SNIa event

Fig. 7.9. Element production in an SNIa event. (From Nomoto et al. 1997.)...

Apart from the fact that they represent the most productive explosions in the sky since the Big Bang, they can also be used as a waymark across space. The depths of the cosmos are lit up by light from SNIa events. At its high point, just one such event shines like abillion suns, on apar with a small galaxy. By virtue of their extraordinary brightness, they can be seen billions of light-years away. By... 

Manganese is an odd element that is underrepresented in halo stars. However, the above explanation does not seem to apply since scandium (Z = 21) and vanadium (Z = 23) do not follow the same trend, no more than does cobalt (Z = 27). It is tempting to deduce from the unusual behaviour of manganese that it is produced by SNIa events and that we are observing a mirror-image phenomenon to the one described for a elements. 

Hydrogen shines by its absence and the optical spectra of SNIa events feature spectral lines of neutral and once ionised elements (Ca, Mg+, S+ and 0+) at the minimum of the light curve. This indicates that the outer layers are composed of intermediate mass elements. SNIa events reach their maximum luminosity after about 20 days. This luminous peak is followed by a sharp drop amounting to three magnitudes per month. Later the light curve falls exponentially at the rate of one magnitude per month. 

Fig. A2.1. Light curves for various SNIa events. The figure shows empirical families of light curves. The brightest shine longer, at the peak of their glory. (From Riess et al. 1998.)...

The important datum for cosmology is precisely the luminosity at the peak of the light curve. It is crucial to be able to establish this maximum value in order to use the SNIa event as a distance indicator. Correctly cahbrated and reproducible hght curves from type la supernovas have become a major tool for determining the local expansion rate and geometrical structure of the Universe (Fig. A2.1). A great deal of effort has been put into producing adequate models of these events over the past few years. 

Through a well-established tradihon, relahvely close SNIa events (z < 0.1) have been used to measure the current local value of the Hubble parameter. Supernova-based cosmology has seen a recent upturn in achvity. The local expansion rate is something hke 60 km s per megaparsec, which corresponds to 18 km s per million light-years. 

Systematic research using wide-held images taken at intervals of three to four weeks have allowed two independent groups, the Supernova Cosmology Project and the High-z Supernova Search Team, to identify more than 50 SNIa events at intermediate redshifts. The Hubble redshift-magnitude diagram has been extended out to z = 1. 

Today, a certain level of diversity has been discovered in SNIa events. Before they can be used as cosmological distance standards, it must be checked that their maximal luminosity has an appropriate value. Otherwise ad hoc corrections must be brought to bear to... 

This correction plays a key role in any cosmological application. Without it, SNIa events could not be used as distance indicators. However, its purely empirical nature remains unsatisfactory to demanding theoretical minds. We would like to be able to explain physically why some explosions are weaker than others, and what effect this has on the appearance of the object. This involves building detailed models of these explosions and the way radiation is hansferred through the expanding envelope, similar to those made to describe atomic bombs or spheres struck by laser beams, which implode before exploding. 

The analysis is based upon 42 SNIa events with redshifts between 0.18 and 0.83, combined with 18 nearby SNIa events with z < 0.1. These were discovered in the context of a systematic investigation, the Calan/Tololo Supernova Study. The distances of those supernovas with the highest redshift are on average 10-15% greater than would be obtained in the case of a low-density universe (flm = 0.2) with no quintessence. 

The distance, and hence the apparent magnitude, is sensitive to both and f A At small z, the expression reduces to the well-known Hubble law. However, at large z, the deviation from Hubble s law grows ever larger. This is why distant SNIa events are so significant in choosing between cosmological models. 

A scenario referred to as a sub-Chandrasekhar-mass supernova envisions a C-O WD capped with a helium layer accreted by a companion, and which explodes as the result of a hydrodynamical burning before having reached the Chandrasekhar limit. This type of explosions may exhibit properties which do not match easily the observed properties of typical SNIa events. It cannot be excluded, however, that they are responsible for some special types of events, depending in particular on the He accretion rate and on the CO-sub-Chandrasekhar WD (SCWD) initial mass (e.g. [85]). Unidimensional simulations of He cataclysmics characterized by suitably selected values of these quantities reach the conclusion that the accreted He-rich layer can detonate. Most commonly, this explosion is predicted to be accompanied with the C-detonation of the CO-SC WD. In some specific cases, however, this explosive burning might not develop, so that a remnant would be left following the He detonation. Multidimensional calculations cast doubt on the nature, and even occurrence, of the C-detonation in CO-SC WD (e.g. [86]). 

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