| 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. |
- Nordin, Jakob, et al.
(författare)
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Evidence for a correlation between the SiII λ4000 width and Type Ia supernova color
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357.
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Tidskriftsartikel (refereegranskat)abstract
- We study the pseudo equivalent width of the SiII λ4000 feature of Type Iasupernovae (SNe Ia) in the redshift range 0.0024 ≤z ≤0.634. We find that this spectral indicator correlateswith the light curve color excess SALT2 c as well as previouslydefined spectroscopic subclasses (Branch types) and theevolution of the SiII λ6150 velocity, i.e., the so called velocitygradient. Based on our study of 55 objects from different surveys, we find indications that the SiII λ4000 spectral indicatorcould provide important information to improve cosmological distancemeasurements with SNe Ia.
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| 5. |
- Nordin, Jakob, et al.
(författare)
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Spectral properties of Type Ia supernovae up to z~0.3
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 526, s. A119-
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Tidskriftsartikel (refereegranskat)abstract
- Aims: Spectroscopic observations of type Ia supernovae obtained at the New Technology Telescope (NTT) and the Nordic Optical Telescope (NOT), in conjunction with the SDSS-II Supernova Survey, are analysed. We use spectral indicators measured up to a month after the lightcurve peak luminosity to characterise the supernova properties, and examine these for potential correlations with host galaxy type, lightcurve shape, colour excess, and redshift. Methods: Our analysis is based on 89 type Ia supernovae at a redshift interval z = 0.05-0.3, for which multiband SDSS photometry is available. A lower-z spectroscopy reference sample was used for comparisons over cosmic time. We present measurements of time series of pseudo equivalent widths and line velocities of the main spectral features in type Ia supernovae. Results: Supernovae with shallower features are found predominantly among the intrinsically brighter slow declining supernovae. We detect the strongest correlation between lightcurve stretch and the Si ii λ4000 absorption feature, which also correlates with the estimated mass and star formation rate of the host galaxy. We also report a tentative correlation between colour excess and spectral properties. If confirmed, this would suggest that moderate reddening of type Ia supernovae is dominated by effects in the explosion or its immediate environment, as opposed to extinction by interstellar dust.
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| 6. |
- Östman, Linda, et al.
(författare)
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NTT and NOT spectroscopy of SDSS-II supernovae
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 526, s. A28-
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Tidskriftsartikel (refereegranskat)abstract
- Context. The Sloan Digital Sky Survey II (SDSS-II) Supernova Survey, conducted between 2005 and 2007, was designed to detect a large number of type Ia supernovae around z ~ 0.2, the redshift "gap" between low-z and high-z supernova searches. The survey has provided multi-band (ugriz) photometric lightcurves for variable targets, and supernova candidates were scheduled for spectroscopic observations, primarily to provide supernova classification and accurate redshifts. We present supernova spectra obtained in 2006 and 2007 using the New Technology Telescope (NTT) and the Nordic Optical Telescope (NOT). Aims: We provide an atlas of supernova spectra in the range z = 0.03-0.32 that complements the well-sampled lightcurves from SDSS-II in the forthcoming three-year SDSS supernova cosmology analysis. The sample can, for example, be used for spectral studies of type Ia supernovae, which are critical for understanding potential systematic effects when supernovae are used to determine cosmological distances. Methods: The spectra were reduced in a uniform manner, and special care was taken in estimating the uncertainties for the different processing steps. Host-galaxy light was subtracted when possible and the supernova type fitted using the SuperNova IDentification code (SNID). We also present comparisons between spectral and photometric dating using SALT lightcurve fits to the photometry from SDSS-II, as well as the global distribution of our sample in terms of the lightcurve parameters: stretch and colour. Results: We report new spectroscopic data from 141 type Ia supernovae, mainly between -9 and +15 days from lightcurve maximum, including a few cases of multi-epoch observations. This homogeneous, host-galaxy subtracted, type Ia supernova spectroscopic sample is among the largest such data sets and unique in its redshift interval. The sample includes two potential SN 1991T-like supernovae (SN 2006on and SN 2007ni) and one potential SN 2002cx-like supernova (SN 2007ie). In addition, the new compilation includes spectra from 23 confirmed type II and 8 type Ib/c supernovae.
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| 7. |
- Abbasi, R., et al.
(författare)
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Calibration and characterization of the IceCube photomultiplier tube
- 2010
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 618:1-3, s. 139-152
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Tidskriftsartikel (refereegranskat)abstract
- Over 5000 PMTs are being deployed at the South Pole to compose the IceCube neutrino observatory. Many are placed deep in the ice to detect Cherenkov light emitted by the products of high-energy neutrino interactions, and others are frozen into tanks on the surface to detect particles from atmospheric cosmic ray showers. IceCube is using the 10-in. diameter R7081-02 made by Hamamatsu Photonics. This paper describes the laboratory characterization and calibration of these PMTs before deployment. PMTs were illuminated with pulses ranging from single photons to saturation level. Parameterizations are given for the single photoelectron charge spectrum and the saturation behavior. Time resolution, late pulses and afterpulses are characterized. Because the PMTs are relatively large, the cathode sensitivity uniformity was measured. The absolute photon detection efficiency was calibrated using Rayleigh-scattered photons from a nitrogen laser. Measured characteristics are discussed in the context of their relevance to IceCube event reconstruction and simulation efforts. (C) 2010 Elsevier B.V. All rights reserved.
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| 8. |
- Abbasi, R., et al.
(författare)
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Search for muon neutrinos from gamma-ray bursts with the IceCube neutrino telescope
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 710:1, s. 346-359
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of searches for high-energy muon neutrinos from 41 gamma-ray bursts (GRBs) in the northern sky with the IceCube detector in its 22 string configuration active in 2007/2008. The searches cover both the prompt and a possible precursor emission as well as a model-independent, wide time window of -1 hr to + 3 hr around each GRB. In contrast to previous searches with a large GRB population, we do not utilize a standard Waxman-Bahcall GRB flux for the prompt emission but calculate individual neutrino spectra for all 41 GRBs from the burst parameters measured by satellites. For all of the three time windows, the best estimate for the number of signal events is zero. Therefore, we place 90% CL upper limits on the fluence from the prompt phase of 3.7 x 10(-3) erg cm(-2) (72 TeV-6.5 PeV) and on the fluence from the precursor phase of 2.3 x 10(-3) erg cm(-2) (2.2-55 TeV), where the quoted energy ranges contain 90% of the expected signal events in the detector. The 90% CL upper limit for the wide time window is 2.7 x 10(-3) erg cm(-2) (3 TeV-2.8 PeV) assuming an E-2 flux.
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| 9. |
- 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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| 10. |
- Hartman, Henrik, et al.
(författare)
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The FERRUM project : experimental and theoretical transition rates of forbidden [Sc II] lines and radiative lifetimes of metastable ScII levels
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 480:2, s. 575-580
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Tidskriftsartikel (refereegranskat)abstract
- Context. In many plasmas, long-lived metastable atomic levels are depopulated by collisions (quenched) before they decay radiatively. In low-density regions, however, the low collision rate may allow depopulation by electric dipole (E1) forbidden radiative transitions, so-called forbidden lines (mainly M1 and E2 transitions). If the atomic transition data are known, these lines are indicators of physical plasma conditions and used for abundance determination. Aims. Transition rates can be derived by combining relative intensities between the decay channels, so-called branching fractions (BFs), and the radiative lifetime of the common upper level. We use this approach for forbidden [Sc II] lines, along with new calculations. Methods. Neither BFs for forbidden lines, nor lifetimes of metastable levels, are easily measured in a laboratory. Therefore, astrophysical BFs measured in Space Telescope Imaging Spectrograph (STIS) spectra of the strontium filament of Eta Carinae are combined with lifetime measurements using a laser probing technique on a stored ion-beam (CRYRING facility, MSL, Stockholm). These quantities are used to derive the absolute transition rates (A-values). New theoretical transition rates and lifetimes are calulated using the CIV3 code. Results. We report experimental lifetimes of the Sc II levels 3d(2) a(3)P(0,1,2) with lifetimes 1.28, 1.42, and 1.24 s, respectively, and transition rates for lines from these levels down to 3d4s a(3)D in the region 8270-8390 angstrom. These are the most important forbidden [Sc II] transitions. New calculations for lines and metastable lifetimes are also presented, and are in good agreement with the experimental data.
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| 11. |
- Blomqvist, Michael, et al.
(författare)
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Constraining dark energy fluctuations with supernova correlations
- 2010
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; 10, s. 018-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate constraints on dark energy fluctuations using type Ia supernovae. If dark energy is not in the form of a cosmological constant, that is if the equation of state is not equal to -1, we expect not only temporal, but also spatial variations in the energy density. Such fluctuations would cause local variations in the universal expansion rate and directional dependences in the redshift-distance relation. We present a scheme for relating a power spectrum of dark energy fluctuations to an angular covariance function of standard candle magnitude fluctuations. The predictions for a phenomenological model of dark energy fluctuations are compared to observational data in the form of the measured angular covariance of Hubble diagram magnitude residuals for type Ia supernovae in the Union2 compilation. The observational result is consistent with zero dark energy fluctuations. However, due to the limitations in statistics, current data still allow for quite general dark energy fluctuations as long as they are in the linear regime.
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| 12. |
- Blomqvist, Michael, et al.
(författare)
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Probing dark energy inhomogeneities with supernovae
- 2008
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; 06, s. 027-
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the possibility of identifying anisotropic and/or inhomogeneous cosmological models using type Ia supernova data. A search for correlations in current type Ia peak magnitudes over a large range of angular scales yields a null result. However, the same analysis limited to supernovae at low redshift shows a feeble anticorrelation at the 2σ level at angular scales θ≈40°. Upcoming data from, e.g., the SNLS (Supernova Legacy Survey) and the SDSS-II (SDSS: Sloan Digital Sky Survey) supernova searches will improve our limits on the size of—or possibly detect—possible correlations also at high redshift at the per cent level in the near future. With data from the proposed SNAP (SuperNova Acceleration Probe) satellite, we will be able to detect the induced correlations from gravitational lensing on type Ia peak magnitudes on scales less than a degree.
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| 13. |
- Blomqvist, Michael, 1980-, et al.
(författare)
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Supernovae as seen by off-center observers in a local void
- 2010
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; 05, s. 006-
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Tidskriftsartikel (refereegranskat)abstract
- Inhomogeneous universe models have been proposed as an alternative explanation for the apparent acceleration of the cosmic expansion that does not require dark energy. In the simplest class of inhomogeneous models, we live within a large, spherically symmetric void. Several studies have shown that such a model can be made consistent with many observations, in particular the redshift-luminosity distance relation for type Ia supernovae, provided that the void is of Gpc size and that we live close to the center. Such a scenario challenges the Copernican principle that we do not occupy a special place in the universe. We use the first-year Sloan Digital Sky Survey-II supernova search data set as well as the Constitution supernova data set to put constraints on the observer position in void models, using the fact that off-center observers will observe an anisotropic universe. We first show that a spherically symmetric void can give good fits to the supernova data for an on-center observer, but that the two data sets prefer very different voids. We then continue to show that the observer can be displaced at least fifteen percent of the void scale radius from the center and still give an acceptable fit to the supernova data. When combined with the observed dipole anisotropy of the cosmic microwave background however, we find that the data compells the observer to be located within about one percent of the void scale radius. Based on these results, we conclude that considerable fine-tuning of our position within the void is needed tofit the supernova data, strongly disfavouring the model from a Copernican principle point of view.
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| 14. |
- 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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| 15. |
- 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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| 16. |
- 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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| 17. |
- 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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| 18. |
- Mattsson, Lars, 1974-, et al.
(författare)
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Dust driven mass loss from carbon stars as a function of stellar parameters I : A grid of solar-metallicity wind models
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 509:1, s. 13-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Knowing how the mass loss of carbon-rich AGB stars depends on stellar parameters is crucial for stellar evolution modelling, as well as for the understanding of when and how circumstellar structures emerge around these stars, e.g., dust shells and so-called detached shells of expelled gas.Aims. The purpose of this paper is to explore the stellar parameter space using a numerical radiation hydrodynamic (RHD) model of carbon-star atmospheres, including a detailed description of dust formation and frequency-dependent radiative transfer, in order to determine how the mass loss of carbon stars changes with stellar parameters.Methods. We have computed a grid of 900 numeric dynamic model atmospheres (DMAs) using a well-tested computer code. This grid of models covers most of the expected combinations of stellar parameters, which are the stellar temperature, the stellar luminosity, the stellar mass, the abundance of condensible carbon, and the velocity amplitude of the pulsation.Results. The resultant mass-loss rates and wind speeds are clearly affected by the choice of stellar temperature, mass, luminosity and the abundance of available carbon. In certain parts of the parameter space there is also an inevitable mass-loss threshold, below which a dust-driven wind is not possible. Contrary to some previous studies, we find a strong dependence on the abundance of free carbon, which turns out to be a critical parameter. Furthermore, we have found that the dust grains that form in the atmosphere may grow too large for the commonly-used small-particle approximation of the dust opacity to be strictly valid. This may have some bearing on the wind properties, although further study of this problem is needed before quantitative conclusions can be drawn.Conclusions. The wind properties show relatively simple dependences on stellar parameters above the mass-loss threshold, while the threshold itself is of a more complicated nature. Hence, we chose not to derive any simplistic mass-loss formula, but rather provide a mass-loss prescription in the form of aeasy-to-use FORTRAN routine. Since this mass-loss routine is based on data coming from an essentially self-consistent model of mass loss, it may therefore serve as a better mass-loss prescription for stellar evolution calculations than empirical formulae. Furthermore, we conclude that there are still some issues that need to be investigated, such as the role of grain-sizes.
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| 19. |
- 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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| 20. |
- 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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| 21. |
- Mattsson, Lars, 1974-, et al.
(författare)
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Intense mass loss from C-rich AGB stars at low metallicity?
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 484:1, s. L5-L8
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Tidskriftsartikel (refereegranskat)abstract
- We argue that the energy injection of pulsations may be of greater importance to the mass-loss rate of AGB stars than metallicity, and that the mass-loss trend with metallicity is not as simple as sometimes assumed. Using our detailed radiation hydrodynamical models that include dust formation, we illustrate the effects of pulsation energy on wind properties. We find that the mass-loss rate scales with the kinetic energy input by pulsations as long as a dust-saturated wind does not occur, and all other stellar parameters are kept constant. This includes the absolute abundance of condensible carbon (not bound in CO), which is more relevant than keeping the C/O-ratio constant when comparing stars of different metallicity. The pressure and temperature gradients in the atmospheres of stars, become steeper and flatter, respectively, when the metallicity is reduced, while the radius where the atmosphere becomes opaque is typically associated with a higher gas pressure. This effect can be compensated for by adjusting the velocity amplitude of the variable inner boundary (piston), which is used to simulate the effects of pulsation, to obtain models with comparable kinetic-energy input. Hence, it is more relevant to compare models with similar energy-injections than of similar velocity amplitude. Since there is no evidence for weaker pulsations in low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity C-stars have lower mass-loss rates, than their more metal-rich counterparts with similar stellar parameters, as long as they have a comparable amount of condensible carbon.
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| 22. |
- Mattsson, Lars, 1974-
(författare)
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On the Winds of Carbon Stars and the Origin of Carbon : A Theoretical Study
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon is the basis for life, as we know it, but its origin is still largely unclear. Carbon-rich Asymptotic Giant Branch (AGB) stars (carbon stars) play an important rôle in the cosmic matter cycle and may contribute most of the carbon in the Galaxy. In this thesis it is explored how the dust-driven mass loss of these stars depends on the basic stellar parameters by computing a large grid of wind models. The existence of a critical wind regime and mass-loss thresholds for dust-driven winds are confirmed. Furthermore, a steep dependence of mass loss on carbon excess is found. Exploratory work on the effects of different stellar metallicities and the sizes of dust grains shows that strong dust-driven winds develop also at moderately low metallicities, and that typical sizes of dust grains affect the wind properties near a mass-loss threshold. It is demonstrated that the mass-loss rates obtained with the wind models have dramatic consequences when used in models of carbon-star evolution. A pronounced superwind develops soon after the star becomes carbon rich, and it therefore experiences only a few thermal pulses as a carbon star before the envelope is lost. The number of dredge-up events and the thermal pulses is limited by a self-regulating mechanism: each thermal pulse dredges up carbon, which increases the carbon excess and hence also the mass-loss rate. In turn, this limits the number of thermal pulses. The mass-loss evolution during a thermal pulse (He-shell flash) is considered as an explanation of the observations of so-called detached shells around carbon stars. By combining models of dust-driven winds with a stellar evolution model, and a simple hydrodynamic model of the circumstellar envelope, it is shown that wind properties change character during a He-shell flash such that a thin detached gas shell can form by wind-wind interaction. Finally, it is suggested that carbon stars are responsible for much of the carbon in the interstellar medium, but a scenario where high-mass stars are major carbon producers cannot be excluded. In either case, however, the carbon abundances of the outer Galactic disc are relatively low, and most of the carbon has been released quite recently. Thus, there may neither be enough carbon, nor enough time, for more advanced carbon-based life to emerge in the outer Galaxy. This lends some support to the idea that only the mid-part of the Galactic disc can be a “Galactic habitable zone”, since the inner parts of the Galaxy are plagued by frequent supernova events that are presumably harmful to all forms of life.
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| 23. |
- Micheva, Genoveva, et al.
(författare)
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The faint outskirts of the blue compact galaxy Haro 11 : is there a red excess?
- 2010
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 405:2, s. 1203-1211
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies of the low surface brightness host of the blue compact galaxy (BCG) Haro 11 have suggested an abnormally red colour of V - K = 4.2 +/- 0.8 for the host galaxy. This colour is inconsistent with any normal stellar population over a wide range of stellar metallicities (Z = 0.001-0.02). Similar though less extreme host colours have been measured for other BCGs and may be reconciled with population synthesis models, provided that the stellar metallicity of the host is higher than that of the ionized gas in the central starburst. We present the deepest V- and K-band observations to date of Haro 11 and derive a new V - K colour for the host galaxy. Our new data suggest a far less extreme colour of V - K = 2.3 +/- 0.2, which is perfectly consistent with the expectations for an old host galaxy with the same metallicty as that derived from nebular emission lines in the star-forming centre.
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| 24. |
- Skoglöv, Erik, 1968-
(författare)
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The Dynamical Spin Vector Evolution of the Asteroids
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The dynamical evolution of the spin axis direction due to gravitational and thermal factors is examined. It is found that the spin axis variations generally are regular and relatively small for the bodies in the asteroid main belt. There are also reasons to believe that this is the case for minor objects beyond the main belt. However, it is found that these regular variations are larger when the orbital inclination of the objects is increased. This effect may explain certain features in the spin vector distribution of the main belt asteroids, not possible to explain by collisional factors. The spin vector evolution of the asteroids in the inner solar system, including the Earth- and Mars-crossing objects, is often subjected to strong forces related to frequencies in the orbital evolution. The variations in the spin vector direction are then very large and often subjected to chaos. The larger frequency related obliquity zones of the Mars-crossers are usually regular while the zones of the Earth-Mars-crossers often are of a chaotic nature. The spin vector evolution of asteroids with comet-like orbits is often chaotic regardless of initial obliquity. For the inner solar system asteroids, it is often possible for an initial prograde spin to turn into a retrograde one, or vice versa, due to the frequency related phenomena. Though some spin vector directions seem to be more probable than other ones over time, there are no indications for an evolution towards a more prograde or a more retrograde spin vector distribution.The effects on the spin vector evolution from the thermal Yarkovsky force are examined for objects with radii larger than 50 m. This force will affect the orbital evolution and thus indirectly affect the spin vector evolution. However, it is found that the studied effects are minor as compared to the gravitationally related ones. This is true both for the diurnal and the seasonal variants of the Yarkovsky force.
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| 25. |
- Sollerman, J., et al.
(författare)
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First-Year Sloan Digital Sky Survey-II (SDSS-II) Supernova Results : Constraints on Nonstandard Cosmological Models
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 703:2, s. 1374-1385
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Tidskriftsartikel (refereegranskat)abstract
- We use the new Type Ia supernovae discovered by the Sloan Digital Sky Survey-II supernova survey, together with additional supernova data sets as well as observations of the cosmic microwave background and baryon acoustic oscillations to constrain cosmological models. This complements the standard cosmology analysis presented by Kessler et al. in that we discuss and rank a number of the most popular nonstandard cosmology scenarios. When this combined data set is analyzed using the MLCS2k2 light-curve fitter, we find that more exotic models for cosmic acceleration provide a better fit to the data than the ΛCDM model. For example, the flat Dvali-Gabadadze-Porrati model is ranked higher by our information-criteria (IC) tests than the standard model with a flat universe and a cosmological constant. When the supernova data set is instead analyzed using the SALT-II light-curve fitter, the standard cosmological-constant model fares best. This investigation of how sensitive cosmological model selection is to assumptions about, and within, the light-curve fitters thereby highlights the need for an improved understanding of these unresolved systematic effects. Our investigation also includes inhomogeneous Lemaître-Tolman-Bondi (LTB) models. While our LTB models can be made to fit the supernova data as well as any other model, the extra parameters they require are not supported by our IC analysis. Finally, we explore more model-independent ways to investigate the cosmic expansion based on this new data set.
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