Mu Herculis 4?

Mu Herculis A (or Aa?)

Mu Herculis A is a yellow-orange star of spectral and luminosity type G5 IV. It has the same mass as Sol (Bedding et al, 1996, page 1157) -- possibly 1.1 Solar-mass according to Professor Kaler's Mu Herculis page, about 1.77 to 1.86 times Sol's diameter, and about 2.2 to 2.7 (with infrared) times its luminosity. The star may be 1.3 to three times as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 30). Useful star catalogue numbers for Star A include: Mu Her, 86 Her, HR 6623, Gl 695 A, Hip 86974, HD 161797, BD+27 2888, SAO 85397, FK5 667, LHS 3326, LDS 1002, LTT 15266, LFT 1374, GC 24138, ADS 10786, and AC 7.

Mu Herculis A (or Aa) appears to have an unseen, close companion (Mu Herculis D or Ab) with as much as a fifth of Sol's mass. In turn, this close binary pair (Aab) has another companion binary of stars (BC). According to the Astronomisches Rechen-Institut's Catalogue of Nearby Stars (ARICNS) notes on Star B, the BC companion binary pair has an observed separation of about 286 AUs (34.0" at a HIPPARCOS parallax of 0.11905+/-0.00062") from primary pair Aab at an orbital inclination of 247° (1955) from the perspective of an observer on Earth. (See an animation of the orbits of Stars Aa, Ab/D, B, C, and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

The orbit of an Earth-like planet (with liquid water) around this star would be centered around 1.6 AU -- somewhat farther than the orbital distance of Mars in the Solar System -- with an orbital period of about 2.0 Earth years. However, Mu Herculis A is unusually bright for its spectral type. It appears to be a subgiant star that is evolving off the main sequence, as it begins to fuse the increasing amounts of helium "ash" mixed with hydrogen at its core. This implies that any Earth-like planet that time to develop life in at an orbital distance when Mu Herculis A was less luminous would probably has been "roasted" dry by this time.

NASA -- larger image

Mu Herculis BC are dim red dwarf stars, like the binary pair
Gliese 623 A (M2.5V) and B (M5.8Ve) -- at lower right.

Mu Herculis B

This is a red main sequence dwarf star of spectral and luminosity type M3.5 V. It has a mass of about 31 (+/- 10) percent of Sol's (James F. Wanner, 1967), about 48 percent of its diameter, and less than 5/1,000th of its luminosity. Star B and its stellar companion C are separated by an "average" semi-major axis) of 11.4 AUs. They have an elliptical orbit (e= 0.18) that swings them between 9.4 and 13.5 AUs apart in an orbit that lasts about 43.2 years (Paul Couteau, 1960). Useful star catalogue numbers for the star include Gl 695 B and LHS 3325.

The orbit of an Earth-like planet (with liquid water) around this star would be centered around 0.07 AU with an orbital period of about eight Earth days, caused it to be tidally locked with Star B.

Mu Herculis C

This is a red main sequence dwarf star of spectral and luminosity type M4 V. It has about 31 (+/- 10) percent of Sol's mass (Wanner, 1967), about four tenths of its diameter, and less than 3/1,000th of its luminosity. Useful star catalogue numbers for the star include HR 6623 C and Gl 695 C.

The orbit of an Earth-like planet (with liquid water) around this star would be centered around 0.05 AU with an orbital period of about eight Earth days, caused it to be tidally locked with Star C.

Mu Herculis D (or Ab?)

Astrometric analysis suggests that Star D is in an elliptical orbit (e= 0.34) around Star Aa and has roughly a fifth of Sol's mass (Wulff Dieter Heintz, 1994, pages 2341 and 2346; and Heintz, 1987, page 1080 -- which Heintz referred to as Stars A and Aa versus the more typical designations Aa and Ab). According to the Astronomisches Rechen-Institut's Catalogue of Nearby Stars (ARICNS), Heintz's 1994 analysis of Mu Herculis Aa also derived an updated period of 65 years which would imply a semi-major axis of just under 17.2 AUs, assuming that the combined mass of Mu Herculis Aab is 1.2 times that of Sol's (which is consistent with Wanner's 1967 estimate of the mass ratio of 0.50 (+/- 0.04) for the binary pair BC -- combined -- to the primary).

In 1994, Cochran and Hatzes also reported possible evidence of a brown dwarf or large Jupiter-class companion around Mu Herculis A based on a steady downward drift of its radial velocities over five years (Mazeh et al, 1996, page 423). More recent application of the highly sensitive radial-velocity techniques of Geoffrey W. Marcy and R. Paul Butler, however, have not as yet confirmed the existence of such a companion object (Cumming et al, 1999).