CM Draconis
Encyclopedia
CM Draconis is an eclipsing binary system approximately 47 light-year
s away in the constellation
of Draco
(the Dragon
). The system consists of two nearly identical red dwarf
stars located in the constellation Draco
. The two stars orbit each other with a period of 1.27 day
s with a separation of 2.7 million kilometres. Along with two stars in the triple system KOI 126, the stars in CM Draconis are the lightest stars with precisely measured masses and radii. Consequently, the system plays an important role in testing stellar structure models for very low mass stars. These comparisons find that models underpredict the stellar radii by approximately 5%. This is attributed to consequences of the stars' strong magnetic activity. According to the system's entry in the Combined General Catalogue of Variable Stars
, at least one of the components is a flare star
and at least one is a BY Draconis variable
. The white dwarf
star GJ 630.1B, located 25.7 arcseconds away shares the same proper motion
as the CM Draconis stars and is thus a true companion star of the system. Given the system's distance of 47 light years, this corresponds to a separation of at least 370 astronomical unit
s between CM Draconis and GJ 630.1B.
s in orbit around the binary from 1994–1999. In the end, the existence of all of the transiting planet candidates suggested by the project was ruled out.
Based on variations in the timing of the system's eclipses, it has been suggested that there may be an object in a circumbinary orbit around the two red dwarf stars. In 2000, it was proposed that a Jovian planet is orbiting the system with a period of 750–1050 days. More recent eclipse measurements suggest there may be a planet with a mass similar to Jupiter in an 18.5-year orbit, or instead a more massive object located further out. The variations are inconsistent with being caused by the white dwarf companion star. No further evidence for the 750–1050 day planet was found and it is considered unlikely that it exists.
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s away in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Draco
Draco (constellation)
Draco is a constellation in the far northern sky. Its name is Latin for dragon. Draco is circumpolar for many observers in the northern hemisphere...
(the Dragon
Dragon
A dragon is a legendary creature, typically with serpentine or reptilian traits, that feature in the myths of many cultures. There are two distinct cultural traditions of dragons: the European dragon, derived from European folk traditions and ultimately related to Greek and Middle Eastern...
). The system consists of two nearly identical red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
stars located in the constellation Draco
Draco (constellation)
Draco is a constellation in the far northern sky. Its name is Latin for dragon. Draco is circumpolar for many observers in the northern hemisphere...
. The two stars orbit each other with a period of 1.27 day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
s with a separation of 2.7 million kilometres. Along with two stars in the triple system KOI 126, the stars in CM Draconis are the lightest stars with precisely measured masses and radii. Consequently, the system plays an important role in testing stellar structure models for very low mass stars. These comparisons find that models underpredict the stellar radii by approximately 5%. This is attributed to consequences of the stars' strong magnetic activity. According to the system's entry in the Combined General Catalogue of Variable Stars
General Catalogue of Variable Stars
The General Catalogue of Variable Stars is a list of variable stars. Its first edition, containing 10,820 stars, was published in 1948 by the Academy of Sciences of the USSR and edited by B. V. Kukarkin and P. P. Parenago. Second and third editions were published in 1958 and 1968; the fourth...
, at least one of the components is a flare star
Flare star
A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to magnetic reconnection in the atmospheres of the stars. The brightness increase is...
and at least one is a BY Draconis variable
BY Draconis variable
BY Draconis variables are main sequence variable stars of late spectral types, usually K or M. The name comes from the archetype for this category of variable star system, BY Draconis. They exhibit variations in their luminosity due to rotation of the star coupled with star spots, and other...
. The 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...
star GJ 630.1B, located 25.7 arcseconds away shares the same proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
as the CM Draconis stars and is thus a true companion star of the system. Given the system's distance of 47 light years, this corresponds to a separation of at least 370 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s between CM Draconis and GJ 630.1B.
Possible planetary system
The system was the subject of a dedicated search for transiting extrasolar planetExtrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
s in orbit around the binary from 1994–1999. In the end, the existence of all of the transiting planet candidates suggested by the project was ruled out.
Based on variations in the timing of the system's eclipses, it has been suggested that there may be an object in a circumbinary orbit around the two red dwarf stars. In 2000, it was proposed that a Jovian planet is orbiting the system with a period of 750–1050 days. More recent eclipse measurements suggest there may be a planet with a mass similar to Jupiter in an 18.5-year orbit, or instead a more massive object located further out. The variations are inconsistent with being caused by the white dwarf companion star. No further evidence for the 750–1050 day planet was found and it is considered unlikely that it exists.