120 research outputs found
Conversion of internal gravity waves into magnetic waves
Asteroseismology probes the interiors of stars by studying oscillation modes at a star's surface. Although pulsation spectra are well understood for solar-like oscillators, a substantial fraction of red giant stars observed by Kepler exhibit abnormally low-amplitude dipole oscillation modes. Fuller et al. (2015) suggest this effect is produced by strong core magnetic fields that scatter dipole internal gravity waves (IGWs) into higher multipole IGWs or magnetic waves. In this paper, we study the interaction of IGWs with a magnetic field to test this mechanism. We consider two background stellar structures: one with a uniform magnetic field, and another with a magnetic field that varies both horizontally and vertically. We derive analytic solutions to the wave propagation problem and validate them with numerical simulations. In both cases, we find perfect conversion from IGWs into magnetic waves when the IGWs propagate into a region exceeding a critical magnetic field strength. Downward propagating IGWs cannot reflect into upward propagating IGWs because their vertical wavenumber never approaches zero. Instead, they are converted into upward propagating slow (Alfvénic) waves, and we show they will likely dissipate as they propagate back into weakly magnetized regions. Therefore, strong internal magnetic fields can produce dipole mode suppression in red giants, and gravity modes will likely be totally absent from the pulsation spectra of sufficiently magnetized stars
Conversion of internal gravity waves into magnetic waves
Asteroseismology probes the interiors of stars by studying oscillation modes at a star's surface. Although pulsation spectra are well understood for solar-like oscillators, a substantial fraction of red giant stars observed by Kepler exhibit abnormally low-amplitude dipole oscillation modes. Fuller et al. (2015) suggest this effect is produced by strong core magnetic fields that scatter dipole internal gravity waves (IGWs) into higher multipole IGWs or magnetic waves. In this paper, we study the interaction of IGWs with a magnetic field to test this mechanism. We consider two background stellar structures: one with a uniform magnetic field, and another with a magnetic field that varies both horizontally and vertically. We derive analytic solutions to the wave propagation problem and validate them with numerical simulations. In both cases, we find perfect conversion from IGWs into magnetic waves when the IGWs propagate into a region exceeding a critical magnetic field strength. Downward propagating IGWs cannot reflect into upward propagating IGWs because their vertical wavenumber never approaches zero. Instead, they are converted into upward propagating slow (Alfvénic) waves, and we show they will likely dissipate as they propagate back into weakly magnetized regions. Therefore, strong internal magnetic fields can produce dipole mode suppression in red giants, and gravity modes will likely be totally absent from the pulsation spectra of sufficiently magnetized stars
Tidally Heated Terrestrial Exoplanets: Viscoelastic Response Models
Tidal friction in exoplanet systems, driven by orbits that allow for durable
nonzero eccentricities at short heliocentric periods, can generate internal
heating far in excess of the conditions observed in our own solar system.
Secular perturbations or a notional 2:1 resonance between a Hot Earth and Hot
Jupiter can be used as a baseline to consider the thermal evolution of
convecting bodies subject to strong viscoelastic tidal heating. We compare
results first from simple models using a fixed Quality factor and Love number,
and then for three different viscoelastic rheologies: the Maxwell body, the
Standard Anelastic Solid, and the Burgers body. The SAS and Burgers models are
shown to alter the potential for extreme tidal heating by introducing the
possibility of new equilibria and multiple response peaks. We find that tidal
heating tends to exceed radionuclide heating at periods below 10-30 days, and
exceed insolation only below 1-2 days. Extreme cases produce enough tidal heat
to initiate global-scale partial melting, and an analysis of tidal limiting
mechanisms such as advective cooling for earthlike planets is discussed. To
explore long term behaviors, we map equilibria points between convective heat
loss and tidal heat input as functions of eccentricity. For the periods and
magnitudes discussed, we show that tidal heating, if significant, is generally
detrimental to the width of habitable zones.Comment: 18 pages, 9 figure
The photometric variability of massive stars due to gravity waves excited by core convection
Massive stars die in catastrophic explosions, which seed the interstellar
medium with heavy elements and produce neutron stars and black holes.
Predictions of the explosion's character and the remnant mass depend on models
of the star's evolutionary history. Models of massive star interiors can be
empirically constrained by asteroseismic observations of gravity wave
oscillations. Recent photometric observations reveal a ubiquitous red noise
signal on massive main sequence stars; a hypothesized source of this noise is
gravity waves driven by core convection. We present the first 3D simulations of
massive star convection extending from the star's center to near its surface,
with realistic stellar luminosities. Using these simulations, we make the first
prediction of photometric variability due to convectively-driven gravity waves
at the surfaces of massive stars, and find that gravity waves produce
photometric variability of a lower amplitude and lower characteristic frequency
than the observed red noise. We infer that the photometric signal of gravity
waves excited by core convection is below the noise limit of current
observations, so the red noise must be generated by an alternative process.Comment: As accepted for publication in Nature Astronomy except for final
editorial revisions. Supplemental materials available online at
https://doi.org/10.5281/zenodo.7764997 . We have also sonified our results to
make them more accessible, see
https://github.com/evanhanders/gmode_variability_paper/blob/main/sound/gmode_sonification.pd
The impact of a fossil magnetic field on dipolar mixed-mode frequencies in sub- and red-giant stars
Stars more massive than M are known to develop a
convective core during the main-sequence: the dynamo process triggered by this
convection could be the origin of a strong magnetic field inside the core of
the star, trapped when it becomes stably stratified and for the rest of its
evolution. The presence of highly magnetized white dwarfs strengthens the
hypothesis of buried fossil magnetic fields inside the core of evolved low-mass
stars. If such a fossil field exists, it should affect the mixed modes of red
giants as they are sensitive to processes affecting the deepest layers of these
stars. The impact of a magnetic field on dipolar oscillations modes was one of
Pr. Michael J. Thompson's research topics during the 90s when preparing the
helioseismic SoHO space mission. As the detection of gravity modes in the Sun
is still controversial, the investigation of the solar oscillation modes did
not provide any hint of the existence of a magnetic field in the solar
radiative core. Today we have access to the core of evolved stars thanks to the
asteroseismic observation of mixed modes from CoRoT, Kepler, K2 and TESS
missions. The idea of applying and generalizing the work done for the Sun came
from discussions with Pr. Michael Thompson in early 2018 before we loss him.
Following the path we drew together, we theoretically investigate the effect of
a stable axisymmetric mixed poloidal and toroidal magnetic field, aligned with
the rotation axis of the star, on the mixed modes frequencies of a typical
evolved low-mass star. This enables us to estimate the magnetic perturbations
to the eigenfrequencies of mixed dipolar modes, depending on the magnetic field
strength and the evolutionary state of the star. We conclude that strong
magnetic fields of 1MG should perturbe the mixed-mode frequency pattern
enough for its effects to be detectable inside current asteroseismic data.Comment: Conference proceeding, in press, 7 pages, 3 figure
Molecular Blocking of CD23 Supports Its Role in the Pathogenesis of Arthritis
BACKGROUND: CD23 is a differentiation/activation antigen expressed by a variety of hematopoietic and epithelial cells. It can also be detected in soluble forms in biological fluids. Initially known as the low-affinity receptor for immunoglobulin E (Fc epsilonRII), CD23 displays various other physiologic ligands such as CD21, CD11b/c, CD47-vitronectin, and mannose-containing proteins. CD23 mediates numerous immune responses by enhancing IgE-specific antigen presentation, regulating IgE synthesis, influencing cell differentiation and growth of both B- and T-cells. CD23-crosslinking promotes the secretion of pro-inflammatory mediators from human monocytes/macrophages, eosinophils and epithelial cells. Increased CD23 expression is found in patients during allergic reactions and rheumatoid arthritis while its physiopathologic role in these diseases remains to be clarified. METHODOLOGY/PRINCIPAL FINDINGS: We previously generated heptapeptidic countrestructures of human CD23. Based on in vitro studies on healthy and arthritic patients' cells, we showed that CD23-specific peptide addition to human macrophages greatly diminished the transcription of genes encoding inflammatory cytokines. This was also confirmed by significant reduction of mediator levels in cell supernatants. We also show that CD23 peptide decreased IgE-mediated activation of both human and rat CD23(+) macrophages. In vivo studies in rat model of arthritis showed that CD23-blocking peptide ameliorates clinical scores and prevent bone destruction in a dose dependent manner. Ex-vivo analysis of rat macrophages further confirmed the inhibitory effect of peptides on their activation. Taken together our results support the role of CD23 activation and subsequent inflammatory response in arthritis. CONCLUSION: CD23-blocking peptide (p30A) prevents the activation of monocytes/macrophages without cell toxicity. Thus, targeting CD23 by antagonistic peptide decreases inflammatory markers and may have clinical value in the treatment of human arthritis and allergic reactions involving CD23
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