The origin of white dwarfs
Two ideas played a key role in explaining the genesis of white dwarfs: the thought
- Triple helium reaction and isothermal cores of red giants
During the evolution of stars, the mainsequence, hydrogen “burns out” - nucleosynthesis with the formation of helium (see Bethe cycle). Such a burnout leads to a cessation of energy release in the central parts of the star, compression and, accordingly, an increase in temperature and density in its core.
Temperature and density rise in the stellar coreleads to conditions in which a new source of thermonuclear energy is activated: helium burnout (triple helium reaction or triple alpha process), characteristic of red giants and supergiants.
- Mass loss and shell shedding by red giants
Nuclear reactions in red giants do not occuronly in the core: as hydrogen burns out in the core, helium nucleosynthesis spreads to the still hydrogen-rich regions of the star, forming a spherical layer at the boundary of the hydrogen-poor and hydrogen-rich regions.
A similar situation arises with the triple helium reaction: as helium burns out in the core, it also concentrates in a spherical layer at the border between helium-poor and helium-rich regions.
The luminosity of stars with such “double-layer”areas of nucleosynthesis increases significantly, reaching about several thousand luminosities of the Sun, while the star “swells”, increasing its diameter to the size of the Earth’s orbit. The helium nucleosynthesis zone rises to the surface of the star: the fraction of mass inside this zone is ~70% of the star's mass.
“Blowing up” is accompanied by a fairly intense outflow of matter from the surface of the star; objects such as protoplanetary nebulae are observed.
The exact mechanisms of mass loss and further envelope ejection for such stars are still unclear, but the following factors can be assumed that can contribute to the envelope loss:
- Due to the extremely high luminosity, the light pressure of the radiation flux of the star on its outer layers becomes significant, which, according to the calculated data, can lead to the loss of the envelope in several thousand years.
- Due to the ionization of hydrogen in the regions lyingbelow the photosphere, strong convective instability can develop. Solar activity has a similar nature, but in the case of red giants, the power of convective flows should significantly exceed the solar one.
- In extended stellar envelopes, they caninstabilities develop, leading to strong oscillatory processes, accompanied by a change in the thermal regime of the star. In fig. 4, density waves of the matter ejected by the star are observed, which may be the consequences of such oscillations.
- Red giants with a "two-layer" thermonuclearthermal pulsations, accompanied by the "switching" of hydrogen and helium thermonuclear sources and intense mass loss, are observed by sources that have passed at a late stage of their evolution to the asymptotic branch of giants.
- Contraction of white dwarfs
Theorists predicted that young whitesdwarfs at an early stage of evolution should shrink. According to calculations, due to gradual cooling, the radius of a typical white dwarf can be reduced by several hundred kilometers in the first million years of its existence.
In 2017, Russian astrophysicists fromThe State Astronomical Institute named after P.K. Sternberg Moscow State University, the Institute of Astronomy of the Russian Academy of Sciences, the Institute of Theoretical and Experimental Physics named after A.I. Alikhanov and the National Institute of Astrophysics (Milan) under the leadership of Professor Sergei Borisovich Popov, for the first time in the world, documented the discovery of a young white dwarf, very rapidly decreasing radius.
Russian scientists and their Italian assistantsstudied the X-ray emission of the binary system HD49798/RX J0648.0-4418, located in the constellation Puppis at a distance of two thousand light years from Earth. The research results were published in the journalMonthly Notices of the Royal Astronomical Societyin February 2018
Properties of white dwarfs
- Chemical composition
The chemical composition of a white dwarf is determined by the stage at which thermonuclear reactions inside the progenitor star ended.
If the mass of the original star is small, 0.08-0.5 massesThe sun, which is not enough to start the burning of helium, then after using up the entire supply of hydrogen such stars become helium white dwarfs with a mass of up to 0.5 solar.
If the original star has a mass of 0.5-8masses of the Sun, then this is enough for a helium flash, the evolution of the star will continue at the red giant phase and will stop only after the helium burns out. The resulting degenerate core of such a star will become a carbon-oxygen white dwarf with a mass of 0.5-1.2 solar.
When the original star has a mass of 8-12 solar masses,this is enough to start burning carbon, the evolution of the star will continue further and the carbon in its interior can be processed into heavier elements, in particular neon and magnesium. And then its final stage in the evolution of such a star can be the formation of an oxygen-neon-magnesium white dwarf with a mass close to the Chandrasekhar limit.
Evolution of white dwarfs
White dwarfs begin their evolution as the exposed degenerate cores of red giants that have shed their shell - that is, as the central stars of young planetary nebulae.
The temperatures of the photospheres of the nuclei of young planetarynebulae are extremely high - for example, the temperature of the central star of the nebula NGC 7293 ranges from 90,000 K (estimated from absorption lines) to 130,000 K (estimated from the X-ray spectrum). At these temperatures, hard ultraviolet and soft X-rays account for most of the spectrum.
At the same time, the observed white dwarfs in theirspectra are predominantly divided into two large groups - “hydrogen” spectral class DA, in the spectra of which there are no helium lines, which make up ~80% of the population of white dwarfs, and “helium” spectral class DB without hydrogen lines in the spectra, making up most of the remaining 20% populations.
The reason for this difference in the composition of white atmospheresdwarfs remained unclear for a long time. In 1984, Iko Iben considered scenarios for the “exit” of white dwarfs from pulsating red giants on the asymptotic giant branch at various pulsation phases.
At a later stage of evolution, red giants withup to ten solar masses, as a result of the "burnout" of the helium core, a degenerate core is formed, consisting mainly of carbon and heavier elements, surrounded by a non-degenerate helium layer source, in which a triple helium reaction takes place.
In an extremely short time (~ 30 years), the luminosityof the helium source increases so much that the combustion of helium goes into convective mode, the layer expands, pushing out the hydrogen layer source, which leads to its cooling and the cessation of hydrogen combustion. After the excess helium is burned out during the flare, the luminosity of the helium layer decreases, the outer hydrogen layers of the red giant are compressed, and a new ignition of the hydrogen layer source occurs.
Astronomical phenomena involving white dwarfs
A feature of the radiation of white dwarfs inX-ray range is the fact that the main source of X-ray radiation for them is the photosphere, which sharply distinguishes them from "normal" stars: the latter emits an X-ray corona heated to several million Kelvin, and the temperature of the photosphere is too low to emit X-rays.
In the absence of accretion, the white luminosity sourcedwarfs are the store of thermal energy of ions in their depths, so their luminosity depends on age. A quantitative theory of cooling of white dwarfs was built in the late 1940s by Professor Samuel Kaplan.
- Accretion on White Dwarfs in Binary Systems
During the evolution of stars of different masses in binarysystems, the rates of evolution of the components are not the same, while a more massive component can evolve into a white dwarf, while a less massive one can remain on the main sequence by this time.
In turn, upon descent in the process of evolutiona less massive component from the main sequence and its transition to the branch of red giants, the size of an evolving star begins to grow until it fills its Roche lobe.
- Nonstationary accretion onto white dwarfs inIf the companion is a massive red dwarf, it leads to the appearance of dwarf novae (stars of the U Gem (UG) type) and nova-like catastrophic variable stars.
- Accretion on white dwarfs with strongmagnetic field is directed to the region of the white dwarf's magnetic poles, and the cyclotron mechanism of radiation of the accreting plasma in the near-polar regions of the dwarf's magnetic field causes a strong polarization of radiation in the visible region (polars and intermediate polars).
- Accretion on hydrogen-rich white dwarfsmatter leads to its accumulation on the surface (consisting mainly of helium) and heating to the temperatures of the helium fusion reaction, which, in the case of the development of thermal instability, leads to an explosion observed as a burst of a new star.
- Sufficiently long and intense accretion ona massive white dwarf leads to its mass exceeding the Chandrasekhar limit and a thermonuclear explosion, observed as a type Ia supernova. An example of such an event is the SN 1572 supernova explosion.
Formation of white dwarf disks
Scientists at the US Planetary Science Institute decidedmystery surrounding the formation of debris disks around white dwarfs. These disks are known to appear only 10-20 million years after the red giant stage.
During the red giant phase, the star loses a significant portion of its mass before becoming a carbon-oxygen white dwarf, about the size of Earth and half the mass of the Sun.
At this time, the orbits of any remaining planetsdestabilized, and asteroids are thrown towards the white dwarf. When they get too close, the tidal forces of the star turn them into dust. Young white dwarfs are expected to form discs quickly, but this is not the case.
It turned out that the delay was explained precisely bytemperature of white dwarfs. They are so hot that any dust will quickly evaporate and dissipate. This evaporation stops only when the surface temperature of the white dwarf cools down to about 27 thousand Kelvin. This is consistent with observational data: discs have been found in dwarfs whose temperatures are below critical.
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