66 research outputs found
Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars
Red giants are evolved stars that have exhausted the supply of hydrogen in
their cores and instead burn hydrogen in a surrounding shell. Once a red giant
is sufficiently evolved, the helium in the core also undergoes fusion.
Outstanding issues in our understanding of red giants include uncertainties in
the amount of mass lost at the surface before helium ignition and the amount of
internal mixing from rotation and other processes. Progress is hampered by our
inability to distinguish between red giants burning helium in the core and
those still only burning hydrogen in a shell. Asteroseismology offers a way
forward, being a powerful tool for probing the internal structures of stars
using their natural oscillation frequencies. Here we report observations of
gravity-mode period spacings in red giants that permit a distinction between
evolutionary stages to be made. We use high-precision photometry obtained with
the Kepler spacecraft over more than a year to measure oscillations in several
hundred red giants. We find many stars whose dipole modes show sequences with
approximately regular period spacings. These stars fall into two clear groups,
allowing us to distinguish unambiguously between hydrogen-shell-burning stars
(period spacing mostly about 50 seconds) and those that are also burning helium
(period spacing about 100 to 300 seconds).Comment: to appear as a Letter to Natur
A multi-site campaign to measure solar-like oscillations in Procyon. II. Mode frequencies
We have analyzed data from a multi-site campaign to observe oscillations in
the F5 star Procyon. The data consist of high-precision velocities that we
obtained over more than three weeks with eleven telescopes. A new method for
adjusting the data weights allows us to suppress the sidelobes in the power
spectrum. Stacking the power spectrum in a so-called echelle diagram reveals
two clear ridges that we identify with even and odd values of the angular
degree (l=0 and 2, and l=1 and 3, respectively). We interpret a strong, narrow
peak at 446 muHz that lies close to the l=1 ridge as a mode with mixed
character. We show that the frequencies of the ridge centroids and their
separations are useful diagnostics for asteroseismology. In particular,
variations in the large separation appear to indicate a glitch in the
sound-speed profile at an acoustic depth of about 1000 s. We list frequencies
for 55 modes extracted from the data spanning 20 radial orders, a range
comparable to the best solar data, which will provide valuable constraints for
theoretical models. A preliminary comparison with published models shows that
the offset between observed and calculated frequencies for the radial modes is
very different for Procyon than for the Sun and other cool stars. We find the
mean lifetime of the modes in Procyon to be 1.29 +0.55/-0.49 days, which is
significantly shorter than the 2-4 days seen in the Sun.Comment: accepted for publication in Ap
Fast core rotation in red-giant stars revealed by gravity-dominated mixed modes
When the core hydrogen is exhausted during stellar evolution, the central
region of a star contracts and the outer envelope expands and cools, giving
rise to a red giant, in which convection occupies a large fraction of the star.
Conservation of angular momentum requires that the cores of these stars rotate
faster than their envelopes, and indirect evidence supports this. Information
about the angular momentum distribution is inaccessible to direct observations,
but it can be extracted from the effect of rotation on oscillation modes that
probe the stellar interior. Here, we report the detection of non-rigid rotation
in the interiors of red-giant stars by exploiting the rotational frequency
splitting of recently detected mixed modes. We demonstrate an increasing
rotation rate from the surface of the star to the stellar core. Comparing with
theoretical stellar models, we conclude that the core must rotate at least ten
times faster than the surface. This observational result confirms the
theoretical prediction of a steep gradient in the rotation profile towards the
deep stellar interior.Comment: to appear as a Letter to Natur
Daily rhythms of the sleep-wake cycle
The amount and timing of sleep and sleep architecture (sleep stages) are determined by several factors, important among which are the environment, circadian rhythms and time awake. Separating the roles played by these factors requires specific protocols, including the constant routine and altered sleep-wake schedules. Results from such protocols have led to the discovery of the factors that determine the amounts and distribution of slow wave and rapid eye movement sleep as well as to the development of models to determine the amount and timing of sleep. One successful model postulates two processes. The first is process S, which is due to sleep pressure (and increases with time awake) and is attributed to a 'sleep homeostat'. Process S reverses during slow wave sleep (when it is called process S'). The second is process C, which shows a daily rhythm that is parallel to the rhythm of core temperature. Processes S and C combine approximately additively to determine the times of sleep onset and waking. The model has proved useful in describing normal sleep in adults. Current work aims to identify the detailed nature of processes S and C. The model can also be applied to circumstances when the sleep-wake cycle is different from the norm in some way. These circumstances include: those who are poor sleepers or short sleepers; the role an individual's chronotype (a measure of how the timing of the individual's preferred sleep-wake cycle compares with the average for a population); and changes in the sleep-wake cycle with age, particularly in adolescence and aging, since individuals tend to prefer to go to sleep later during adolescence and earlier in old age. In all circumstances, the evidence that sleep times and architecture are altered and the possible causes of these changes (including altered S, S' and C processes) are examined
COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study
Background:
The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms.
Methods:
International, prospective observational study of 60â109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms.
Results:
âTypicalâ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (â€â18 years: 69, 48, 23; 85%), older adults (â„â70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each Pâ<â0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country.
Interpretation:
This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men
Cross-modality attentional modulation of pain and audition
This study evaluated the effect of attention on pain- and auditory-evoked cortical activity in humans using positron emission tomography (PET). Regional cerebral blood flow (rCBF) was measured while subjects detected changes in thermal intensity or auditory frequency. Subjects rated pain on a visual analogue scale. Using statistical brain maps of the pain-related activity, directed searches were conducted in contralateral insular (IC), anterior cingulate (ACC), primary (S1) and secondary (S2) somatosensory cortices---regions consistently found to be activated by painful stimuli.Pain intensity was rated higher in the thermal than in the auditory task. Likewise, whereas in the thermal task there were significant pain-related rCBF increases in S1 and S2, none of these regions had significant pain-evoked rCBF increases when subjects performed the auditory task. Only rCBF in S1 was significantly correlated with the pain intensity ratings generated during the thermal and auditory tasks. These results suggest that changes in S1 cortical activity may be involved in attentional modulation of pain
The Potential of Mean Force concept for bridging (length and time) scales in the modeling of complex porous materials
We introduce the concept of Potential of Mean Force, PMF, as a way to implement upscaling modeling from the nano-scale to micron-scale. A PMF is a free energy function representing in an effective way the interactions between objects (cement hydrates, clay platelets, etc.) at thermodynamics conditions. The PMF is therefore the key piece of information allowing to coarse-grained Physical-Chemistry information in a meso-scale model formulation. The use of PMF offers a huge computational advantage as it allows a straight up-scaling to the meso-scale while keeping essential interactions information that are the hallmark of Physical-Chemistry processes. Such a coarse-grained modeling integrates atomistic response into inter-particle potentials that fully propagate molecular scale information all the way to the meso-scale
The Potential of Mean Force concept for bridging (length and time) scales in the modeling of complex porous materials
We introduce the concept of Potential of Mean Force, PMF, as a way to implement upscaling modeling from the nano-scale to micron-scale. A PMF is a free energy function representing in an effective way the interactions between objects (cement hydrates, clay platelets, etc.) at thermodynamics conditions. The PMF is therefore the key piece of information allowing to coarse-grained Physical-Chemistry information in a meso-scale model formulation. The use of PMF offers a huge computational advantage as it allows a straight up-scaling to the meso-scale while keeping essential interactions information that are the hallmark of Physical-Chemistry processes. Such a coarse-grained modeling integrates atomistic response into inter-particle potentials that fully propagate molecular scale information all the way to the meso-scale
T1725 BMP Signaling is Essential for the Organogenesis, Glandular Architecture and Specification of Gastric Epithelium in the Mouse
Feasibility of a multigroup Boltzmann-Fokker-Planck solution for electron beam dose calculations
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