA is the first instrument dedicated to the
observations of free oscillations of Jupiter. Principles and theoretical
performance have been presented in paper I. This second paper describes the
data processing method, the real instrumental performance and presents the
first results of a Jovian observation run, lead in 2005 at Teide Observatory.
Methods. SYMPA is a Fourier transform spectrometer which works at fixed optical
path difference. It produces Doppler shift maps of the observed object.
Velocity amplitude of Jupiter's oscillations is expected below 60 cm/s. Results
Despite light technical defects, the instrument demonstrated to work correctly,
being limited only by photon noise, after a careful analysis. A noise level of
about 12 cm/s has been reached on a 10-night observation run, with 21 % duty
cycle, which is 5 time better than previous similar observations. However, no
signal from Jupiter is clearly highlighted.Comment: 13 pages, 26 figure
HD 46375: seismic and spectropolarimetric analysis of a young Sun hosting a Saturn-like planet
HD 46375 is known to host a Saturn-like exoplanet orbiting at 0.04 AU from
its host star. Stellar light reflected by the planet was tentatively identified
in the 34-day CoRoT run acquired in October-November 2008. We constrain the
properties of the magnetic field of HD 46375 based on spectropolarimetric
observations with the NARVAL spectrograph at the Pic du Midi observatory. In
addition, we use a high-resolution NARVAL flux spectrum to contrain the
atmospheric parameters. With these constraints, we perform an asteroseismic
analysis and modelling of HD 46375 using the frequencies extracted from the
CoRoT light curve. We used Zeeman Doppler imaging to reconstruct the magnetic
map of the stellar surface. In the spectroscopic analysis we fitted isolated
lines using 1D LTE atmosphere models. This analysis was used to constrain the
effective temperature, surface gravity, and chemical composition of the star.
To extract information about the p-mode oscillations, we used a technique based
on the envelope autocorrelation function (EACF). From the Zeeman Doppler
imaging observations, we observe a magnetic field of ~5 gauss. From the
spectral analysis, HD 46375 is inferred to be an unevolved K0 type star with
high metallicity [Fe/H]=+0.39. Owing to the relative faintness of the star
(m_hip=8.05), the signal-to-noise ratio is too low to identify individual
modes. However, we measure the p-mode excess power and large separation Delta
nu_0=153.0 +/- 0.7 muHz. We are able do constrain the fundamental parameters of
the star thanks to spectrometric and seismic analyses. We conclude that HD
46375 is similar to a young version of Alpha-CenB. This work is of special
interest because of its combination of exoplanetary science and
asteroseismology, which are the subjects of the current Kepler mission and the
proposed PLATO mission.Comment: Accepted in Astronomy & Astrophysics. 8 pages, 9 figure
The CoRoT target HD175726: an active star with weak solar-like oscillations
Context. The CoRoT short runs give us the opportunity to observe a large
variety of late-type stars through their solar-like oscillations. We report
observations of the star HD175726 that lasted for 27 days during the first
short run of the mission. The time series reveals a high-activity signal and
the power spectrum presents an excess due to solar-like oscillations with a low
signal-to-noise ratio. Aims. Our aim is to identify the most efficient tools to
extract as much information as possible from the power density spectrum.
Methods. The most productive method appears to be the autocorrelation of the
time series, calculated as the spectrum of the filtered spectrum. This method
is efficient, very rapid computationally, and will be useful for the analysis
of other targets, observed with CoRoT or with forthcoming missions such as
Kepler and Plato. Results. The mean large separation has been measured to be
97.2+-0.5 microHz, slightly below the expected value determined from solar
scaling laws.We also show strong evidence for variation of the large separation
with frequency. The bolometric mode amplitude is only 1.7+-0.25 ppm for radial
modes, which is 1.7 times less than expected. Due to the low signal-to-noise
ratio, mode identification is not possible for the available data set of
HD175726. Conclusions. This study shows the possibility of extracting a seismic
signal despite a signal-to-noise ratio of only 0.37. The observation of such a
target shows the efficiency of the CoRoT data, and the potential benefit of
longer observing runs.Comment: 8 pages. Accepted in A&
Seismic and spectroscopic characterization of the solar-like pulsating CoRoT target HD 49385
The star HD 49385 is the first G-type solar-like pulsator observed in the
seismology field of the space telescope CoRoT. The satellite collected 137 days
of high-precision photometric data on this star, confirming that it presents
solar-like oscillations. HD 49385 was also observed in spectroscopy with the
NARVAL spectrograph in January 2009. Our goal is to characterize HD 49385 using
both spectroscopic and seismic data. The fundamental stellar parameters of HD
49385 are derived with the semi-automatic software VWA, and the projected
rotational velocity is estimated by fitting synthetic profiles to isolated
lines in the observed spectrum. A maximum likelihood estimation is used to
determine the parameters of the observed p modes. We perform a global fit, in
which modes are fitted simultaneously over nine radial orders, with degrees
ranging from l=0 to l=3 (36 individual modes). Precise estimates of the
atmospheric parameters (Teff, [M/H], log g) and of the vsini of HD 49385 are
obtained. The seismic analysis of the star leads to a clear identification of
the modes for degrees l=0,1,2. Around the maximum of the signal (nu=1013
microHz), some peaks are found significant and compatible with the expected
characteristics of l=3 modes. Our fit yields robust estimates of the
frequencies, linewidths and amplitudes of the modes. We find amplitudes of
about 5.6 +/- 0.8 ppm for radial modes at the maximum of the signal. The
lifetimes of the modes range from one day (at high frequency) to a bit more
than two days (at low frequency). Significant peaks are found outside the
identified ridges and are fitted. They are attributed to mixed modes.Comment: 13 pages, 14 figures, accepted in A&
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