121 research outputs found

    Stellar Convective Penetration: Parameterized Theory and Dynamical Simulations

    Get PDF
    Most stars host convection zones in which heat is transported directly by fluid motion, but the behavior of convective boundaries is not well-understood. Here, we present 3D numerical simulations that exhibit penetration zones: regions where the entire luminosity could be carried by radiation, but where the temperature gradient is approximately adiabatic and convection is present. To parameterize this effect, we define the "penetration parameter" , which compares how far the radiative gradient deviates from the adiabatic gradient on either side of the Schwarzschild convective boundary. Following Roxburgh and Zahn, we construct an energy-based theoretical model in which controls the extent of penetration. We test this theory using 3D numerical simulations that employ a simplified Boussinesq model of stellar convection. The convection is driven by internal heating, and we use a height-dependent radiative conductivity. This allows us to separately specify and the stiffness of the radiative&ndash;convective boundary. We find significant convective penetration in all simulations. Our simple theory describes the simulations well. Penetration zones can take thousands of overturn times to develop, so long simulations or accelerated evolutionary techniques are required. In stars, we expect , and in this regime, our results suggest that convection zones may extend beyond the Schwarzschild boundary by up to &sim;20%&ndash;30% of a mixing length. We present a MESA stellar model of the Sun that employs our parameterization of convective penetration as a proof of concept. Finally, we discuss prospects for extending these results to more realistic stellar contexts. &nbsp;</p

    The photometric variability of massive stars due to gravity waves excited by core convection

    Full text link
    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

    Full text link
    Stars more massive than ∌1.3\sim 1.3 M⊙_\odot 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 ∌\sim 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

    Get PDF
    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

    Experience Feedback Committee: a management tool to improve patient safety in mental health

    Full text link
    BACKGROUND: A management tool, called the Experience Feedback Committee, has been applied for patient safety and successfully used in medical departments. The purpose of this study was to analyse the functioning of an Experience Feedback Committee in a psychiatric department and to explore its contribution to the particular issues of patient safety in mental health. METHODS: We conducted a descriptive study based on all the written documents produced by the Experience Feedback Committee between March 2010 and January 2013. The study was conducted in Grenoble University Hospital in France. We analysed all reported incidents, reports of meetings and event analysis reports. Adverse events were classified according to the Conceptual Framework for the International Classification for Patient Safety. RESULTS: A total of 30 meetings were attended by 22 professionals including seven physicians and 12 paramedical practitioners. We identified 475 incidents reported to the Experience Feedback Committee. Most of them (92 %) had no medical consequence for the patient. Eleven incidents were investigated with an analysis method inspired by civil aviation security systems. Twenty-one corrective actions were set up, including eight responses to the specific problems of a mental health unit, such as training to respond to situations of violence or management of suicide attempts. CONCLUSIONS: The Experience Feedback Committee makes it possible to involve mental healthcare professionals directly in safety management. This tool seems appropriate to manage specific patient safety issues in mental health. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12991-015-0062-2) contains supplementary material, which is available to authorized users
    • 

    corecore