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Spectral Properties of Type Ia Supernovae and Implications for Cosmology
Stockholm University, Faculty of Science, Department of Physics. (CoPS)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2011. , 81 p.
Keyword [en]
cosmology, spectroscopy, supernovae
National Category
Physical Sciences Astronomy, Astrophysics and Cosmology Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-56785ISBN: 978-91-7447-300-1OAI: oai:DiVA.org:su-56785DiVA: diva2:413054
Public defence
2011-05-27, lecture room FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Accepted. Available from: 2011-05-05 Created: 2011-04-27 Last updated: 2011-04-29Bibliographically approved
List of papers
1. Quantifying Systematic Uncertainties in Supernova Cosmology
Open this publication in new window or tab >>Quantifying Systematic Uncertainties in Supernova Cosmology
2008 (English)In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, Vol. 02, 008- p.Article in journal (Refereed) Published
Abstract [en]

Observations of Type Ia supernovae used to map the expansion history of the Universe suffer from systematic uncertainties that need to be propagated into the estimates of cosmological parameters. We propose an iterative Monte Carlo simulation and cosmology fitting technique (SMOCK) to investigate the impact of sources of error upon fits of the dark energy equation of state. This approach is especially useful to track the impact of non-Gaussian, correlated effects, e.g. reddening correction errors, brightness evolution of the supernovae, K-corrections, gravitational lensing, etc. While the tool is primarily aimed at studies and optimization of future instruments, we use the Gold data-set in Riess et al (2007 Astrophys. J. 659 98) to show examples of potential systematic uncertainties that could exceed the quoted statistical uncertainties.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-16774 (URN)10.1088/1475-7516/2008/02/008 (DOI)000254151100008 ()
Available from: 2008-12-23 Created: 2008-12-23 Last updated: 2011-04-27Bibliographically approved
2. NTT and NOT spectroscopy of SDSS-II supernovae
Open this publication in new window or tab >>NTT and NOT spectroscopy of SDSS-II supernovae
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2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 526, A28- p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
methods: observational, techniques: spectroscopic, supernovae: general, surveys, cosmology: observations
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics; Astronomy
Identifiers
urn:nbn:se:su:diva-56782 (URN)10.1051/0004-6361/201015704 (DOI)000286458400040 ()
Available from: 2011-04-27 Created: 2011-04-27 Last updated: 2012-01-25Bibliographically approved
3. Spectral properties of Type Ia supernovae up to z~0.3
Open this publication in new window or tab >>Spectral properties of Type Ia supernovae up to z~0.3
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2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 526, A119- p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
methods: data analysis, techniques: spectroscopic, supernovae: general, cosmology: observations, line: profiles
National Category
Astronomy, Astrophysics and Cosmology Astronomy, Astrophysics and Cosmology
Research subject
Physics; Astronomy
Identifiers
urn:nbn:se:su:diva-56783 (URN)10.1051/0004-6361/201015705 (DOI)000286458400131 ()
Available from: 2011-04-27 Created: 2011-04-27 Last updated: 2012-01-25Bibliographically approved
4. Evidence for a correlation between the SiII λ4000 width and Type Ia supernova color
Open this publication in new window or tab >>Evidence for a correlation between the SiII λ4000 width and Type Ia supernova color
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(English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357Article in journal (Refereed) Accepted
Abstract [en]

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.

Keyword
cosmology distance scale dust extinction - supernovae: general
National Category
Astronomy, Astrophysics and Cosmology Astronomy, Astrophysics and Cosmology
Research subject
Physics; Astronomy
Identifiers
urn:nbn:se:su:diva-56779 (URN)
Available from: 2011-04-27 Created: 2011-04-27 Last updated: 2011-05-11Bibliographically approved

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