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Novae and Supernovae


Abstract

 

Keywords: nova, supernova, pulsar, black hole, white dwarf

Introduction

Nova (plural novae) comes from the Latin word meaning new. To the ancients, a nova was a new star - the Chinese called them ‘guest stars’ - which suddenly appeared where no star was visible before. In fact, these are not newly formed stars (star formation requires many thousands, if not millions, of years, and is usually hidden from our view inside a molecular cloud) but stars which suddenly become very much brighter. The star was there before, only too faint to be seen with the naked eye.

Stars brighten suddenly in response to a number of phenomena, the most spectacular being supernova explosions, which completely disrupt the star. Other types of outburst do not, however. For example, the famous Southern Hemisphere object, Eta Carinae has undergone several giant outbursts in the last centuries. A Sumerian recording of a ‘new star’ in 3000 B.C. is possibly attributable to Eta Carinae (Naeye 1997). In 1837, Eta Carinae flared up, peaking second only to Sirius at magnitude -0.8, in 1843. It remained at first magnitude for around 20 years, but has since settled back around 6 to 8.

"Once every second, somewhere in the universe a massive star is disrupted in a supernova explosion" (Janka 2002, p. 1134). They represents a huge outpouring of energy, which can be powered either by the gravitational energy released during the core collapse of a massive star or by the nuclear energy released by explosive thermonuclear reaction, colloquially known as ‘burning’ although there is no combustion involved.

Supernova explosions are visible even when they occur in distant galaxies. The largest supernovae, sometimes called hypernovae, are believed to be responsible for the formerly enigmatic gamma ray bursts sometimes observed. This link was strongly established when a particularly nearby example, known as 2003dh, appeared in Leo on 29 March 2003 (Schilling 2003, p. 1860).

 
 

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Classification of Nova Phenomena

include table from paper

Type I Hydrogen lines absent from optical spectrum.

Type Ia

Characterised by the presence of a deep Si II absorption line near wavelength 6150 and late-time spectra dominated by strong blends of Fe emission lines.

Type Ib

Si II absorption line absent; early-time spectra characterised by moderately strong He I absorption lines, especially at 5876.

Type Ic

Si II absorption line absent; He I absorption lines weak or absent.
Type II Hydrogen lines present from optical spectrum.
   
   
   
Table 1: Types of supernovae. (After Nomoto et al. 1997.)

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Type Ia Supernovae

Characteristics

Luminosity

Composition

Size Range and Distribution

Interpretation: Thought to result from the explosion of massive binary stars. There are spectroscopic and photometric indications that the progenitor stars of type Ia supernovae are white dwarfs that are composed of C + O with strongly degenerate electrons. Such stars are formed from intermediate mass stars, < 8 M, ###

"White dwarfs composed of C+O are formed from intermediate mass stars (M < 8 Mo, where Mo is the mass of our sun), undergo cooling, and eventually become dark matter as they evolve towards fainter luminosities. In a close binary system, the white dwarf evolves differently because the companion star expands to transfer matter to the white dwarf; the accreting white dwarfs are rejuvenated and, in certain cases, undergo thermonuclear explosions to give rise to SNe Ia. Theoretically, the Ch [Chandrasekhar mass] white dwarf models and the sub-Ch models have been considered to explain the origin of SNe Ia [Branch et al. 1995; Renzini 1996]. Various evolutionary scenarios have been proposed, including (i) merging of double C+O white dwarfs with a combined mass exceeding the Ch limit (a DD scenario) [ref. 8 - not on my hardcopy; check PDF] and (ii) accretion of H or He by mass transfer from a binary companion at a relatively high rate (an SD scenario) [refs. 7 & 9 - not on my hardcopy; check PDF]" (Nomoto et al., pp. 1378-1379).

Thermonuclear reactions power the expansion of the core and eventual disruption of the star, but not the luminosity of the expanding gas. The energy source for the latter is provided by the slow radioactive decay sequence 56Ni 56Co 56Fe (Gamezo et al. 2002, p. 77).

The supernova occurs when the white dwarf has accreted sufficient mass from its companion to trigger an explosion. However, the progenitor systems and hydrodynamical models are still controversial.

(After Nomoto et al. 1997.)

Type Ia supernova explosions are caused by the complete thermonuclear disruption of a white dwarf. Model studies attempting to elucidate the explosion mechanism have been limited to spherically symmetric one-dimensional models. Recently, Gamezo et al. (2002, p. 77) developed a three-dimensional model which may herald a new era of model complexity in supernova studies.

Occurence/Examples

Type Ib Supernovae

Characteristics

Luminosity

Composition

Size Range and Distribution

Interpretation: Thought to result from the explosion of massive binary stars.

Occurence/Examples

Type Ic Supernovae

Characteristics

Luminosity

Composition

Size Range and Distribution

Interpretation: Thought to result from the explosion of massive binary stars.

Occurence/Examples

Type II Supernovae

Characteristics

Luminosity

Composition

Size Range and Distribution

Interpretation: Thought to result from the explosion of massive single stars.

Occurence/Examples

Remnants

e.g. Pulsar B1620-26 in the Messier 4 globular cluster

References

Branch, D.; Livio, M.; Yungelson, L.R.; Boffi, F.R.; Baron, E. 1995: Publ. Astron. Soc. Pac. 107: 717.

Gamezo, Vadim N.; Khokhlov, Alexei M.; Oran, Elaine S.; Chtchelkanova, Almadena Y.; Rosenberg, Robert O. 2003: Thermonuclear Supernovae: Simulations of the Deflagration Stage and their Implications. Science 299 (5603): p. 77-81.

Janka, H.-Th. 2002: The Secrets Behind Supernovae. Science 297: 1134-1135.

Naeye 1997: Astronomy or Sky & Telescope article.

Nomoto, Ken'ichi; Iwamoto, Koichi; Kishimoto, Nobuhiro 1997: Type Ia Supernovae: Their Origin and Possible Applications in Cosmology. Science, 276: 1378-1382.

Renzini, A. 1996: In McCray, R.; Wang, Z. (eds.) 1996: IAU Colloquium 145. Supernovae and supernova remnants. Cambridge, pp. 77-85.

Schilling, Govert 2003: Astronomers Nail Down Origin of Gamma Ray Bursts. Science, 300: 1860.


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