Groombridge 34 AB

Medialab, © ESA 2002



Larger illustration of
the Darwin Mission.



Astronomers have identified
Groombridge 34 AB as prime targets
for NASA's optical SIM and the ESA's
infrared Darwin missions (Star A),
both now indefinitely postponed.

Due to Groombridge 34 AB's proximity to Sol, the system has been an object of intense interest among astronomers. It was selected as a "Tier 1" target star for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as three Earth-masses within two AUs of its host star (and so some summary system information and images of Groombridge 34 A and 34 B may still be available from the SIM Teams), but the SIM project manager announced on November 8, 2010 that the mission was indefinitely postponed due to withdrawal of NASA funding. Astronomers are also hoping to use the ESA's Darwin group of infrared interferometers to analyze the atmospheres of any rocky planet found in the "habitable zone" (HZ) around Groombridge 34 A for evidence of Earth-type life (Lisa Kaltenegger, 2005).

AB Binary Star System

The precise orbital elements of Groombridge 34 A and B may be uncertain. Based on photographs taken between 1937 and 1970, Sarah Lee Lippincott reported in 1971 that star A and B are separated by an "average" distance of 147 times the Earth-Sun distance (AU) (of a semi-major axis) in a circular orbit (e= 0.00) of about 2,600 years, in contrast to Josef Hopmann's (1890-1975) earlier report in 1958 of an elliptical orbit (e= 0.25) with an orbital period of 3,000 years and an "average" distance of 157 AU (of a semi-major axis) that varies between 118 and 196 AU.

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	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Earth)	Diameter (Earth)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
AB Mass Center	0.0	...	...	...	...	...	...	...	...
Groombridge 34 A	25.5	2,600	0.00	61.4	125,000	37.1	...	...	...
Planet b?	0.072	0.031	0.12	61.4	=>5.4	>1.0	...	...	...
Inner H.Z. Edge A?	0.150	0.083	0	61.4	...	...	...	...	...
Outer H.Z. Edge A?	0.293	0.228	0	61.4	...	...	...	...	...
Groombridge 34 B	121.3	2,600	0.00	61.4	26,000	20.7	...	...	...
Inner H.Z. Edge B?	0.065	0.041	0	61.4	...	...	...	...	...
Outer H.Z. Edge B?	0.126	0.111	0	61.4	...	...	...	...	...

Groombridge 34 A

This cool and dim, main sequence red dwarf (M1.5 Vne) may have about 37.5 to 48.6 percent of Sol's mass (Howard et al, 2014; RECONS; and Berger et al, 2006, Table 5, based on Delfosse et al, 2000), 34 to 39 percent of its diameter (Howard et al, 2014), and some 2.2 percent of its luminosity and 2.9 percent of its theoretical bolometric luminosity (Howard et al, 2014), correcting for infrared output (NASA Star and Exoplanet Database, derived using exponential formula from Kenneth R. Lang, 1980). Based on the abundance of iron to hydrogen, Star A appears to be 35 to 48 percent as enriched as Sol in "metals" -- elements heavier than hydrogen and helium (Howard et al, 2014; Berger et al, 2006, Table 5). Some useful catalogue numbers for this star are: GX And, Gl 15 A, Hip 1475, HD 1326 A, BD+43 44 A, SAO 36248, LHS 3, LTT 10108, LFT 31, G 171-47, and ADS 246 A.

Habitable Zone for Star A

Planet "b"

Groombridge 34 B

This even cooler and dimmer, main sequence red dwarf star (M3.5 Vne) may have only 16.3 percent of Sol's mass (RECONS), 19 percent of its diameter, and 42/100,000th of its luminosity and percent of its luminosity. Useful catalogue numbers for this star include: GQ And, Gl 15 B, HD 1326 B, BD+43 44 B, G 171-48, LHS 4, LTT 10109, LFT 32, and ADS 246 B.

Jeffrey L. Linsky, JILA, STScI, NASA




Larger image.




Like Gliese 752 B, Groombridge 34 B is
so small, with less than 20 percent of Sol's
mass, that it can transport core heat only
through convection, unlike larger larger red
dwarf stars like Gliese 752 A (more).

Habitable Zone for Star B

With a spectral type of M3.5, Ross 154 can be used as a proxy for Groombridge 34 B (M3.5). According to one type of model calculations performed for the NASA Star and Exoplanet Database, the inner edge of Ross 154's habitable zone should be located very close to the star, at around 0.065 AU from the star, while the outer edge lies around 0.126 AUs. Accounting for the great infrared output of M-stars like Ross 154, the equivalent orbital distance for an Earth-type planet be only around 0.096 AUs. At slightly less than a fourth of Mercury's orbital distance in the Solar System, however, the rotation of the planet could become tidally locked with the star so that one side would have perpetual daylight with the other in darkness. Assuming that Groombridge 34 B has about 16.3 percent of Sol's mass, a small Earth-type rocky planet would complete its orbit the star in about 27 days.

Hunt for Stellar and Substellar Companions

In 1947, Alfred H. Joy (page 101) reported finding that Groombridge 34 A had an observed range in the radial velocity of 26 km/sec and so may be a spectroscopic binary star. However, no significant velocity variations were detected in subsequent studies (Pettersen and Griffin, 1980; Marcy and Benitz, 1989; and Henry and McCarthy, 1990, last page). A recent search for faint companions to nearby stars using the Hubble Space Telescope's wide field planetary camera found no evidence of a large Jupiter or brown dwarf sized object (Schroeder et al, 2000).

Arnold O. Benz, Institute of Astronomy, ETH Zurich

High resolution and jumbo images (Benz et al, 1998).

Groombridge 34 AB are both flare stars, like UV Ceti
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.

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 starss 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 Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (such as Groombridge 34 A or B) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf itself, doubling its brightness or more.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a 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 (about 0.08 AU for Groombridge 34 A with a period of 13.5 days or about 0.02 for Groombridge 34 B with a period of 3.8 days), which makes flares even more dangerous around such stars. In addition, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.

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