Epsilon Lupi

EPS LUP (Epsilon Lupi). From a splatter of modestly bright stars southwest of Scorpius howls Lupus, the celestial Wolf. Among them is a confusing multiple system, a bright third magnitude (3.37) class B (B2) subgiant-dwarf (but see below) 512 light years away (give or take 56) that is a member of the Upper Centaurus-Lupus association of O and B stars. Eps Lup appears first as spectroscopic binary in a very short 4.56 day orbit. Farther out, a fraction of a second of arc away, is Eps Lup B. Individual magnitudes of 3.56 and 5.05 add to magnitude 3.31, which we'll adopt. Observation of an even closer star (Eps Lup Ab) orbiting the spectroscopic pair could not be replicated, so it is ignored. Then nearly half a minute of arc away lies ninth magnitude (9.1) Epsilon C. While an early nineteenth century observation suggests the AC pair is quickly drawing apart (hence is a line of sight pair), "nearly all the 20 other measures from 1837 to 2010 have hovered around 26 seconds," so (though there is yet some doubt) "C" probably belongs to the system. Then there is 16th magnitude Eps Lup D. With but one modern measure that places it 40 seconds of arc away, we can probably assume that this one is indeed a coincidence. When all shakes out, Epsilon Lupi seems to be a fine hierarchical quadruple star. From a sequence of observations, Eps Lupi B orbits the inner spectroscopic pair every 737 years at an average separation of 221 Astronomical Units, a rather high eccentricity running them between 42 and 400 AU apart. They were last closest in 2010. Kepler's laws give a total mass to the three (the tight spectroscopic pair and Eps B) of 19.9 solar masses.

With a long period of 737 years, Epsilon Lupi B is seen to go around Epsilon A (at the cross, the star itself a close binary) with an average separation of 221 Astronomical Units, a high eccentricity taking them between 42 and 400 AU apart. The green points were observed by traditional means, while the blue points are from recent and far more accurate data secured by a technique called "speckle interferometry," which eliminates the obscuring effects of stellar twinkling. In reality, the two stars go around a common center of mass set on a line between them but much closer to more-massive Epsilon-A. The true orbital semimajor axis and focus are offset from those of the apparent ellipse because of the 118 degree tilt of the orbit to the plane of the sky. (Courtesy of A. Tokovinin, B. D. Mason, and W. I. Hartkopf, in the Astronomical Journal.)

With a small 0.2 magnitude correction for interstellar dust, a temperature of 20,800 Kelvin, and an assumption that the spectroscopic pair consists of equally bright components (which is a leap, but what can one do?), each shines with the light of 2700 Suns. Theory yields 7.5 solar masses apiece and suggests great youth. Kepler's Laws show that they must orbit just 0.11 AU apart. With an assumed temperature of 15,500 Kelvin, Eps B radiates 860 solar luminosities, which gives it a mass of 4.5 Suns, the inner three then adding to 19.5, remarkably close to the value of 19.9 found from the orbit. Rashly assuming equality, radii of 4.0 times that of the Sun coupled with a projected equatorial rotation speed of 153 kilometers per second give rotation periods under 1.4 days, well below synchrony with the orbital period (which will probably be achieved). From its brightness, Eps C is a class F0 dwarf. At least 4100 AU from the inner AB trio, and with an assumed mass of 1.5 Suns, C then must take at least 60,000 years to orbit. All in all it's a fine mess to deal with, especially given that Eps A probably does NOT consist of equal components. But given the various uncertainties, the adoption of other brightness ratios is an exercise in futility. The inner pair is just under the limit at which stars blow up as supernovae. If left alone, they would die as a close double white dwarf. If drawn closer by evolution, the system is a candidate for a white dwarf (Type Ia) supernova, caused in one scenario by stars that merge and overflow the white dwarf (Chandrasekhar) support limit of 1.4 Suns. Given the seemingly vast number of uncertainties involving Eps Lup, who knows? (Thanks to Bill Hartkopf for discussion and the above quote.)

Written byJim Kaler 6/05/15. Return to STARS.