| 1. |
- Nordin, Jakob, 1979-
(författare)
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Spectral Properties of Type Ia Supernovae and Implications for Cosmology
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Type Ia supernovae can, for a short period of time, reach the same brightness as an entire galaxy. They are responsible for the creation of a large fraction of all heavy elements and can be used, as standard candles, to prove that the expansion of the universe is accelerating. Yet, we do not fully understand them. A basic picture where Type Ia supernovae are caused by thermonuclear explosions of white dwarfs is generally accepted, but the details are still debated. These unknowns propagate into systematic uncertainties in the estimates of cosmological parameters. A Monte Carlo framework, SMOCK, designed to model this error propagation, is presented. Evolution with time/distance and the nature of reddening are studied as the dominant astrophysical uncertainties. Optical spectra of Type Ia supernovae contain a wealth of information regarding the nature of these events, and can be used both to understand supernovae and to limit the systematic uncertainties in cosmological parameter estimates. We have reduced spectra observed with the Nordic Optical Telescope and the New Technology Telescope in conjunction with the SDSS-II supernova survey, and compared spectral properties (pseudo-Equivalent Widths and line velocities) of this sample with local supernovae.We have further studied possible systematic difficulties in such comparisons between nearby and distant supernovae, caused by noise and host galaxy contamination.Taking such uncertainties into account, we find a tentative evolution in supernova properties with redshift, compatible with expected demographic changes. Correlations with light curve shape found by other studies are confirmed. A tentative correlation with light curve colour is also presented. The latter could indicate an intrinsic component of the observed reddening, i.e. independent of interstellar dust in the host galaxy.
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| 2. |
- Gerlee, Philip, 1980, et al.
(författare)
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Scientific Models : Red Atoms, White Lies and Black Boxes in a Yellow Book
- 2016
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Bok (övrigt vetenskapligt/konstnärligt)abstract
- A zebrafish, the hull of a miniature ship, a mathematical equation and a food chain - what do these things have in common? They are examples of models used by scientists to isolate and study particular aspects of the world around us. This book begins by introducing the concept of a scientific model from an intuitive perspective, drawing parallels to mental models and artistic representations. It then recounts the history of modelling from the 16th century up until the present day. The iterative process of model building is described and discussed in the context of complex models with high predictive accuracy versus simpler models that provide more of a conceptual understanding. To illustrate the diversity of opinions within the scientific community, we also present the results of an interview study, in which ten scientists from different disciplines describe their views on modelling and how models feature in their work. Lastly, it includes a number of worked examples that span different modelling approaches and techniques. It provides a comprehensive introduction to scientific models and shows how models are constructed and used in modern science. It also addresses the approach to, and the culture surrounding modelling in different scientific disciplines. It serves as an inspiration for model building and also facilitates interdisciplinary collaborations by showing how models are used in different scientific fields. The book is aimed primarily at students in the sciences and engineering, as well as students at teacher training colleges but will also appeal to interested readers wanting to get an overview of scientific modelling in general and different modelling approaches in particular.
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| 3. |
- Gerlee, Philip, 1980, et al.
(författare)
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Scientific Models
- 2016
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Bok (övrigt vetenskapligt/konstnärligt)abstract
- A zebrafish, the hull of a miniature ship, a mathematical equation and a food chain - what do these things have in common? They are examples of models used by scientists to isolate and study particular aspects of the world around us. This book begins by introducing the concept of a scientific model from an intuitive perspective, drawing parallels to mental models and artistic representations. It then recounts the history of modelling from the 16th century up until the present day. The iterative process of model building is described and discussed in the context of complex models with high predictive accuracy versus simpler models that provide more of a conceptual understanding. To illustrate the diversity of opinions within the scientific community, we also present the results of an interview study, in which ten scientists from different disciplines describe their views on modelling and how models feature in their work. Lastly, it includes a number of worked examples that span different modelling approaches and techniques. It provides a comprehensive introduction to scientific models and shows how models are constructed and used in modern science. It also addresses the approach to, and the culture surrounding modelling in different scientific disciplines. It serves as an inspiration for model building and also facilitates interdisciplinary collaborations by showing how models are used in different scientific fields. The book is aimed primarily at students in the sciences and engineering, as well as students at teacher training colleges but will also appeal to interested readers wanting to get an overview of scientific modelling in general and different modelling approaches in particular.
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| 4. |
- Scott, Pat, 1982-
(författare)
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Searches for Particle Dark Matter : Dark stars, dark galaxies, dark halos and global supersymmetric fits
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The identity of dark matter is one of the key outstanding problems in both particle and astrophysics. In this thesis, I describe a number of complementary searches for particle dark matter. I discuss how the impact of dark matter on stars can constrain its interaction with nuclei, focussing on main sequence stars close to the Galactic Centre, and on the first stars as seen through the upcoming James Webb Space Telescope. The mass and annihilation cross-section of dark matter particles can be probed with searches for gamma rays produced in astronomical targets. Dwarf galaxies and ultracompact, primordially-produced dark matter minihalos turn out to be especially promising in this respect. I illustrate how the results of these searches can be combined with constraints from accelerators and cosmology to produce a single global fit to all available data. Global fits in supersymmetry turn out to be quite technically demanding, even with the simplest predictive models and the addition of complementary data from a bevy of astronomical and terrestrial experiments; I show how genetic algorithms can help in overcoming these challenges.
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| 5. |
- Festin, Leif
(författare)
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The faintest stars : A study of white, red and brown dwarfs
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The local space density of the faintest stars, including M dwarfs, white dwarfs and brown dwarfs has been studied by two different methods.By using dark nebulae as opaque outer screens, the luminosity functions of M dwarfs and white dwarfs were studied as volume-limited and dynamically unbiassed foreground samples. The surveyed volume corresponds to 464 pc3 in the solar neighbourhood and contains 21 M dwarfs and 7 white dwarfs. The derived M-dwarf luminosity function is consistent with previous findings, showing no substantial upturnbeyond Mv = 16. The white dwarf space density is consistent with a 20% fraction of the dynamical matter in the solar neighbourhood residing in white dwarfs. This is in line with the most recent independent results obtained with different methods, making the white dwarfs the best explanation to the Milky Way dark matter at present.The brown dwarfs were studied in the Pleiades open cluster. The distance and age of the Pleiades make the rapidly fading brown dwarfs still rather bright and easy to detect. 850 arcmin2 were covered in a deep RIJK survey. Nine new possible cluster members were discovered, four of which are below the brown dwarf limit. The faintest of these has an estimated mass of 0.040 M. and is thereby the lowest mass brown dwarf identified in the Pleiades cluster at present. The derived Pleiades substellar luminosity function is consistent with a mass function index between 0 and 1, making the brown dwarfs unlikely to contribute more than a few percent to the cluster mass, which is also consistent with recent dynamical results.
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| 6. |
- Gunnarsson, Marcus, 1971-
(författare)
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Gas Production in Distant Comets
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Molecular spectroscopy at radio wavelengths is a tool well suited for studying the composition and outgassing kinematics of cometary comae. This is particularly true for distant comets, i.e. comets at heliocentric distances greater than a few AU, where the excitation of molecules is inefficient other than for rotational energy levels. At these distances, water sublimation is inefficient, and cometary activity is dominated by outgassing of carbon monoxide.An observing campaign is presented, where the millimeter-wave emission from CO in comet 29P/Schwassmann-Wachmann 1 has been studied in detail using the Swedish-ESO Submillimetre Telescope (SEST). Coma models have been used to analyse the spectra. The production of CO is found to have two separate sources, one releasing CO gas on the nuclear dayside, and one extended source, where CO is produced from coma material, proposed to be icy dust grains.Radio observations of many molecules in comet C/1995 O1 (Hale-Bopp) have been carried out in a long-term international effort using several radio telescopes. An overview of the results is presented, describing the evolution of the gas production as the comet passed through the inner Solar system. Spectra recorded using the SEST, primarily of CO, for heliocentric distances from 3 to 11 AU are analysed in detail, also using coma models.The concept of icy grains constituting the extended source discovered in comet 29P/Schwassmann-Wachmann 1 is examined by theoretical modelling of micrometre-sized ice/dust particles at 6 AU from the Sun. It is shown that that such grains can release their content of volatiles on timescales similar to that found for the extended source.
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| 7. |
- Hidalgo-Gámez, Ana Maria
(författare)
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A study of possible chemical inhomogeneities of dwarf irregular galaxies and the influence on the Z-L relationship
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A study on the chemical abundances of dwarf irregular galaxies (dI's) is presented in this work. This type of galaxies have typically low metallicity. First of all, a definition of dI is proposed, in order to obtain a homogeneous group of targets.Due to the small size of the dI galaxies and the absence of strong dynamical processes a homogeneous chemical composition is normally assumed. The main argument is that the gas in the interstellar medium of the galaxies are probably not too much affected by internal dynamics and could therefore be well mixed. This hypothesis has been investigated. Optical spectra for various star-forming regions (HII regions) in five dI's have been obtained and analysed. For all the HII regions, the chemical abundances of the whole region have been determined. Spectra of four of these HII regions are of very high quality which allow an additional study of possible variations of the chemical abundances within the regions. It could be concluded that the existence of variations in the chemical abundances, especially at large scales, may depend on thephysical properties of the galaxy and its environment.Another important aspect to study is the behaviour of the dI galaxies in the metallicity-luminosity plane. A deep study of previous relationships for this kind of objects were performed as well as the possible sources of dispersion from this relationship. With the results on the chemical abundances of eight galaxies and the latest distance determinations a new, weak, relationship between the metallicity and the luminosity was derived. The influence of the environment and other physical properties of the galaxies have also been studied.
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| 8. |
- Lagerros, Johan S. V.
(författare)
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Thermal physics of asteroids
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thermal infrared and microwave emission from asteroids have many applications. Fundamental physical properties can be investigated, which have implications for the geophysical evolution of the asteroids, and the early history of the solar system.A new thermophysical model of asteroids is presented. A number of physical processes previously neglected in the Standard Thermal Model are now considered. The new model predicts the thermal emission of asteroids, from mid-infrared to microwave wavelengths. The irregular shapes of asteroids are modelled in detail, and the spin state of the asteroids is taken into account, whereby it is possible to calculate model thermal lightcurves. The heat conduction into the surface material in general lowers the mid-infrared flux from main-belt asteroids. The model is able to explain the observed "beaming" of the emission into the solar direction, under the assumption that it is caused by the small-scale surface roughness. Subsurface scattering processes are considered when calculating the directional- and wavelength-dependent emissivity.The model is applied to a large database of observations in the wavelength range of 7-2 000 µm. The purpose is to derive the thermophysical properties of ten asteroids, selected as calibration targets for the instruments on board the Infrared Space Observatory. The results indicate very rough and porous surfaces, with low levels of heat conduction. The emissivity variessignificantly with wavelength.
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| 9. |
- Mattsson, Lars, 1974-, et al.
(författare)
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Dust Driven Mass Loss from Carbon Stars as Function of Stellar Parameters II : Effects of Relaxing the Small Particle Approximation
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Context. It is well-established that the winds of carbon-rich AGB stars (carbon stars) can be driven by radiation pressure on grains of amorphous carbon and collisional transfer of momentum to the gas. This has been demonstrated by different numerical wind models including time-dependent dust formation, where it has been assumed that the dust grains that form never grow to sizes comparable to wavelengths around the stellar flux maximum (or beyond), which simplifies the treatment of grain opacities considerably. It is not clear, however, if this small particle approximation (SPA) is always valid. Aims. In this paper we explore the effects of relaxing the SPA by considering a few less severe approximations for the radiation pressure efficiency, which include the effects of grain sizes. The purpose of the study is mainly to establish when the SPA can be applied and to quantify the possible errors that may occur when it does not hold. Methods. We have computed wind models with time-dependent dust formation and grain-size dependent opacities, where (1) the radiation pressure efficiency is approximated using grain sizes based on various means of the grain size distribution, and (2) where the problem is simplified by assuming a single dust-grain size. Results. It is shown that in critical cases, the effect of grain sizes can be significant. Mass-loss rates may increase by a factor of two, or more, and wind speeds may be an order of magnitude higher. Furthermore, all models with grain-size dependent opacities that have resultant winds appear to have much lower degrees of dust condensation, compared to corresponding SPA models. Consequently, the "dust-loss rates" are much lower in the new models. However, for well-developed dust-driven winds, where the dust formation has saturated, the effect of grain sizes on the mass-loss rate and wind speed is almost negligible. Conclusions. We conclude that the SPA is, under many circumstances, a reasonable simplification in models of carbon star mass loss. However, critical cases do exist, where especially the effects of momentum transfer due to scattering become significant. It is therefore uncertain whether previous results for winds in the transition regions (between no dust-driven mass loss and well-developed winds) are quantitatively correct. However, we argue that the SPA is a reasonable simplification for strong dust-driven winds. Furthermore, we note that there are other effects (not yet included in the model) than those of grain-size dependent opacities, which may become important in the critical wind regime.
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| 10. |
- Mattsson, Lars, 1974-, et al.
(författare)
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Effects of Carbon-excess dependent Mass Loss and Molecular Opacities on Models of C-star Evolution
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We present models of stellar evolution for a 2Msun-star of initial metallicity Z = 0.01 (scaled solar), with focus on the carbon-star phase, the effects of increasing carbon excess on mass loss and molecular opacities. We employ a new, state-of-the-art theoretical mass-loss prescription for dust-driven winds, which takes effects of the carbon excess into account and implement a set of composition-dependent molecular opacities, which incorporates the changes due to the evolution of the carbon excess. This work should be regarded as an exploratory and comparative study. The stellar evolution models were computed using the MESA code-package (http://mesa.sourceforge.net), which includes a new fast and efficient code for stellar evolution, but still needs further calibration before quantitative comparisons with observations can be made. We find that the development of the carbon excess is controlling much of the carbon-star evolution. A very pronounced superwind is forming and the termination of the AGB happens soon after the star becomes carbon rich, in fact, after only 4-5 thermal pulses, which is much less than in existing models.
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