TAU TAU (Tau Tauri). How can you resist a star with a name like Tau Tau? It's certainly unique. (Don't confuse it with T Tauri, a faint protostar that is still developing, or with TT Tau, a variable carbon star.) Just to the east of the Hyades (but not a part of the cluster) in Taurus, at a distance of 398 light years (give or take 43), Tau Tauri is a multiple mess, certainly triple, possibly quadruple. The dominating star is a fourth magnitude (4.3) class B (B3) dwarf with a temperature of 18,700 Kelvin. It's joined by an obvious seventh magnitude (7.0) A1 dwarf companion, Tau Tauri B. In the 19th century, Smythe and Chambers called the pretty pair "bluish white and lilac." For more than two centuries Tau B has held its separation from Tau A at 63 seconds of arc, the two obviously trekking through space together, joined by a fragile gravitational bond. But now things get tricky. At a separation of about 0.1 seconds of arc is another seventh magnitude companion (Tau Ab, rendering the bright star Aa), and possibly at much less separation (0.0005 seconds) another (call it Ac), both detected by lunar occultation. The inner of the close pair may be related to spectroscopic observation of a companion with a 2.96 day period, but neither has been confirmed. The outer, however, Tau Ab, has been well enough followed for an orbit to have been constructed. At an average separation of 35 Astronomical Units, Tau Ab takes 57.9 years to go around Tau Aa, an eccentricity of 0.36 taking them between 22 and 47 AU apart. They were last closest together in February of 1980. Kepler's laws give a combined mass (which may include Ac) of 12.4 times that of the Sun.
Tau Tau Tau Tauri Ab closely orbits much brighter Tau Tau Aa (at the cross) with a period of 57.9 years, a semimajor axis (the dash-dot line) of 35 Astronomical Units, and an eccentricity of 0.36. Only 0.1 to 0.2 seconds of arc apart, they are studied by lunar occultation and interferometry. The stars have yet to orbit once since first observed. As time proceeds, the orbital parameters will be refined. In reality both stars orbit a common center of mass between them. The offsets of the true semimajor axis from that of the observed ellipse and Tau Tau Aa from the its focus are caused by the 49 degree tilt of the orbit against the plane of the sky and the orientation of the ellipse within the orbital plane. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

The stars are dimmed 0.3 magnitudes by interstellar dust. Assuming the inner companion (Tau Ac) is insignificant, the primary star, Tau Aa, shines with the light (including invisible ultraviolet) of 1420 Suns, which gives it a radius of 3.6 times solar. That and a typically fast rotation speed of more than 187 kilometers per second show the star to be rotating in under a day. With luminosity and temperature, theory gives a mass of 6.1 Suns. From its absolute magnitude, Ab is an A2 dwarf. A supposed temperature of 9100 Kelvin leads to a luminosity of 29 Suns, a radius of 2.1 solar, and a mass of 2.2 Suns, which adds to a total of 8.3 times that of the Sun, notably less than that derived from the orbit. A small adjustment of the mean orbital radius to 30 AU could take care of that, but so could some mass from the spectroscopic companion, Ac. With its short period, Ac would orbit at only a few hundredths of an Astronomical Unit from Aa, terribly and almost unrealistically close to its surface. But wait, there's more! Distant Tau B, the A1 dwarf companion, is closely similar to Tau Ab, with a luminosity of 29 Suns, a mass of 2.2 Suns, and a radius of 1.9 times solar. With an equatorial rotation speed of at least 100 kilometers per second, it too takes under a day to spin. With a physical separation of at least 7800 AU, it must take over 200,000 years to orbit the inner triple. We could continue by assuming the unconfirmed magnitude of Tau Ac, which reduces Tau A to a luminosity of 850 Suns and a mass of 5.9, but increases the system mass (as noted) to give better agreement with the orbital sum. However, one could go on forever with adjustments and speculations; the basic structure of fascinating Tau Tau is clear, and perhaps it's best to quit and wait for additional observations.

Written by Jim Kaler 3/07/14. Return to STARS.