KIS Astrophysical Colloquium 2017

The colloquium usually takes place on Thursdays at 11:30 if not stated otherwise.

Upcoming Talks:
November 30 Tobias Felipe, Instituto de Astrofisica de Canarias, La Laguna, Spain: Sunspot light bridges: 3D structure and associated events
Light bridges are bright elongated structures that usually appear in the umbrae of sunspots. They exhibit a weaker and more horizontal field strength in comparison with the surrounding umbra, whose field lines meet above the light bridge and form a canopy structure. The chromosphere above light bridges shows plenty of dynamic phenomena, and an accurate characterization of the light bridge magnetic topology is key for understanding these events. In this talk, I will show the three-dimensional magnetic and thermodynamic structure of two light bridges inferred from spectropolarimetric observations obtained with the GREGOR Infrared Spectrograph of the GREGOR telescope. The results show evidence of vigorous convection. The convective flows can drag and bend the field lines, producing reversals in the field orientation. One of the light bridges was observed during the occurrence of a C-class flare in the same active region. Using simultaneous observations with the GREGOR Fabry-Pérot Interferomenter, the High-resolution Fast Imager (both instruments attached to GREGOR), and Atmospheric Imaging Assembly, we have identified photospheric and chromospheric brightenings, heating events, and changes in the Stokes profiles associated with the flare eruption and the subsequent arrival of an ejected plasmoid to the light bridge. These features are interpreted as the result of reconnection events driven by the interaction of the plasma blob with the light bridge magnetic field.
December 7 Emre Isik, Max-Planck-Institut für Sonnensystemforschung, Göttingen (Germany) & Feza Gürsey Center for Physics and Mathematics, Bogazici University, Istanbul (Turkey): Modelling brightness variations in Sun-like stars
With the advent of high-precision space-borne stellar photometry, it has become essential to improve modelling of stellar magnetic activity in rotational time scales. In the first part of this talk, I will discuss the importance of forward modelling when testing the performance of maximum-entropy inversions of Kepler light curves. In the second part, I will present our new models for “young suns” with rotation rates between 1 and 8 times the solar rate. The model provides the latitudinal distribution of emergence latitudes and tilt angles of spot groups, the subsequent surface flux transport, and makes some predictions about the global and latitudinal coverage of starspots.
January 25, 2018 Kostas Kokkotas, Universität Tübingen:
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February 6, 2018 Manolis Georgoulis, Science Academy of Athens, Greece:
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February 1, 2018 Jürg Beer, EAWAG, Zürich, Schweiz:
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February 8, 2018 Sebastian Wolf, Universität Kiel:
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Past Talks:
February 9 Prof. Dr. Dagmar Schmauks, Technical University, Berlin: Extra-Terrestrial Life in Science Fiction
Deutsch Ein erkenntnistheoretischer Aspekt von Science Fiction ist ihr Versuch, unsere erd-, gattungs-, kultur- und epochenspezifischen Vorurteile zu erhellen, also sozusagen eine Außenansicht der Welt des homo sapiens zu bieten. Ein grundlegendes Thema hierbei ist das Zusammentreffen mit außerirdischen Lebensformen. Schlichte Autoren kombinieren nur bekannte Fakten neu, so dass die Außerirdischen irdischen Kraken ähneln oder Stielaugen besitzen. Kreativere Autoren hingegen entwerfen Lebewesen, die kein Vorbild in der irdischen Evolution haben. Einfache Lebensformen werden oft unabsichtlich oder gezielt ausgerottet (schließlich hat sich der Mensch schon öfters als „invasiver Neobiont“ erwiesen). Intelligente Außerirdische sind aus dramaturgischen Gründen oft technologisch weit überlegen, was meist zu spektakulären Kämpfen und selten zu friedlicher Koexistenz führt. Die interessantesten Visionen entwerfen Lebewesen, die eine ganz andere Biologie besitzen (etwa silizium-basiert), die keine Individuen in unserem Sinne sind (etwa Lems Gallert-Ozean Solaris) oder die gar nicht auf Planeten leben (Hoyles Schwarze Wolke). Kontaktversuche mit Außerirdischen sind ein Prüfstein jeder Kommunikationstheorie. Optimistische Texte beschreiben Telepathie oder universelle Übersetzungscomputer, andere machen interessante Vorschläge für den Aufbau gegenseitiger Verständigung. Abschließend werden mögliche Vorteile und Gefahren kosmischer Kontakte skizziert. Und vielleicht sollten wir uns erst einmal um bessere Kontakte mit unseren intelligenten Mitgeschöpfen auf der Erde bemühen? English An epistemological aspect of science fiction is its attempt to elucidate our prejudices related to earth, species, culture, and epoch, thus delivering an external view on the world of homo sapiens. One basic topic is in the contact with extraterrestrial lifeforms. Simple writers only combine known facts, so that the aliens resemble terrestrial octopuses or possess stalk eyes. More creative authors, however, design creatures without example in terrestrial evolution. Primitive extraterrestrial lifeforms are often exterminated inadvertently or purposefully (after all, humans on earth have already proved themselves to be “invasive neobiota”). Due to dramaturgical reasons, intelligent aliens frequently possess a highly superior technology, usually leading to spectacular battles and rarely to friendly coexistence. The most interesting visions describe living beings that have a completely different biology (such as silicon-based), that are not individuals in our sense (Lems gelatinous ocean Solaris) or are not bound to planets (Hoyles Black Cloud). Contact attempts with extraterrestrials are a touchstone of every communication theory. Optimistic texts describe telepathy or universal translation computers, while others make interesting suggestions for building up mutual understanding. Finally, some possible advantages and dangers of cosmic contacts are sketched. And maybe we should initially try to improve our contacts with our intelligent fellow creatures on earth?
March 2 Travis Metcalfe, Space Science Institute, Boulder, USA: A stellar context for the solar activity cycle
Precise photometry from the Kepler space telescope allows not only the measurement of rotation in solar-type field stars, but also the determination of reliable masses and ages from asteroseismology. These critical data have recently provided the first opportunity to calibrate rotation-age relations for stars older than the Sun. The evolutionary picture that emerges is surprising: beyond middle-age the efficiency of magnetic braking is dramatically reduced, implying a fundamental change in angular momentum loss beyond a critical Rossby number (Ro~2). We have compiled chromospheric activity measurements for the sample of Kepler asteroseismic targets that were used to establish the new rotation-age relations. We use these data along with a sample of well-characterized solar analogs from the Mount Wilson HK survey to develop a qualitative scenario connecting the evolution of chromospheric activity to a fundamental shift in the character of differential rotation. We conclude that the Sun may be in a transitional evolutionary phase, and that its magnetic cycle might represent a special case of stellar dynamo theory.
March 23 Ansgar Reiners, Universität Göttingen: Exoplanets and solar convection: Motivating advances in high accuracy spectroscopy
Extrasolar planet research has matured from lucky discovery mode to planet characterization and the systematic search for planetary systems. Among the major drivers today is the search for planets around low-mass stars, which include our immediate neighborhood where discoveries inspired groundbreaking ideas like sending spaceprobes to Proxima b. One of the limitations for current and next generation planet surveys is the moving stellar plasma that can introduce Doppler shifts far larger than the radial velocity signal from planets. This convective blueshift is difficult to study on other stars, but a lot can be learned from the Sun. I will give an overview about exoplanetary research and spectrograph development, and I will show how Doppler programs provide (even more) motivation for high accuracy solar spectroscopy.
April 4 Reza Rezai, Instituto de Astrofisica de Canarias: Structure of sunspot light bridges in the chromosphere and transition region
Light bridges are elongated structures with enhanced intensity embedded in sunspot umbra and pores. Using IRIS spectral data, we study properties of a sample of 60 light bridges in the chromosphere and transition region. Traces of photospheric light bridges are found in the near ultra violet Mg II h and k lines as well as in the far ultra violet emission lines of the O I, C II, Si IV and O IV. These set of spectral lines cover the entire chromosphere and lower part of the transition region. We created a sample of mean values for the continuum and line intensities, line shifts and widths. A majority of photospheric light bridges in our sample have an associated enhanced emission in the chromosphere and transition region. In most cases, morphology of light bridges in the chromosphere and transition region are similar to the photospheric light bridge such that a 1-1 correspondence can be established. We measured a systematic shift between position of the light bridges in the transition region and photosphere, which illustrates the three-dimensional structure of light bridges, where the Si IV signal originates from about 3.5 Mm above the photosphere. The line widths increases with the formation temperature in the light bridge atmosphere, similar to quiet Sun. The line profiles have distinctly different shapes in the light bridge and nearby umbra suggesting that the umbral scattered light does not play an important role. Light bridges show red shift in the chromosphere and the transition region. We find the observed co-temporal and co-spatial emission enhancements above photospheric light bridges is a systematic phenomena suggesting that light bridges extend well beyond the photospheric boundary.
April 13 Anish Amarsi, Max-Planck-Institut für Astronomie, Heidelberg: 3D non-LTE spectral line-formation in the Sun and other late-type stars
The majority of spectroscopic studies of late-type stars are based on one-dimensional (1D) hydrostatic model atmospheres and local thermodynamic equilibrium (LTE). It has recently become possible to relax these assumptions, with the development of grids of ab-initio 3D hydrodynamic model stellar atmospheres, efficient codes for 3D multi-level non-LTE radiative transfer, and realistic models for inelastic low-energy collisions. Comparisons with solar spectra, and in particular with solar centre-to-limb variations, can be used to test this new generation of models. In this talk I shall describe recent 3D non-LTE spectral line-formation calculations for oxygen and iron in the solar photosphere, and discuss progress towards applying the 3D non-LTE methodology to other late-type stars.
April 20 Jorge Sanchez Almeida, Instituto de Astrofisica de Canarias, Spain: What are galaxies feeding from?
Galaxies are born in the very early Universe. Depending on their properties (mainly on their mass and environment), most of them are still forming today. Galaxies are systems that transform gas into stars, and they are still doing so despite the fact that the galaxies exhaust their gas reservoir in a very short time-scale. The question arises as to where the gas that keep them alive comes from. The answer is clear from the point of view of the numerical simulations of galaxy formation: metal-poor gas is continuously accreted from the cosmic web filaments (e.g., Dekel et al. 2009). However, the hypothetical cosmological gas inflow has been extremely difficult to detect observationally (e.g., Sanchez Almeida et al. 2014). The talk will describe the theoretical framework that explains the formation of galaxies in a cosmological context, the key role played by the accretion of cosmic web gas, and the evidence that we have of the process at work. (Some of this evidence has been gathered by our group; e.g., Sanchez Almeida et al. 2015.) In a sense, the situation is similar the 'quiet revolution' that happened in Solar Physics around the year 2000, when the quiet sun magnetic fields were theoretically predicted (Cattaneo 1999) but when observations only hinted at their existence (e.g., Sanchez Almeida & Martinez Gonzalez 11).
May 4 Alexei Pevtsov, National Solar Observatory, Boulder, USA and University of Oulu, Finland: “Old” data – new science or why do we need long-term synoptic programs
It is not uncommon to see the references to recent (cycle 24) solar activity as “exceptionally low” or as an “extended long-term decline”. But how would we know that the current level of activity is unusual if we did not have historical data taken over many cycles? How would we know, for example, what are the strongest field strengths in sunspots and how they change with time or that the amplitude of next cycle could be defined by the strength of polar field in previous cycle if we did not have long-term records of solar activity? Truly, synoptic observations feed future research to solve issues that may not be identified at the time when data are acquired. In my talk, I will discuss the current state of long-term synoptic programs and present results of my recent projects on reconstructing the solar activity using historical data.
June 1 Eberhard Wiehr, Göttingen: Macro-shifts induced by prominence micro-structures
The resonance lines of neutral sodium and singly-ionized strontium were observed with the VTT in solar prominences. The spectral aspects of the two emissions appear surprisingly similar, although strontium is ionized clearly above 8000 K, where sodium is no more neutral. The ratio of both emissions is a measure for the electron density, n_e, in prominences, which is found to range near 10^11 per cm^3 without marked variation with height above the limb or with time. The deduction of n_e from the emission ratio depends on the line-broadening, which is ambiguous in prominences, due to a width excess of lines from ions over lines from neutrals. This establishes earlier findings and seems to be related to an excess of macro-shifts of lines from ions, which may indicate a coupling to the prominence magnetic field. Such an explanation is, however, hardly compatible with the similar aspect of both emission spectra. We discuss a possible influence of unresolved fine-structures with different conditions for the population of neutrals and ions.
June 14 Zdzislaw Musielak, University of Texas at Arlington, USA: Detection of Exomoons
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June 22 Robert Jedicke, University of Hawaii, USA: Super catastrophic disruption of asteroids at small perihelion distances
Most near-Earth objects (NEOs) were derived from the asteroid belt as small objects that drifted via non-gravitational thermal forces into resonant escape routes that then push them onto planet-crossing orbits. The dynamics of how these objects evolve in the inner solar system is well understood and has been used to predict the orbital nature and size-frequency distribution of the NEO population. However, as NEO discoveries have increased problems have emerged with the existing NEO models. For example, many NEOs should be on orbits that closely approach the Sun yet few have been discovered. In addition, even though the NEO population is roughly an even mix of low-albedo (<10%) and high-albedo (>10%) asteroids, the characterized NEOs with small perihelion distances show a strong preference for high albedos. Our new NEO model shows that the deficit is likely produced by the super-catastrophic disruption of a substantial fraction of the small and mid-sized NEOs when they reach perihelion distances of a few tens of solar radii.
July 13 Arnold Hanslmeier, Universität Graz, Österreich: Gravitational lensing - what can we learn about quasars
We describe principles of gravitational lensing, macrolensing and microlensing. The physics of Quasars is shortly discussed and then quasar macro- and microlensing by galaxies is described. Own observations consist of photometric observations and spectroscopic observations. We show how from the lensing parameters quantities like the Hubble constant, the mass of the lensing galaxy, the size of the accretion disc and the mass of the supermassive black hole at the center of the quasar can be derived.
July 26 P. Venkatakrishnan, Indian Institute of Astrophysics: Interferometry of Solar Features: Preliminary Experiments with MAST
MAST (Multi Application Solar Telescope) is a recently commissioned 50 cm aperture solar telescope of the Udaipur Solar Observatory. This telescope has 4 ports for backend instruments. The interferometric experiments are being conducted at one of these ports. Interferometry of solar features is an useful technique to look for very small structures in the solar atmosphere. In this talk, I shall first briefly introduce MAST and then describe two experiments. The first experiment was tried by deploying a Fizeau mask at a convenient pupil plane of the telescope. The second experiment used a Golay mask for interferometric imaging of solar fine structures. After presenting some preliminary results, I will discuss the relative merits of Fizeau and Golay masks and also the utility of these techniques for discovering new features using larger telescopes like GREGOR.
August 10 Nobumitsu Yokoi, University Tokyo: Flow generation by helicity and angular-momentum transport in the Sun
Turbulent helicity (velocity–vorticity correlation) represents breaking mirror-symmetry in turbulence. With the aid of an analytical statistical theory for inhomogeneous turbulence, an expression of the Reynolds stress in non-mirror-symmetric turbulence is obtained from the fundamental hydrodynamic equations. It is shown that the helicity gradient enters the Reynolds stress as the coupling coefficient of the mean absolute vorticity Ω* (system rotation and mean relative vorticity). Using this analytical expression, a turbulence model (helicity model) is constructed, which is validated by direct numerical simulations (DNS) of rotating turbulence with helical forcing. This result implies that inhomogeneous turbulent helicity coupled with the mean absolute vorticity Ω* may induce a global flow in the direction of Ω*. This effect is generic in rotating turbulence with inhomogeneous helicity, and is expected to be relevant to several astro- and geo-physical flows, which include cyclones and solar convective motion. The angular-momentum transport in the solar convection zone is discussed from the view point of this helicity effect.
September 26 Matthias Rempel, High Altitude Observatory (Boulder USA): Simulations of the coupled photosphere/corona system: Realistic MHD simulations of flux emergence and flares
Coupling the photosphere and corona requires to deal with a large separation of time scales. While typical photospheric time scales of interest range from minutes (granulation) to days (active region flux emergence), numerical time steps in the corona can be very small due to Alfven velocities exceeding 100,000 km/s and very efficient heat conduction. To cope with these challenges we present a recently developed version of the MURaM radiative MHD code that includes coronal physics in terms of optically thin radiative loss and field aligned heat conduction. The code employs the "Boris correction" (semi-relativistic MHD with a reduced speed of light) and a hyperbolic treatment of heat conduction, which allow for efficient simulations of the photosphere/corona system by avoiding the severe time-step constraints arising from Alfven wave propagation and heat conduction. We demonstrate that this approach can be used even in dynamic phases such as a flare by dynamically adjusting the "reduced speed of light" accordingly. We consider a setup in which a flare is triggered by flux emergence into a pre-existing bipolar active region. Synthetic observables of EUV, soft and hard X-ray emission reproduce several well known features of solar flares, such as the fast rise and slow decay of GOES soft X-ray flux, hard X-ray emission from the corona with extended powerlaws and temperature dependent Doppler shifts in the flare ribbons. We demonstrate that these findings are robust by comparing simulations computed with different values of the saturation heat flux as well as the "reduced speed of light". In a second setup we couple the MURaM code to a global convective dynamo simulation that self-consistently produces magnetic flux concentrations that rise towards the solar surface. We track one of these flux emergence events and study active region scale flux emergence over a time-span of 50 hours, showing the steady transformation of a quiet sun corona to an active sun corona. During this time span we find more than 20 flares, out of which 2 reach M-class and lead to coronal mass ejections.
October 5 Ana Belén Griñón-Marín, Instituto de Astrofisica de Canarias (Spain) & High Altitude Observatory (USA): Temporal Evolution of 3 Light Bridges in the Active Region NOAA 12049 Using Gregor Data
Light bridges are bright elongated structures that are seen in the umbrae of sunspots. They can show filamentary or granular shapes, depending on their physical properties. Their lifetimes are shorter than that of the sunspots that host them and they are very dynamic. These structures have drawn attention since the end of the 19th century. In this talk we present a spectropolarimetric study of the light bridges in the sunspot associated to the NOAA 12049. The data come from the GRegor Infrared Spectrograph (GRIS,Collados et al. 2012) installed at the GREGOR Telescope (Schmidt et al. 2012) in the Teide Observatory (Canary Islands), which has a high spatial resolution (around 0.35"). The full Stokes vector was recorded in a high magnetically sensitive FeI line in the 1.5 micron range. The SIR inversion code (Ruiz Cobo & del Toro Iniesta 1992) has been used in order to recover the various atmospheric physical properties. We will show what we have learned about the physical properties of these puzzling structures.
November 22 14:00 Gedenkkolloquium zu Ehren von Joachim Staiger
14:00 – 14:30 Kaffee
14:30 – 14:45 Svetlana Berdyugina / Oskar von der Lühe (KIS)
14:40 – 15:15 Arnold Hanslmeier (Universität Graz)
15:30 – 15:45 Thomas Rimmele (NSO)
15:45 – 16:00 Markus Knobloch (KIS)
16:00 – 16:15 Markus Roth (KIS)
16:15 – 17:00 Freunde und Familie