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Institute
- Institut für Physik und Astronomie (183) (remove)
The characterization of exoplanets applying high-resolution transmission spectroscopy ini- tiated a new era making it possible to trace atmospheric signature at high altitudes in exoplanet atmospheres and to determine atmospheric properties which enrich our under- standing of the formation and evolution of the solar system. In contrast to what is observed in our solar system, where gaseous planets orbit at wide orbits, Jupiter type exoplanets were detected in foreign stellar systems surrounding their host stars within few days, in close orbits, the so called hot- and ultra-hot Jupiters. The most well studied ones are HD209458b and HD189733b, which are the first exoplanets where absorption is detected in their atmospheres, namely from the alkali line sodium. For hot Jupiters, the resonant alkali lines are the atmospheric species with one of the strongest absorption signatures, due to their large absorption cross-section. However, al- though the alkali lines sodium and potassium were detected in low-resolution observations for various giant exoplanets, potassium was absent in different high-resolution investiga- tions in contrast to sodium. The reason for this is quite puzzling, since both alkalis have very similar physical and chemical properties (e.g. condensation and ionization proper- ties). Obtaining high-resolution transit observations of HD189733b and HD209458b, we were able to detect potassium on HD189733b (Manuscript 1), which was the first high-resolution detection of potassium on an exoplanet. The absence of potassium on HD209458b could be reasoned by depletion processes, such as condensation or photo-ionization or high-altitude clouds. In a further study (Manuscript II), we resolved the potassium line and compared this to a previously detected sodium absorption on this planet. The comparison showed, that the potassium lines are either tracing different altitudes and temperatures compared to the sodium lines, or are depleted so that the planetary Na/K- ratio is way larger than the stellar one. A comparison of the alkali lines with synthetic line profiles showed that the sodium lines were much broader than the potassium lines, probably being induced by winds. To investigate this, the effect of zonal streaming winds on the sodium lines on Jupiter-type planets is investigated in a further study (Manuscript III), showing that such winds can significantly broaden the Na- lines and that high-resolution observations can trace such winds with different properties. Furthermore, investigating the Na-line observations for different exoplanets, I showed that the Na-line broadening follows a trend with cooler planets showing stronger line broadening and so hinting on stronger winds, matching well into theoretical predictions. Each presented manuscript depends on the re- sults published within the previous manuscript, yielding a unitary study of the exoplanet HD189733b. The investigation of the potassium absorption required to account for different effects: The telluric lines removal and the effect of center-to-limb variation (see Manuscript I), the residual Rossiter-Mc-Laughlin effect (see Manuscript II) and the broadening of spectral lines on a translucent atmospheric ring by zonal jet streams (see Manuscript III). This thesis shows that high-resolution transmission spectroscopy is a powerful tool to probe sharp alkali line absorption on giant exoplanet atmospheres and to investigate on the properties and dynamics of hot Jupiter type atmospheres.
Partial synchronous states appear between full synchrony and asynchrony and exhibit many interesting properties. Most frequently, these states are studied within the framework of phase approximation. The latter is used ubiquitously to analyze coupled oscillatory systems. Typically, the phase dynamics description is obtained in the weak coupling limit, i.e., in the first-order in the coupling strength. The extension beyond the first-order represents an unsolved problem and is an active area of research. In this paper, three partially synchronous states are investigated and presented in order of increasing complexity. First, the usage of the phase response curve for the description of macroscopic oscillators is analyzed. To achieve this, the response of the mean-field oscillations in a model of all-to-all coupled limit-cycle oscillators to pulse stimulation is measured. The next part treats a two-group Kuramoto model, where the interaction of one attractive and one repulsive group results in an interesting solitary state, situated between full synchrony and self-consistent partial synchrony. In the last part, the phase dynamics of a relatively simple system of three Stuart-Landau oscillators are extended beyond the weak coupling limit. The resulting model contains triplet terms in the high-order phase approximation, though the structural connections are only pairwise. Finally, the scaling of the new terms with the coupling is analyzed.
Global quantum thermometry
(2021)
A paradigm shift in quantum thermometry is proposed. To date, thermometry has relied on local estimation, which is useful to reduce statistical fluctuations once the temperature is very well known. In order to estimate temperatures in cases where few measurement data or no substantial prior knowledge are available, we build instead a method for global quantum thermometry. Based on scaling arguments, a mean logarithmic error is shown here to be the correct figure of merit for thermometry. Its full minimization provides an operational and optimal rule to postprocess measurements into a temperature reading, and it establishes a global precision limit. We apply these results to the simulated outcomes of measurements on a spin gas, finding that the local approach can lead to biased temperature estimates in cases where the global estimator converges to the true temperature. The global framework thus enables a reliable approach to data analysis in thermometry experiments.
Context. The intergalactic medium (IGM) is believed to contain the majority of baryons in the universe and to trace the same dark matter structure as galaxies, forming filaments and sheets. Ly alpha absorbers, which sample the neutral component of the IGM, have been extensively studied at low and high redshift, but the exact relation between Ly alpha absorption, galaxies, and the large-scale structure is observationally not well constrained.Aims. In this study, we aim at characterising the relation between Ly alpha absorbers and nearby over-dense cosmological structures (galaxy filaments) at recession velocities Delta v <= 6700 km s(-1) by using archival observational data from various instruments.Methods. We analyse 587 intervening Ly alpha absorbers in the spectra of 302 extragalactic background sources obtained with the Cosmic Origins Spectrograph (COS) installed on the Hubble Space Telescope (HST). We combine the absorption line information with galaxy data of five local galaxy filaments from the V8k catalogue.Results. Along the 91 sightlines that pass close to a filament, we identify 215 (227) Ly alpha absorption systems (components). Among these, 74 Ly alpha systems are aligned in position and velocity with the galaxy filaments, indicating that these absorbers and the galaxies trace the same large-scale structure. The filament-aligned Ly alpha absorbers have a similar to 90% higher rate of incidence (d?/dz=189 for log N(HI) >= 13.2) and a slightly shallower column density distribution function slope (-beta=-1.47) relative to the general Ly alpha population at z=0, reflecting the filaments' matter over-density. The strongest Ly alpha absorbers are preferentially found near galaxies or close to the axis of a filament, although there is substantial scatter in this relation. Our sample of absorbers clusters more strongly around filament axes than a randomly distributed sample would do (as confirmed by a Kolmogorov-Smirnov test), but the clustering signal is less pronounced than for the galaxies in the filaments.
The study of exoplanet atmospheres showed large diversity compared to the planets in our Solar system. Especially Jupiter-type exoplanets orbiting their host star in close orbits, the so-called hot and ultra-hot Jupiters, have been studied in detail due to their enhanced atmospheric signature. Due to their tidally locked status, the temperature difference between the day- and nightside triggers atmospheric winds that can lead to various fingerprints in the observations. Spatially resolved absorption lines during transit such as sodium (Na) could be a good tracer for such winds. Different works resolved the Na absorption lines on different exoplanets which show different line widths. Assuming that this could be attributed to such zonal jet streams, this work models the effect of such winds on synthetic absorption lines. For this, transiting Jupiter-type planets with rotational velocities similar to hot and ultra-hot Jupiter are considered. The investigation shows that high wind velocities could reproduce the broadening of Na-line profiles inferred in different high-resolution transit observations. There is a tendency that the broadening values decrease for planets with lower equilibrium temperature. This could be explained by atmospheric drag induced by the ionization of alkali lines that slow down the zonal jet streams, favouring their existence on hot Jupiter rather than ultra-hot Jupiter.
Wolf-Rayet stars are amongst the rarest but also most intriguing massive stars. Their extreme stellar winds induce famous multiwavelength circumstellar gas nebulae of various morphologies, spanning from circles and rings to bipolar shapes. This study is devoted to the investigation of the formation of young, asymmetric Wolf-Rayet gas nebulae and we present a 2.5-dimensional magneto-hydrodynamical toy model for the simulation of Wolf-Rayet gas nebulae generated by wind-wind interaction. Our method accounts for stellar wind asymmetries, rotation, magnetization, evolution, and mixing of materials. It is found that the morphology of the Wolf-Rayet nebulae of blue supergiant ancestors is tightly related to the wind geometry and to the stellar phase transition time interval, generating either a broadened peanut-like or a collimated jet-like gas nebula. Radiative transfer calculations of our Wolf-Rayet nebulae for dust infrared emission at 24 mu m show that the projected diffuse emission can appear as oblate, bipolar, ellipsoidal, or ring structures. Important projection effects are at work in shaping observed Wolf-Rayet nebulae. This might call a revision of the various classifications of Wolf-Rayet shells, which are mostly based on their observed shape. Particularly, our models question the possibility of producing pre-Wolf-Rayet wind asymmetries, responsible for bipolar nebulae like NGC 6888, within the single red supergiant evolution channel scenario. We propose that bipolar Wolf-Rayet nebulae can only be formed within the red supergiant scenario by multiple/merged massive stellar systems, or by single high-mass stars undergoing additional, e.g. blue supergiant, evolutionary stages prior to the Wolf-Rayet phase.
The High Energy Stereoscopic System (H.E.S.S.) is an array of five imaging atmospheric Cherenkov telescopes located in the Khomas Highland of Namibia. H.E.S.S. operates in a wide energy range from several tens of GeV to several tens of TeV, reaching the best sensitivity around 1 TeV or at lower energies. However, there are many important topics – such as the search for Galactic PeVatrons, the study of gamma-ray production scenarios for sources (hadronic vs. leptonic), EBL absorption studies – which require good sensitivity at energies above 10 TeV. This work aims at improving the sensitivity of H.E.S.S. and increasing the gamma-ray statistics at high energies. The study investigates an enlargement of the H.E.S.S. effective field of view using events with larger offset angles in the analysis. The greatest challenges in the analysis of large-offset events are a degradation of the reconstruction accuracy and a rise of the background rate as the offset angle increases. The more sophisticated direction reconstruction method (DISP) and improvements to the standard background rejection technique, which by themselves are effective ways to increase the gamma-ray statistics and improve the sensitivity of the analysis, are implemented to overcome the above-mentioned issues. As a result, the angular resolution at the preselection level is improved by 5 - 10% for events at 0.5◦ offset angle and by 20 - 30% for events at 2◦ offset angle. The background rate at large offset angles is decreased nearly to a level typical for offset angles below 2.5◦. Thereby, sensitivity improvements of 10 - 20% are achieved for the proposed analysis compared to the standard analysis at small offset angles. Developed analysis also allows for the usage of events at large offset angles up to approximately 4◦, which was not possible before. This analysis method is applied to the analysis of the Galactic plane data above 10 TeV. As a result, 40 sources out of the 78 presented in the H.E.S.S. Galactic plane survey (HGPS) are detected above 10 TeV. Among them are representatives of all source classes that are present in the HGPS catalogue; namely, binary systems, supernova remnants, pulsar wind nebulae and composite objects. The potential of the improved analysis method is demonstrated by investigating the more than 10 TeV emission for two objects: the region associated with the shell-type SNR HESS J1731−347 and the PWN candidate associated with PSR J0855−4644 that is coincident with Vela Junior (HESS J0852−463).
Filaments are omnipresent features in the solar chromosphere, one of the atmospheric layers of the Sun, which is located above the photosphere, the visible surface of the Sun. They are clouds of plasma reaching from the photosphere to the chromosphere, and even to the outer-most atmospheric layer, the corona. They are stabalized by the magnetic field. If the magnetic field is disturbed, filaments can erupt as coronal mass ejections (CME), releasing plasma into space, which can also hit the Earth. A special type of filaments are polar crown filaments, which form at the interface of the unipolar field of the poles and flux of opposite magnetic polarity, which was transported towards the poles. This flux transport is related to the global dynamo of the Sun and can therefore be analyzed indirectly with polar crown filaments. The main objective of this thesis is to better understand the physical properties and environment of high-latitude and polar crown filaments, which can be approached from two perspectives: (1) analyzing the large-scale properties of high-latitude and polar crown filaments with full-disk Hα observations from the Chromospheric Telescope (ChroTel) and (2) determining the relation of polar crown and high-latitude filaments from the chromosphere to the lower-lying photosphere with high-spatial resolution observations of the Vacuum Tower Telescope (VTT), which reveal the smallest details.
The Chromospheric Telescope (ChroTel) is a small 10-cm robotic telescope at Observatorio del Teide on Tenerife (Spain), which observes the entire Sun in Hα, Ca IIK, and He I 10830 Å. We present a new calibration method that includes limb-darkening correction, removal of non-uniform filter transmission, and determination of He I Doppler velocities. Chromospheric full-disk filtergrams are often obtained with Lyot filters, which may display non-uniform transmission causing large-scale intensity variations across the solar disk. Removal of a 2D symmetric limb-darkening function from full-disk images results in a flat background. However, transmission artifacts remain and are even more distinct in these contrast-enhanced images. Zernike polynomials are uniquely appropriate to fit these large-scale intensity variations of the background. The Zernike coefficients show a distinct temporal evolution for ChroTel data, which is likely related to the telescope’s alt-azimuth mount that introduces image rotation. In addition, applying this calibration to sets of seven filtergrams that cover the He I triplet facilitates determining chromospheric Doppler velocities. To validate the method, we use three datasets with varying levels of solar activity. The Doppler velocities are benchmarked with respect to co-temporal high-resolution spectroscopic data of the GREGOR Infrared Spectrograph (GRIS). Furthermore, this technique can be applied to ChroTel Hα and Ca IIK data. The calibration method for ChroTel filtergrams can be easily adapted to other full-disk data exhibiting unwanted large-scale variations. The spectral region of the He I triplet is a primary choice for high-resolution near-infrared spectropolarimetry. Here, the improved calibration of ChroTel data will provide valuable context data.
Polar crown filaments form above the polarity inversion line between the old magnetic flux of the previous cycle and the new magnetic flux of the current cycle. Studying their appearance and their properties can lead to a better understanding of the solar cycle. We use full-disk data of the ChroTel at Observatorio del Teide, Tenerife, Spain, which were taken in three different chromospheric absorption lines (Hα 6563 Å, Ca IIK 3933 Å, and He I 10830 Å), and we create synoptic maps. In addition, the spectroscopic He I data allow us to compute Doppler velocities and to create synoptic Doppler maps. ChroTel data cover the rising and decaying phase of Solar Cycle 24 on about 1000 days between 2012 and 2018. Based on these data, we automatically extract polar crown filaments with image-processing tools and study their properties. We compare contrast maps of polar crown filaments with those of quiet-Sun filaments. Furthermore, we present a super-synoptic map summarizing the entire ChroTel database. In summary, we provide statistical properties, i.e. number and location of filaments, area, and tilt angle for both the maximum and declining phase of Solar Cycle 24. This demonstrates that ChroTel provides a
promising dataset to study the solar cycle.
The cyclic behavior of polar crown filaments can be monitored by regular full-disk Hα observations. ChroTel provides such regular observations of the Sun in three chromospheric wavelengths. To analyze the cyclic behavior and the statistical properties of polar crown filaments, we have to extract the filaments from the images. Manual extraction is tedious, and extraction with morphological image processing tools produces a large number of false positive detections and the manual extraction of these takes too much time. Automatic object detection and extraction in a reliable manner allows us to process more data in a shorter time. We will present an overview of the ChroTel database and a proof of concept of a machine learning application, which allows us a unified extraction of, for example, filaments from ChroTel data.
The chromospheric Hα spectral line dominates the spectrum of the Sun and other stars. In the stellar regime, this spectral line is already used as a powerful tracer of magnetic activity. For the Sun, other tracers are typically used to monitor solar activity. Nonetheless, the Sun is observed constantly in Hα with globally distributed ground-based full-disk imagers. The aim of this study is to introduce Hα as a tracer of solar activity and compare it to other established indicators. We discuss the newly created imaging Hα excess in the perspective of possible application for modelling of stellar atmospheres. In particular, we try to determine how constant is the mean intensity of the Hα excess and number density of low-activity regions between solar maximum and minimum. Furthermore, we investigate whether the active region coverage fraction or the changing emission strength in the active regions dominates time variability in solar Hα observations. We use ChroTel observations of full-disk Hα filtergrams and morphological image processing techniques to extract the positive and negative imaging Hα excess, for bright features (plage regions) and dark absorption features (filaments and sunspots), respectively. We describe the evolution of the Hα excess during Solar Cycle 24 and compare it to other well established tracers: the relative sunspot number, the F10.7 cm radio flux, and the Mg II index. Moreover, we discuss possible applications of the Hα excess for stellar activity diagnostics and the contamination of exoplanet transmission spectra. The positive and negative Hα excess follow the behavior of the solar activity over the course of the cycle. Thereby, positive Hα excess is closely correlated to the chromospheric Mg II index. On the other hand, the negative Hα excess, created from dark features like filaments and sunspots, is introduced as a tracer of solar activity for the first time. We investigated the mean intensity distribution for active regions for solar minimum and maximum and found that the shape of both distributions is very similar but with different amplitudes. This might be related with the relatively stable coronal temperature component during the solar cycle. Furthermore, we found that the coverage fraction of Hα excess and the Hα excess of bright features are strongly correlated, which will influence modelling of stellar and exoplanet atmospheres.
High-resolution observations of polar crown and high-latitude filaments are scarce. We present a unique sample of such filaments observed in high-resolution Hα narrow-band filtergrams and broad-band images, which were obtained with a new fast camera system at the VTT. ChroTel provided full-disk context observations in Hα, Ca IIK, and He I 10830 Å. The Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) provided line-of-sight magnetograms and ultraviolet (UV) 1700 Å filtergrams, respectively. We study filigree in the vicinity of polar crown and high-latitude filaments and relate their locations to magnetic concentrations at the filaments’ footpoints. Bright points are a well studied phenomenon in the photosphere at low latitudes, but they were not yet studied in the quiet network close to the poles. We examine size, area, and eccentricity of bright points and find that their morphology is very similar to their counterparts at lower latitudes, but their sizes and areas are larger. Bright points at the footpoints of polar crown filaments are preferentially located at stronger magnetic flux concentrations, which are related to bright regions at the border of supergranules as observed in UV filtergrams. Examining the evolution of bright points on three consecutive days reveals that their amount increases while the filament decays, which indicates they impact the equilibrium of the cool plasma contained in filaments.
Active Galactic Nuclei (AGN) are considered to be the main powering source of active galaxies, where central Super Massive Black Holes (SMBHs), with masses between 106 and 109 M⊙ gravitationally pull the surrounding material via accre- tion. AGN phenomenon expands over a very wide range of luminosities, from the most luminous high-redshift quasars (QSOs), to the local Low-Luminosity AGN (LLAGN), with significantly weaker luminosities. While "typical" luminous AGNs distinguish themselves by their characteristical blue featureless continuum, the Broad Emission Lines (BELs) with Full Widths at Half Maximum (FWHM) in order of few thousands km s1, arising from the so-called Broad Line Region (BLR), and strong radio and/or X-ray emission, detection of LLAGNs on the other hand is quite chal- lenging due to their extremely weak emission lines, and absence of the power-law continuum. In order to fully understand AGN evolution and their duty-cycles across cosmic history, we need a proper knowledge of AGN phenomenon at all luminosi- ties and redshifts, as well as perspectives from different wavelength bands.
In this thesis I present a search for AGN signatures in central spectra of 542 local (0.005 < z < 0.03) galaxies from the Calar Alto Legacy Integral Field Area (CALIFA) survey. The adopted aperture of 3′′ × 3′′ corresponds to central ∼ 100 − 500 pc for the redshift range of CALIFA. Using the standard emission-line ratio diagnostic diagrams, we initially classified all CALIFA emission-line galaxies (526) into star- forming, LINER-like, Seyfert 2 and intermediates. We further detected signatures of the broad Hα component in 89 spectra from the sample, of which more than 60% are present in the central spectra of LINER-like galaxies. These BELs are very weak, with luminosities in range 1038 − 1041 erg s−1, but with FWHMs between 1000 km s−1 and 6000 km s−1, comparable to those of luminous high-z AGN. This result implies that type 1 AGN are in fact quite frequent in the local Universe. We also identified additional 29 Seyfert 2 galaxies using the emission-line ratio diagnostic diagrams.
Using the MBH − σ∗ correlation, we estimated black hole masses of 55 type 1 AGN from CALIFA, a sample for which we had estimates of bulge stellar velocity dispersions σ∗. We compared these masses to the ones that we estimated from the virial method and found large discrepancies. We analyzed the validity of both meth- ods for black hole mass estimation of local LLAGN, and concluded that most likely virial scaling relations can no longer be applied as a valid MBH estimator in such low-luminosity regime. These black holes accrete at very low rate, having Edding- ton ratios in range 4.1 × 10−5 − 2.4 × 10−3. Detection of BELs with such low lumi- nosities and at such low Eddington rates implies that these LLAGN are still able to form the BLR, although with probably modified structure of the central engine.
In order to obtain full picture of black hole growth across cosmic time, it is es- sential that we study them in different stages of their activity. For that purpose, we estimated the broad AGN Luminosity Function (AGNLF) of our entire type 1 AGN sample using the 1/Vmax method. The shape of AGNLF indicates an apparent flattening below luminosities LHα ∼ 1039 erg s−1. Correspondingly we estimated ac- tive Black Hole Mass Function (BHMF) and Eddington Ration Distribution Function (ERDF) for a sub-sample of type 1 AGN for which we have MBH and λ estimates. The flattening is also present in both BHMF and ERDF, around log(MBH) ∼ 7.7 and log(λ) < 3, respectively. We estimated the fraction of active SMBHs in CALIFA by comparing our active BHMF to the one of the local quiescent SMBHs. The shape of
the active fraction which decreases with increasing MBH, as well as the flattening of AGNLF, BHMF and ERDF is consistent with scenario of AGN cosmic downsizing.
To complete AGN census in the CALIFA galaxy sample, it is necessary to search for them in various wavelength bands. For the purpose of completing the census we performed cross-correlations between all 542 CALIFA galaxies and multiwavelength surveys, Swift – BAT 105 month catalogue (in hard 15 - 195 keV X-ray band), and NRAO VLA Sky Survey (NVSS, in 1.4 GHz radio domain). This added 1 new AGN candidate in X-ray, and 7 in radio wavelength band to our local LLAGN count.
It is possible to detect AGN emission signatures within 10 – 20 kpc outside of the central galactic regions. This may happen when the central AGN has recently switched off and the photoionized material is spread across the galaxy within the light-travel-time, or the photoionized material is blown away from the nucleus by outflows. In order to detect these extended AGN regions we constructed spatially resolved emission-line ratio diagnostic diagrams of all emission-line galaxies from the CALIFA, and found 1 new object that was previously not identified as AGN.
Obtaining the complete AGN census in CALIFA, with five different AGN types, showed that LLAGN contribute a significant fraction of 24% of the emission-line galaxies in the CALIFA sample. This result implies that AGN are quite common in the local Universe, and although being in very low activity stage, they contribute to large fraction of all local SMBHs. Within this thesis we approached the upper limit of AGN fraction in the local Universe and gained some deeper understanding of the LLAGN phenomenon.
As society paves its way towards device miniaturization and precision medicine, micro-scale actuation and guided transport become increasingly prominent research fields, with high potential impact in both technological and clinical contexts. In order to accomplish directed motion of micron-sized objects, as biosensors and drug-releasing microparticles, towards specific target sites, a promising strategy is the use of living cells as smart biochemically-powered carriers, building the so-called bio-hybrid systems. Inspired by leukocytes, native cells of living organisms efficiently migrating to critical targets as tumor tissue, an emerging concept is to exploit the amoeboid crawling motility of such cells as mean of transport for drug delivery applications.
In the research work described in this thesis, I synergistically applied experimental, computational and theoretical modeling approaches to investigate the behaviour and transport mechanism of a novel kind of bio-hybrid system for active transport at the micro-scale, referred to as cellular truck. This system consists of an amoeboid crawling cell, the carrier, attached to a microparticle, the cargo, which may ideally be drug-loaded for specific therapeutic treatments.
For the purposes of experimental investigation, I employed the amoeba Dictyostelium discoideum as crawling cellular carrier, being a renowned model organism for leukocyte migration and, in general, for eukaryotic cell motility. The performed experiments revealed a complex recurrent cell-cargo relative motion, together with an intermittent motility of the cellular truck as a whole. The evidence suggests the presence of cargoes on amoeboid cells to act as mechanical stimulus leading cell polarization, thus promoting cell motility and giving rise to the observed intermittent dynamics of the truck. Particularly, bursts in cytoskeletal polarity along the cell-cargo axis have been
found to occur in time with a rate dependent on cargo geometrical features, as particle diameter. Overall, the collected experimental evidence pointed out a pivotal role of cell-cargo interactions in the emergent cellular truck motion dynamics. Especially, they can determine the transport capabilities of amoeboid cells, as the cargo size significantly impacts the cytoskeletal activity and repolarization dynamics along the cell-cargo axis, the latter responsible for truck displacement and reorientation.
Furthermore, I developed a modeling framework, built upon the experimental evidence on cellular truck behaviour, that connects the relative dynamics and interactions arising at the truck scale with the actual particle transport dynamics. In fact, numerical simulations of the proposed model successfully reproduced the phenomenology of the cell-cargo system, while enabling the prediction of the transport properties of cellular trucks over larger spatial and temporal scales. The theoretical analysis provided a deeper understanding of the role of cell-cargo interaction on mass transport, unveiling in particular how the long-time transport efficiency is governed by the interplay between the persistence time of cell polarity and time scales of the relative dynamics stemming from cell-cargo interaction. Interestingly, the model predicts the existence of an optimal cargo size, enhancing the diffusivity of cellular trucks; this is in line with previous independent experimental data, which appeared rather counterintuitive and had no explanation prior to this study.
In conclusion, my research work shed light on the importance of cargo-carrier interactions in the context of crawling cell-mediated particle transport, and provides a prototypical, multifaceted framework for the analysis and modelling of such complex bio-hybrid systems and their perspective optimization.
Human migration is often studied using gravity models. These models, however, have known limitations, including analytic inconsistencies and a dependence on empirical data to calibrate multiple parameters for the region of interest. Overcoming these limitations, the radiation model has been proposed as an alternative, universal approach to predicting different forms of human mobility, but has not been adopted for studying migration. Here we show, using data on within-country migration from the USA and Mexico, that the radiation model systematically underpredicts long-range moves, while the traditional gravity model performs well for large distances. The universal opportunity model, an extension of the radiation model, shows an improved fit of long-range moves compared to the original radiation model, but at the cost of introducing two additional parameters. We propose a more parsimonious extension of the radiation model that introduces a single parameter. We demonstrate that it fits the data over the full distance spectrum and also-unlike the universal opportunity model-preserves the analytical property of the original radiation model of being equivalent to a gravity model in the limit of a uniform population distribution.
The majority of baryons in the Universe is believed to reside in the intergalactic medium (IGM). This makes the IGM an important component in understanding cosmological structure formation. It is expected to trace the same dark matter distribution as galaxies, forming structures like filaments and clusters. However, whereas galaxies can be observed to be arranged along these large-scale structures, the spatial distribution of the diffuse IGM is not as easily unveiled. Absorption line studies of quasar (QSO) spectra can help with mapping the IGM, as well as the boundary layer between IGM and galaxies: the circumgalactic medium (CGM). By studying gas in the Local Group, as well as in the IGM, this study aims to get a better understanding of how the gas is linked to the large-scale structure of the local Universe and the galaxies residing in that structure.
Chapter 1 gives an introduction to the CGM and IGM, while the methods used in this study are explained in Chapter 2. Chapter 3 starts on a relatively small cosmological scale, namely that of our Local Group, which includes i.a. the Milky Way (MW) and the M31. Within the CGM of the MW, there exist denser clouds, some of which are infalling while others are moving away from the Galactic disc. To study these clouds, 29 QSO spectra obtained with the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope (HST) were analysed. Abundances of Si II, Si III, Si IV, C II, and C IV were measured for 69 HVCs belonging to two samples: one in the direction of the LG’s barycentre and the other in the anti-barycentre direction. Their velocities range from -100 ≥ vLSR ≥ -400 km/s for the barycentre sample and between +100 ≤ vLSR ≤ +300 km/s for the anti-barycentre sample. By using Cloudy models, these data could then be used to derive gas volume densities for the HVCs. Because of the relationship between density and pressure of the ambient medium, which is in turn determined by the Galactic radiation field, the distances of the HVCs could be estimated. From this, a subsample of absorbers located in the direction of M31 was found to exist outside of the MW’s virial radius, their low densities (log nH ≤ -3.54) making it likely for them to be part of the gas in between the MW and M31. No such low-density absorbers were found in the anti-barycentre sample. Our results thus hint at gas following the dark matter potential, which would be deeper between the MW and M31 as they are by far the most massive members of the LG.
From this bridge of gas in the LG, this study zooms out to the large-scale structure of the local Universe (z ~ 0) in Chapter 4. Galaxy data from the V8k catalogue and QSO spectra from COS were used to study the relation between the galaxies tracing large-scale filaments and the gas existing outside of those galaxies. This study used the filaments defined in Courtois et al. (2013). A total of 587 Lyman α (Lyα) absorbers were found in the 302 QSO spectra in the velocity range 1070 - 6700 km/s. After selecting sightlines passing through or close to these filaments, model spectra were made for 91 sightlines and 215 (227) Lyα absorbers (components) were measured in this sample. The velocity gradient along each filament was calculated and 74 absorbers were found within 1000 km/s of the nearest filament segment.
In order to find whether the absorbers are more tied to galaxies or to the large-scale structure, equivalent widths of the Lyα absorbers were plotted against both galaxy and filament impact parameters. While stronger absorbers do tend to be closer to either galaxies or filaments, there is a large scatter in this relation. Despite this large scatter, this study found that the absorbers do not follow a random distribution either. They cluster less strongly around filaments than galaxies, but stronger than random distributions, as confirmed by a Kolmogorov-Smirnov test.
Furthermore, the column density distribution function found in this study has a slope of -β = 1.63±0.12 for the total sample and -β =1.47±0.24 for the absorbers within 1000 km/s of a filament. The shallower slope for the latter subsample could indicate an excess of denser absorbers within the filament, but they are consistent within errors. These values are in agreement with values found in e.g. Lehner et al. (2007); Danforth et al. (2016).
The picture that emerges from this study regarding the relation between the IGM and the large-scale structure in the local Universe fits with what is found in other studies: while at least part of the gas traces the same filamentary structure as galaxies, the relation is complex. This study has shown that by taking a large sample of sightlines and comparing the data gathered from those with galaxy data, it is possible to study the gaseous large-scale structure. This approach can be used in the future together with simulations to get a better understanding of structure formation and evolution in the Universe.
The evolution of life on Earth has been driven by disturbances of different types and magnitudes over the 4.6 million years of Earth’s history (Raup, 1994, Alroy, 2008). One example for such disturbances are mass extinctions which are characterized by an exceptional increase in the extinction rate affecting a great number of taxa in a short interval of geologic time (Sepkoski, 1986). During the 541 million years of the Phanerozoic, life on Earth suffered five exceptionally severe mass extinctions named the “Big Five Extinctions”. Many mass extinctions are linked to changes in climate
(Feulner, 2009). Hence, the study of past mass extinctions is not only intriguing, but can also provide insights into the complex nature of the Earth system. This thesis aims at deepening our understanding of the triggers of mass extinctions and how they affected life. To accomplish this, I investigate changes in climate during two of the Big Five extinctions using a coupled climate model.
During the Devonian (419.2–358.9 million years ago) the first vascular plants and vertebrates evolved on land while extinction events occurred in the ocean (Algeo et al., 1995). The causes of these formative changes, their interactions and their links to changes in climate are still poorly understood. Therefore, we explore the sensitivity of the Devonian climate to various boundary conditions using an intermediate-complexity climate model (Brugger et al., 2019). In contrast to Le Hir et al. (2011), we find only a minor biogeophysical effect of changes in vegetation cover due to unrealistically high soil albedo values used in the earlier study. In addition, our results cannot support the strong influence of orbital parameters on the Devonian climate, as simulated with a climate model with a strongly simplified ocean model (De Vleeschouwer et al., 2013, 2014, 2017). We can only reproduce the changes in Devonian climate suggested by proxy data by decreasing atmospheric CO2. Still, finding agreement between the evolution of sea surface temperatures reconstructed from proxy data (Joachimski et al., 2009) and our simulations remains challenging and suggests a lower δ18O ratio of Devonian seawater. Furthermore, our study of the sensitivity of the Devonian climate reveals a prevailing mode of climate variability on a timescale of decades to centuries. The quasi-periodic ocean temperature fluctuations are linked to a physical mechanism of changing sea-ice cover, ocean convection and overturning in high northern latitudes.
In the second study of this thesis (Dahl et al., under review) a new reconstruction of atmospheric CO2 for the Devonian, which is based on CO2-sensitive carbon isotope fractionation in the earliest vascular plant fossils, suggests a much earlier drop of atmo- spheric CO2 concentration than previously reconstructed, followed by nearly constant CO2 concentrations during the Middle and Late Devonian. Our simulations for the Early Devonian with identical boundary conditions as in our Devonian sensitivity study (Brugger et al., 2019), but with a low atmospheric CO2 concentration of 500 ppm, show no direct conflict with available proxy and paleobotanical data and confirm that under the simulated climatic conditions carbon isotope fractionation represents a robust proxy for atmospheric CO2. To explain the earlier CO2 drop we suggest that early forms of vascular land plants have already strongly influenced weathering. This new perspective on the Devonian questions previous ideas about the climatic conditions and earlier explanations for the Devonian mass extinctions.
The second mass extinction investigated in this thesis is the end-Cretaceous mass extinction (66 million years ago) which differs from the Devonian mass extinctions in terms of the processes involved and the timescale on which the extinctions occurred. In the two studies presented here (Brugger et al., 2017, 2021), we model the climatic effects of the Chicxulub impact, one of the proposed causes of the end-Cretaceous extinction, for the first millennium after the impact. The light-dimming effect of stratospheric sulfate aerosols causes severe cooling, with a decrease of global annual mean surface air temperature of at least 26◦C and a recovery to pre-impact temperatures after more than 30 years. The sudden surface cooling of the ocean induces deep convection which brings nutrients from the deep ocean via upwelling to the surface ocean. Using an ocean biogeochemistry model we explore the combined effect of ocean mixing and iron-rich dust originating from the impactor on the marine biosphere. As soon as light levels have recovered, we find a short, but prominent peak in marine net primary productivity. This newly discovered mechanism could result in toxic effects for marine near-surface ecosystems. Comparison of our model results to proxy data (Vellekoop et al., 2014, 2016, Hull et al., 2020) suggests that carbon release from the terrestrial biosphere is required in addition to the carbon dioxide which can be attributed to the target material. Surface ocean acidification caused by the addition of carbon dioxide and sulfur is only moderate. Taken together, the results indicate a significant contribution of the Chicxulub impact to the end-Cretaceous mass extinction by triggering multiple stressors for the Earth system.
Although the sixth extinction we face today is characterized by human intervention in nature, this thesis shows that we can gain many insights into future extinctions from studying past mass extinctions, such as the importance of the rate of change (Rothman, 2017), the interplay of multiple stressors (Gunderson et al., 2016), and changes in the carbon cycle (Rothman, 2017, Tierney et al., 2020).
Das Manuskript dient der Vorbereitung der Prüfung der Fachkunde zum Strahlenschutz für Lehrer. Es enthält wichtige Grundlagen der Kernphysik, insbesondere die Eigenschaften der Alpha-, Beta-, Gamma-, Neutronen- und Röntgenstrahlen. Es folgt eine kurze Beschreibung des Einflusses der Strahlung auf belebte Materie. Wichtige Paragrafen der Strahlenschutzverordnung werden beschrieben. Eine Aufgabensammlung dient zur Illustration und Übung.
Gravitational-wave (GW) astrophysics is a field in full blossom. Since the landmark detection of GWs from a binary black hole on September 14th 2015, fifty-two compact-object binaries have been reported by the LIGO-Virgo collaboration. Such events carry astrophysical and cosmological information ranging from an understanding of how black holes and neutron stars are formed, what neutron stars are composed of, how the Universe expands, and allow testing general relativity in the highly-dynamical strong-field regime. It is the goal of GW astrophysics to extract such information as accurately as possible. Yet, this is only possible if the tools and technology used to detect and analyze GWs are advanced enough. A key aspect of GW searches are waveform models, which encapsulate our best predictions for the gravitational radiation under a certain set of parameters, and that need to be cross-correlated with data to extract GW signals. Waveforms must be very accurate to avoid missing important physics in the data, which might be the key to answer the fundamental questions of GW astrophysics. The continuous improvements of the current LIGO-Virgo detectors, the development of next-generation ground-based detectors such as the Einstein Telescope or the Cosmic Explorer, as well as the development of the Laser Interferometer Space Antenna (LISA), demand accurate waveform models. While available models are enough to capture the low spins, comparable-mass binaries routinely detected in LIGO-Virgo searches, those for sources from both current and next-generation ground-based and spaceborne detectors must be accurate enough to detect binaries with large spins and asymmetry in the masses. Moreover, the thousands of sources that we expect to detect with future detectors demand accurate waveforms to mitigate biases in the estimation of signals’ parameters due to the presence of a foreground of many sources that overlap in the frequency band. This is recognized as one of the biggest challenges for the analysis of future-detectors’ data, since biases might hinder the extraction of important astrophysical and cosmological information from future detectors’ data. In the first part of this thesis, we discuss how to improve waveform models for binaries with high spins and asymmetry in the masses. In the second, we present the first generic metrics that have been proposed to predict biases in the presence of a foreground of many overlapping signals in GW data.
For the first task, we will focus on several classes of analytical techniques. Current models for LIGO and Virgo studies are based on the post-Newtonian (PN, weak-field, small velocities) approximation that is most natural for the bound orbits that are routinely detected in GW searches. However, two other approximations have risen in prominence, the post-Minkowskian (PM, weak- field only) approximation natural for unbound (scattering) orbits and the small-mass-ratio (SMR) approximation typical of binaries in which the mass of one body is much bigger than the other. These are most appropriate to binaries with high asymmetry in the masses that challenge current waveform models. Moreover, they allow one to “cover” regions of the parameter space of coalescing binaries, thereby improving the interpolation (and faithfulness) of waveform models. The analytical approximations to the relativistic two-body problem can synergically be included within the effective-one-body (EOB) formalism, in which the two-body information from each approximation can be recast into an effective problem of a mass orbiting a deformed Schwarzschild (or Kerr) black hole. The hope is that the resultant models can cover both the low-spin comparable-mass binaries that are routinely detected, and the ones that challenge current models. The first part of this thesis is dedicated to a study about how to best incorporate information from the PN, PM, SMR and EOB approaches in a synergistic way. We also discuss how accurate the resulting waveforms are, as compared against numerical-relativity (NR) simulations. We begin by comparing PM models, whether alone or recast in the EOB framework, against PN models and NR simulations. We will show that PM information has the potential to improve currently-employed models for LIGO and Virgo, especially if recast within the EOB formalism. This is very important, as the PM approximation comes with a host of new computational techniques from particle physics to exploit. Then, we show how a combination of PM and SMR approximations can be employed to access previously-unknown PN orders, deriving the third subleading PN dynamics for spin-orbit and (aligned) spin1-spin2 couplings. Such new results can then be included in the EOB models currently used in GW searches and parameter estimation studies, thereby improving them when the binaries have high spins. Finally, we build an EOB model for quasi-circular nonspinning binaries based on the SMR approximation (rather than the PN one as usually done). We show how this is done in detail without incurring in the divergences that had affected previous attempts, and compare the resultant model against NR simulations. We find that the SMR approximation is an excellent approximation for all (quasi-circular nonspinning) binaries, including both the equal-mass binaries that are routinely detected in GW searches and the ones with highly asymmetric masses. In particular, the SMR-based models compare much better than the PN models, suggesting that SMR-informed EOB models might be the key to model binaries in the future. In the second task of this thesis, we work within the linear-signal ap- proximation and describe generic metrics to predict inference biases on the parameters of a GW source of interest in the presence of confusion noise from unfitted foregrounds and from residuals of other signals that have been incorrectly fitted out. We illustrate the formalism with simple (yet realistic) LISA sources, and demonstrate its validity against Monte-Carlo simulations. The metrics we describe pave the way for more realistic studies to quantify the biases with future ground-based and spaceborne detectors.
'Complex systems are information processors' is a statement that is frequently made. Here we argue for the distinction between information processing-in the sense of encoding and transmitting a symbolic representation-and the formation of correlations (pattern formation/self-organisation). The study of both uses tools from information theory, but the purpose is very different in each case: explaining the mechanisms and understanding the purpose or function in the first case, versus data analysis and correlation extraction in the latter. We give examples of both and discuss some open questions. The distinction helps focus research efforts on the relevant questions in each case.
High-resolution observations of polar crown and high-latitude filaments are scarce. We present a unique sample of such filaments observed in high-resolution H alpha narrow-band filtergrams and broad-band images, which were obtained with a new fast camera system at the Vacuum Tower Telescope (VTT), Tenerife, Spain. The Chromospheric Telescope (ChroTel) provided full-disk context observations in H alpha, CaiiK, and Hei 10830 angstrom. The Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) provided line-of-sight magnetograms and ultraviolet (UV) 1700 angstrom filtergrams, respectively. We study filigree in the vicinity of polar crown and high-latitude filaments and relate their locations to magnetic concentrations at the filaments' footpoints. Bright points are a well studied phenomenon in the photosphere at low latitudes, but they were not yet studied in the quiet network close to the poles. We examine size, area, and eccentricity of bright points and find that their morphology is very similar to their counterparts at lower latitudes, but their sizes and areas are larger. Bright points at the footpoints of polar crown filaments are preferentially located at stronger magnetic flux concentrations, which are related to bright regions at the border of supergranules as observed in UV filtergrams. Examining the evolution of bright points on three consecutive days reveals that their amount increases while the filament decays, which indicates they impact the equilibrium of the cool plasma contained in filaments.
Ground-based astronomy is set to employ next-generation telescopes with apertures larger than 25 m in diameter before this decade is out. Such giant telescopes observe their targets through a larger patch of turbulent atmosphere, demanding that most of the instruments behind them must also grow larger to make full use of the collected stellar flux. This linear scaling in size greatly complicates the design of astronomical instrumentation, inflating their cost quadratically. Adaptive optics (AO) is one approach to circumvent this scaling law, but it can only be done to an extent before the cost of the corrective system itself overwhelms that of the instrument or even that of the telescope. One promising technique for miniaturizing the instruments and thus driving down their cost is to replace some, or all, of the free space bulk optics in the optical train with integrated photonic components.
Photonic devices, however, do their work primarily in single-mode waveguides, and the atmospherically-distorted starlight must first be efficiently coupled into them if they are to outperform their bulk optic counterparts. This is doable by two means: AO systems can again help control the angular size and motion of seeing disks to the point where they will couple efficiently into astrophotonic components, but this is only feasible for the brightest of objects and over limited fields of view. Alternatively, tapered fiber devices known as photonic lanterns — with their ability to convert multimode into single-mode optical fields — can be used to feed speckle patterns into single-mode integrated optics. They, nonetheless, must conserve the degrees of freedom, and the number of output waveguides will quickly grow out of control for uncorrected large telescopes. An AO-assisted photonic lantern fed by a partially corrected wavefront presents a compromise that can have a manageable size if the trade-off between the two methods is chosen carefully. This requires end-to-end simulations that take into account all the subsystems upstream of the astrophotonic instrument, i.e., the atmospheric layers, the telescope, the AO system, and the photonic lantern, before a decision can be made on sizing the multiplexed integrated instrument.
The numerical models that simulate atmospheric turbulence and AO correction are presented in this work. The physics and models for optical fibers, arrays of waveguides, and photonic lanterns are also provided. The models are on their own useful in understanding the behavior of the individual subsystems involved and are also used together to compute the optimum sizing of photonic lanterns for feeding astrophotonic instruments. Additionally, since photonic lanterns are a relatively new concept, two novel applications are discussed for them later in this thesis: the use of mode-selective photonic lanterns (MSPLs) to reduce the multiplicity of multiplexed integrated instruments and the combination of photonic lanterns with discrete beam combiners (DBCs) to retrieve the modal content in an optical waveguide.
Proteins of halophilic organisms that accumulate molar concentrations of KCl in their cytoplasm have much higher content in acidic amino acids than proteins of mesophilic organisms. It has been proposed that this excess is necessary to maintain proteins hydrated in an environment with low water activity: either via direct interactions between water and the carboxylate groups of acidic amino acids or via cooperative interactions between acidic amino acids and hydrated cations, which would stabilize the folded protein. In the course of this Ph.D. study, we investigated these possibilities using atomistic molecular dynamics simulations and classical force fields. High quality parameters describing the interaction between K+ and carboxylate groups present in acidic amino acids are indispensable for this study. We first evaluated the quality of the default parameters for these ions within the widely used AMBER ff14SB force field for proteins and found that they perform poorly. We propose new parameters, which reproduce solution activity derivatives of potassium acetate solutions up to 2 mol/kg and the distances between potassium ions and carboxylate groups observed in x-ray structures of proteins. To understand the role of acidic amino acids in protein hydration, we investigated this aspect for 5 halophilic proteins in comparison with 5 mesophilic ones. Our results do not support the necessity of acidic amino acids to keep folded proteins hydrated. Proteins with a larger fraction of acidic amino acids indeed have higher hydration levels. However, the hydration level of each protein is identical at low (b_KCl = 0.15 mol/kg) and high (b_KCl = 2 mol/kg) KCl concentration. It has also been proposed that cooperative interactions between acidic amino acids with nearby hydrated cations stabilize the folded protein and slow down its solvation shell; according to this theory, the cations would be preferentially excluded from the unfolded structure. We investigate this possibility through extensive free energy calculation simulations. We find that cooperative interactions between neighboring acidic amino acids exist and are mediated by the ions in solution but are present in both folded and unfolded structures of halophilic proteins. The translational dynamics of the solvation shell is barely distinguishable between halophilic and mesophilic proteins; therefore, such a cooperative effect does not result in unusually slow solvent dynamics as has been suggested.
Partial synchronous states exist in systems of coupled oscillators between full synchrony and asynchrony. They are an important research topic because of their variety of different dynamical states. Frequently, they are studied using phase dynamics. This is a caveat, as phase dynamics are generally obtained in the weak coupling limit of a first-order approximation in the coupling strength. The generalization to higher orders in the coupling strength is an open problem. Of particular interest in the research of partial synchrony are systems containing both attractive and repulsive coupling between the units. Such a mix of coupling yields very specific dynamical states that may help understand the transition between full synchrony and asynchrony. This thesis investigates partial synchronous states in mixed-coupling systems. First, a method for higher-order phase reduction is introduced to observe interactions beyond the pairwise one in the first-order phase description, hoping that these may apply to mixed-coupling systems. This new method for coupled systems with known phase dynamics of the units gives correct results but, like most comparable methods, is computationally expensive. It is applied to three Stuart-Landau oscillators coupled in a line with a uniform coupling strength. A numerical method is derived to verify the analytical results. These results are interesting but give importance to simpler phase models that still exhibit exotic states. Such simple models that are rarely considered are Kuramoto oscillators with attractive and repulsive interactions. Depending on how the units are coupled and the frequency difference between the units, it is possible to achieve many different states. Rich synchronization dynamics, such as a Bellerophon state, are observed when considering a Kuramoto model with attractive interaction in two subpopulations (groups) and repulsive interactions between groups. In two groups, one attractive and one repulsive, of identical oscillators with a frequency difference, an interesting solitary state appears directly between full and partial synchrony. This system can be described very well analytically.