Browsing by discipline "Astronomi"

Estimates for asteroid masses are based on gravitational perturbations on the orbits of other objects such as Mars, spacecraft, or other asteroids and/or their satellites. In the case of asteroid-asteroid perturbations, this leads to a 13-dimensional inverse problem where the aim is to derive the mass of the perturbing asteroid and six orbital elements for both the perturbing asteroid and the test asteroid using astrometric observations. We have developed and implemented three different mass estimation algorithms utilizing asteroid-asteroid perturbations into the OpenOrb asteroid-orbit-computation software: the very rough 'marching' approximation, in which the asteroid orbits are fixed at a given epoch, reducing the problem to a one-dimensional estimation of the mass, an implementation of the Nelder-Mead simplex method, and most significantly, a Markov-Chain Monte Carlo (MCMC) approach. We introduce each of these algorithms with particular focus on the MCMC algorithm, and present example results for both synthetic and real data. Our results agree with the published mass estimates, but suggest that the published uncertainties may be misleading as a consequence of using linearized mass-estimation methods. Finally, we discuss remaining challenges with the algorithms as well as future plans.

This Master’s Thesis deals with stellar magnetic activity, both as it is seen in the Sun, our closest star, and in other, more extreme examples of stellar activity. For the Sun, much higher quality data is available, and for a much longer time, so it remains by far the best studied star, although stars, that are much more magnetically active, have been discovered. This thesis will review the most common forms of magnetic activity, as observed both in the Sun and in other stars. A special focus is given to BY Draconis -type stars. These are young stars, whose photometric brightness variations are caused by large, cool starspots, similar to the sunspots seen on the Sun. A particular BY Draconis -star, V889 Herculis, is analysed in detail, using spectroscopic and photometric methods. From spectroscopy, by the Doppler imaging method, a surface temperature map can be constructed for the star. From photometry, both short-term and long-term variations of the star can be studied. During the 20 years time span of the photometric data, the star seems to have gone through similar activity cycles as the well known 11 year solar cycle. The aim of this thesis is to compare solar and stellar activity in general, and specifically V889 Her to the Sun. Based on the analysis, the basic properties of V889 Her, seen in previous studies, are confirmed: a large polar spot is present on its surface, and there are clear changes in its activity. As the stellar parameters, like mass and surface temperature, of V889 Her are very similar to the Sun, and it is also a single star, the main difference is age. As a young star, V889 Her is still magnetically much more active than the Sun today, but probably it will continue to become more and more similar to the present Sun, losing its high activity with increasing age. Similarly, V889 Her shows us what the Sun most likely was like billions of years ago.

Filamentary structures appear to be very common in molecular clouds which are known to be the birth sites of stars. These interstellar filaments of gas and dust can have a dominant role in star formation. According to the current understanding of star formation, large-scale supersonic MHD turbulence present in the clouds tends to form these narrow elongated structures which are then broken into dense clumps through gravitational instability. The aim of my Master's thesis is to study the formation of stars by first reviewing the classical stability analysis of the most common self-gravitating structures subject to harmonic disturbances, having an emphasis on cylindrical symmetry which can be used to describe the behaviour of filamentary structures. Especially the density distribution, the critical line mass and the stability criterion (Jeans length) for self-gravitating isothermal cylinder are presented. Then the theory is applied to an individual target, a filament known as Taurus Molecular Cloud–1 (TMC–1). The goal is to investigate if the observed structures are in agreement with theoretical models and if stars can be formed in the filament through gravitational instability. By examining the properties of the cloud it is possible to find out when gravity (instead of external pressure) begins to dominate the evolution of the cloud. Important supportive forces, thermal pressure and turbulence, can be studied by observing standard molecular line tracers as ammonia. Ammonia NH3(1,1) and NH3(2,2) inversion lines are measured with the Effelsberg 100-m telescope in order to derive e.g. gas kinetic temperatures and non-thermal velocity dispersions along the TMC–1 filament. Furthermore, supplementing the molecular line data with the data from SCUBA 850 micron dust emission map of the cloud (Nutter et al., 2008) and assuming that the gas kinetic temperature equals to the dust temperature (Tkin = Tdust), hydrogen column densities can be calculated. Moreover, the cross-sectional column density profiles in several places along the filament can be fitted with the classical model of a hydrostatic isothermal cylinder by Stodolkiewicz (1963). The model fits roughly to the cross-sectional column density profiles. The fragment lengths or the projected clump separations along the ‘backbone’ of the filament (longitudinal profile) are derived from the intensity maxima seen in the SCUBA data. The observed fragment lengths along the filament are compared with the Jeans lengths and the fastest growing modes of disturbance predicted from Stodolkiewicz's model in order to investigate the star-forming potential of TMC-1. The observed clump separations are in a rough agreement with the local theoretical Jeans lengths. Moreover, the longitudinal SCUBA intensity profile is expanded into Fourier series, and the wavelengths corresponding to the dominating amplitudes are calculated. These wavelengths are compared with theoretical Jeans lengths in order to investigate if the perturbations travelling in the filament can cause collapse or if they are only ‘sound waves’. Two wavelengths, lambda = 0.23 pc and lambda = 0.30 pc, exceed the local theoretical Jeans length and hence may cause collapse within the filament and trigger star formation in the fragments.

In this Master's thesis, I have reviewed the theory of galaxy formation and evolution from primordial fluctuations to present-day galaxies. I have also examined the procedure for creating initial conditions for cosmological Zoom-in simulations. The main aim of this thesis was to perform groundwork in the field of Zoom-in simulations at the Department of Physics of the University of Helsinki for my future PhD studies. I have prepared a total number of 10 high-resolution multi-mass Zoom-in initial conditions, five of which contains dark matter and baryons and the rest consisting of dark matter only. The Zoom-in initial conditions were calculated using the P-GenIC N-body initial conditions code using the Alcyone cluster at the Department of Physics in July and August 2013. The computation of the Zoom-in initial conditions took approximately 41 days in wall-clock time using 24 processors. The ten Zoom-in simulations were run on the supercomputer Sisu at CSC, the Finnish IT Center for Science in September and October 2013 using the N-body simulation code GADGET-3. The code calculated the Newtonian gravitational force using the TreePM method. The gas dynamics was computed with the SPH algorithm and the properties of the interstellar medium and star formation were modeled using a statistical subgrid model. The simulation runs took approximately from one hour to one week of wall-clock time using 64 processors. The ten simulated galaxies were searched for contamination from low-resolution particles. The high-resolution regions of the galaxies were not contaminated, validating the preparation method of the Zoom-in initial conditions. The basic properties of the galaxies at z=0 were analyzed, including virial radii and virial masses, circular rotation curves and density profiles. Finally, star formation histories and colors of the simulated galaxies were extracted. The properties of the simulated galaxies were consistent with previous studies in the field of Zoom-in simulations (Naab et al. 2007, Johansson et al. 2012). Thus, I conclude that the aims set for this Master's thesis were successfully completed. After the groundwork has been conducted, it is convenient to move to future studies in the field of cosmological structure formation simulations. The focus of the future work will be in improving the simulation codes and in developing more realistic astrophysical feedback models. Increased spatial resolution and improved feedback models of the simulations can lead to significant progress on the most demanding challenges in the field of cosmological structure formation simulations.

This dissertation develops and introduces a new interplanetary (IP) shock database. The database includes currently IP shocks and their related parameters for four years: 1998, 2000, 2003 and 2008 identified using the data provided by the Advance Composition Explorer(ACE) spacecraft. IP shocks have a crucial role in space weather as they are responsible for the most effective particle acceleration in the solar corona and solar wind. These particles are hazardous for satellites and space-embedded technology. Thus, studying of IP shocks has received considerable attention in recent decades and to date various lists and databases have been developed and introduced to study IP shocks. This database provides an important opportunity to advance the understanding of IP shocks. In the future the database will be extended to have a comprehensive coverage of observations from the spacecraft that have been monitoring the solar wind (e.g. Wind, ACE, STEREO) it brings users a new system for quick analysis of IP shock properties. One can filter IP shocks from the database by date and several key shock parameters, e.g., shock type, magnetosonic speed, etc. This thesis will introduce theory behind IP shocks, the structure of the database and how IP shock candidates were selected. In addition, a statical analysis of ACE IP shocks for 1998, 2000, 2003 and 2008 are performed.

The visual structure of galaxies, their morphologies, reveal a lot of information about the ongoing physical processes in galaxies. Galaxies interact gravitationally with each other, so the effects of the environment can be studied via galaxy structure. Galaxy structure can be studied by fitting analytical light profile functions to the observed images. The fitting is based on an assumption that the shape of the galaxy can be described by certain analytical forms. This kind of parametric studies can be done using the GALFIT software. As a result, these fitted models give values for the fundamental parameters describing galaxy size, shape and brightness. In this Master's thesis I introduce GALFIT as a tool for structural studies and morphological classifications of galaxies. I have gathered a sample of group member spiral galaxies in multiwavelength Hubble Space Telescope CANDELS survey data. I have completed detailed multicomponent and multiwavelength fits for 9 galaxies from the sample of 73 galaxies. The results are used to study stellar mass-to-light ratios and star formation using galaxy colors. Also a variety of asymmetry measurements are calculated for a subsample of these 9 galaxies. The results imply that the fitting process should be unified to measure similar comparable parameters for the entire sample. I have used my knowledge in GALFIT in a study of galaxies in the infalling groups observed in LoCuSS survey by completing bulge-to-disk decomposition fitting for a sample of several hundred galaxies. The sample galaxies are classified based on the Sérsic indices from my fits and tests verify the classification. The results imply that the effect of mass-quenching is negligible in the infalling galaxy groups. The results also imply that the star-forming galaxies are more elongated than the passive ones, and that the bulge luminosity of the star-forming galaxies rise towards the center of the cluster, however with a low statistical significance. GALFIT is capable of producing images based on the initial input parameter values if it's not used to fit existing data. I have applied this feature to simulate mock observational data of fields covered in galaxies for the Euclid mission. The images accompanied with catalogs of coordinates, magnitudes and position angles of the objects are used by the Euclid group in Helsinki as independent test data for developing validation tools for Euclid data simulations.

In this study, I have investigated interstellar molecular clouds and specifically three fields mapped with the Planck satellite and Herschel Space Observatory at radio and sub-millimeter wavelengths: G131.65+9.75, G108.28+16.68, and G161.55-9.30. In these fields, four cloud cores were selected for closer investigation. The molecular line observations at 13CO, C18O, and N2H+ were made with the 20-m radio telescope in Onsala Space Observatory, Sweden. Also, data from Herschel Space Observatory was used. The molecular line observations were used for the study of cloud morphology and kinematics. The hydrogen column densities were estimated from both the molecular line and the dust continuum observations. Radiative transfer models were created for both the continuum and line data. The molecular line emission maxima were approximately where the dust continuum emission maxima were. The molecular line observations indicate that the studied cores are quite stable and show low turbulence. No clear detections of outflows were found in any of the cores. The column densities estimated from the dust continuum observations were 1 - 5 times the column densities derived from the molecular line observations. The column densities derived from the molecular line observations were also considerably lower than what is expected for protostellar cores. When comparing the masses estimated from the molecular line observations to virial and Bonnor-Ebert masses, the mass estimates were considerably lower and it seems that the cores are not on the verge of collapsing. The column density estimates derived from dust emission, however, are in line with the ones expected from prestellar cores and even in the light of the molecular line observations, there is no reason to doubt these data. Clearly, some more research in this area is needed to confirm, for example, the molecular depletion as an explanation of the apparent discrepancy between the continuum and line data

Havaittavaan maailmankaikkeuteen lähes tasaisesti jakaantunut ja noin 380 000 vuotta sitten syntynyt kosminen mikroaaltotaustasäteily on merkittävin yksittäinen havaittavan kosmologian tiedonlähde. Tämä taustasäteily on matkannut sen syntyhetkestä lähtien halki maailmankaikkeuden laajenemishistorian aina herkkiin mittalaitteisiimme saakka, ja täten se on sekä suora havaintoikkuna nuoreen maailmankaikkeuteen, että tärkeä havaintoaineisto maailmankaikkeuden laajenemishistoriasta. Vuonna 1992 NASA lähetti matkaan ensimmäisen taustasäteilyn epäisotropioita kartoittavan COBE-satelliitin, jonka mittaustulosten ja tätä seuranneiden satelliittimissioiden, WMAPin ja Planckin, ansiosta nykykosmologiasta on tullut täsmätiedettä. Vuonna 2009 laukaistun Planck-satelliitin perintö kosmologialle tulee olemaan unohtumaton. Jotta sen tarkat mittaustulokset saadaan luotettaviksi, tulee satelliitin havaintokeilat tuntea hyvin, sillä keiloista aiheutuvat efektit ovat suuria systemaattisten virheiden lähteitä. Havaintokeiloilla tarkoitetaan satelliitin yksittäisen detektorin, tai vaihtoehtoisesti useampien detektorien, spatiaalista vastetta, johon on huomioitu satelliitin skannausstrategiasta, datan käsittelystä, detektorista ja optiikasta aiheutuvat efektit. Tavallisesti nämä havaintokeilat on tapana esittää harmonisessa avaruudessa, mitatun tai simuloidun datan jatkoanalyysiä varten, jolloin niistä käytetään nimitystä skalaari-ikkunafunktio. Näiden skalaari-ikkunafunktioiden on todettu sisältävän ylimääräisiä systemaattisia efektejä, erityisesti lämpötila- ja polarisaatiosignaalin välistä vuotoa, joiden oletetaan olevan seurausta satelliitin samaan syöttötorveen kytkettyjen kahden detektorin välisten havaintokeilojen eroista. Tässä työssä esitetään uudenlainen malli, jolla voidaan vähentää kosmisen mikroaaltotaustasäteilyn epäisotropioita kartoittavan satelliitin instrumenteista ja skannausstrategiasta aiheutuvia ylimääräisiä efektejä, erityisesti signaalivuotoa lämpötilan ja polarisaation välillä. Vaikka tätä mallia on esitelty tässä tutkielmassa erityisesti Planck-satelliitin matalan taajuuden instrumenttien (engl. Low Frequency Instrument, LFI) taustasäteilyä koskevan signaalivuodon eristämiseen, soveltuu se käytettäväksi niin Planckin korkean taajuuden instrumenteille kuin myös signaalivuodon eristämiseen etualan säteilystä. Tästä mallista käytetään nimitystä matriisi-ikkunafunktio, ja se on vuonna 2015 kehitetty Helsingin yliopiston Planck-tutkimusryhmän yhteistyönä. Tässä tutkielmassa tarkastellaan CMB Monte Carlo —simulaatioiden avulla, miten havaintokeilojen erilainen muoto eri detektorien välillä vaikuttaa vuotokomponenttien suuruuteen. Erityisesti matriisi-ikkunafunktiomallin simulaatioilla näytetään, miten nämä signaalivuodot saadaan tehokkaasti eristettyä omiksi komponenteikseen, ja miten matriisi-ikkunafunktiota voidaan hyödyntää todenmukaisen taivaan lämpötilan ja polarisaation kulmatehospektrien rekonstruoimiseen. Näistä rekonstruoiduista taivaan kulmatehospektreistä voidaan puolestaan selvittää nykykosmologialle merkittävät kosmologiset parametrit, jotka karakterisoivat maailmankaikkeuden rakennetta, syntyä ja kehitystä.

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.

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.