Browsing by Subject "Observational Astronomy"
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(2024)Aims. In this thesis, we review the history in studying the evolution path of magnetic white dwarfs and explain the longstanding questions and debates over their magnetic origin. We intend to find magnetic white dwarfs in various forms (isolated or with companions) from the spectral database of LAMOST DR7 to complement the current magnetic white dwarf catalogue. We then move on to compare our results with some commonly accepted scenarios regarding their magnetic origin. Methods. Low-resolution spectra is the main source in this study, we intend to locate signs of Zeeman-splitting in spectra of isolated white dwarfs, measure separations of substructures due to Zeeman-splitting and estimate magnetic field strength. Magnetic white dwarfs in binary or multiple systems are found by seeking signs of cyclotron radiation due to mass transfer and particle movement in magnetic fields. Photometric survey from Transiting Exoplanet Survey Satellite (TESS) was used to fold periodic light curves for targets of interest, in order to further study the nature of our candidates, especially the ones that are believed to coexist with companions. Results. We identified 31 isolated magnetic white dwarfs in the LAMOST DR7 database by the discovery of Zeeman-splitting components. Their estimated magnetic field strength ranging from below 1 mega gauss (MG) to a scale ten times larger. Two Polars/Intermediate Polars were found with both Zeeman-splitting components and broad Balmer emissions usually seen in cataclysmic variables. We also discovered two candidates of detached magnetic binaries. These systems are believed to be the progenitors of polars or intermediate polars. Despite their rarity, these candidates serve as vital hints in clarifying the ongoing debates concerning the magnetic field origins in white dwarfs.
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(2022)Over the last thirty years more than five thousand exoplanets have been discovered around a wide variety of stellar objects. Most exoplanets have been discovered using the transit method, which relies on observing the periodic brightness changes of stars as their planet transits in front of them. The discovery efficiency of these planets has been strongly enhanced with the advent of space telescopes dedicated to the discovery of planets using the transit method. Planetary signals in the photometric data of active stars can be challenging to find, as the surface features of the stars combined with their rotation might produce signals which are orders of magnitude stronger than those caused by the planetary transit. The question of what statistical methods should be applied to account for the innate variability of stars in order to identify the transits of exoplanets in the lightcurves of active stars is being investigated in this thesis. I test a number of statistical methods in order to combat stellar activity and to identify planetary transit signals. The rotation period of the star is investigated using the Lomb-Scargle and likelihood ratio periodograms. Starspot induced variability is approximated with a number of sinusoids, with periods based on the star's rotation period. Additional stellar activity is filtered out using autoregressive and moving average models. Model fittings are performed with least squares fitting, and using samples generated by the Adaptive Metropolis algorithm. After the lightcurve has been detrended for stellar activity, the likelihoods of planetary transit signals are assessed with a box-fitting algorithm. Models are compared with the Bayesian and Akaike information criteria. Planetary characteristics are then estimated by modeling the shape of the transit lightcurve. These methods are tested and performed on the lightcurve of HD~110082, a highly active young star with one confirmed planetary companion, based on the observations of the TESS space telescope. I find that stellar activity is sufficiently filtered out with a model containing four sinusoid signals. The signal corresponding to the planet is confirmed by the box fitting algorithm, agreeing with results available in scientific literature.
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(2024)The thesis discusses the observations of linear polarization occurring on the surface of near-Sun asteroid (NSA) (3200) Phaethon. As part of the research, new observational data from the Nordic Optical Telescope (NOT) are analyzed. The data were obtained in 2019, a few weeks after the perihelion passage. The scientific goal of these observations was to explain the partially conflicting results of linear polarization in earlier literature. The conflicting degrees of linear polarization in large phase angles (the Sun - the object - the observer) covered different hemispheres of Phaethon, which may be explained by the differences in surface regolith size distribution. A new pipeline was created to analyze the data. The pipeline was used to calculate the Stokes parameters of Phaethon based on the fluxes in the data frames obtained by using the ALFOSC (Alhambra Faint Object Spectrograph and Camera). The pipeline is directly applicable to other optical linear polarization data observed using ALFOSC. However, it is also applicable to sidereal observations or non-sidereal observations conducted with other instruments with relatively minor modifications. Features of the pipeline include the estimation of the uncertainty and validity of the observations by classical error propagation, as well as the possible dependence of the results based on the diameter of the circular aperture. The results of the analysis are compared to the prior observations of Phaethon presented in the literature. First, the variation of linear polarization is modeled as a function of phase angle using the Lumme-Muinonen Function (LMF) with different initial conditions. The analysis results are in line with previous results, although the data points in large phase angles have relatively large uncertainty estimates. Second, the existence of latitudinal correlation is evaluated, as such analysis is enabled by the sufficiently large total number of observations and a pole solution presented in earlier literature. No clear evidence of latitudinal correlation was found. Third, the sufficiently stable sub-observer latitude during the NOT observations allows the search for a correlation between linear polarization and rotational phase. There are consistent variation features as a function of the rotational phase, but they are classified as hints of detection in terms of statistical uncertainties (<5σ). In addition to analyzing the variation of linear polarization, a statistical summary of the orbital parameters of near-Sun asteroids is presented. To complete the thesis, a credibility analysis of all the reported results is performed.
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