Colloquia

Turbulent convection in stellar envelopes is critical to heat transport and dynamo activity. Modeling it well has proven surprisingly difficult, and recent solar and stellar observations have raised questions about our understanding of the dynamics of both the deep solar convection and the mean structure of the upper layers of convective stellar envelopes.  In particular, the amplitude of low wavenumber convective motions in both local area radiative magnetohydrodynamic and global spherical shell magnetohydrodynamic simulations of the Sun appear to be too high. In global simulations this results in weaker than needed rotational constraint and consequent non solar-like differential rotation profiles. In deep local area simulations it yields strong horizontal flows in the photosphere on scales much larger than the observed supergranulation, leaving the origin of the solar supergranular scale enigmatic. The problem is not confined to the Sun. When comparing computed oscillation frequencies to observations, mixing length models of stellar convection show too sharp a transition to the interior adiabatic gradient. This contributes to what asteroseismologists call the `surface effect' correction.

 We suggest that there is a common solution to these problems: convective motions in stellar envelopes are even more nonlocal than numerical models suggest. Small scale photospherically driven motions dominate convective transport even at depth, descending through a very nearly adiabatic or possible even somewhat subadiabatic deep convection zone. Convection of this form may meet Rossby number constraints set by global scale motions, and implies that the solar supergranulation is the largest buoyantly driven scale of motion in the Sun. We test this hypothesis using a suite of three-dimensional stellar atmosphere models, and can use it to both recover their mean stratification and estimate the supergranular scale on other stars

Galactic Archeology is today a vibrant field of research. The adoption and launch of the Gaia astrometric satellite by ESA has resulted in many spectroscopic Galactic surveys that aim to complement the Gaia data with information (for the fainter Gaia stars) about stellar elemental abundances, radial velocities, and stellar parameters. This results in multi-dimensional data sets which will allow us to put the Milky Way stellar populations into a much broader galactic context, eg by comparing with models and galaxies at large look-back times. In this talk I will review a selection of recent exciting developments in Galactic Archeaology found via on-going surveys as well as look to the future and see what surveys like 4MOST and WEAVE will bring.  The proposed surveys will be put into a wider context of past, on-going and future spectroscopic surveys and how this can all be combined to understand the Milky Way as a galaxy.

This fundamental question has challenged space scientists for decades. At temperatures of several million degrees, the corona is hundreds of times hotter than the solar surface, and heat cannot simply flow upward against the temperature gradient. (The same is true on other stars.) It is widely believed that the energy responsible for the extreme temperatures is extracted from stressed magnetic fields that permeate the corona. This likely occurs in the form of small impulsive energy bursts called nanoflares, but the details of how they work are still a matter of vigorous debate. Understanding these details is crucial, since the basic mechanisms are central to many phenomena--on the Sun, within the heliosphere, and throughout the universe. I will review our current understanding of the coronal heating problem from both the observational and theoretical perspectives.

Star formation and supermassive black hole growth in galaxies appear to be self limiting. The mechanisms for self regulation are known as /feedback/. Cosmic rays, the relativistic particle component in interstellar and intergalactic plasma, are among the agents of feedback. Because cosmic rays are virtually collisionless in the plasma environments of interest, their interaction with the ambient medium is primarily mediated by large scale magnetic fields and kinetic scale plasma waves. Because kinetic scales are much smaller than global scales, this interaction is most conveniently described by fluid models. In this paper I discuss the kinetic theory and the classical theory of cosmic ray hydrodynamics (CCRH) which follows from assuming cosmic rays interact only with self excited waves. I generalize CCRH to generalized cosmic ray hydrodynamics (GCRH), which accommodates interactions with extrinsic turbulence, present examples of cosmic ray feedback in galaxies and galaxy clusters, and assess where progress is needed.

Models of the Thermally Pulsing Asymptotic Giant Branch (TP-AGB) stellar evolutionary phase play a critical role across astrophysics, from the chemical composition of meteorites in the pre-solar nebula up to galaxy evolution in the high-redshift Universe. In spite of its importance, the modelling of TP-AGB is still affected by large uncertainties which propagate into the field of extragalactic astronomy, impinging on the predicting power of current population synthesis models of galaxies in terms of their basic properties such as ages, masses and chemical enrichment. In this context I will review recent advances and ongoing efforts toward a physically-sound TP-AGB calibration that, moving beyond the classical use of the Magellanic Cloud clusters, combines increasingly refined TP-AGB stellar models with exceptionally high-quality data for resolved TP-AGB stars in nearby galaxies.

The discovery of Quantum Physics gave rise to one of the most important scientific and technological revolutions experienced by mankind. It triggered, for instance, the discovery of lasers, semiconductors, or nuclear power. In the last few years we are experiencing a second "Quantum Revolution", where the most exotic features of Quantum Physics can not only be confirmed, but also have major technological consequences. In particular, new cryptographic and computational opportunities are emerging, which will be impossible to reach with any other technology. Nowadays, there exists an extensive international effort to build quantum computers, cryptographic systems, as well as other devices. In this talk I will explain the basics of all those devices, their potential applications, as well as the status of that international effort and its prospects of giving rise to powerful technologies.

After a basic introduction into asteroseismology for the non-expert, we emphasize how to achieve practical applications of the technique based on uninterrupted high-precision data from space. We show how series of detected and identified oscillation modes allow to deduce details of the interior physics of stars that are impossible to unravel in any other way. We highlight the most recent findings on the interior rotation and chemical mixing of stars with a convective core and illustrate how these affect stellar evolution theory. We reveal the power of combining Gaia and asteroseismic data for stellar physics, galactic archeology, and exoplanet studies. Finally, we provide an outlook for future projects in asteroseismology to illustrate the bright future of this research domain.

Global snow and ice cover (the "cryosphere") plays a major role in global climate and hydrology through a range of complex interactions and feedbacks, the best known of which is the ice - albedo feedback.  Snow and ice cover undergo marked seasonal and long term changes in extent and thickness. The perennial elements - the major ice sheets and permafrost - play a role in present-day regional and local climate and hydrology, but the large seasonal variations in snow cover and sea ice are of importance on continental to hemispheric scales. The characteristics of these variations, especially in the Northern Hemisphere, and evidence for recent trends in snow and sea ice extent are discussed.

The Cosmic Microwave Background (CMB), the fossil light of the BigBang, is the oldest light that one can ever hope to observe in ourUniverse. The CMB provides us with a direct image of the Universe whenit was still an "infant" - 380,000 years old - and has enabled us to obtaina wealth of cosmological information, such as the composition, age,geometry, and history of the Universe. Yet, can we go further and learnabout the primordial universe, when it was much younger than 380,000years old, perhaps as young as a tiny fraction of a second? If so, thisgives us a hope to test competing theories about the origin of theUniverse at ultra high energies. In this talk I present the results from theWilkinson Microwave Anisotropy Probe (WMAP) satellite that Icontributed, and then discuss the recent results from the Plancksatellite (in which I am not involved). Finally, I discuss future prospectson ourquest to probe the physical condition of the very early Universe.

The realism of hydrodynamical simulations of the formation and evolution of galaxies has improved considerably in recent years. I will try to give some insight into the reasons behind this success, focusing in particular on the importance of subgrid models and the associated limitations. I will also present recent results from the cosmological EAGLE simulations as well as from higher-resolution simulations of individual galaxies.