44 (i) Boötis 3

All three stars of the 44 Boötis system are similar to Sol in size, brightness, and color. The annual proper motion of the system is about 40" in PA 274°, and it's radial velocity is around 24 km per second (15 miles per second) in approach. It is visible to the naked eye. All three are believed to be more than a billion years old (Alan Hale, 1994, pp. 312 and 314).

44 (i) Boötis A

This star is a yellowish main sequence dwarf star of spectral and luminosity type F5-G1 Vn (Nikolic et al, 1997; based on Frans van't Veer, 1971; and Kurpinska and van't Veer, 1970; versus Hill et al, 1989, page 89). It may be as massive as (or slightly more so than) Sol, with about the same diameter -- 1.03 to 1.05 percent Solar (Johnson and Wright, 1983, page 683; and Hill et al, 1989) and around 1.14 times its luminosity. Useful star catalogue numbers and designations for 44 Boötis A include: 44 Boo, i Boo, 44i Boo, HR 5618*, Gl 575 A, Hip 73695, HD 133640, BD+48 2259, SAO 45357, Struve 1909 A, and ADS 9494 A.

From the perspective of an observer on Earth, the orbit of Star A and the BC tight binary pair exhibit a very elongated and narrow ellipse whose separation has varied from 4.7" in 1880 to less than 0.4" in 1969 (Kaj Aage Gunnar Strand, 1937; A. Gennaro, 1940; L. Bennendijk, 1955; Worley and Heintz, 1983; and Wulff Dieter Heintz, 1963 to 1997; among others). According to new measurements (Staffan Soderhjelm, 1999) found in the new Sixth Catalog of Visual Orbits of Binary Stars, Stars A and B are separated by an "average distance" of about 48.5 AUs (semi-major axis of 3.8" with a HIPPARCOS distance estimate of 41.6 ly), or more than the average of orbital distance of Pluto in the Solar System. They move in a highly elliptical orbit (e= 0.55) that takes about 206 years to complete. Their orbit is inclined about 84° from the perspective of an observer on Earth. These elements are similar to Heintz's 1997 elements of: P=220.0 years; a=3.70"; e= 0.451; and i=83.7 (Wulff Dieter Heintz, 1997). (See an animation of the orbits of Stars A, B, and C and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

44 (i) Boötis B

This star is a yellow-orange main sequence dwarf star of spectral and luminosity type G2 V (Nikolic et al, 1997; and Hill et al, 1989). This star may have around the same mass as Sol, 87 to 89 percent of its diameter (Johnson and Wright, 1983, page 683; and Hill et al, 1989), and as little as 54 percent of its luminosity. Useful catalogue numbers for the star include: Gl 575 B, Struve 1909 B, and ADS 9494 B.

© Carla Maceroni
(used with permission)


Stars B and C form a
W-Ursae-Majoris-type,
shallow contact binary
whose orbital motion
results in eclipses
every three hours
(more discussion).

44 Boötis is classified as an eclipsing variable of W Ursae Majoris type (that also resembles U Pegasi) because Star B has a double-lined, spectroscopic companion that is close enough to be considered a (weak thermal) shallow contact binary (Hill et al, 1989, page 96; and Jan Schilt, 1926). Since the outer gas envelopes of the stars are in contact (overflowing their Roche lobes), they essentially share a common photosphere despite having two distinct nuclear-burning cores. Indeed, Stars B and C are separated by only some 0.008 AU, around three quarters of a million miles (more than one million km) or about three times the distance between the Earth and its Moon. They are revolving in a highly circular orbit (e~ 0) that takes only 6.427 hours to complete. Moreover, from the perspective of an observer on Earth, Stars B and C eclipse each other twice at every revolution (every three hours). (See an animation of the orbits of Stars A, B, and C and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

Melissa Weiss, Chandra X-Ray Observatory

Larger image.

The tight BC pair is relatively bright in x-rays
near their poles (and seems to be transferring
mass), with each star passing in front of the
other every three hours. (The red arrow points
to orbital direction, while the graph plots
x-ray emission from Neon ions -- more).

X-ray emission from stellar coronal material has been observed around Stars B and C with the Chandra X-Ray Observatory (press release; Brickhouse et al, 2001; and Nikolic et al, 1997; among others). According to the Yale Bright Star Catalogue's notes entry for HR 5618, a variation in the light curve for this close spectroscopic binary pair appears to be caused by mass transfer, which is supported by observations of gaseous streams between the stars. Eclipsing variables of this type may develop into eruptive "dwarf novae" similar to U Geminorum and SS Cygni, and U Pegasi has been observed to exhibit flares or eruptions of small amplitude that may presage more violent activity at a later stage of evolution. (More discussion on W Ursae Majoris type binaries is available from: Maceroni and van't Veer, 1996.)

44 (i) Boötis C

This star is a yellow-orange main sequence dwarf star of possibly spectral and luminosity type G V (Nikolic et al, 1997; and Hill et al, 1989), or later spectral type. This star probably has less mass than Sol, as little as 66 percent of its diameter (Hill et al, 1989), and significantly lower luminosity than Star B. Useful catalogue numbers for the star include Gl 575 C and NS 1503+4739 C.

Hunt for Substellar Companions

Since at least one of the stars of 44 Boötis is fairly similar to our Sun, some speculate whether the system might contain planets that harbor life. The distance from Star A where an Earth-type planet would be "comfortable" with liquid water is centered around 1.07 AU -- just beyond the orbital distances of Earth in the Solar System, with an orbital period of more than an Earth year. For close-orbiting Stars B and C, the liquid water zone may be centered around 0.73 AU -- between the orbital distances of Venus and Earth, with an orbital period around half a year. Astronomers would find it very difficult to detect an Earth-type planet around either of these stars using present methods.