Recent Submissions

  • Journal Article

    Multiscale Modeling of Dyadic Structure-Function Relation in Ventricular Cardiac Myocytes 

    Cosi, Filippo G.; Giese, Wolfgang; Neubert, Wilhelm; Luther, Stefan; Chamakuri, Nagaiah; Parlitz, Ulrich; Falcke, Martin
    Biophysical Journal 2019; 117(12) p.2409-2419
    Cardiovascular disease is often related to defects of subcellular components in cardiac myocytes, specifically in the dyadic cleft, which include changes in cleft geometry and channel placement. Modeling of these pathological changes requires both spatially resolved cleft as well as whole cell level descriptions. We use a multiscale model to create dyadic structure-function relationships to explore the impact of molecular changes on whole cell electrophysiology and calcium cycling. This multiscale model incorporates stochastic simulation of individual L-type calcium channels and ryanodine receptor channels, spatially detailed concentration dynamics in dyadic clefts, rabbit membrane potential dynamics, and a system of partial differential equations for myoplasmic and lumenal free Ca2+ and Ca2+-binding molecules in the bulk of the cell. We found action potential duration, systolic, and diastolic [Ca2+] to respond most sensitively to changes in L-type calcium channel current. The ryanodine receptor channel cluster structure inside dyadic clefts was found to affect all biomarkers investigated. The shape of clusters observed in experiments by Jayasinghe et al. and channel density within the cluster (characterized by mean occupancy) showed the strongest correlation to the effects on biomarkers.
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  • Journal Article

    Chemotaxis in external fields: Simulations for active magnetic biological matter 

    Codutti, Agnese; Bente, Klaas; Faivre, Damien; Klumpp, Stefan
    PLOS Computational Biology 2019; 15(12): Art. e1007548
    The movement of microswimmers is often described by active Brownian particle models. Here we introduce a variant of these models with several internal states of the swimmer to describe stochastic strategies for directional swimming such as run and tumble or run and reverse that are used by microorganisms for chemotaxis. The model includes a mechanism to generate a directional bias for chemotaxis and interactions with external fields (e.g., gravity, magnetic field, fluid flow) that impose forces or torques on the swimmer. We show how this modified model can be applied to various scenarios: First, the run and tumble motion of E. coli is used to establish a paradigm for chemotaxis and investigate how it is affected by external forces. Then, we study magneto-aerotaxis in magnetotactic bacteria, which is biased not only by an oxygen gradient towards a preferred concentration, but also by magnetic fields, which exert a torque on an intracellular chain of magnets. We study the competition of magnetic alignment with active reorientation and show that the magnetic orientation can improve chemotaxis and thereby provide an advantage to the bacteria, even at rather large inclination angles of the magnetic field relative to the oxygen gradient, a case reminiscent of what is expected for the bacteria at or close to the equator. The highest gain in chemotactic velocity is obtained for run and tumble with a magnetic field parallel to the gradient, but in general a mechanism for reverse motion is necessary to swim against the magnetic field and a run and reverse strategy is more advantageous in the presence of a magnetic torque. This finding is consistent with observations that the dominant mode of directional changes in magnetotactic bacteria is reversal rather than tumbles. Moreover, it provides guidance for the design of future magnetic biohybrid swimmers.
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  • Journal Article

    Effect of Hydrogen Charging on Pop-in Behavior of a Zr-Based Metallic Glass 

    Tian, Lin; Tönnies, Dominik; Hirsbrunner, Moritz; Sievert, Tim; Shan, Zhiwei; Volkert, Cynthia A.
    Metals 2020; 10(1): Art. 22
    In this work, structural and mechanical properties of hydrogen-charged metallic glass are studied to evaluate the e ect of hydrogen on early plasticity. Hydrogen is introduced into samples of a Zr-based (Vit 105) metallic glass using electrochemical charging. Nanoindentation tests reveal a clear increase in modulus and hardness as well as in the load of the first pop-in with increasing hydrogen content. At the same time, the probability of a pop-in occurring decreases, indicating that hydrogen hinders the onset of plastic instabilities while allowing local homogeneous deformation. The hydrogen-induced sti ening and hardening is rationalized by hydrogen stabilization of shear transformation zones (STZs) in the amorphous structure, while the improved ductility is attributed to the change in the spatial correlation of the STZs.
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  • Journal Article

    Nonlinear Loading-Rate-Dependent Force Response of Individual Vimentin Intermediate Filaments to Applied Strain 

    Block, Johanna; Witt, Hannes; Candelli, Andrea; Peterman, Erwin J. G.; Wuite, Gijs J. L.; Janshoff, Andreas; Köster, Sarah
    Physical Review Letters 2017; 118(4): Art. 048101
    The mechanical properties of eukaryotic cells are to a great extent determined by the cytoskeleton, a composite network of different filamentous proteins. Among these, intermediate filaments (IFs) are exceptional in their molecular architecture and mechanical properties. Here we directly record stress-strain curves of individual vimentin IFs using optical traps and atomic force microscopy. We find a strong loading rate dependence of the mechanical response, supporting the hypothesis that IFs could serve to protect eukaryotic cells from fast, large deformations. Our experimental results show different unfolding regimes, which we can quantitatively reproduce by an elastically coupled system of multiple two-state elements.
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  • Journal Article

    In vivo optochemical control of cell contractility at single‐cell resolution 

    Kong, Deqing; Lv, Zhiyi; Häring, Matthias; Lin, Benjamin; Wolf, Fred; Großhans, Jörg
    EMBO reports 2019; 20(12): Art. e47755
    The spatial and temporal dynamics of cell contractility plays a key role in tissue morphogenesis, wound healing, and cancer invasion. Here, we report a simple optochemical method to induce cell contractions in vivo during Drosophila morphogenesis at single-cell resolution. We employed the photolabile Ca2+ chelator o-nitrophenyl EGTA to induce bursts of intracellular free Ca2+ by laser photolysis in the epithelial tissue. Ca2+ bursts appear within seconds and are restricted to individual target cells. Cell contraction reliably followed within a minute, causing an approximately 50% drop in the cross-sectional area. Increased Ca2+ levels are reversible, and the target cells further participated in tissue morphogenesis. Depending on Rho kinase (ROCK) activity but not RhoGEF2, cell contractions are paralleled with non-muscle myosin II accumulation in the apico-medial cortex, indicating that Ca2+ bursts trigger non-muscle myosin II activation. Our approach can be, in principle, adapted to many experimental systems and species, as no specific genetic elements are required.
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  • Journal Article

    Dynamic Neural Fields with Intrinsic Plasticity 

    Strub, Claudius; Schöner, Gregor; Wörgötter, Florentin; Sandamirskaya, Yulia
    Frontiers in Computational Neuroscience 2017; 11: Art. 74
    Dynamic neural fields (DNFs) are dynamical systems models that approximate the activity of large, homogeneous, and recurrently connected neural networks based on a mean field approach. Within dynamic field theory, the DNFs have been used as building blocks in architectures to model sensorimotor embedding of cognitive processes. Typically, the parameters of a DNF in an architecture are manually tuned in order to achieve a specific dynamic behavior (e.g., decision making, selection, or working memory) for a given input pattern. This manual parameters search requires expert knowledge and time to find and verify a suited set of parameters. The DNF parametrization may be particular challenging if the input distribution is not known in advance, e.g., when processing sensory information. In this paper, we propose the autonomous adaptation of the DNF resting level and gain by a learning mechanism of intrinsic plasticity (IP). To enable this adaptation, an input and output measure for the DNF are introduced, together with a hyper parameter to define the desired output distribution. The online adaptation by IP gives the possibility to pre-define the DNF output statistics without knowledge of the input distribution and thus, also to compensate for changes in it. The capabilities and limitations of this approach are evaluated in a number of experiments.
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  • Journal Article

    Adaptive Control Strategies for Interlimb Coordination in Legged Robots: A Review 

    Aoi, Shinya; Manoonpong, Poramate; Ambe, Yuichi; Matsuno, Fumitoshi; Wörgötter, Florentin
    Frontiers in Neurorobotics 2017; 11: Art. 39
    Walking animals produce adaptive interlimb coordination during locomotion in accordance with their situation. Interlimb coordination is generated through the dynamic interactions of the neural system, the musculoskeletal system, and the environment, although the underlying mechanisms remain unclear. Recently, investigations of the adaptation mechanisms of living beings have attracted attention, and bio-inspired control systems based on neurophysiological findings regarding sensorimotor interactions are being developed for legged robots. In this review, we introduce adaptive interlimb coordination for legged robots induced by various factors (locomotion speed, environmental situation, body properties, and task). In addition, we show characteristic properties of adaptive interlimb coordination, such as gait hysteresis and different time-scale adaptations. We also discuss the underlying mechanisms and control strategies to achieve adaptive interlimb coordination and the design principle for the control system of legged robots.
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  • Journal Article

    Band structure evolution during the ultrafast ferromagnetic-paramagnetic phase transition in cobalt 

    Eich, Steffen; Plötzing, Moritz; Rollinger, Markus; Emmerich, Sebastian; Adam, Roman; Chen, Cong; Kapteyn, Henry Cornelius; Murnane, Margaret M.; Plucinski, Lukasz; Steil, Daniel; et al.
    Stadtmüller, BenjaminCinchetti, MirkoAeschlimann, MartinSchneider, Claus M.Mathias, Stefan
    Science Advances 2017; 3(3): Art. e1602094
    The evolution of the electronic band structure of the simple ferromagnets Fe, Co, and Ni during their well-known ferromagnetic-paramagnetic phase transition has been under debate for decades, with no clear and even contradicting experimental observations so far. Using time- and spin-resolved photoelectron spectroscopy, we can make a movie on how the electronic properties change in real time after excitation with an ultrashort laser pulse. This allows us to monitor large transient changes in the spin-resolved electronic band structure of cobalt for the first time. We show that the loss of magnetization is not only found around the Fermi level, where the states are affected by the laser excitation, but also reaches much deeper into the electronic bands. We find that the ferromagnetic-paramagnetic phase transition cannot be explained by a loss of the exchange splitting of the spin-polarized bands but instead shows rapid band mirroring after the excitation, which is a clear signature of extremely efficient ultrafast magnon generation. Our result helps to understand band structure formation in these seemingly simple ferromagnetic systems and gives first clear evidence of the transient processes relevant to femtosecond demagnetization.
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  • Journal Article

    Drebrin-like protein DBN-1 is a sarcomere component that stabilizes actin filaments during muscle contraction 

    Butkevich, Eugenia; Bodensiek, Kai; Fakhri, Nikta; von Roden, Kerstin; Schaap, Iwan A. T.; Majoul, Irina; Schmidt, Christoph F.; Klopfenstein, Dieter R.
    Nature Communications 2015; 6(1): Art. 7523
    Actin filament organization and stability in the sarcomeres of muscle cells are critical for force generation. Here we identify and functionally characterize a Caenorhabditis elegans drebrin-like protein DBN-1 as a novel constituent of the muscle contraction machinery. In vitro, DBN-1 exhibits actin filament binding and bundling activity. In vivo, DBN-1 is expressed in body wall muscles of C. elegans. During the muscle contraction cycle, DBN-1 alternates location between myosin- and actin-rich regions of the sarcomere. In contracted muscle, DBN-1 is accumulated at I-bands where it likely regulates proper spacing of α-actinin and tropomyosin and protects actin filaments from the interaction with ADF/cofilin. DBN-1 loss of function results in the partial depolymerization of F-actin during muscle contraction. Taken together, our data show that DBN-1 organizes the muscle contractile apparatus maintaining the spatial relationship between actin-binding proteins such as α-actinin, tropomyosin and ADF/cofilin and possibly strengthening actin filaments by bundling.
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  • Journal Article

    Game of Zones: how actin-binding proteins organize muscle contraction 

    Butkevich, Eugenia; Klopfenstein, Dieter R.; Schmidt, Christoph F.
    Worm 2016; 5(2): Art. e1161880
    Locomotion of C. elegans requires coordinated, efficient transmission of forces generated on the molecular scale by myosin and actin filaments in myocytes to dense bodies and the hypodermis and cuticle enveloping body wall muscles. The complex organization of the acto-myosin scaffold with its accessory proteins provides a fine-tuned machinery regulated by effectors that guarantees that sarcomere units undergo controlled, reversible cycles of contraction and relaxation. Actin filaments in sarcomeres dynamically undergo polymerization and depolymerization. In a recent study, the actin-binding protein DBN-1, the C. elegans ortholog of human drebrin and drebrin-like proteins, was discovered to stabilize actin in muscle cells. DBN-1 reversibly changes location between actin filaments and myosin-rich regions during muscle contraction. Mutations in DBN-1 result in mislocalization of other actin-binding proteins. Here we discuss implications of this finding for the regulation of sarcomere actin stability and the organization of other actin-binding proteins.
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  • Journal Article

    Three-Dimensional Spatiotemporal Pulse-Train Solitons 

    Lahav, Oren; Kfir, Ofer; Sidorenko, Pavel; Mutzafi, Maor; Fleischer, Avner; Cohen, Oren
    Physical Review X 2017; 7(4)
    Experimental realization of three-dimensional spatiotemporal solitons, which were proposed several decades ago, is still considered a “grand challenge” in nonlinear science. Here, we present experimental observation of 3D optical spatiotemporal pulse-train solitons. A spatially bright temporally dark pulse-train beam is trapped in a bulk medium that supports two types of nonlinearities: slowly responding saturable self-focusing that collectively self-trap the beam in the transverse directions and fast self-phase modulation that self-localizes each dark notch temporally (longitudinally). This work opens the possibility for experimental investigations of various soliton phenomena, including soliton interaction in 3D, formation of multimode spatiotemporal solitons, and envisioning new entities like partially coherent spatiotemporal solitons.
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  • Journal Article

    Efficient phenotypic sex classification of zebrafish using machine learning methods 

    Hosseini, Shahrbanou; Simianer, Henner; Tetens, Jens; Brenig, Bertram; Herzog, Sebastian; Sharifi, Ahmad Reza
    Ecology and Evolution p.1-12
    Sex determination in zebrafish by manual approaches according to current guidelines relies on human observation. These guidelines for sex recognition have proven to be subjective and highly labor‐intensive. To address this problem, we present a methodology to automatically classify the phenotypic sex using two machine learning methods: Deep Convolutional Neural Networks (DCNNs) based on the whole fish appearance and Support Vector Machine (SVM) based on caudal fin coloration. Machine learning techniques in sex classification provide potential efficiency with the advantage of automatization and robustness in the prediction process. Furthermore, since developmental plasticity can be influenced by environmental conditions, we have investigated the impact of elevated water temperature during embryogenesis on sex and sex‐related differences in color intensity of adult zebrafish. The estimated color intensity based on SVM was then applied to detect the association between coloration and body weight and length. Phenotypic sex classifications using machine learning methods resulted in a high degree of association with the real sex in nontreated animals. In temperature‐induced animals, DCNNs reached a performance of 100%, whereas 20% of males were misclassified using SVM due to a lower color intensity. Furthermore, a positive association between color intensity and body weight and length was observed in males. Our study demonstrates that high ambient temperature leads to a lower color intensity in male animals and a positive association of male caudal fin coloration with body weight and length, which appears to play a significant role in sexual attraction. The software developed for sex classification in this study is readily applicable to other species with sex‐linked visible phenotypic differences.
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  • Journal Article

    Effects of small‐scale dynamo and compressibility on the Λ effect 

    Käpylä, Petri J.
    Astronomische Nachrichten 2019; 340(8) p.744-751
    The Λ effect describes a rotation‐induced nondiffusive contribution to the Reynolds stress. It is commonly held responsible for maintaining the observed differential rotation of the Sun and other late‐type stars. Here, the sensitivity of the Λ effect to small‐scale magnetic fields and compressibility is studied by means of forced turbulence simulations either with anisotropic forcing in fully periodic cubes or in density‐stratified domains with isotropic forcing. Effects of small‐scale magnetic fields are studied in cases where the magnetic fields are self‐consistently generated by a small‐scale dynamo. The results show that small‐scale magnetic fields lead to a quenching of the Λ effect which is milder than in cases where also a large‐scale field is present. The effect of compressibility on the Λ effect is negligible in the range of Mach numbers from 0.015 to 0.8. Density stratification induces a marked anisotropy in the turbulence and a vertical Λ effect if the forcing scale is roughly two times larger than the density scale height.
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  • Journal Article

    Water vapor detection in the transmission spectra of HD 209458 b with the CARMENES NIR channel 

    Sánchez-López, A.; Alonso-Floriano, F. J.; López-Puertas, M.; Snellen, I. A. G.; Funke, B.; Nagel, E.; Bauer, F. F.; Amado, P. J.; Caballero, J. A.; Czesla, S.; et al.
    Nortmann, L.Pallé, E.Salz, M.Reiners, A.Ribas, I.Quirrenbach, A.Anglada-Escudé, G.Béjar, V. J. S.Casasayas-Barris, N.Galadí-Enríquez, D.Guenther, E. W.Henning, Th.Kaminski, A.Kürster, M.Lampón, M.Lara, L. M.Montes, D.Morales, J. C.Stangret, M.Tal-Or, L.Sanz-Forcada, J.Schmitt, J. H. M. M.Zapatero Osorio, M. R.Zechmeister, M.
    Astronomy & Astrophysics 2019; 630: Art. A53
    Aims. We aim at detecting water vapor in the atmosphere of the hot Jupiter HD 209458 b and perform a multi-band study in the near infrared with CARMENES. Methods. The water vapor absorption lines from the atmosphere of the planet are Doppler-shifted due to the large change in its radial velocity during transit. This shift is of the order of tens of km s−1, whilst the Earth’s telluric and the stellar lines can be considered quasi-static. We took advantage of this shift to remove the telluric and stellar lines using SYSREM, which performs a principal component analysis including proper error propagation. The residual spectra contain the signal from thousands of planetary molecular lines well below the noise level. We retrieve the information from those lines by cross-correlating the residual spectra with models of the atmospheric absorption of the planet. Results. We find a cross-correlation signal with a signal-to-noise ratio (S/N) of 6.4, revealing H2O in HD 209458 b. We obtain a net blueshift of the signal of –5.2 −1.3+2.6 km s−1 that, despite the large error bars, is a firm indication of day- to night-side winds at the terminator of this hot Jupiter. Additionally, we performed a multi-band study for the detection of H2O individually from the three near infrared bands covered by CARMENES. We detect H2O from its 0.96–1.06 μm band with a S/N of 5.8, and also find hints of a detection from the 1.06–1.26 μm band, with a low S/N of 2.8. No clear planetary signal is found from the 1.26–1.62 μm band. Conclusions. Our significant H2O signal at 0.96–1.06 μm in HD 209458 b represents the first detection of H2O from this band individually, the bluest one to date. The unfavorable observational conditions might be the reason for the inconclusive detection from the stronger 1.15 and 1.4 μm bands. H2O is detected from the 0.96–1.06 μm band in HD 209458 b, but hardly in HD 189733 b, which supports a stronger aerosol extinction in the latter, in line with previous studies. Future data gathered at more stable conditions and with larger S/N at both optical and near-infrared wavelengths could help to characterize the presence of aerosols in HD 209458 b and other planets.
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  • Journal Article

    Proposal to Detect Dark Matter using Axionic Topological Antiferromagnets 

    Marsh, David J. E.; Fong, Kin Chung; Lentz, Erik W.; Šmejkal, Libor; Ali, Mazhar N.
    Physical Review Letters 2019; 123(12): Art. 121601
    Antiferromagnetically doped topological insulators (ATI) are among the candidates to host dynamical axion fields and axion polaritons, weakly interacting quasiparticles that are analogous to the dark axion, a long sought after candidate dark matter particle. Here we demonstrate that using the axion quasiparticle antiferromagnetic resonance in ATIs in conjunction with low-noise methods of detecting THz photons presents a viable route to detect axion dark matter with a mass of 0.7 to 3.5 meV, a range currently inaccessible to other dark matter detection experiments and proposals. The benefits of this method at high frequency are the tunability of the resonance with applied magnetic field, and the use of ATI samples with volumes much larger than 1  mm^{3}.
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  • Journal Article

    Understanding stellar activity-induced radial velocity jitter using simultaneous K2 photometry and HARPS RV measurements 

    Oshagh, M.; Santos, N. C.; Figueira, P.; Barros, S. C. C.; Donati, J.-F.; Adibekyan, V.; Faria, J. P.; Watson, C. A.; Cegla, H. M.; Dumusque, X.; et al.
    Hébrard, E.Demangeon, O.Dreizler, S.Boisse, I.Deleuil, M.Bonfils, X.Pepe, F.Udry, S.
    Astronomy & Astrophysics 2017; 606: Art. A107
    One of the best ways to improve our understanding of the stellar activity-induced signal in radial velocity (RV) measurements is through simultaneous high-precision photometric and RV observations. This is of prime importance to mitigate the RV signal induced by stellar activity and therefore unveil the presence of low-mass exoplanets. The K2 Campaign 7 and 8 fields of view were located in the southern hemisphere, and provided a unique opportunity to gather unprecedented simultaneous high-precision photometric observation with K2 and high-precision RV measurements with the HARPS spectrograph to study the relationship between photometric variability and RV jitter. We observed nine stars with different levels of activity, from quiet to very active. We first probed the presence of any meaningful relation between measured RV jitter and the simultaneous photometric variation, and also other activity indicators (such as BIS, FWHM, log R′HK, and F8) by evaluating the strength and significance of the monotonic correlation between RVs and each indicator. We found that for the case of very active stars, strong and significant correlations exist between almost all the observables and measured RVs; however, when we move towards lower activity levels the correlations become random, and we could not reach any conclusion regarding the tendency of correlations depending on the stellar activity level. Except for the F8 whose strong correlation with RV jitter persists over a wide range of stellar activity level, and thus our result suggests that F8 might be a powerful proxy for activity-induced RV jitter over a wide range of stellar activity. Moreover, we examine the capability of two state-of-the-art modeling techniques, namely the FF′ method and SOAP2.0, to accurately predict the RV jitter amplitude using the simultaneous photometric observation. We found that for the very active stars both techniques can predict the amplitude of the RV jitter reasonably well; however, at lower activity levels the FF′ method underpredicts the RV jitter amplitude.
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  • Journal Article

    Recovery of subsurface profiles of supergranular flows via iterative inversion of synthetic travel times 

    Bhattacharya, Jishnu; Hanasoge, Shravan M.; Birch, Aaron C.; Gizon, Laurent
    Astronomy & Astrophysics 2017; 607: Art. A129
    Aims. We develop a helioseismic inversion algorithm that can be used to recover subsurface vertical profiles of two-dimensional supergranular flows from surface measurements of synthetic wave travel times. Methods. We carried out seismic wave-propagation simulations with a two-dimensional section of a flow profile that resembles an average supergranule and a starting model that only has flows at the surface. We assumed that the wave measurements are entirely without realization noise for the purpose of our test. We expanded the vertical profile of the supergranule stream function on a basis of B-splines. We iteratively updated the B-spline coefficients of the supergranule model to reduce the travel-time differences observed between the two simulations. We performed the exercise for four different vertical profiles peaking at different depths below the solar surface. Results. We are able to accurately recover depth profiles of four supergranule models at depths up to 8−10 Mm below the solar surface using f−p4 modes under the assumption that there is no realization noise. We are able to obtain the peak depth and the depth of the return flow for each model. Conclusions. A basis-resolved inversion performs significantly better than an inversion in which the flow field is inverted at each point in the radial grid. This is an encouraging result and might act as a guide in developing more realistic inversion strategies that can be applied to supergranular flows in the Sun.
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  • Journal Article

    Measuring solar active region inflows with local correlation tracking of granulation 

    Löptien, B.; Birch, A. C.; Duvall, T. L.; Gizon, L.; Proxauf, B.; Schou, J.
    Astronomy & Astrophysics 2017; 606: Art. A28
    Context. Sixteen years ago local helioseismology detected spatially extended converging surface flows into solar active regions. These flows play an important role in flux-transport models of the solar dynamo. Aims. We aim to validate the existence of the inflows by deriving horizontal flow velocities around active regions with local correlation tracking of granulation. Methods. We generate a six-year time series of full-disk maps of the horizontal velocity at the solar surface by tracking granules in continuum intensity images provided by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Results. On average, active regions are surrounded by inflows extending up to 10° from the center of the active region of magnitudes of 20–30 m/s, reaching locally up to 40 m/s, which is in agreement with results from local helioseismology. By computing an ensemble average consisting of 243 individual active regions, we show that the inflows are not azimuthally symmetric, but converge predominantly towards the trailing polarity of the active region with respect to the longitudinally and temporally averaged flow field.
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  • Journal Article

    Three planets around HD 27894 

    Trifonov, T.; Kürster, M.; Zechmeister, M.; Zakhozhay, O. V.; Reffert, S.; Lee, M. H.; Rodler, F.; Vogt, S. S.; Brems, S. S.
    Astronomy & Astrophysics 2017; 602: Art. L8
    Aims. Our new program with HARPS aims to detect mean motion resonant planetary systems around stars which were previously reported to have a single bona fide planet, often based only on sparse radial velocity data. Methods. Archival and new HARPS radial velocities for the K2V star HD 27894 were combined and fitted with a three-planet self-consistent dynamical model. The best-fit orbit was tested for long-term stability. Results. We find clear evidence that HD 27894 is hosting at least three massive planets. In addition to the already known Jovian planet with a period Pb≈ 18 days we discover a Saturn-mass planet with Pc≈ 36 days, likely in a 2:1 mean motion resonance with the first planet, and a cold massive planet (≈5.3 MJup) with a period Pd ≈ 5170 days on a moderately eccentric orbit (ed = 0.39). Conclusions. HD 27894 is hosting a massive, eccentric giant planet orbiting around a tightly packed inner pair of massive planets likely involved in an asymmetric 2:1 mean motion resonance. HD 27894 may be an important milestone for probing planetary formation and evolution scenarios.
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  • Journal Article

    Turbulent gas accretion between supermassive black-holes and star-forming rings in the circumnuclear disk 

    Chamani, Wara; Dörschner, Stephanie; Schleicher, Dominik R. G.
    Astronomy & Astrophysics 2017; 602: Art. A84
    While supermassive black-holes are known to co-evolve with their host galaxy, the precise nature and origin of this co-evolution is not clear. In this paper we explore the possible connection between star formation and black-hole growth in the circumnuclear disk (CND) to probe this connection in the vicinity close to the black-hole. We adopt here a circumnuclear disk model developed by previous works and explore both the dependence on the star formation recipe as well as the role of the gravitational field, which can be dominated by the central black-hole, the CND itself or the host galaxy. A specific emphasis is put on the turbulence regulated star formation model to explore the impact of a realistic star formation recipe. It is shown that this model helps to introduce realistic fluctuations in the black-hole and star formation rate, without overestimating them. Consistent with previous works, we show that the final black-hole masses are rather insensitive to the masses of the initial seeds, even for seed masses of up to 106M⊙. In addition, we apply our model to the formation of high-redshift quasars, as well as to the nearby system NGC 6951, where a tentative comparison is made in spite of the presence of a bar in the galaxy. We show that our model can reproduce the high black-hole masses of the high-redshift quasars within a sufficiently short time, provided a high mass supply rate from the host galaxy. In addition, it reproduces several of the properties observed in NGC 6951. With respect to the latter system, our analysis suggests that supernovae (SN) feedback may be important in creating the observed fluctuations in the star formation history as a result of negative feedback effects.
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