Barnard's Star

© Steve Quirk, Views from Frog Rock
(used with permission).





Apparent motion of Barnard's Star over
25 years.

Medialab, © ESA 2002



Larger illustration of
the Darwin Mission.



Astronomers have identified
Barnard's Star as a prime target
for NASA's optical SIM and the
ESA's infrared Darwin missions,
both now indefinitely postponed.

The Star

A very cool and dim, main sequence red dwarf (M3.8 Ve), Barnard's Star has less than 17 percent of Sol's mass (RECONS estimate), 15 to 20 percent of its diameter (Dawson and de Robertis, 2004; and Ochsenbein and Halbwachs, 1982, page 529), around 4/10,000th of its visual luminosity -- 0.00346 of its bolometric luminosity (Dawson and de Robertis, 2004), and between 10 and 32 percent of its abundance of elements heavier than hydrogen -- "metallicity" (John E. Gizis, 1997, page 820). According to calculations by Dr. Sten Odenwald, substituting Barnard's Star for Sol would give the Earth such a dim and very red Sun that it would only be 100 times brighter than the Full Moon, and so the planet would freeze solid at the surface. With less than 20 percent of Sol's mass, moreover, Barnard's Star is so small that it can transport core heat to its surface only through convection, unlike larger red dwarf stars like Gliese 752 A -- also known as Wolf 1055 A or Van Biesbroeck's Star (more).

Jeffrey L. Linsky, JILA, STScI, NASA




Larger image.




Like Gliese 752 B, Wolf 359 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).

Unlike Sol, Barnard's appears to be an old star that formed before the galaxy became much enriched with heavy elements (Monet et al, 1992, page 655). Its high space motion and sub-Solar metallicity suggests that the star is an intermediate Population II star," somewhere between a Halo and a disk star (Kürster et al, 2003; and John E. Gizis, 1997). Moreover, its low x-ray luminosity and presumed rotation period of 130.4 days also indicate that it is an old, inactive red dwarf. While the star may be as much as 11 or 12 billion years old (Ken Croswell, 2005), it may last another another 40 billion years or more before cooling into a black dwarf.

NASA -- larger image

Barnard's is a dim red dwarf star, like Gliese
623 A (M2.5V) and B (M5.8Ve) at lower right.
(See a 2MASS Survey image of Barnard's Star
from the NASA Star and Exoplanet Database.)

A small star spot that was shrinking in size may have been observed on Barnard's recently with the Hubble Space Telescope (Benedict et al, 1998). Small periodic variations have been observed which probably result from the "rotational modulation of starspots" (Benedict et al, 1998) and which suggest that the old star rotates slowly (only around once every 130 days) and so should not produce significant flares with regularity from rotational magnetic activity. On July 17, 1998, a hot bluish flare lasting at least an hour may have brightened the star by a half magnitude or more (from its magnitude of 9.55), where the flare's temperature was at least 8,000 Kelvin which would be more than double the star's temperature of 3,100 degrees Kelvin (Ken Croswell, 2005 and Paulson et al, 2006). Barnard's has been given the variable star designation V2500 Ophiuchus as well as the New Suspected Variable star designated NSV 9910. Some other useful star catalogue numbers include: V2500 Oph, Gl 699, Hip 87937, BD+04 3561a, LHS 57, LTT 15309, LFT 1385, G 140-24, Vys/McC 799, and Munich 15040.

INES, LAEFF, ESA




Larger illustration.





Although Barnard's star may be
around 10 billion years old, it
is still burning core hydrogen
as a small and cool member
of the main sequence.

Habitable Zone

In order to be warmed sufficiently have liquid water at the surface, an Earth-type rocky planet would have to be located very close to such a cool and dim red dwarf star like Barnard's, at around 0.034 to 0.082 AU, the Earth-Sun distance (Kürster et al, 2003). At such close distances, such a planet can easily become tidally locked -- with one side in perpetual day -- and race around the star in 5.75 to 21.5 (or three weeks). According to one type of model calculations performed for the NASA Star and Exoplanet Database, the inner edge of Barnard's habitable zone should be located a little further out from the star, at around 0.056 AU from the star, and the outer edge would lie at around 0.109 AUs. Some astronomers have suggested that any rocky planets that formed around Barnard's are likely to be sparse in the heavier elements of the atomic table, and that there may be a greater probability of gas giants made mostly of hydrogen and helium in cold, outer orbits.

Astronomers have long sought to find perturbations ("wobbles") in this star's motion that could be due to planet-sized companions. During the late 1960s, Peter van de Kamp (1901-1995) announced the detection of possibly two coplanar and corevolving planets, whose estimated masses were fine-tuned in 1982 to be about 0.7 and 0.5 of Jupiter's mass with orbital periods of 12 and 20 years, respectively, in "approximately" circular orbits based on astrometric positions obtained from 1938 to 1981 (van de Kamp, 1982). Until his death in 1995, Van de Kamp devoted most of his life (at at Sproul Observatory at (Swarthmore College) to analyzing over 2,000 plates of Barnard's Star that he and his students had taken from 1938 through 1981.

Neither planet was ever verified, however, and more recent observations with the Hubble Space Telescope have failed to yield supporting evidence for a large Jupiter or brown dwarf sized object (Schroeder et al, 2000). Some astronomers suspected that van de Kamp's data were distorted by the cleaning and remounting of the telescope lens at Sproul, 25 years after he began his observations. In 1995, George G. Gatewood (director of the University of Pittsburgh's Allegheny Observatory) suggested that, while brown dwarfs exceeding Jupiter's mass by more than 10 times could not exist around Barnard's Star, planets having a mass smaller than Jupiter's may possibly be present. Subsequent astrometric measurements set even more stringent upper mass limits of 2.1 down to 0.37 Jupiter-masses for orbital periods of 50 up to 600 days (Benedict et al, 1999). (For more information about the search for planets around Barnard's through astrometric perturbation methods, go to George Bell's summary of Barnard's Star and van de Kamp's Planets.)

Between 2000 and March 2006, a team of astronomers engaged in several years of high-precision radial velocity observations of Barnard's star that set even stricter limits on any large planets in circular orbits around this small star (Kürster et al, 2006; and Kürster et al, 2003). Within 1.8 AUs around Barnard's star, the data up through 2005 appear to exclude planets with minimum masses greater than five (4.9) Earth-masses and a true mass greater than Uranus'. In addition, a cold debris or dust disk has not been detected (Lestrade et al, 2010; and Gautier et al, 2007).

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