Meteors are light events appearing in the sky, looking like strips of light. A meteor occurs when a space object, a meteoroid, enters the earth’s atmosphere at high velocity and starts to glow. Meteor triangulation is an important part in astronomical research, by mapping streams of material in our solar system the knowledge about the solar system is extended. By making measurements with meteor cameras and calculate the meteors trajectories with triangulation methods, the meteors origin in space can be derived. Previous research has shown that by including time measurements, and not only use triangulation techniques with spatial measurements, the trajectory for a meteor can be calculated more accurately. This project seeks to explore if including the time information from the Swedish meteor data would create more precise solutions for the meteor’s paths through the atmosphere. This was done by investigating how the time variable can be implemented and running the Swedish 2020 Perseid data through the algorithm that implements the time variable. The accuracy of the methods is determined by how exact the Perseids radiant position is calculated and also by looking at the computed velocities for the meteor events. The results from this project show that the time variable should be considered in the Swedish meteor triangulation algorithm. With inclusion of the time variable, the radiant position of the Perseids can be more closely determined and the velocities for the meteors are in the expected range for Perseid meteors. But it turns out that the timestamps between the meteor cameras are not synchronized, which means that time offsets between the stations are required. / Meteorer är ljusfenomen som kan ses på natthimlen, de ser ut som ljusstreck och kallas även stjärnfall. En meteor inträffar när ett objekt från rymden, en meteoroid, kommer in i jordatmosfären med hög hastighet och börjar glöda. Meteortriangulering är en viktig del i astronomisk forskning, genom att kartlägga strömmar av material i solsystemet kan kunskapen om vårt solsystem breddas. Meteorers ursprung kan beräknas genom att göra mätningar med meteorkameror, för att sedan med trianguleringsmetoder beräkna dess banor genom atmosfären.  Tidigare forskning har visat att man kan få mer noggranna bestämningar för meteorspår genom att inkludera tidmätningar i beräkningarna, i Sverige används just nu endast trianguleringsmetoder som bygger på spatiella mätningar. Målet med det här projektet är att undersöka om inkludering av tidmätningar från den svenska meteordatan kan ge mer exakta lösningar för meteorspår. Detta undersöktes genom att kolla på hur tidsvariabeln kan integreras i den svenska trianguleringsalgoritmen och sedan testköra svensk perseiddata från år 2020. Precisionen i metoderna avgörs genom att kolla på hur exakt perseidernas radiantposition kan bestämmas och genom att kolla på meteorernas beräknade hastigheter i atmosfären. Resultaten från det här projektet visar att tidsvariabeln borde tas i beaktning i den svenska trianguleringsalgoritmen. Genom att inkludera tidsvariabeln kan radiantpositionerna bestämmas mer exakt och hastigheterna är inom det förväntade spannet för perseidmeteorer. Men det visade sig att tidmätningarna mellan de olika kamerorna är osynkroniserade, så tidsoffsets måste inkluderas i algoritmen.

By including dissipative fluxes in the description, this thesis extends previous results regarding first order perturbations on homogeneous and hypersurface orthogonal locally rotationally symmetric (LRS) class II cosmologies using the 1 + 1 + 2 covariant split of spacetime. Whereas previous works consider perturbations of perfect fluid type, perturbations pertaining to heat flux and fluid viscosity are here studied with the aim to ascertain their effect on the evolution of the fluid vorticity. The studied perturbations include scalar, vector, and tensor modes, and are harmonically decomposed to yield a system of ordinary differential equations. These equations, originating from the Bianchi identities, the Ricci identities for certain preferred vector fields, and the thermodynamic Eckart theory, then decouple into two independent systems. These separately closed systems, with four and eight remaining variables respectively, describe the evolution of perturbations pertaining to the Weyl tensor and the fluid shear, vorticity, heat flow, energy density, and number density. From the final system of equations it is seen that the inclusion of heat flux and fluid viscosity has the possibility to yield mechanisms for generating vorticity, even if this vorticity vanishes initially. This is in contrast to the case of barotropic perfect fluids, for which it can be shown that vorticity perturbations cannot be generated. The validity of the results presented here can be questioned, as the Eckart theory, which violates causality, is employed to describe the detailed thermodynamic properties of the fluid. However, on time scales much larger than the relaxation times of the fluid, it should still provide a decent description of the dissipative phenomena, provided that certain couplings between the dissipative fluxes can be neglected.

In this Licentiate thesis I review the properties of the solar atmosphere and the diagnostic value ofdifferent spectral lines in the visible and ultraviolet (UV) along with the millimeter (mm) continua in theelectromagnetic spectrum of the Sun.While the solar atmosphere has been routinely observed in high-resolution from ground-based opti-cal telescopes such as the Swedish Solar Telescope (SST), and more recently in the UV from space tele-scopes such as the Interface Region Imaging Spectrograph (IRIS), radio observations lag behind despitetheir great usefulness. This is likely to change thanks to the Atacama Large Millimeter Array (ALMA)that only started observing the Sun in 2016 with a few limitations, but the first results are promising.ALMA observations probe the solar chromosphere at different heights by tuning into slightly differentfrequencies at potentially milliarcsecond scales if the full array is able to operate with the longest base-lines. This new spectral window onto the Sun is expected to advance various fields of research suchas wave propagation and oscillations in the chromosphere, thermal structure of filaments/prominences,triggering of flares and microflares, and more generally chromospheric and coronal heating, because themm-intensities can be modelled by simply assuming local-thermodynamic equilibrium.In da Silva Santos et al. (2018) we find that coordinated observations from SST, IRIS and ALMA willpermit us to estimate with greater accuracy the full thermodynamical state of the plasma as a functionof optical depth based on experiments with a snapshot of a three-dimensional magnetohydrodynamicsimulation of the Sun’s atmosphere. Particularly, the mm-continuum improves the accuracy of inferredtemperatures in the chromosphere. Here we expand on the Why and How this can be done. The goal isto better constrain the temperature stratification in the solar atmosphere in order to understand chromo-spheric heating.

Solar plage has been the topic of many studies since its initial description in the mid 19th century, but as of yet it has not been understood to the point where we can reproduce all aspects of these active regions in quasi-realistic numerical models. To a large extent, this is caused by an incomplete understanding of the magnetic structure that drives the activity in these areas. Detailed measurements have been done of the magnetic field configuration of plage in the photosphere since the late 20th century, but only a handful of papers have managed to make any measurements at all in the higher situated chromosphere, despite the fact that the magnetic field vector of plage is important in understanding chromospheric magnetic fields in general, as well as the heating processes of the higher atmosphere. In Pietrow et al. (2020) we add to these measurements by introducing what is to our knowledge the first full Stokes inversion of chromospheric plage, which allowed us to estimate the magnetic field vector at an optical depth of logτ = -3.5. The obtained value is |B| = 440 ± 90 G in the plage with an inclination of 10° ± 16° with respect to the local vertical. Our reported magnetic field strength matches with a recent study by Morosin et al. (2020), but is higher by a factor of two or more compared to previous studies that measured the field using other methods. Additionally we measure an average magnetic field strength of |B| = 300 ± 50 G in a fibrillar region close to the plage. In this thesis we explore the difficulties of measuring this magnetic field vector. Since plage exists in a complex environment, we will begin with a general description of the structure and properties of the solar atmosphere and the layers from which it is composed, as well as review the types of active regions that can be found in the solar atmosphere. Our focus then narrows to the chromosphere, the diagnostic properties of spectral lines that are sensitive to this layer (mainly the \cair line), plage regions, and plage chromospheric magnetic fields. Additionally, we touch upon the theory of radiative transfer and how physical characteristics of the atmosphere can be inferred from polarised light. We also give attention to the observing process with the Swedish 1-m Solar Telescope (SST) and the workings of the reduction pipeline and post-reduction methods as well as the process spectropolarimetric inversions. Finally, once we have understood why and how this project has been done, we summarize our findings and compare them to current literature.

No description available.

Developing efficient and accurate numerical methods to simulate dynamics of physical systems has been an everlasting challenge in computational physics. Physics-Informed Neural Networks (PINNs) are neural networks that encode laws of physics into their structure. Utilizing auto-differentiation, they can efficiently solve partial differential equations (PDEs) by minimizing the loss function at certain points within the domain of interest. The remarkable efficiency exhibited by these networks when solving PDEs positions them as ideal solvers for simulating complex systems.  In this pioneering work, we take a first step towards simulating galaxy dynamics using PINNs by solving the gravitational Poisson equation. We initially substantiate the capacity of PINNs to solve the gravitational Poisson equation for the simple Hernquist (Hernquist, 1990) radial density profile, and for the parametric Dehnen (Dehnen, 1993) radial density profile. Following this, we extended our study to encompass a more complex axisymmetric density profile describing a Thick Exponentiel Disk.  The capacity of PINNs to generate comparatively accurate results has been validated with an average error of 1.71% and 3.75% respectively for the spherically symmetric Hernquist and Dehnen models. While for the axisymmetric thick exponentiel disk model the PINN demonstrated an average relative error of 0.36% with a maximum error of just 0.99% after fine-tuning the PINN’s hyperparameters. Although this model typically relies on the two coordinates R and z along with the ratio η of the model’s scale lengths, the PINN is here trained using a fixed, predetermined value of η.  Drawing upon the outcomes of the grid search implemented for the thick exponen- tial disk model, we provide a succinct examination of how the hyperparameters of the PINN impact the relative error. Given the limited quantity of datapoints, we refrain from formulating definitive conclusions, yet we do exhibit certain discernible patterns. Specifically, we demonstrate that the hyperbolic tangent (tanh) activation function con- sistently outperforms other activation functions in the context of our model. Addition- ally, it appears that augmenting the depth of the network offers superior error reduction in comparison to increasing its width, reinforcing the importance of architectural con- siderations in the optimization of Physics-Informed Neural Networks  Our results show clear advantages of PINNs over regular solvers in terms of efficiency. Despite the success of the two-parameter PINN for the thick exponential disk, further work is required to confirm its extension to three dimensions. This pioneering research offers a promising foundation for further developments in the field, and demonstrates the genuine practical utility of PINNs for simulating complex systems such as galaxies.

A core-collapse supernova (CCSN) is an astronomical explosion that indicates the death of a massive star. The iron core of the star collapses into either a neutron star or a black hole while the rest of the material is expelled at high velocities. Supernovae (SNe) are important for the chemical evolution of the Universe because a large fraction of the heavier elements such as oxygen, silicon, and iron are liberated by CCSN explosions. Another important role of SNe is that the ejected material seed the next generation of stars and planets. From observations, it is clear that a large fraction of all massive stars undergoes SN explosions, but describing how SNe explode has remained a challenge for many decades. The attached papers focus on comparing theoretical predictions with observations, primarily observations of SN 1987A. The compact remnant in SN 1987A has not yet been detected and we have investigated how a compact object can remain hidden in the ejecta (Paper I and II). Because of the high opacity of the metal-rich ejecta, the direct X-ray observations are not very constraining even for potentially favorable viewing angles. However, the combined observations still strongly constrain fallback accretion and put a limit on possible pulsar wind activity. The thermal surface emission from a neutron star is consistent with the observations if our line of sight is dust-obscured, and only marginally consistent otherwise. Future observations provide promising opportunities for detecting the compact object. We have also compared the most recent three-dimensional neutrino-driven SN models that are based on explosion simulations with early X-ray and gamma-ray observations of SN 1987A (Paper III). The models that are designed to match SN 1987A fit the data well, but not all tensions can be explained by choosing a suitable viewing angle. More generally, the asymmetries do not affect the early emission qualitatively and different progenitors of the same class result in similar early emission. We also find that the progenitor metallicity is important for the low-energy X-ray cuto↵. Current instruments should be able to detect this emission from SNe at distances of 3–10 Mpc, which correspond to distances slightly beyond the Local Group. / En kärnkollapssupernova (CCSN) är en astronomisk explosion som indikerar slutet av en massiv stjärnas liv. Stjärnans järnkärna kollapsar antingen till en neutronstjärna eller ett svart hål medan resten av materialet slungas iväg med höga hastigheter. Supernovor (SNe) är viktiga för Universums kemiska utveckling eftersom en stor andel av alla tyngre element såsom syre, kisel, och järn frigörs i CCSN-explosioner. Ytterligare en viktig roll för SNe är att nästa generations stjärnor och planeter bildas av det utkastade materialet. Från observationer är det tydligt att en stor andel av alla massiva stjärnor genomgår SN-explosioner, men att förklara hur SNe exploderar har kvarstått som en utmaning under flera decennier. De bifogade artiklarna fokuserar på att jämföra teoretiska förutsägelser med observationer, primärt observationer av SN 1987A. Det kompakta objektet i SN 1987A har ännu inte blivit detekterat och vi har undersökt hur ett kompakt objekt kan förbli dolt i ejektat (Paper I och II). De direkta röntgenobservationerna är inte så begränsande även längs potentiellt gynsamma siktlinjer på grund av det metallrika ejektats höga opacitet. Däremot begränsar kombinationen av alla observationer starkt ackretion och sätter en gräns för möjlig pulsarvindsaktivitet. Den termiska ytstrålningen från en neutronstjärna är konsistent med observationerna om vår siktlinje är skymd av stoft, och bara marginellt konsistent annars. Framtida observationer utgör lovande möjligheter för att detektera det kompakta objektet. Vi har också jämfört de senaste tredimensionella neutrinodrivna SN-modellerna, som är baserade på explosionssimuleringar, med tidiga röntgen- och gamma-observationer av SN 1987A (Paper III). SN 1987A-modellerna passar datan väl, men alla diskrepanser kan inte förklaras av ett lämpligt val av observationsvinkel. Generellt så påverkar inte asymmetrierna den tidiga emissionen kvalitativt och olika föregångarstjärnor av samma kategori resulterar i likartad strålning. Vi finner också att föregångarstjärnans metallisitet är viktig för egenskaperna av lågenergiröntgenstrålningen. Befintliga instrument borde kunna detektera denna emission på 3--10 Mpc, vilket motsvarar avstånd lite bortom den Lokala galaxhopen. / <p>Examintor: Professor Mark PearceQC 20190121</p>

Recent detections of massive quasars at z &gt; 6 has proven to be an interesting challengefor cosmology given the short time frame that these objects have to reach a certainmass via conventional methods (e.g., galaxy mergers). Direct collapse black holes(DCBHs) are theorized black hole seeds that propose a neat solution to the formationof supermassive black holes (SMBHs) by z ≳ 6. During this report, we followed thefirst photometric method developed to identify these black hole seeds, adapting it tothe James Webb Space Telescope (JWST ) photometric system to facilitate its usefor upcoming data. Two DCBH models were used to discard nearby objects (e.g.,cool stars, brown dwarfs and giant exoplanets) as possible interlopers, and allowed usto identify a possible confusion between DCBH signatures and high-redshift galaxies(z = 6) within an age range of 0.8-0.9 Gyr. This confusion could grow larger if wereconsider the analysis adding dust to the galaxy models we used

<p>The properties of Mercury's regolith have been investigated at optical and near-infrared wavelengths with high-resolution imaging, photometric, and spectroscopic observations with the Swedish Vacuum Solar Telescope and the Nordic Optical Telescope. </p><p>A new global optical map at a spatial resolution of 200 km shows that the well known (from Mariner 10) and poorly known hemispheres are indistinguishable with respect to the distribution, number density, and morphological parameters of bright albedo features. This indicates a globally uniform recent (<3 Gyr) geologic evolution, a compositionally very homogeneous surface and the absence of a lunar-like mare–terrae albedo dichotomy.</p><p>It is found that the spectrum of Mercury is linear, strongly sloped, lacks detectable absorption features and displays a unique relation between the continuum slope and photometric geometry. Mercury has a photometrically smoother surface than the average near-side Moon, and is 10–15% fainter and 50% more back scattering in the V-band. Unlike the case for the Moon, the average single-particle backscattering anisotropy increases with wavelength.</p><p>Intimate regolith mixing models are used to determine a probable surface composition of predominantly Ca-rich labradorite plagioclase feldspar with minor low-iron enstatitic orthopyroxene, and rule out high-iron pyroxenes or olivines as other than insignificant constituents. Abundances of FeO ~1.2 wt%, TiO₂ ~0 wt%, and submicroscopic metallic iron ~0.1–0.3 wt% are found for the average surface. This implies an optically active grain size of 15–30 μm that is a factor of two smaller than for the Moon.</p><p>A numerical integration study shows that hermeocentric orbits with semi-major axes <30 mercurian radii for elliptic retrograde, and circular prograde, object are stable for durations in excess of 4.5 Myr. The weak gravitational scattering effect of Mercury indicates that re-impacting particles may have been important for the early evolution of its crust.</p>

<p>Comets are the most primordial and least evolved bodies in the Solar System. As such, they are unique sources of information regarding the early history of the Solar System. However, little is known about cometary nuclei since they are very difficult to observe due to the obscuring coma. Indirect methods are therefore often used to extract knowledge about nucleus parameters such as size, shape, density, material strength, and rotational properties. For example, tidal and non-tidal splitting of cometary nuclei can provide important information about nuclear densities and material strengths, but only if the criteria for mechanical stability are well known. Masses and densities of cometary nuclei can also be obtained by studying orbital modifications due to non-gravitational forces, but only if the thermophysics of comets can be modelled accurately. </p><p>A detailed investigation is made regarding the mechanical stability of small Solar System bodies. New expressions for the Roche distance are derived, as functions of the size, shape, density, material strength, rotational period, and spin axis orientation of a body. The critical rotational period for centrifugal breakup in free space is also considered, and the resulting formulae are applied to comets for which the size, shape and rotational period have been estimated observationally, in order to place constraints on their densities and material strengths. </p><p>A new thermophysical model of cometary nuclei is developed, focusing on two rarely studied features - layer absorption of solar energy, and parallel modelling of the nucleus and innermost coma. Sophisticated modelling of radiative transfer processes and the kinetics of gas in thermodynamic non-equilibrium form the basis for this work. The new model is applied to Comet 19P/Borrelly, and its density is estimated by reproducing the non-gravitational changes of its orbit.</p>