In 2011 the Nobel Prize in Physics was awarded for discovery of dark energy. The observational data of Type Ia supernova explosions played the primary role in this discovery. The distances measured using the SN IA events led 2011 Nobel Laureates to measure the expansion rate of the universe, and hence predict the existence of dark energy.
Since a long time, the amount of lithium observed in the universe was a great mystery. The famous lithium abundance problem was that the observed amount of lithium was over 160% more than theoretically expected. Where did all this lithium come from? A very recent study led by Summner Starrfield pointed at the lithium isotopes produced in the white dwarf novae explosion. This could be the final key to unlock the lithium abundance puzzle.
Thus, the study of Violent white dwarfs has led us to important discoveries and to solve some significant puzzles of the universe.
The second article in the series of Stellar Explosions, it looks into the physics of white dwarf novae and supernovae explosions. Stay tuned for more!
What prevents white dwarfs from collapsing under gravity?
White dwarfs are stellar corpses composed of either helium, Carbon-Oxygen or Oxygen-Neon-Magnesium cores. Majority of them consist of oxygen core. About 5 billion years from now our sun too, would have exhausted its hydrogen fuel and become a red giant. Eventually, it will shed its outer layers and turn into a carbon-oxygen white dwarf.
All main sequence stars (like the sun) are supported against gravity by the ideal gas pressure (some massive stars have a significant fraction of radiation pressure too), but the White dwarf are supported by electron degeneracy pressure. Electron degeneracy arises from the Pauli Exclusion principle, which says no two electrons (no two fermions in general) can have the same set of quantum numbers.
So as the lower energy quantum states fill up, the electrons are forced to occupy higher energy states. Higher energy states give rise to higher momenta. Relativistic electrons, moving at speeds tending to the speed of light, occupies the highest possible energy states. The maximum number of possible electronic states is thus limited by the speed of light, which in turn limits the maximum mass that a white dwarf could support against gravity.
White Dwarfs are supported by electron degeneracy pressure
As calculated by S. Chandrasekhar the white dwarfs cannot have more mass than 1.37 \(M_{\odot}\).
S. Chandrashekar received Nobel Prize in Physics in 1983 for estimating the White Dwarf Mass Limit
The Nova Explosions
Novae Explosions are explosive expulsions of stellar mass from a white dwarf accreting matter from its main-sequence or red giant companion. Observationally we know that the majority of stars are born in the binary or triple system or in large stellar clusters. Only a few, like the sun, are isolated.
In a binary system, when one of the star, of intermediate mass (0.6-8.0 \(M_{\odot}\)) runs out of nuclear fuel, it sheds its outer mass layers, exposing an inner core of carbon-oxygen (helium or oxygen-neon-magnesium in cases depending upon the extent to which elements are burnt inside).
Mass exchange takes place in stars which lie within some given radius for gravitational effects to be prominent enough. A white dwarf can accrete mass from its binary partner when they both lie within a calculated region, known as Roche lobe.
Illustration of Roche lobe of a giant star and it’s white dwarf companion.
The extra accreted mass increases the gravitational pressure on the inner degenerate core. This leads to an increase in the surface temperature, and at a threshold of \(7\times10^{8}\) K the accreted hydrogen starts fusing. The fusion of hydrogen turns into a runaway thermonuclear reaction and the outer surface layer of white dwarfs explodes. A huge amount of mass is expelled out, in the explosion known as the ‘Nova’.
Novae are quite common, on an average ~50 novae are predicted to occur every year in our galaxy, of which ~35 are detected.
Classical novae are powered by proton- proton (p-p) chain reaction. These novae eject about \(10^{-3}\) \(M_{\odot}\), with velocity of 1500km/s. The extreme temperature conditions (\(1 \times 10^{7}\) K) also allows run-away fusion reactions of hydrogen and helium to produce isotopes of Be, Li, C, O, Si in considerable amounts.
Illustration of p-p chain reaction.
Novae are further classified into three different kinds:
- Classical novae - A white dwarf accretes in a binary system with a main sequence star.
- Recurrent novae - A white dwarf with mass greater than 1.1 \(M_{\odot}\), and accreting from the binary companion ignites a nova with a rate of about 1 novae per century, making it a recurrent explosion.
- Sym Novae - A white dwarf accreting from its red giant companion.
Light curve of a nova explosion. The maximum occurs between 3-5 days, and declines over 10-15 days.
Recently a study led by Summner Starrfield provided evidence that the novae produces Be-7 which decays into lithium with half life of 53 days. These lithium produced by novae, could possibly solve the lithium abundance problem mentioned in the beginning of this article.
Supernovae of White Dwarfs - The SN Ia Explosions
Progenitor System of SN Ia
Even though we currently have catalogued more than 10000 white dwarfs and are carrying out computationally advanced simulations, the progenitor system of SN Ia is not specifically known. Current research in the field points to the following two possibilities:
- A white dwarf accreting from its main sequence or red giant binary.
- Collision of two white dwarfs.
Double Degenerate Scenario
In the double degenerate system, the following possibilities of explosions exist.
- The primary white dwarf could accrete mass from its binary white dwarf, gain mass over time and explode at Chandrashekar limit.
- Both the white dwarfs could explode as they collide and merge.
- It is also possible that the fusion of accreted helium/hydrogen on outer layers could trigger a detonation event that ruptures the entire carbon-oxygen core.
Artistic illustration of two colliding white dwarfs.
Single Degenerate Scenario
In the Single degenerate case, the white dwarfs accretes the mass till it reaches 1.37 \(M_{\odot}\). The rise in temperature ignites an uncontrolled thermonuclear reaction, and deflagrates the white dwarf at subsonic speeds. The deflagration is observed to turn into a detonation event, the physics behind such transformation not yet known.
In this system it is important that the mass of the white dwarf increases over time. The rate of mass loss through novae explosions must be lower than the rate at which the mass increases by accretion. Therefore, it is hypothesized that white dwarfs with more than 1.1 \(M_{\odot}\), undergoing recurrent novae, are most likely to increase their mass up to the Chandrashekar limit and then explode.
Explosion of white dwarf due to mass accretion from main sequence star.
SN Ia explosions or are broadly characterized by
- Light curves that peaks on the 10th day and emits huge amounts of energy over the span of 15-20 days.
- Spectra that lacks hydrogen and helium, but shows strong emission lines of Be, Li, Si and Iron group elements like nickel, cobalt.
Light curves of SN Ia. First image shows the uncalibrated light curves.
In all ideal single degenerate models, the white dwarf explodes at 1.37 \(M_{\odot}\), releasing \(10^{51}\) ergs of nuclear energy. As all of them explode at Chandrashekhar limit, they all produce the same energy. Thus, their light curves when calibrated, can be used to determine extragalactic distances. This makes the SN Ia a probe/standard candle to determine cosmological distances. The SN Ia are probes into the expansion history of the universe; they led the 2011 Physics Nobel Laureate to determine the acceleration rate of the universe. It is hence, cosmologically important to determine the progenitor system on SN Ia.
A major aspect of current day white dwarf research also focuses on the impact of novae mass ejecta on galaxy dynamics, elements synthesized in novae and SN Iae, and also on the existence of sub-Chandrashekar explosions and super-Chandrashekhar white dwarfs.
References
- Srinivasan, Ganesan. Life and Death of the Stars. Springer Science & Business Media, 2014.
- Branch, David, and J. Craig Wheeler. Supernova Explosions. Springer, 2017.
- Alsabti, Athem W., and Paul Murdin, editors. Handbook of Supernovae. Springer International Publishing, 2016.
