AM Canum Venaticorum (AM CVn) is a hydrogen-deficient

cataclysmic variable In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since ones with an outburst brightness visible to ...

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

in the constellation of

Canes Venatici Canes Venatici () is one of the IAU designated constellations, 88 constellations designated by the International Astronomical Union (IAU). It is a small northern constellation that was created by Johannes Hevelius in the 17th century. Its name is ...

. It is the type star of its class of variables, the AM Canum Venaticorum stars. The system consists of a

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

gaining matter via an

accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...

from a semi-degenerate or white dwarf companion.

Observations

During 1939–40, a survey for faint

s was carried out using an

Schmidt telescope Schmidt may refer to: * Schmidt (surname), including list of people with the surname * Schmidt (singer) (born 1990), German pop and jazz singer * Schmidt (lunar crater), a small lunar impact crater * Schmidt (Martian crater), a crater on Mars * ...

Palomar observatory Palomar Observatory is an astronomical research observatory in San Diego County, California, United States, in the Palomar Mountain Range. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observat ...

. Part of the survey was made around the

north galactic pole The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane parallel to an ap ...

in order to exclude stars of

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...

s O, B, and A, as these higher mass, shorter-lived stars tend to be concentrated along the plane of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

where new star formation occurs. Out of the stars observed, a list of faint blue stars was constructed by Milton L. Humason and

Fritz Zwicky Fritz Zwicky (; ; February 14, 1898 – February 8, 1974) was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and ...

in 1947, with their blue hue suggesting a relatively high

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

. The 29th star on their list, HZ 29, was found to have the most peculiar

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...

out of the set. It displayed an absence of

hydrogen line The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...

s, but broad, diffuse lines of neutral (non-

ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...

) helium. This was interpreted as a hydrogen-deficient white dwarf. In 1962, this star was observed with a photoelectric detector and was found to vary in magnitude over a period of 18 minutes. The

light curve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...

of the variation displayed a double sinusoid pattern. Later, a flickering behavior was observed, which suggested a

mass transfer Mass transfer is the net movement of mass from one location (usually meaning stream, phase, fraction or component) to another. Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane filtration, ...

Distance

The distance of AM CVn has been difficult to determine. It is too faint to have a measured

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...

parallax, too distant to have a reliable precise parallax determined by other means, and too rare to have its parameters known by comparison with other objects. Calibration against other

cataclysmic variables In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since ones with an outburst brightness visible t ...

yields a distance of . Other estimates of its distance, by comparison to models of its accretion disk, give and . One ground-based measurement of its absolute parallax gave a distance of . Derivation of a relative parallax, by comparison to the estimated parallaxes of three comparison stars, using the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

Fine Guidance Sensor gives a very large distance of .

Gaia Data Release 2 The ''Gaia'' catalogues are star catalogues created using the results obtained by ''Gaia'' space telescope. The catalogues are released in stages that will contain increasing amounts of information; the early releases also miss some stars, especia ...

gives a parallax of , leading to a distance of . This value gives the system a lower luminosity and accretion rate, closer to what would be expected by accretion disk models.

Description

The model developed to explain the observations was that AM Canum Venaticorum is a binary system consisting of a pair of white dwarfs in a close orbit. The primary is a more massive white dwarf composed of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

, whereas the secondary is a less massive white dwarf made of

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

, with no hydrogen but traces of heavier elements. At the unexpectedly large distance found by the HST, the secondary would be a semi-

degenerate Degeneracy, degenerate, or degeneration may refer to: Arts and entertainment * Degenerate (album), ''Degenerate'' (album), a 2010 album by the British band Trigger the Bloodshed * Degenerate art, a term adopted in the 1920s by the Nazi Party i ...

object such as subdwarf B star. Gravitational wave radiation is causing a loss of

angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...

in the orbit, leading to the transfer of helium from the secondary to the primary as the two draw closer. This transfer is occurring because the secondary is overflowing its

Roche lobe In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential, wit ...

—a tear drop shaped lobe created by the gravitational interaction between the two stars. The mass transfer rate between the two stars is estimated as about solar masses per year, which is creating an

around the companion white dwarf. The energy output from the mass flow onto this accretion disk is actually the primary contributor to the visual luminosity of this system; outshining both of the stellar components. The temperature of this disk is about 30,000 K. High speed photometry of the system shows multiple periods of variation in the luminosity. The main period of seconds (17^m 8.73^s) is the orbital period of the pair. A secondary period of seconds (17^m 31^s) is believed to be caused by a

superhump In astronomy, a superhump is a periodic brightness variation in a cataclysmic variable star system, with a period within a few percent of the orbital period of the system. History Superhumps were first seen in SU Ursae Majoris (SU UMa) stars, a ...

—an elevated outburst in the signal that occurs with a period slightly longer than the orbital period. The superhump may be the result of an elongation of the accretion disk in combination with

precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In othe ...

. The elliptical disk precesses about the white dwarf over a time interval much longer than the orbital period, causing a slight change in the orientation of the disk over each orbit.

Flares

Normally AM CVn only exhibits magnitude variations of 0.05. However, AM CVn star systems such as this are

nova A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...

-like objects that are known to randomly generate intense flares in luminosity. AM Canum Venaticorum displayed just such flaring behavior twice during the period 1985–1987, with these flares showing rapid fluctuations in luminosity. A 1986 flare caused an increase in magnitude of up to Δm = and lasted for 212 seconds. The amount of energy released during this event is estimated as

erg The erg is a unit of energy equal to 10−7joules (100 nJ). It originated in the Centimetre–gram–second system of units (CGS). It has the symbol ''erg''. The erg is not an SI unit. Its name is derived from (), a Greek word meaning 'work' o ...

. These flashes are caused by the brief

thermonuclear fusion Thermonuclear fusion is the process of atomic nuclei combining or “fusing” using high temperatures to drive them close enough together for this to become possible. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the re ...

of helium being accumulated along an outer shell by the primary.

References

External links

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_video) {{DEFAULTSORT:AM_Canum_Venaticorum Canes_Venatici.html" ;"title="Chandra X-ray Observatory
video)">Chandra X-ray Observatory">A Tour of AM CVn (Chandra X-ray Observatory
video) {{DEFAULTSORT:AM Canum Venaticorum Canes Venatici">Chandra X-ray Observatory
video)">Chandra X-ray Observatory">A Tour of AM CVn (Chandra X-ray Observatory
video) {{DEFAULTSORT:AM Canum Venaticorum Canes Venatici AM CVn stars Objects with variable star designations, Canum Venaticorum, AM White dwarfs