Struve 2398 AB

Due to Struve 2398 AB's proximity to Sol, the system has been an object of high interest among astronomers. Stars A and B have been selected as "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 Struve 2398 A and Struve 2398 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.

AB Binary Star System

There are two stars, Struve 2398 A and B, that orbit each other at an "average" distance of about 56 times the Earth-Sun distance -- 42 astronomical unit (AUs) of an orbital semi-major axis -- which is roughly Pluto's orbital distance in the Solar System. The separation of the two components was first measured in 1832 by Friedrich Georg Wilhelm von Struve (1793-1864), who became director of Russia's Dorpat Observatory in 1817 and founded and directed the Pulkovo Observatory in 1837, surveyed 120,000 stars from 1819 to 1827, published an extensive monograph of Halley's Comet based on observations in 1835 and his findings on 2,640 double stars in 1837, and measured the parallax of Vega from 1835 to 1838. While now often named in his honor, Struve's stars were originally numbered after the Greek letter "Sigma," so that this binary pair was designated as Sigma 2398.

Although Struve 2398 A and its stellar companion B are known to have an eccentric orbit, the specific orbital elements may still be uncertain. In 1958, Wilhelm F. Rabe (1893-1958) derived a period of 346 years with an average distance of 42 AUs (based on updated parallax) and an eccentricity of 0.55, so that the stars swing between 19.0 and 65.3 AUs apart. Using photographic plates from 1918 to 1987, however, (Wulff Dieter Heintz, 1987) subsequently calculated a period of 408 years with an eccentricity of 0.53, and an average distance of 56 AUs where the pair swings between 26 and 86 AUs apart.

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	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Solar)	Diameter (Solar)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
AB Mass Center	0.0	...	...	...	...	...	...	...	...
Struve 2398 A	23.8	408	0.53	66.0	0.34-0.35	0.34	...	...	0.45-0.49
Inner H.Z. Edge A	0.123	0.073	0	66.0	...	...	...	...	...
Outer H.Z. Edge A	0.240	0.199	0	66.0	...	...	...	...	...
Struve 2398 B	32.0	408	0.53	66.0	0.26-0.27	0.27	...	...	0.42-0.45
Inner H.Z. Edge B	0.091	0.054	0	66.0	...	...	...	...	...
Outer H.Z. Edge B	0.146	0.054	0	66.0	...	...	...	...	...

Struve 2398 A

This star is probably a main-sequence red dwarf star of spectral and luminosity type M3.0 V, although it is still recorded as orange as K5 in some catalogues. The star has about 34 to 35 percent of Sol's mass (Jenkins et al, 2009; and RECONS), 34 percent of its diameter (NASA Star and Exoplanet Database, derived from the power law formula of Kenneth R. Lang, 1980), and 27/10,000th of its visual luminosity and 1.5 percent of its bolometric luminosity (NASA Star and Exoplanet Database, derived from the exponential formula of Kenneth R. Lang, 1980). Star A appears to be about 45 to 49 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Jenkins et al, 2009). A chromospherically active star (López-Santiago et al, 2010; and Bonfils et al, 2005, see GJ 725 A in Table 5), Struve 2398 A has been given the New Suspected Variable designation of NSV 11288. Useful star catalogue numbers for Struve 2398 A include: ADS 11632, Gl 725 A, Hip 91768, HD 173739, BD+59 1915 A, SAO 31128, G 227-46, G 229-13, LHS 58, LTT 15522, LDS 1466, and LFT 1431.

Habitable Zone around Star A

Accounting for infrared heating, the distance from Struve 2398 A where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.17 AU (SIM Project. At that distance from the star, such a planet would have an orbital period of around 98 days, and so it may become tidally locked in orbit so that one side stays in perpetual daylight while the other lies in darkness. According to alternative calculations performed for the NASA Star and Exoplanet Database, the HZ would be centered a little farther out at 0.182 AU, with the inner edge of Star A's habitable zone at around 0.123 AU and the outer edge at around 0.240 AUs.

Struve 2398 B

This star is probably a main-sequence red dwarf star of spectral and luminosity type M3.5 V, although it is still recorded as orange as K5 in some catalogues. It has about 26 to 27 percent of Sol's mass (Jenkins et al, 2009; and RECONS), 27 percent of its diameter (NASA Star and Exoplanet Database, derived from the power law formula of Kenneth R. Lang, 1980), and about 13/10,000th of its visual luminosity and 0.3 percent of its bolometric luminosity (NASA Star and Exoplanet Database, derived from the exponential formula of Kenneth R. Lang, 1980). Star B appears to be about 42 to 45 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Jenkins et al, 2009; and Bonfils et al, 2005, see GJ 725 A in Table 5). In addition to being a chromospherically active star (López-Santiago et al, 2010), Struve 2398 B is also a known flare star. Useful star catalogue numbers for Struve 2398 B include: Gl 725 B, Hip 91772, HD 173740, BD+59 1915 B, SAO 31129, G 227-47, and LHS 59.

Arnold O. Benz, Institute of Astronomy, ETH Zurich

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

Struve 2398 B is a flare star, like 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.

Habitable Zone around Star B

Accounting for infrared heating, the distance from Struve 2398 B where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.15 AU (SIM Project. At that distance from the star, such a planet would have an orbital period of only around 39 days and so would likely be tidally locked with perpetual daylight on one side. According to alternative calculations performed for the NASA Star and Exoplanet Database, the HZ would be centered a little closer in at 0.134 AU, with the inner edge of Star A's habitable zone at around 0.091 AU and the outer edge at around 0.177 AUs.

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 Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (such as Struve 2398 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 0.15 AU for Struve 2398 B), 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.

Hunt for Substellar Companions and Debris disk

A recent search for faint companions using the Hubble Space Telescope found no supporting evidence for a large Jupiter or brown dwarf sized object (Schroeder et al, 2000). No significant, cold debris disk was detected around either Star A or B (Lestrade et al, 2009).

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