AM CVn star
Encyclopedia
An AM CVn star, or AM Canum Venaticorum star, is a type of cataclysmic variable star
. These binary stars
are named after their type star, AM Canum Venaticorum
. In these variables, a white dwarf
accretes hydrogen
-poor matter from a compact companion star. These binaries have extremely short orbital periods (shorter than about one hour), and are predicted to be strong sources of gravitational radiation, strong enough to be detected with the Laser Interferometer Space Antenna
.
. Their ultra-short orbital periods of less than about 70 minutes indicate that both the donor star and accretor star in the binary are degenerate
or semi-degenerate objects. Where the accretor is always a white dwarf, the donor star can be either a (helium or hybrid) white dwarf, a low-mass helium star or an evolved main-sequence
star. Although all three types of donor star have been predicted to exist, observations suggest that only helium-star donors may have been found so far.
AM CVn stars with a white-dwarf donor can be formed when a binary consisting of a white dwarf and a low-mass giant
evolve through a common-envelope
(CE) phase. The outcome of the CE will be a double white-dwarf binary. Through the emission of gravitational radiation, the binary loses angular momentum
, which causes the binary orbit to shrink. When the orbital period has shrunk to about 5 minutes, the least-massive (and the largest) of the two white dwarfs will fill its Roche lobe
and start mass transfer to its companion. Soon after the onset of mass transfer, the orbital evolution will reverse and the binary orbit will expand. It is in this phase, after the period minimum, that the binary is most likely to be observed.
AM CVn stars with a helium-star donor are formed in a similar way, but in this case the giant that causes the common envelope is more massive and produces a helium star rather than a second white dwarf. A helium star is more expanded than a white dwarf, and when gravitational radiation brings the two stars into contact, it is the helium star which will fill its Roche lobe and start mass transfer, at an orbital period of roughly 10 minutes. As in the case of a white-dwarf donor, the binary orbit is expected to 'bounce' and start expanding soon after mass transfer is started, and we should typically observe the binary after the period minimum.
The third type of potential donor in an AM CVn system is the evolved main-sequence star. In this case, the secondary star does not cause a common envelope, but fills its Roche lobe near the end of the main sequence (terminal-age main sequence or TAMS). An important ingredient for this scenario is magnetic braking
, which allows efficient angular-momentum loss from the orbit and hence a strong shrinkage of the orbit to ultra-short periods. The scenario is rather sensitive to the initial orbital period; if the donor star fills its Roche lobe too long before the TAMS the orbit will converge, but bounce at periods of 70-80 minutes, like ordinary CVs. If the donor starts mass transfer too long after the TAMS, the mass-transfer rate will be high and the orbit will diverge. Only a narrow range of initial periods, around this bifurcation period will lead to the ultra-short periods that are observed in AM CVn stars. The process of bringing the two stars into a close orbit under the influence of magnetic braking is called magnetic capture. AM CVn stars formed this way may be observed either before or after the period minimum (which can lie anywhere between 5 and 70 minutes, depending on exactly when the donor star filled its Roche lobe) and are assumed to have some hydrogen on their surface.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
. These binary stars
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
are named after their type star, AM Canum Venaticorum
AM Canum Venaticorum
AM Canum Venaticorum is a cataclysmic variable star in the constellation of Canes Venatici. It is the type star of its class of variables, the AM CVn stars....
. In these variables, a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
accretes hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
-poor matter from a compact companion star. These binaries have extremely short orbital periods (shorter than about one hour), and are predicted to be strong sources of gravitational radiation, strong enough to be detected with the Laser Interferometer Space Antenna
Laser Interferometer Space Antenna
The Laser Interferometer Space Antenna is a planned space mission to detect and accurately measure gravitational waves from astronomical sources. LISA was originally conceived as a joint effort between the United States space agency NASA and the European Space Agency...
.
Properties
AM CVn stars differ from most other cataclysmic variables (CVs) in the lack of hydrogen lines from their spectraSpectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
. Their ultra-short orbital periods of less than about 70 minutes indicate that both the donor star and accretor star in the binary are degenerate
Degenerate matter
Degenerate matter is matter that has such extraordinarily high density that the dominant contribution to its pressure is attributable to the Pauli exclusion principle. The pressure maintained by a body of degenerate matter is called the degeneracy pressure, and arises because the Pauli principle...
or semi-degenerate objects. Where the accretor is always a white dwarf, the donor star can be either a (helium or hybrid) white dwarf, a low-mass helium star or an evolved main-sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
star. Although all three types of donor star have been predicted to exist, observations suggest that only helium-star donors may have been found so far.
Formation scenarios
The three possible types of donor stars each have a different formation scenario.AM CVn stars with a white-dwarf donor can be formed when a binary consisting of a white dwarf and a low-mass giant
Giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...
evolve through a common-envelope
Common envelope
In astronomy, a common envelope refers to a short-lived phase in the evolution of a binary star in which the largest of the two stars has initiated unstable mass transfer to its companion star. Mass transfer is unstable when the radius of the donor star expands more rapidly or shrinks less...
(CE) phase. The outcome of the CE will be a double white-dwarf binary. Through the emission of gravitational radiation, the binary loses angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
, which causes the binary orbit to shrink. When the orbital period has shrunk to about 5 minutes, the least-massive (and the largest) of the two white dwarfs will fill its Roche lobe
Roche lobe
The Roche lobe is the region of space around a star in a binary system within which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material can escape the gravitational pull of the star. If the star is in a binary system then the material...
and start mass transfer to its companion. Soon after the onset of mass transfer, the orbital evolution will reverse and the binary orbit will expand. It is in this phase, after the period minimum, that the binary is most likely to be observed.
AM CVn stars with a helium-star donor are formed in a similar way, but in this case the giant that causes the common envelope is more massive and produces a helium star rather than a second white dwarf. A helium star is more expanded than a white dwarf, and when gravitational radiation brings the two stars into contact, it is the helium star which will fill its Roche lobe and start mass transfer, at an orbital period of roughly 10 minutes. As in the case of a white-dwarf donor, the binary orbit is expected to 'bounce' and start expanding soon after mass transfer is started, and we should typically observe the binary after the period minimum.
The third type of potential donor in an AM CVn system is the evolved main-sequence star. In this case, the secondary star does not cause a common envelope, but fills its Roche lobe near the end of the main sequence (terminal-age main sequence or TAMS). An important ingredient for this scenario is magnetic braking
Magnetic braking
Magnetic braking is a theory explaining the loss of solar angular momentum due to material getting captured by the solar magnetic field and thrown out at great distance from the surface of the Sun.-The problem:...
, which allows efficient angular-momentum loss from the orbit and hence a strong shrinkage of the orbit to ultra-short periods. The scenario is rather sensitive to the initial orbital period; if the donor star fills its Roche lobe too long before the TAMS the orbit will converge, but bounce at periods of 70-80 minutes, like ordinary CVs. If the donor starts mass transfer too long after the TAMS, the mass-transfer rate will be high and the orbit will diverge. Only a narrow range of initial periods, around this bifurcation period will lead to the ultra-short periods that are observed in AM CVn stars. The process of bringing the two stars into a close orbit under the influence of magnetic braking is called magnetic capture. AM CVn stars formed this way may be observed either before or after the period minimum (which can lie anywhere between 5 and 70 minutes, depending on exactly when the donor star filled its Roche lobe) and are assumed to have some hydrogen on their surface.