At the end of the lives of low- to intermediate mass stars they can be found on the asymptotic giant branch (AGB). The AGB phase ends when the entire circumstellar envelope (CSE) is blown away in a superwindphase, in the end creating a planetary nebula. It is unknown what shapes the CSE and the planetary nebula. Binarity is a favored theory. In order to test this theory the CSE around the star R Aquarii has been studied using the emission from different molecules observed with ALMA. R Aquarii is a nearby binary system and therefore easy to study. The system consists of a Mira variable on the AGB and a hot white dwarf. It was found that only in the emission from the 12CO J=3–2 transition were the CSE resolved enough for any structure to be seen. The morphology was irregular and no clear symmetry was seen. A spot in the same molecular line was detected at high velocities (v = -23 km/s) relative to the star at a projected distance of 7 arcsec south of R Aqr. Line profiles for 12CO and 13CO follow the same shape but differs in magnitude, indicating that they can be found in the same structure. A mass loss rate of 6.5·10-7 solar masses per year was calculated for R Aquarii using line intensities obtained from the line profile of 12CO. The morphology and kinematics of the CO CSE of R Aquarii are discussed within the limitations of the current data set. More observations with better resolution are needed to better understand the morphology of the CSE of R Aquarii and draw firm conclusions.

The presence of dark matter is inferred at scales ranging from rotations of galaxies to imprints in the CMB – the Big Bang after-glow. The nature of dark matter is, however, still unknown as no detection other than the gravitational one has been made. This thesis presents two analyses searching for a neutrino signal from dark matter annihilations in the Milky Way. The first analysis searched for an excess of νμ charged current events with directions from the central region of the dark matter halo and, was focused on low energy events, thus probing low dark matter particle masses. Approximately 319 days of data collected with the 79-string configuration of the IceCube detector was used in the analysis. Despite a large deficit in the number of observed events the data were found to be consistent with background and upper limits were set on <σⱴ>. At the time of the analysis these limits were the strongest set by a neutrino experiment below 100 GeV. The second analysis was performed on a data sample originally used in an unfolding analysis of the atmospheric and astrophysical neutrino spectra. The data consisted of contained cascade events above 1 TeV collected with the 79-string configuration and the completed detector in the 86-string configuration during two years of data-taking. The limits set by this analysis were more constraining by up to a factor of 10 compared to previous IceCube analyses, and the most competitive limits are set assuming a Burkert halo profile. These two analyses prompted the development of a signal subtraction likelihood method to address the problem of signal contamination in background estimates based on scrambled data. Additionally a study concerning future extensions of IceCube in the Gen2 project is presented. The cascade reconstruction performance was examined and compared for different proposed detector extensions.

Through the spectral synthesis model YGGDRASIL, developed by E. Zackris- son et al. 2011, luminosities for the chemically pristine population III galaxies are retrieved and compared to the capabilities of the upcoming infrared telescopes, in particluar the Wide Field InfraRed Survey Telescope (WFIRST). In order to push the very faint galaxies into the detectable regime of the telescope, magnification by gravitational lensing needs to be introduced. The probabilities for the nec- essary magnifications at different redshifts in combination with the depth of the telescope are translated into required minimum number densities and minimum formation rates for detecting one object per survey area. Both photometric and spectroscopic detections are investigated and compared to limits predicted by theory. Three different initial mass functions (IMF) for the stars that make up the galaxy are used, two that are top-heavy, i.e centers around stellar masses of 10 − 500M⊙, and one that resembles the IMF that are used for stars of younger generations, with its distribution peak at below one solar mass. The most optimistic results comes from the model that focusses on a galaxy of very heavy stars (with a total galaxy mass of 105 − 106M⊙), that yields minimum number densities for photometric detection at z > 10 that are lower or comparable to what has been predicted by theory. When the calculations are concerned with spectroscopy, the minimum number densities goes up and very massive galaxies (107M⊙) are required to reach the predicted limits at z > 10 and to enable detection by WFIRST. A comparison between the upcoming infrared telescopes; WFIRST, James Webb Space Telescope (JWST) and Euclid, are performed with WFIRST as the strongest candidate for photometry and JWST as the preferred instrument for spectroscopy, with a strong dependence on the survey area.

Magnetic fields are important for multiple physical processes in and around stars, for these reasons improving the understanding of how they are generated and maintained is of great value. In this work the magnetic field structure of the eclipsing binary UV Piscium is investigated. This is done by utilising the Zeeman-Doppler Imaging technique that reconstructs stellar magnetic maps by combining the information of how the magnetic field affects spectral lines with the rotational modulation of spectral lines. In order to improve the signal-to-noise ratio the least squares deconvolution technique was used to combine multiple spectral lines into an average line profile. The high resolution circular polarisation observations analysed in this work were taken by the ESPaDOnS spectograph at the Canada-France-Hawaii Telescope during August and September of 2016. We reconstructed detailed magnetic field maps and obtained the average magnetic field strengths of 137G for the primary and 88G for the secondary, which is not unusual values for stars of this type. The methods used are however likely to underestimate the magnetic field strengths. This is because the lack of linear polarisation profiles likely results in systematic underestimation of magnetic field strengths, especially meridional components. Another issue that became apparent in this work is that in eclipsing binaries, without linear polarisation observations, there is a degeneracy between the different hemispheres, resulting in further uncertainties in the determination of surface magnetic field geometry. We also found that there is indication of surface evolution on the time scale of months as some observations taken around fifty days earlier were could not be phased with the main data set.

The open cluster Messier 67 is known to have chemical composition, metallicity and age (~ 4 Gyr) close to the Sun. Therefore, it is advantageous for stellar physical studies and of stellar evolution, in particular for solar like stars within the cluster. This work considers three such stars, the formerly studied solar twin M67-1194 and two more recently suggested solar twins M67-1787 & 2018. Most solar twins show a ratio of volatile to refractory elements that systematically depart from the Sun’s. Our targets do not follow this trend as closely. Their composition is closer to the Sun and they are, therefore, exquisite targets for studies of stellar evolution within the cluster. However, their solar likeness also provides studies regarding the origin and evolution of the Solar system. The stellar ages of the solar twins are established through a chemical clock [Y/Mg] and via stellar isochrones from BaSTI. The latter age assessment of the solar twins is supplemented with the analysis of two subgiant stars M67-1442 & 1844. We approach the isochrone-based method using spectroscopically, astrometrically and photometrically derived parameters. The different ages of the stars and methods thus estimate the age of the cluster itself. The chemical ages of the stars suggest a cluster age of 4.56  ±0.44 Gyr and the isochrone-based estimates suggests a cluster age within the range 3.30-5.51 Gyr. Our results thus affirm and imply a near solar age of the cluster.

No description available.

This thesis was carried out in two parts. The stellar spectral data was used from the Gaia-ESO survey. The data used was fromthe public archive as well as data received from Dr. Recio-Blanco at Observatoire Cote D'Azure. 1) I performed anomaly detection using unsupervised random forests, by applying the concept of weirdness scores to identify outliers. 2) Using spectral data along with physical parameters of objects in the galactic bulge of the Gaia-ESO survey, I built a variational autoencoder neural network to reconstruct stellar spectra and explore latent features learning physical parameters by themselves.

SN2020qlb (ZTF20abobpcb) is an extensively sampled hydrogen-poor superluminous supernova (SLSN-I) that is among the most luminous (max. Mg = -22:25+/-0:01 magnitudes) and long-rising (77.1 days from explosion to maximum) in a category of the brightest and longest rising SNe currently known in the Universe. SN2020qlb exhibits clear light-curve undulations, a phenomenon seen in other SLSNe but whose physical origin is still a mystery. This Master of Science Thesis discusses both the potential power source of these immense explosions as well as the power mechanisms behind the observed light-curve undulations. A particularly large set of photometric data in both the visible and ultraviolet ranges and covering the first 410 days after explosion, as well as 10 spectra are available for analysis. The explosion date is constrained to +/-0.28 days; the phase and magnitude of the peak luminosity are determined; light-curves for each telescope/filter combination are constructed; the g-r color evolution is plotted; the photospheric temperature and radius evolutions are estimated; the bolometric light-curve is constructed and compared with known power source models; and the rest-frame spectral evolution is plotted for analysis. SN2020qlb is found to meet all the known criteria to be a SLSN-I. A radioactive power source model based on the decay of 56Ni is rejected due to unphysical parameter results. A source model based on the dipole spindown energy deposition of a magnetar fits well the bulk portion of the bolometric light-curve with physically possible parameter values. Two full periods of about 32+/-6 day undulations are found on top of the bulk light-curve after subtracting the smooth model values. Evidence for them is also seen in each filter light-curve. Hypothetical power sources for these striking characteristics from the literature are analyzed and discussed. In summary, a magnetar source for the bulk of the light curve is favored. An external source for the undulations is favored, e.g. interaction with variations in the progenitor star’s circumstellar material, with a caveat that an explanation involving the break-down of model assumptions cannot be ruled out. Together they are favored to explain the entire light-curve of SN2020qlb.

Failed supernovae are the implosive final fates of massive stars, where ablack hole is formed. During the collapse, the proto-neutron star emits a huge number of neutrinos, and when the black hole is finally formed, it engulfs theneutrino-emitting material and the signal is cut off. Inspired by the recent work of Nagakura & Hotokezaka (2020), this thesis improves on some parts of theirs imulation work and further explores the neutrino signal from failed supernovae, using a supercomputer to perform Monte Carlo simulations. In particular, we realized the neutrino flux’ time evolution around black hole formation hasn’t previously been studied well, and so it is investigated here, as well as the plausibility of measuring the black hole mass through the shape of the decay. A new component of the signal is presented, an echo of neutrinos emitted before black hole formation that, due to scattering in supersonic material around the black hole, arrive with a time delay of up to 15 ms, and with a significantly higher average energy, for heavy lepton neutrinos around 50 MeV.

Modelling stellar structures and comparing them with observationsis a very important step when it comes to verifying our theories aboutstellar evolution. Three-dimensional reconstruction is therefore impor-tant and in the case of certain stellar types it makes for a large portionof the ongoing research.For this project, three dierent objects and their three-dimensionalmodels were selected for 3D-printing. The systems in question areEta Carinae, 1 Gruis and HD 101584 and the reason behind thischoice is the fact that each object showcases a dierent phase and/orprocess of stellar evolution approaching or during the planetary nebula(PN) stage. On top of that, these objects have been observed using atechnique that allows us to deduce their 3D structure.The three-dimensional models and prints allow us to nd features(such as axial symmetries) that give us more information about themovements inside the system and their consequences on how the stellarstructure has and will evolve.