General Seminar (2024)
current | 2024 | 2023 | 2022 | 2021 | 2020 | 2019 | 2018 | 2017
General Seminar takes place every Monday at 11.15 in Piwnice, radioastronomy seminar room.
Remote participation via the BigBlueButton (BBB).
8 January 2024
Cancelled
15 January 2024
“Interstellar medium of spiral galaxies heated by magnetic reconnection?”
dr Marek Weżgowiec (Astronomical Observatory, Jagiellonian University)
Abstract:
In several face-on spiral galaxies a similar spiral structure of the ordered magnetic fields have been observed using sensitive radio polarimetry. These so-called magnetic arms are ‘phase-shifted’ with the spiral arms and their nature is not fully understood. This behaviour of the magnetic fields might be explained by magnetic reconnection that would increase their ordering. Theoretical studies show that magnetic reconnection at the same time can efficiently heat the low-density interstellar medium. To study both effects in more detail we used the XMM-Newton X-ray data and the VLA/Effelsberg radio polarimetric data for several galaxies in which the magnetic arms were found. We compared the properties of the magnetic fields (strengths and energy densities) and of the hot gas (temperatures and energy densities) in selected areas of galactic disks. The analysis revealed signatures of possible increase in thermal energy of the hot gas that can be attributed to the action of magnetic reconnection effects. This seems to be more easily detected in the inter-arm regions than in the spiral arms of galaxies. Our studies also suggest that such ‘reconnection heating’ may be also efficient in galactic halos, where the density of the ISM is lowest.
22 January 2024
“Chemically-homogeneously evolving stars in the dwarf galaxy IZw18?”
dr Dorottya Szécsi (Institute of Astronomy, Nicolaus Copernicus University)
Abstract:
Chemically-homogeneously evolving stars have been proposed to account for several exotic phenomena, including gamma-ray bursts, gravitational-wave emissions and certain types of supernovae. Here I show how these stars can account for observations of the metal-poor dwarf galaxy, I Zwicky 18. Applying new PoWR synthetic spectral models to predict the photonionizing flux and the strength of observable stellar lines from a I Zw 18-like population, we compare the results to all available optical and UV data from the literature. We show that the measurements are consistently explained by our models. The source of the stellar emission, such as the UV C-IV (λ1550 Å) line, is a handful of WN and/or WO-type Wolf–Rayet stars (skipping the WC phase completely, which contradicts what was previously assumed), while the high He-II ionizing flux is accounted for by the chemically-homogeneously evolving stars which happen to be extremely hot.
29 January 2024
“Towards a holistic understanding of sub-Neptunes.”
dr Rafael Luque (Department of Astronomy and Astrophysics, University of Chicago)
Abstract:
The nature and origin of sub-Neptune-sized planets is arguably the hottest debate in the field of exoplanets nowadays. While absent in the Solar System, they are the most common planet type in the Galaxy. Multiple models (gas dwarfs, water worlds, Hycean planets) appear to explain current observational evidence from mass-radius measurements and demographic analyses. JWST promises to break those degeneracies, but the first robust results are yet to be published. In the meantime, the discovery and characterization of benchmark targets amenable for these studies is the top priority. In this talk, I will give an overview of the questions surrounding the origin of the “Radius gap”, recent sub-Neptune discoveries such as the 6-planet resonant chain orbiting HD 110067, and the latest efforts in the community to detect molecular species in their atmospheres.
5 February 2024
“Populations of short-period super-Earths, sub-Neptunes, and Neptune-sized planets and prospects for their atmospheric characterisation”
dr Grzegorz Nowak (Institute of Astronomy, Nicolaus Copernicus University)
Abstract:
Space-based, high-precision photometric missions to detect transiting planets, such as CoRoT, Kepler, Kepler-K2 and TESS, have revolutionised our understanding of extrasolar planets, particularly for the smallest planets of 1-4 Earth radii. Probably the most important result we owe to the original Kepler mission is a bimodal structure in the radius distribution of the close-in (Pp < 100 d) small planets, with a gap around 1.7 R_Earth that separates the two main classes of small planets: presumably rocky super-Earths with radii around 1.2 R_Earth, and sub-Neptunes with radii around 2.4 R_Earth. Detailed studies of the population of small planets with short periods require precise dynamical mass measurements via radial velocities. These measurements, together with radius, provide information about the bulk densities of the planets. However, measurements of mass, radius, and mean density alone are insufficient to answer all questions about their origin, initial formation location, and internal composition. But since different classes of small planets should have different atmospheres with different observable signatures, their atmospheric studies should help us a lot to answer the above questions. In this talk, I am going to present recent radial velocity follow-up results from TESS planetary systems with small planets and prospects for their atmospheric characterisation. These results include two warm Neptunes transiting HIP 9618, four sub-Neptunes orbiting the K dwarf TOI-1246, TOI-1416 - a system with a super-Earth planet with a period of 1.07 days, and TOI-1136 - a new TESS system with six confirmed transiting sub-Neptunes and Neptunes in resonance, and a seventh single-transiting planet candidate around a young (~700 Myr), active G5 dwarf.
26 February 2024
“Tangle of Chaos in Standing Gravitational Waves”
MSc Syed Naqvi (Astronomical Observatory, Jagiellonian University)
Abstract:
Standing waves, well-studied in mechanical and electromagnetic domains, have been studied within the Einstein-Rosen spacetime, representing cylindrical standing gravitational waves. Our study aimed to analyze the behavior of test particles’ orbits within this spacetime. We observed a sensitivity to initial conditions in the trajectories of these particles. Additionally, we discover the fractal associated with this system. Examining the Poincare map, we identified intricate crossings of stable and unstable manifolds at hyperbolic fixed points. A chaotic heteroclinic network further characterized the complex dynamics of massive test particles. Analogous to phenomena such as mechanical vibrations generating Chladni figures and the intricate shapes of Faraday waves, gravitational standing waves produce complex patterns through the interactions of test particles.
4 March 2024
“Clouds vs Cherenkov telescopes: problems and solutions”
prof. dr hab. Julian Sitarek (Computer Simulations Laboratory, University of Łódź)
Abstract:
Very-high-energy (VHE >~100 GeV) gamma rays are absorbed in Earth’s atmosphere and thus cannot be detected directly on Earth. Their fluxes are also typically too low to efficiently study them with satellite instruments. A VHE gamma ray entering the atmosphere initiates an electromagnetic cascade that induces faint flashes of blueish Cherenkov light. Such flashes can be then detected by Imaging Atmospheric Cherenkov Telescopes registering images of passage of individual gamma rays through the atmosphere. The usage of atmosphere as a part of the detector allows us to achieve a collection area of gamma rays over two orders of magnitude higher than the physical size of the detector. But it also introduces systematic errors connected with the atmosphere’s transparency. In particular, cloud presence during the observations can significantly affect the data. In this seminar I will cover different methods used to correct the influence of the clouds. I will show how lack of such a correction introduces bias in the energy estimation of gamma rays. I will present how the affected images of showers are degraded and thus can be confused with background events, lowering the collection area of the telescope. Finally, I will show a novel method of correcting the influence of the clouds already at the image level, and discuss the possibility of measuring the parameters of a cloud directly with the observations by the Cherenkov telescopes.
11 March 2024
“Maneage: a proof-of-concept for rigorous reproducible research papers”
prof. dr hab. Boudewijn Roukema (Institute of Astronomy, Nicolaus Copernicus University)
Abstract:
In astronomy, both galactic and extragalactic, we often want to check what other astronomers have done in the final version of their peer-reviewed, published, article, so that we can compare it directly to our own work or so that we can do followup work. In practice, details that are missing from the paper, such as the lack of input parameters, the lack of software versions, missing information about which specific libraries and compilers were used, and how the data were analysed, can force us to spend a month or longer to do this. The idea of reproducible quantitative research papers is that using the same observational or theoretical data or parameters, and the exact same method as the authors, we should be able to calculate identical results, rather than speculate about what the authors did. Astronomers have introduced an approach to this called “Maneage”, which aims to yield peer-reviewed, published papers whose results are reproducible either immediately or a decade later by scientists with basic astronomical software skills (shell, make, gcc). This seminar will present an overview of Maneage (Akhlaghi+2021, CiSE 23, 82 DOI:10.1109/MCSE.2021.3072860, arXiv:2006.03018).
18 March 2024
“Magnetic flux saturation mechanism at accreting black holes”
dr hab. Krzysztof Nalewajko, prof. CAMK PAN (Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences)
Abstract:
Black holes (BH) acquire relativistic magnetospheres by accreting magnetized gas. Once they collect significant magnetic flux across the horizon, aided by the spin they can drive powerful relativistic jets by the Blandford-Znajek mechanism. Large enough BH magnetic flux backreacts on the accretion flow, which has been described in terms of arresting or choking. Magnetic flux eruptions have been identified as the mechanism of BH magnetic flux saturation. These eruptions can potentially dissipate a large fraction of magnetic energy in the BH magnetosphere by means of relativistic magnetic reconnection, accelerating particles and producing flares of non-thermal radiation. We analyze the results of 3D general-relativistic ideal magnetohydrodynamic (GRMHD) numerical simulations of accretion flows onto magnetically saturated Kerr BHs, focusing on the initiation of magnetic flux eruptions.
25 March 2024
“Exploring exoplanets with radial velocity observations”
MSc Hannah Osborne (Mullard Space Science Laboratory, University College London, UK)
Abstract:
The first confirmed exoplanet orbiting a main sequence star was detected by the radial velocity (RV) method, and in the nearly 30 years since then the technique has continued to improve. In this talk I will highlight some of the benefits of using RVs to study exoplanets, as well as some of the struggles ? particularly from an observational perspective. I will also present some recent work using RV observations to characterise a newly discovered small planet which sits inside the small planet radius valley and could have a water-world composition. Finally, I will give an overview of what we can expect with future RV observations and the potential implications for exoplanets more generally.
8 April 2024
“Constrained simulations from peculiar velocities”
dr Simon Pfeifer (Leibniz-Institute for Astrophysics Potsdam, Germany)
Abstract:
Constrained simulations aim to reproduce the real Universe not just in a statistical sense but also by matching the positions and properties of observed structures. I will give a broad overview of the field of constrained simulations, and in particular the methodology of the CLUES collaboration, which uses peculiar velocities of galaxies as constraints. I will show some of the achievements from recent years and discuss the next and ongoing projects.
15 April 2024
“The old Milky Way: investigating the past of our Galaxy”
dr hab. Rodolfo Smiljanic, prof. CAMK PAN (Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences)
Abstract:
Our knowledge of how the Milky Way formed and evolved is experiencing an on-going revolution, thanks to the overwhelming amount of data produced by several large stellar surveys; Gaia being the most extraordinary example of this. The study of old metal-poor stars is crucial in this context, as long-lived low-mass stars keep in their atmospheres the chemical composition of the interstellar medium at the time and place of their birth. The combination of chemical abundances with stellar orbits, in large samples of metal-poor stars, is revealing the complicated history of mergers that took place in the early stages of the Milky Way. In this seminar, I will summarise on-going work that is being done in my group, with the aim of better understanding the past of the Milky Way. I will discuss how chemical differences are being used to distinguish accreted stars from those formed in situ, the precise dating of the last major merger suffered by the Milky Way, and what accreted stars can tell about the site of r-process nucleosynthesis. I will finish with some perspective for the future, mentioning new surveys and instruments that will help sustain the revolution in this field.
22 April 2024
“Orbital analysis of multiple systems with jovian planets”
prof. dr hab. Krzysztof Goździewski (Institute of Astronomy, Nicolaus Copernicus University)
Abstract:
In the past thirty years, a category of planetary systems that includes large Jupiter-like planets has been identified. The gravitational interactions between the components of these systems have a measurable impact on orbital models that describe the observational time series, particularly through radial velocity, astrometric, and chronometric methods. The aim of this presentation is to introduce systems that contain Jovian planets for which it is particularly important to base the orbital model on physical and mathematical principles. These principles are generally described as the N-body problem. In this context, we will analyse a template planetary system HD160691. Iterative studies of such systems provide information on the orbital architecture, distribution of orbital parameters, and planet masses, which may become increasingly important for extended observational intervals as new data becomes available and modelling methods are refined.
29 April 2024
“Understanding exoplanets through their host stars”
prof. Vincent Van Eylen (Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, UK)
Abstract:
During the past 30 years over 5000 exoplanets have been discovered. I review what is known about these planets and their formation. I show how most of what is known about these planets is inferred indirectly through their host stars, making it crucial to understand stellar properties to learn about exoplanet systems. I discuss the architecture of exoplanet systems, and highlight a fascinating paradigm shift in exoplanet science: the realisation that small planets come in two distinct flavours, i.e., super-Earth and sub-Neptune planets, separated by a radius valley. The valley is key to understanding the formation of small planets, as its location and slope as a function of orbital period are shaped by stellar radiation, disk properties, and planet composition. I show what can be learned through detailed modelling of Kepler and TESS transiting planets, and what has been learned through improving stellar parameters using spectroscopy, Gaia, and asteroseismology. Turning to giant planets, I show how TESS observations have been used to infer the occurrence of Jupiters for different types of stars, and what this implies for planet formation models. Finally, I show how homogeneous studies of stellar properties such as mass, radius, and age, are crucial to better understand the diversity of exoplanets and discuss the important role of the upcoming PLATO mission in this regard.
6 May 2024
“The binary context of blue large-amplitude pulsators”
prof. dr hab. Andrzej Pigulski (Astronomical Institute, Faculty of Physics and Astronomy, University of Wrocław, Poland)
Abstract:
Blue large-amplitude pulsators (BLAPs) form a distinctive group of pulsating stars discovered a few years ago and now have about 70 identified members. These stars exhibit pulsation periods ranging from 15 to 60 minutes, characterised by non-sinusoidal light curves with peak-to-peak amplitudes of up to 0.4 mag in the V band. They pulsate radially. In the Hertzsprung-Russell (H-R) diagram, BLAPs are located between hot massive main-sequence stars and hot subdwarfs. Revealing the evolutionary status of BLAPs is presently a big challenge. To address this problem, several different evolutionary scenarios have been proposed. Crucial to these scenarios is the assumption that these stars are or were the members of binary systems. In my talk, I will summarise the current findings related to the binarity of BLAPs and discuss them in the context of the proposed evolutionary scenarios.