Supernova

Stellar explosion that expels much or all of the stellar material with great force, driving a blast wave into the surrounding space, and leaving a supernova remnant. Supernovae are classified based on the presence or absence of features in their optical spectra taken near maximum light. They were first categorized in 1941 by Rudolph Minkowski, who divided them into those that showed H in their spectra (Type II), and those that did not (Type I). Type I supernovae were further sub-divided in the 1980s based on the presence or absence of Si and He in their spectra. Type Ia supernovae have obvious Si absorption at 6150 Å. Type Ib have no Si but show He emission lines, and Type Ic display neither Si nor He.


Image source: http://astronomy.swin.edu.au/cosmos/S/Supernova+Classification.

Type Ia supernovae can be found anywhere and in any type of galaxy, but Type Ib and Type Ic supernovae occur primarily in populations of massive stars, similar to Type IIs. It is now known that Type II, Type Ib and Type Ic supernovae result from the core-collapse of massive stars, while Type Ia supernovae are the thermonuclear explosions of white dwarfs.


Image source: http://www.ifa.hawaii.edu/~barnes/ast110_06/tooe.html#[13].

Type Ia Supernova (snia)
Result of the explosion of a carbon-oxygen white dwarf in a binary system. They are the brightest of all supernovae with an absolute magnitude of -19.5 at maximum light, occur in all galaxy types, and are characterized by a silicon absorption feature (rest wavelength = 6355 angstroms) in their maximum light spectra. They can eject material at speeds of the order of 10,000 km/s and outshine an entire galaxy at their peak brightness.
Both the nature of Type Ia progenitors and the manner in which the star explodes are still uncertain. It is generall accepted that as a white dwarf gains mass from its companion, it contracts and increases its temperature and density. As the mass approaches the Chandrasekhar limit of 1.4 Msun, temperature and pressure in the interior of the star increase until a burning front forms, where C is fused into Fe and Ni almost instantaneously. However, iIf the accretion model is right, the white dwarf ought to emit an abundance of X-rays for ~10 million years before it explodes. Recent (2010) measurmeents reveal that X-ray emissions from five nearby elliptical galaxies and the central region of the Andromeda spiral galaxy are one-thirtieth to one-fiftieth the amount expected in the accretion model. An alternative model is that type Ia supernovae form by merger of white dwarf stars.

The most popular theory is that the white dwarf undergoes a delayed detonation, where the burning front is initially subsonic (deflagration = “burning”) but later becomes supersonic (detonation). Observed explosion energies and observed amounts of unburnt C and O rule out a pure deflagration scenario. The fact that the whole star is not burnt to Fe and Ni rules out a pure detonation scenario. However, variations in explosion energies and the distributions of elements observed in Type Ia supernovae can be modeled by altering when the transition from deflagration to detonation occurs.

The B-band light curves of all SNIa look the same. There is an initial very rapid increase in luminosity, where the brightness of the supernova can change by up to 3 magnitudes in 15 days, that ends at maximum light. After reaching a maximum, brightness declines fairly rapidly (~0.087 mag/day) for the next 3–4 weeks. About a month after maximum light, the decline rate changes to a steady ~0.015 mag/day, dominated by the radioactive decay of 56Co.

Originally it was thought that every SNIa had the same peak brightness; although close to the truth, this is not quite correct. SNIa exhibit maximum brightnesses that range from about +1.5 to -1.5 magnitudes around a typical SNIa. The decline rate of the brightness after maximum light is correlated with the width of the maximum and the peak brightness of the supernova. Brightness can be standardized by applying the luminosity-decline rate relation. This is done using one of three light curve fitting techniques: the Δm15 method (shown below), the Multicoloured Light Curve Shapes method, or the Stretch method.


Image source: http://astronomy.swin.edu.au/cms/astro/cosmos/T/Type+Ia+Supernova+Light+Curves.

Each uses a standard or series of standard light curves to determine how under- or over-luminous a supernova compared to a “typical” SNIa. Astronomers then correct for the luminosity difference before using the supernova as a distance indicator. Consequently, SNIa are very precise distance indicators that can be observed over large distances. They have been instrumental in narrowing down the value of the Hubble Constant, and were the objects used to discover the accelerating universe.

Type Ib Supernova (snib)
Type originally lumped with Type Ia (SNIa) and Type Ic (SNIc) supernovae due to the similar appearance of their light curves. Type Ib supernovae were recognized as a separate class based on the absence of the Si absorption feature typical of Type Ia spectra, and the presence of He absorption which is absent in Type Ic spectra at maximum light. In addition, late-time spectra of SNIb spectra are dominated by intermediate mass elements whereas SNIa spectra are dominated by heavy elements.

SNIb have much more in common with SNII and SNIc than SNIa. They are primarily found in arms of spiral galaxies close to HII regions, and emit radiation at radio wavelengths indicating that circumstellar material surrounded the progenitor star. Some SNII have been observed to transition to SNIb at late times. These observations lead to a model where SNIb result from the core-collapse of a massive star; the difference between SNII and SNIb is that the Type Ib progenitor lost its outer H layer before the collapse.

Type Ic Supernova (snic)
Supernova subtype recognized in 1987. Whereas, Type Ia supernova result from the explosion of a white dwarf, Types Ib and Ic supernovae result from core-collapse of a massive star and are similar to Type II supernovae (SNII). Some astronomers label both Type Ib supernova and SNIc supernova as SNIb/c, because they have similar light curves, spectral evolution and radio properties. The only observable difference between the two types is the lack of He in the spectra of SNIc.


Image source: http://astronomy.swin.edu.au/cms/astro/cosmos/T/Type+Ic+Supernova.

Progenitor stars of SNIc are probably similar to those of SNII supernovae. However, whereas the progenitors of SNII retain H and He envelopes prior to explosion, and SNIb retain He envelopes, SNIc appear to have lost both, resulting in an almost featureless spectrum. The light curves of SNIc are very similar to those of SNIb and SNIa. Like SNIb, the light curves tend to be 1.0–1.5 magnitudes fainter than those of typical SNIa supernova. However, it is generally not possible to distinguish between SNIa, SNIb and SNIc based on light curve shape alone. Spectra (preferably at maximum light) are required to correctly classify new supernovae.

Type Ii Supernova (snii)
Explosive death of a >4 Msun star. Type II supernovae (SNII) were recognized as a distinct type of supernova in the early 1940s. They are distinguished by H emission lines in their spectra, and light curve shapes significantly different than those of Type I supernovae. SNII are sub-classified depending on whether their light curves show linear decline after the maximum (SNII-L) or a plateau phase where the brightness remains constant for an extended period of time (SNII-P). The peak brightnesses of all SNII are several magnitudes fainter than that of Type Ia supernovae. SNII-P show large variability in maximum brightness; peak brightnesses of SNII-L are nearly uniform at 2.5 magnitudes fainter than SNIa supernovae.

SNII are only found in regions of star formation, indicating that they result from core-collapse of massive stars. The difference between the three types of core-collapse supernovae is whether they have retained their outer envelopes of H and He before the explosion. The progenitors of SNII retain both H and He layers (progenitors of Type Ib supernovae have lost their H envelope but retain the He envelope; progenitors of Type Ic supernovae have lost both H and He envelopes before the core-collapse). Unlike Type Ia supernova, SNII tend to form shells of ejected stellar material around either a neutron star (core mass <3 Msun), or a black hole. The ejected material is rich in heavy elements synthesized during the explosion, making SNII one of the principal sources for heavy elements in the Universe.

The first evidence that a core-collapse supernova has occurred is a massive burst of neutrinos. A shock wave emerges from the star a few hours later, releasing electromagnetic radiation initially as a UV flash. The supernova becomes visible at optical wavelengths as it expands. Brightness increases as the surface area increases combined with a relatively slow temperature decrease.


Image source: http://astronomy.swin.edu.au/cms/astro/cosmos/T/Type+II+Supernova+Light+Curves.

The peak in brightness occurs as temperature of the outer layers starts to decrease more rapidly. At this point, Type II supernovae may be divided into two classes based on the shape of their light curves. SNII-L (Linear) supernovae show a fairly rapid, linear decay after maximum light. SNII-P (Plateau) supernovae remain bright for an extended time after maximum. The lack of plateaus in SNII-L probably arises because SNII-L have much smaller hydrogen envelopes than SNII-P. The peak brightness of SNII-L are nearly uniform at ~2.5 magnitudes fainter than a Type Ia supernova. However, the peak brightnesses of SNII-P show large variations due to differences in the radii of the progenitor stars. The end of the light curve is a radioactive tail, powered by the conversion of 56Co into 56Fe; it has the same shape for all core-collapse supernovae.