ETD

Archivio digitale delle tesi discusse presso l'Università di Pisa

Tesi etd-09112011-183513


Tipo di tesi
Tesi di laurea magistrale
Autore
SASDELLI, MICHELE
URN
etd-09112011-183513
Titolo
A characterization of overluminous type Ia supernovae using radiation transport models
Dipartimento
SCIENZE MATEMATICHE, FISICHE E NATURALI
Corso di studi
FISICA
Relatori
relatore Prof. Mazzali, Paolo
Parole chiave
  • SN 1999ee
  • abundances of elements
  • type Ia supernovae
  • SN 1999aa
  • SN 1991T
Data inizio appello
10/10/2011
Consultabilità
Non consultabile
Data di rilascio
10/10/2051
Riassunto
The first observed supernovae were discovered over two thousands years ago by careful Chinese astronomers. These objects are so luminous that if they explode nearby in our galaxy they can be even visible during daylight. Moreover, among supernovae, type Ia are the most luminous class and are among the brightest objects in the present day universe. This class of supernovae has gathered much interest in recent years because of their use as standard candles to cosmological distances. They have become a major tool to measure the geometrical structure of the universe.
Although type Ia supernovae have been studied for some decades and a lot of effort has been expended on modelling these objects the mechanism of the explosion and even the progenitors are still not well determined. Using radiation transport codes for modelling the spectrum of these objects in order to find the internal structure of the ejecta after the explosion proved to be a reliable method. It could also help to discern among the possible candidates.
The type Ia is a very homogeneous class of objects compared with the others classes of supernovae. Nevertheless within them there are important differences. Using the light curve they can be arranged in a one-parameter sequence according to the decline rate after maximum with the slower declining being the more luminous. But spectroscopically a one-parameter sequence is not enough to characterise type Ia supernovae.
Among the slow declining type Ia supernovae there are some with spectra that look ''normal'' and others with a peculiar pre-maximum spectrum that become normal in the post-maximum phase. This peculiar supernovae are well represented by the well observed SN 1991T. This supernova is characterized by deep FeIII lines, the weakness of the usual SiII and CaII lines and an hot continuum bluer than usual in the early spectrum.
In the spectral series of a supernova it is ''encoded'' a lot of information about the actual structure of the supernova ejecta. Using radiation transport codes it is possible to reproduce the spectra finding the distribution of elements along the radius independently of the explosion mechanism. In the thesis we used the Monte Carlo spectrum synthesis code developed by Mazzali and Lucy. With this code it is possible to reproduce the spectra in the photospheric phase, hence deduce the distribution of elements in the outer layers of the ejecta.
The goals of the thesis are understanding the differences among the structures of the peculiar overluminous type Ia and the normal ones, and then, comparing with some explosion models, guess the more suitable explosion mechanism for the peculiar ones. A question that we tried to answer is whether there is a continuity of property between peculiar and normal type Ia or they belong to distinct groups. In order to do this we analyse the SN 1999aa that, accordingly to the appearance of the spectrum, is in the middle between the SN 1991T and normal ones as the SN 1999ee or the SN 2003du.
The thesis is organised as follow:
Chapter 1 is a brief review of the field of type Ia supernovae focusing on what is important to understand the thesis.
First of all we describe the most important empirical evidences about type Ia starting with the classification of the supernovae and with the differences among type Ia and the other classes. Subsequently we show the characteristic of the 1991T-like supernovae. We also briefly discuss the progenitor candidates and the proposed explosion mechanism. Finally we point out the importance of understanding the type Ia SNe due to their use as standard candles.
Chapter 2 contains a short description of the physics of the radiation transport that is used in the Monte Carlo code used in the thesis.We show the approximations employed in the code and its limits and then we describe the modelling strategy used to fit the spectra.
Chapter 3 illustrates the modelling procedure of the early spectra of the peculiar SN 1991T. We show the spectra computed assuming two different luminosities of the object. For the best luminosity we display the changes of using a delayed detonation density profile instead of the ''standard'' W7 density profile. Finally we show the resulting distributions of elements of the best model. This abundances distribution are the main result of this work.
Chapter 4 reflects the structure of the previous chapter but with the analysis of the SN 1999aa.
Chapter 5 shows the analysis of the luminous normal SN 1999ee.
Chapter 6 is used to make some comparisons. First of all we compare the peculiar supernovae of which we have modelled the spectra. We compare then these peculiar type Ia supernovae with the normal SN 1999ee and with the SN 2003du already analysed with this method. Finally we try to make some constrains on explosion models. The resulting distributions of elements suggest that a strong delayed detonation explosion could be the mechanism originating peculiar overluminous type Ia.
File