Solar and Stellar Astrophysics (astro-ph.SR)

Abstract: Gravity waves are generated by turbulent subsurface convection overshooting or penetrating locally into a stably stratified medium. While propagating energy upwards, their characteristic negative phase shift over height is a well-recognized observational signature. Since their first detailed observational detection and estimates of energy content, a number of studies have explored their propagation characteristics and interaction with magnetic fields and other wave modes in the solar atmosphere. Here, we present a study of the atmospheric gravity wave dispersion diagrams utilizing intensity observations that cover photospheric to chromospheric heights over different magnetic configurations of quiet-Sun (magnetic network regions), a plage, and a sunspot as well as velocity observations within the photospheric layer over a quiet and a sunspot region. In order to investigate the propagation characteristics, we construct two-height intensity - intensity and velocity-velocity cross-spectra and study phase and coherence signals in the wavenumber-frequency dispersion diagrams and their association with background magnetic fields. We find signatures of association between magnetic fields and much reduced coherence and phase shifts over height from intensity-intensity and velocity-velocity phase and coherence diagrams, both indicating suppression/scattering of gravity waves by the magnetic fields. Our results are consistent with the earlier numerical simulations, which indicate that gravity waves are suppressed or scattered and reflected back into the lower solar atmosphere in the presence of magnetic fields.

Abstract: In this article we review small-scale dynamo processes that are responsible for magnetic field generation on scales comparable to and smaller than the energy carrying scales of turbulence. We provide a review of critical observation of quiet Sun magnetism, which have provided strong support for the operation of a small-scale dynamo in the solar photosphere and convection zone. After a review of basic concepts we focus on numerical studies of kinematic growth and non-linear saturation in idealized setups, with special emphasis on the role of the magnetic Prandtl number for dynamo onset and saturation. Moving towards astrophysical applications we review convective dynamo setups that focus on the deep convection zone and the photospheres of solar-like stars. We review the critical ingredients for stellar convection setups and discuss their application to the Sun and solar-like stars including comparison against available observations.

Abstract: The Gaia colour-magnitude diagram reveals a striking separation between hydrogen-atmosphere white dwarfs and their helium-atmosphere counterparts throughout a significant portion of the white dwarf cooling track. However, pure-helium atmospheres have Gaia magnitudes that are too close to the pure-hydrogen case to explain this bifurcation. To reproduce the observed split in the cooling sequence, it has been shown that trace amounts of hydrogen and/or metals must be present in the helium-dominated atmospheres of hydrogen-deficient white dwarfs. Yet, a complete explanation of the Gaia bifurcation that takes into account known constraints on the spectral evolution of white dwarfs has thus far not been proposed. In this work, we attempt to provide such a holistic explanation by performing population synthesis simulations coupled with state-of-the-art model atmospheres and evolutionary calculations that account for element transport in the envelopes of white dwarfs. By relying on empirically grounded assumptions, these simulations successfully reproduce the bifurcation. We show that the convective dredge-up of optically invisible traces of carbon from the deep interior is crucial to account for the observations. Neither the convective dilution/mixing of residual hydrogen nor the accretion of hydrogen or metals can be the dominant drivers of the bifurcation. Finally, we emphasize the importance of improving the equation of state of partially ionized carbon in warm dense helium, a key input for our predictions of the amount of dredged-up carbon.