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- Folatelli, Gastón, 1973-
(författare)
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Type Ia Supernova Cosmology : Quantitative Spectral Analysis
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Type Ia supernovae have been successfully used as standardized candles to study the expansion history of the Universe. In the past few years, these studies led to the exciting result of an accelerated expansion caused by the repelling action of some sort of dark energy. This result has been confirmed by measurements of cosmic microwave background radiation, the large-scale structure, and the dynamics of galaxy clusters. The combination of all these experiments points to a “concordance model” of the Universe with flat large-scale geometry and a dominant component of dark energy.However, there are several points related to supernova measurements which need careful analysis in order to doubtlessly establish the validity of the concordance model. As the amount and quality of data increases, the need of controlling possible systematic effects which may bias the results becomes crucial. Also important is the improvement of our knowledge of the physics of supernovae events to assure and possibly refine their calibration as standardized candle.This thesis addresses some of those issues through the quantitative analysis of supernova spectra. The stress is put on a careful treatment of the data and on the definition of spectral measurement methods. The comparison of measurements for a large set of spectra from nearby supernovae is used to study the homogeneity and to search for spectral parameters which may further refine the calibration of the standardized candle. One such parameter is found to reduce the dispersion in the distance estimation of a sample of supernovae to below 6%, a precision which is comparable with the current lightcurve-based calibration, and is obtained in an independent manner. Finally, the comparison of spectral measurements from nearby and distant objects is used to test the possibility of evolution with cosmic time of the intrinsic brightness of type Ia supernovae.
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| 2. |
- Garavini, G, et al.
(författare)
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Spectroscopic Observations and Analysis of the Peculiar SN 1999aa
- 2004
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Ingår i: Astronomical Journal. - : The American Astronomical Society.. - 0004-6256 .- 1538-3881. ; 128, s. 387-404
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Tidskriftsartikel (refereegranskat)abstract
- We present an extensive new time series of spectroscopic data of the peculiar SN 1999aa in NGC 2595. Ourdata set includes 25 optical spectra between 11 and +58 days with respect to B-band maximum light, providingan unusually complete time history. The early spectra resemble those of an SN 1991T–like object but with arelatively strong Ca H and K absorption feature. The first clear sign of Si ii k6355, characteristic of Type Iasupernovae, is found at day 7, and its velocity remains constant up to at least the first month after B-bandmaximum light. The transition to normal-looking spectra is found to occur earlier than in SN 1991T, suggestingSN 1999aa as a possible link between SN 1991T–like and Branch-normal supernovae. Comparing the observationswith synthetic spectra, doubly ionized Fe, Si, and Ni are identified at early epochs. These are characteristicof SN 1991T–like objects. Furthermore, in the day 11 spectrum, evidence is found for an absorptionfeature that could be identified as high velocity C ii k6580 or H. At the same epoch C iii k4648.8 at photosphericvelocity is probably responsible for the absorption feature at 4500 8. High-velocity Ca is found aroundmaximum light together with Si ii and Fe ii confined in a narrow velocity window. Implied constraints onsupernovae progenitor systems and explosion hydrodynamic models are briefly discussed.
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| 3. |
- Garavini, Gabriele, 1972-
(författare)
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Type Ia Supernovae : Homogeneity and Diversity
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of type Ia supernovae as distance estimators has shown that about 75% of the energy content of the universe has a negative equation of state parameter and thus, drives the acceleration of the universe. Constraining the exact nature of this energy is one of the main goals in cosmology. As the statistics of observed high-redshift supernovae increases, systematic effects become the limiting factor to pursue such investigations, thus deeper understanding of the physical properties of SNe is of great importance.In this thesis we investigate spectral homogeneity and diversity of local and high redshift supernovae. Special emphasis has been given to the analysis of optical spectra of local peculiar supernovae 1999aa and 1999ac. The study of the spectra of SN 1999aa pointed out that this SN could be a link between the extreme peculiar SN 1991T and normal SNe. Moreover, the identification of a high velocity component of Ca II and possibly of a low velocity component of C III suggests some degree of asphericity in the ejecta of this supernova. Evidence for a deflagration of a C+O white dwarf was found in the early spectra of SN 1999ac.The spectral proprieties of a vast sample of local SNe are also studied by means of newly introduced spectral indicators. These were used to possibly improve the intrinsic spread of SN peak magnitudes to 0.15 mag, independently of light curve parameters.The first quantitative comparison between local and high redshift supernova is carried out. No evidence for extreme peculiar sub-luminous SNe was found in our data set including 13 SNe with redshift range z=0.279-0.912. Furthermore, SN2002fd (z=0.279) was found to show spectral characteristics similar to SN 1991T/SN 1999aa-like supernovae.We also present a feasibility study of the Hubble diagram in rest frame I-band up to z~0.5, and show the possibility to probe the presence of intergalactic dust, which could possibly mimic the effect of dark energy in the Hubble diagram.
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| 6. |
- Nobili, Serena, 1971-
(författare)
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Tests for systematic effects in supernova cosmology
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Type~Ia supernovae are used as standard candles to measure the energy density components of the universe. This led to the new paradigm in cosmology: only about 30\% of the universe is made by ordinary pressure-less matter, the rest is associated with an unknown form of energy with a negative equation of state parameter, called dark energy, able to drive the accelaration of the universe. The importance of this discovery requires to fully understand and control the possible systematic effects affecting both current and future measurements, aiming at probing the equation of state parameter, i.e. the nature of dark energy.In this thesis, we tackle systematic effects involved in several aspects of supernova cosmology. Studies of supernova colours are used for investigating the homogeneity of the standard candle and to improve the spectral templates used for $K$-corrections. We have measured the intrinsic colour dispersion and assessed its correlation between different epochs of supernova evolution. We develop a technique for fitting the $I$-band ligthcurve and present studies of correlations of its properties with the SN luminosity. Moreover, we present a pioneer study of restframe $I$-band Hubble diagram extended at redshift $\sim$ 0.5. This is found to be a valuable complementary tool for cosmological studies, and the results found are consistent with the concordance model of the universe, though the uncertainties are large. Presence of grey dust in the intergalactic medium is investigated both using the $I$-band Hubble diagram and supernova colour excess. Although the low statistics of the high redshift sample used do not allow to draw firm conclusions, both methods are tested and shown to be useful for probing the presence of intergalactic dust. The hypothesis of supernova population drift is tested in two different ways, both studying restframe $I$-band lightcurve properties and by comparing spectra of high redshift with those of nearby SNe. One distant supernova SN~2002fd (z=0.279) shows spectral similarities with 1991T/1999aa like objects. No signs of evolution in supernova properties is found in these studies, strengthening our confidence in the measured cosmological parameters.
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