CM Draconis 3

CM Draconis Aa has a close, double-lined spectroscopic companion Ab, with which it forms a non-contact, eclipsing binary, and a wide common-proper-motion companion B. Stars Aab and B have an observed separation of about 420 AUs (25.7"). On the other hand, stars Aa and Ab have are separated by only 3.76 solar radii, which is about 14.9 times the radius of CM Draconis Aa (Claud H. Lacy, 1977), and have a mutual orbital period of just under 1.27 days (Deeg et al, 1998; Metcalfe et al, 1996; and Claud H. Lacy, 1977). According to a TEP page on CM Draconis, their close orbit is high circular (e= 0.0050 +/- 0.0015) and inclined by 89.8° from the perspective of an observer on Earth. (See an animation of the orbits of the two stars and their potentially habitable zone, with a table of basic orbital and physical characteristics.)

The Aab system has a low flare rate of 0.025 per hour (Deeg et al, 1998). Both flare stars Aab are defined as BY Dra type variables, with CM Draconis as their variable star designation. Some useful star catalogue numbers are: CM Dra A, GJ 630.1 A, G 226-16, G 225-67, LHS 421, and LP 101-15.

Arnold O. Benz, Institute of Astronomy, ETH Zurich
-- high resolution and jumbo images (Benz et al, 1998).

CM Draconis Aab may both be flare stars, like much
younger UV Ceti (Luyten 726-8 B) shown flaring at left.
UV Ceti is an extreme example of a flare star that can
boost its brightness by five times in less than a minute,
then fall somewhat slower back down to normal
luminosity within two or three minutes before flaring
suddenly again after several hours.

CM Draconis Ab (or "B" or "C")

This main sequence red dwarf star (M4.5 V) may have only 21 percent of Sol's mass and 6.2 percent of its diameter (see TEP page on CM Draconis).

H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA

CM Draconis B is a white dwarf (a remnant stellar core which enriched its binary companions, Stars Aab,
with elements heavier than hydrogen when it cast off its outer gas layers) like planetary nebula NGC 2440.

CM Draconis B (or "C")

This even dimmer, main sequence white dwarf star (DQ8 /VII) may have only 18/100,000th of Sol's luminosity. The stellar remnant has cooled, become dimmer over time, and is presumed to be relatively old (perhaps around nine billion years like its binary companions). In theory, it has around 0.6 to 1.4 times Sol's mass with only a planet-sized diameter (e.g., around one percent of the Sun's). Useful catalogue numbers include: CM Dra B, GJ 630.1 B, G 226-15, G 225-68, EG 258, LHS 422, and LP 101-16.

Hunt for Substellar Companions

© Lynette Cook

(Artwork from Extrasolar Planets - Collection III, used with permission)


View from an asteroid of tightly orbiting red dwarfs with a tidally-locked planet in the system's liquid water zone. The hypothetical planet's darkside ice cap is backlit from a nearby white dwarf in the distance.

In 1996, two teams of astronomers announced the possible detection of a planetary transit eclipse of the close binary pair CM Draconis Aab (Guinan et al, 1998; Martin and Deeg, 1996; and Guinan et al, 1996). The initial discovery of a brown-dwarf-sized object was disputed (Deeg et al, 1998). Eventually, it was determined that no substellar objects around CM Draconis larger than about three Jupiter-masses are orbiting closer than one AU (Doyle et al, 2000). As of mid 2000, further observations and analysis suggested that one (or two) planet(s) of at least 2.5 Earth diameters across in a 21- or a 26-day orbit and a farther out giant planet in the range of 1.5 to three Jupiter masses at an orbital distance of 1.1 to 1.45 AU around the close-orbiting binary's barycenter (Deeg et al, 2000).

For low-mass eclipsing binary stars, the method of eclipse minimum timing allows astronomers to search for smaller masses than those feasible with radial velocity measurements. Some astronomers, however, have recently detected starspots which can result in false non-periodic detections and also a long-term variation in "seasonal" averages with a period of several years which deserves further study (Hargis et al, 2000). In any case, a circumbinary orbital distance from CM Draconis Aab where an Earth-type planet would be comfortable with liquid water would be centered around 0.3 AU, with a "year" of 18 to 35 days. (Doyle et al, 2000; in Scientific American). See an animation of the orbits of the two stars and their potentially habitable zone, with a table of basic orbital and physical characteristics.

Life Around a Flare Star

Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on our Sun every so often, the amount of energy released in a solar flare is small compared to the total amount of energy that Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (CM Draconis) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf does normally.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be unlikely because their planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (just over 0.1 AU for CM Draconis Aab), which makes flares even more dangerous around such stars. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.