Browsing by discipline "Tähtitiede"

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.

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.

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.

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.

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.