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Browsing by discipline "Tähtitiede"

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  • Pentikäinen, Hanna (2013)
    The interpretation of asteroid spectra is closely tied to surface structure and composition. Asteroid surfaces are usually assumed to be covered with a regolith, which is a mixture of mineral grains ranging from micrometers to centimeters in size. The inverse problem of deducing the characteristics of the grains from the scattering of light (e.g., using photometric and polarimetric observations) is difficult. Meteorite spectroscopy can be a valuable alternative source of information considering that unweathered meteoritic 'falls' are almost pristine samples of their parent bodies. Reflectance spectra of 18 different meteorites were measured with the Finnish Geodetic Institute Field Goniospectrometer (FIGIFIGO) covering a wavelength range of 350-2500 nm and a zenith angle of reflection range of ± 60 degrees (Suomalainen et al., Sensors 9, 3891, 2009). Principal Component Analysis (PCA) performed on the spectra tends to separate the undifferentiated ordinary chondrites and the differentiated achondrites. The measurements expand the database of reflectance spectra obtained by Paton et al. (JQSRT 112, 1803, 2012) and Gaffey (NASA PDS, 2001). The spectra of meteorites found in all the data sets are consistent. For the FIGIFIGO measurements, the principal components suggest a discrimination between the spectra from ordinary chondrites of petrologic grades 5 and 6. The distinction is not present when the data are supplemented with the spectra from the two other data sets obtained with differing measuring techniques. Single-scattering albedos for meteoritic fundamental scatterers were derived with a Monte Carlo radiative-transfer model. In the derivation, realistic scattering phase functions were utilized. The functions were obtained by fitting triple Henyey-Greenstein functions to the scattering matrix elements of olivine powder for two different size distributions (Muñoz et al., A&A 360, 777, 2000; JQSRT 113, 565, 2012). The simulated reflectances for different scattering phase functions were fitted to the measured meteorite spectra. The single-scattering albedos for the analyzed ordinary chondrites range from 0.65 to 0.9, which is in line with the largest single-particle albedo values of 0.50 ± 0.25 for the ordinary chondrite Bjurböle measured by Piironen et al. (PSS 46, 937, 1998). Based on the analysis, meteorites with higher petrologic grades have higher single-scattering albedos across the wavelength range. Using the larger single scatterer results in a lower albedo but a wider range in albedo values.
  • Damsted, Sanna (2020)
    Galaxy clusters are the largest tightly gravitationally bound structures in the Universe. They are abundant and uniformly distributed in the sky, which makes them excellent targets for studying the history and properties of our Universe. They can be used to study some of the burning questions in astrophysics and cosmology. For example, how have the large scale structures evolved from billions of years ago to how we see them today, and what are the best values for parameters like matter densities in our cosmological theories. Photometric galaxy cluster surveys have been conducted for decades, but now is the dawn of large scale spectroscopic surveys. Spectroscopy gives more precise redshift values for galaxies and galaxy clusters than photometric observations, which in turn makes other astrophysical parameters like mass estimation of galaxy clusters much more reliable. The SPIDERS galaxy cluster survey is the largest X-ray detected, spectroscopic, visually validated survey conducted to date. It improves the precision of galaxy cluster redshifts by a factor of 10. The precision of redshift has a direct improvement on other distance related parameters calculated by using galaxy clusters. The SPIDERS value added catalogue, which came out of the survey, is a tremendous achievement and will benefit astrophysicists and cosmologist around the world. The catalogue is the result of the work of the SPIDERS team of experts, and it is freely available online. This thesis explains how the SPIDERS survey was conducted; it’s phases, algorithms and the science behind it. I give many examples of the data processing and visual validation of targets, and explain the results and the significance of having such a large and precise data set.
  • Fedorets, Grigori (2013)
    The investigation of rotation of asteroids is an important means to deriving information about their physical properties and processes using ground-based instrumentation. The rotation rate distribution imply the significance of non-gravitational forces depending on the size of the asteroid. The absence of certain rotation rates in the asteroid population poses constraints on the inner structure of asteroids of different sizes. The majority of rotation periods of asteroids solved from photometric lightcurves belong to fast rotators. The observation time required for obtaining an accurate period estimate is substantially longer for a slowly rotating asteroid than for a rapidly rotating asteroid. Therefore, lightcurve surveys with limited observation time which allocate the same amount of time for all asteroids, are expected to only produce accurate periods for asteroids with short rotation periods. This thesis concentrates on various aspects of asteroid rotation. Methods used to obtain asteroid rotation periods and shape models are discussed. Also, physical mechanisms affecting the rotation of asteroids are considered. The focus is on the main-belt objects and slowly rotating bodies. The Thousand Asteroid Lightcurve Survey (Masiero et al. 2009) carried out with the Canada-France-Hawaii Telescope was one of the first systematic asteroid lightcurve surveys. One of the objects selected for follow-up was Hungaria asteroid (39420) 2084 T-2. Masiero et al. (2009) quoted a rotation period of 105 ± 21 h which is a relatively inaccurate solution, especially compared to the solutions for the fast rotators. In this work we present an updated period fit as well as a possible convex shape solution (Kaasalainen \& Torppa 2001, Kaasalainen et al. 2001) for the asteroid. The possibility that the investigated asteroid is a close binary or a non-principal-axis rotator is also discussed.
  • Mannfors, Emma (2019)
    Star formation in 53 Galactic fields, selected from the Planck Catalog of Galactic Cold Clumps, has been studied using continuum submillimeter observations from the Herschel space telescope (PACS instrument 70, 100, and 160 μm, SPIRE instrument 250, 350, and 500 μm) and the James Clerk Maxwell Telescope SCUBA-2 instrument (850 μm). Fields are located at galactic latitudes between -20 and +37, and distances of 0.1-4.5 kpc. Dense clumps have been extracted from the 850-μm data using the FellWalker clumpfinding algorithm. Properties of the dust have been found by fitting a modified blackbody function to SPIRE data and clump masses have been estimated using dust properties. Whether clumps are gravitationally bound has been estimated using Bonnor-Ebert analysis. Finally, young stellar objects (YSO) from four catalogs have been associated with the clumps. Clumps are characterized as protostellar, prestellar or starless with protostellar sources having at least one YSO spatially related to them. Prestellar clumps are gravitationally bound while starless clumps are not. Virial analysis has been performed and compared to the results of Bonnor-Ebert analysis to study the role of turbulence in these regions. FellWalker analysis found 529 dense clumps, 147 of which had sufficient data for mass estimation. Mass and radius of clumps is strongly correlated with distance. Temperature also shows a slight increase in more distant fields. These effects are due to the resolution of the SCUBA-2 instrument. Of these 147 clumps, from Bonnor-Ebert analysis 91 are protostellar, 55 prestellar, and 1 starless. The starless clump is the coldest and smallest of the whole sample. Prestellar clumps have an average temperature of 13.2 ± 1.0 K, and a column density of (1.5 ± 0.8)x 10^(22) cm^(-2). Protostellar clumps are on average larger and hotter, but have lower column density than other types of clumps. Virial analysis, which includes estimates of non-thermal support, found 91 protostellar, 17 prestellar, and 39 starless clumps, showing a strong effect of turbulence. Virial prestellar clumps are the largest, coldest and densest of the sample. These clumps represent a diverse sample of Galactic star forming regions, from high-latitude nearby clumps to distant massive clouds.
  • Järvenpää, Anni (2018)
    During the nearly a century the approximate structure of the Local Group has been known, many methods for estimating the masses of both the individual galaxies within it and the group as a whole have been constructed. Still, to this day, estimates from different credible sources vary by a factor of 2–3. In this master's thesis I construct a new model for estimating the combined mass of the Milky Way and Andromeda galaxies based on the kinematics of the galaxy pair and properties of the Hubble flow surrounding it, aiming to improve on the accuracy of the timing argument. The model is based on a sample of subhalo catalogues from cosmological dark matter only simulations containing a system resembling the Local Group. From these catalogues, I identified the Local Group analogues based on the presence of a pair of dark matter haloes with mutual kinematics and masses resembling those of the Milky Way and Andromeda galaxies in the Local Group, with no other massive objects in the vicinity. For the Hubble flow fitting I used an algorithm for automatically choosing the best range of objects to include in the fit. The surrounding Hubble flows showed clear signs of anisotropy and existence of substructures within the flow. In order to capture the properties of these structures, I clustered the subhaloes within 1.5 to 5.0 Mpc from the Milky Way analogue in each simulation using the DBSCAN clustering algorithm. I then fitted separate Hubble flows for subhaloes outside clusters, within each cluster and within each cluster with all members less massive than 8*10^11 solar masses in each simulation. I used twelve variables to construct the model for predicting the Local Group mass. Nine of these were the Hubble constants, Hubble flow zero point distances and velocity dispersions around the fit measured separately for all haloes, clustered haloes and haloes outside clusters. The remaining three consisted of the radial and tangential velocity components and the distance of the Andromeda Galaxy analogue as seen from the Milky Way analogue. I split the data set consisting of 119 subhalo catalogues into a training set with approximately 60% of the whole data set (71 subhalo catalogues) and a test set containing the remaining subhalo catalogues. I then extracted the principal components of the training set and selected the two first to be used in predicting the mass. A linear regression model was fitted to these components using 10-fold cross-validation. The error of the resulting model was estimated by applying the model on the test set and comparing its predictions to the known masses of the subhalo pair. The obtained root-mean-square error was 1.06*10^12 solar masses. This is a clear improvement over the timing argument, which had a root-mean-square error of 1.42*10^12 solar masses in the same data set.
  • Kupri, Artem (2014)
    In this Master's thesis, I present for the first time the consideration of the effect of uncertainty associated with the astrophysical modeling of lens systems on the derived value of Omega_Lambda. I show that strong gravitational lensing, in the Universe described by the LCDM model, can be used for determination of a value of the vacuum energy density. The models used in this work are based on differences between the observed stellar velocity dispersions and the actual stellar velocity dispersions of elliptical and lenticular galaxies, which play a role of lenses in lensing systems. It will be shown that the accuracy of observations is very important in such kind of calculations. I present an own derivation of the formulas used in this work for determination of Omega_Lambda, which was never done in a reference work [Schwab et al., 2010]. Moreover, in difference from [Schwab et al., 2010], I will represent these formulas in the correct form. In this work, I use 30 lensing systems, observed by the Sloan Digital Sky Survey and the Hubble Space Telescope. I calculate the angular diameter distances of lensing and background galaxies and combine them with the stellar velocity dispersion measurements, taking into account the total mass density, the luminosity density and the anisotropy of the lensing galaxy. Finally, I have performed chi-squared test to identify the acceptable model. The value of the vacuum energy density determined in this thesis, by using a restricted sample (26 galaxy lensing systems), has realistic confidence intervals, much larger than previously published. I show that the strong constraints on the value of Omega_Lambda determined in some other similar works ([Schwab et al., 2010] and [Cao et al., 2012]) depend on inclusion of systems that are not described by the model.
  • Uusitalo, Joonas (2016)
    In 2012, a sudden and significant increase in the radiocarbon 14C abundance of tree rings at around AD 775 was found by Miyake et al (2012). Since then, various explanations for the cause of the event have been offered. These include a supernova explosion, a short gamma-ray burst, a comet collision with the Sun causing an energetic burst, a comet disintegration in the Earth's atmosphere and an especially energetic solar flare. Even though there have been a lot of studies considering the event, the definite cause is still unclear. Most of the knowledge regarding the event comes from 14C measurements made from trees that grew in various locations. These include Japan, Germany, USA and Russia. To further increase the understanding of the event and its cause, we have measured the event from a subfossil Lapland tree. This measurement is unique, since the other measurements come from locations significantly further from the magnetic pole. Thus, they might be affected by different atmospheric and geomagnetic dynamics. To date, there has not been a considerable effort in trying to quantify possible differences in the various measurements. Only the maximum increase from the background value has been considered. However, it might not be the most robust indicator of the event intensity, since it is susceptible to statistical fluctuations. For this reason, we have adopted a peak fitting method to better quantify the various properties of different measurements. Special interest was put into calculating the area under the curve of the fit to get a more robust indicator of the event intensity. Here we report that the measurement from a Lapland tree shows a significantly stronger 14C signal than what has been found earlier. Furthermore, our peak analysis demonstrates that there is a clear dependency between the latitude, where the trees have grown, and the intensity of the 14C signal, indicating that higher latitude trees have stronger signals. The connection is even more evident when, instead of the latitude, the distance from the North magnetic pole is used. It is known that the production of 14C by charged particles is significantly higher near the polar regions due to geomagnetic effects. Hence, a solar proton event is consistent with the observed latitude effects, whereas a gamma-ray burst or an atmospheric comet disintegration is not. Therefore, a solar origin is strongly implicated. These findings have a societal significance, since a solar storm poses a considerable threat to various infrastructures. We advice that the AD 775 event should be used as a new worst-case scenario when evaluating different risk mitigation strategies.
  • Cole, Elizabeth (2011)
    Thermal instability (hereafter TI) is investigated in numerical simulations to determine its effect on the growth and efficiency of the dynamo processes. The setup used is a three-dimensional periodic cube of a size several times the correlation length of the interstellar turbulence. The simulations are designed to model the interstellar medium without any shear or rotation, to isolate the effect of TI. Hydrodynamical and nonhelical simulations are run for comparison to determine the effects the magnetic field has upon the gas itself. Turbulence is simulated by external helical forcing of varying strength, which is known to create a large-scale dynamo of alpha squared-type. The nonhelical cases are also explored in an attempt to create a small-scale dynamo at high Rm, but no dynamo action could be detected in the range of Rm ~ 30 – 150. The hydrodynamical simulations reproduce the tendency of the gas to separate into two phases if an unstable cooling function is present. The critical magnetic Reynolds number of the large-scale dynamo was observed to be almost twice as large for the unstable versus stable cooling function, indicating that the dynamo is harder to excite when TI is present. The efficiency of the dynamo as measured by the ratio of magnetic to kinetic energy was found to increase for the unstable case at higher forcing. The results of the runs from this thesis are part of a larger project studying dynamo action in interstellar flows.
  • Liljedahl, Lasse (2017)
    To understand the formation of disk galaxies it is also important to understand different feedback mechanisms that affect the formation process. Without a feedback process to delay star formation the disk galaxies should not have ongoing star formation in the present day Universe. However, this is not the case since star formation is still taking place. For example, in the Milky Way the star formation rate is still ~1 solar mass per year. Moreover, during the formation process most of the gas inside galaxies is not bound into stars. Instead when disk galaxies form inside a dark matter halo there is much more baryonic matter initially available in gaseous form than in stars. This contradicts the basic CDM model, according to which most of the gas should cool down and form stars in the absence of feedback. The goal of this thesis is to first introduce the theory behind disk galaxy formation and the feedback mechanisms affecting the galaxy formation process with the main focus being on the supernova feedback. After introducing the theory the aim is to compare how supernova feedback affects the formation of a massive Milky Way-like galaxy and a less massive dwarf galaxy using a simulation code developed by Efstathiou (2000). For both galaxies four cases are simulated. Two of them represent a basic galaxy formation model presented in this thesis. One observes a situation in which the galaxy would have a very high star formation efficiency and the second concentrates on a slightly refined model including some parameters, which are ignored in the basic model. The work conducted in this thesis proves that supernova feedback may work throughout the galaxy's lifetime and causes a significant portion of the gas to escape the galaxy. This also shows that supernova driven feedback might be a reason why disk galaxies in the present day Universe still have ongoing star formation. Also the analytic model is surprisingly realistic and produces results which not only explain why there still is star formation in the present day disk galaxies, but also why the stellar mass in disk galaxies is lower than what is predicted by the basic CDM model. In dwarf galaxies with circular speed 70 km/s the ejected gas mass may be up to 60% of the total initial gas mass and in a high star formation case the ejected gas mass may be equal to the final stellar mass. Dwarf galaxies are also more sensitive to changes in the initial parameters compared to massive galaxies. In more massive galaxies with circular speed 280 km/s the ejected gas mass is smaller, but still may be 20% of the total gas mass. Another result was that massive galaxies are not very sensitive to changes in the initial conditions and the effects of supernova feedback. Finally, in the massive galaxies gas may join a galactic fountain, which was not observed in the dwarf galaxies, in which the gas was lost.
  • Rantakylä, Julia (2020)
    Active longitudes are areas, where star spot activity is centered in and reappears on a periodic manner. Star spots are cooler areas on the star surface, caused by rising magnetic field lines inhibiting the flow of the convective region. The ways to observe active longitudes is limited, but in some stars the phenomenon has clearly been present, as Lehtinen et al. (2016) has showed. One of the observation methods is to analyse the primary and secondary minima epochs of the star’s light curve relative to its orbital period. Time series analyses are tools to gather these phases from light curves. Here two different methods were used to analyse a RS CVn binary member IM Pegasi. Continuous Period Search (CPS) (Lehtinen et al. 2011) defines an adaptive,single periodic model to a moving window of observations, allowing the light curve to contain sudden changes. Discrete Chi-square Method (DCM) (Jetsu 2020)) applies a multiperiod, polynomial-trended model to fit to the data with constant parameters, assuming all changes in the light curve are part of periodic changes. Using these two methods the light curve of IM Pegasi is studied in order to determine if there could be active longitudes present. Four data segments were chosen to be further analysed with DCM based on the CPS results. One of the segments showed a flip-flop effect in the CPS phase results, which was showed to be apparent based on the successful DCM performance. Two segments, which had rather steady phase trend in the CPS results, performed well with the DCM analysis. The fourth segment, which showed strong migrating of the secondary minima phase in CPS analysis, had problems performing with DCM as a whole segment. The primary periodicity is detected in both CPS and DCM withing good limits of agreement. The DCM dual-periodic model results in all four segments indicate of an additional, more fragile irregular structure in the star, like separated dynamo waves.