27 CMA (27 Canis Majoris). Best known by its Flamsteed number, 27 Canis Majoris lies in southern Canis Major just a degree and a quarter east of bright Delta CMa. A class B3 "giant" (but see below), it's listed as fifth magnitude (4.66) in the Bright Star Catalogue but with a note that it varies between 4.3 and 4.65, which allows a variable-star name, EW CMa. Actually a close double with its similar members just a tenth or two of a second of arc apart, the individuals are given magnitudes in a later catalogue of 5.39 and 4.92. Oddly the secondary is the brighter, which suggests that it underwent a modest eruption event that brought the pair to fourth magnitude (4.38), which is within the bounds of the known variation. None of this is surprising given that 27 CMa is also a "Be" emission star with a surrounding unstable disk (presumably around the star that is now the brighter). Indeed, it's a "shell star" in which the disk is more or less edge-on to the line of sight. Using the above individual magnitudes, adjusting for a tenth of a magnitude of absorption by interstellar dust, adopting a distance from parallax of 1735 light years (with a large uncertainty of 295), and a temperature of 18,340 Kelvin (measured for the fainter but applied to both), we find respective luminosities of 8240 and 12,700 Suns and radii of 9.0 and 11.2 times solar. The measures of projected rotation velocity range from 139 kilometers per second to a more modern 220. If the higher velocity is applied to the brighter star we get a rotation period of under 2.1 days. In any case the stars are clearly spinning fast enough to prevent separation of elements and the weird chemical abundances that go with it. Theory yields masses of 8.5 and 9.2 times solar and shows that if the stars are not giants, they are at least subgiants in which core hydrogen fusion has just quit. 27 CMa is also listed as a spectroscopic binary with a period of 0.262 days (the component involved not known), but that might be confused with its subtle 0.09-day Beta Cephei type of oscillation of one of the stars. There are sufficient data on the bright pair to allow for an orbit to be constructed, though the time base is small and the uncertainties high. With a mean separation of 95 Astronomical Units, the two would take 118.5 years to around each other, a high eccentricity moving them between 24 and 165 AU apart. They were closest together in early 1971. The orbital tilt to the plane of the sky is calculated to be 80 degrees, consistent with the "shell star" designation.

27 CMa So little of the 119-year orbit of 27 Canis Majoris has been observed that the uncertainties are still high. A fitted ellipse (in which one star is mathematically made to go about the other, the two in actuality orbiting a common center of mass) gives a mean separation of 95 Astronomical Units (some three times Neptune's distance from the Sun), a high eccentricity, and a high inclination, which (with orbital orientation) is why the principal star (at the cross) is not at the orbit's elliptical focus. Note from the angular scale how close the two are. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

Kepler's Laws, however, give them a combined mass of 60 times that of the Sun, which is far too high, so something is way off with the calculated orbit. If we adopt the period of 118.5 years, then the lower evolutionary masses (those from luminosity and temperature) give a mean separation of 63 AU, 1.5 times smaller. Given the small number of accurate orbital observations, such a difference in calculated masses is, again, no surprise. Further observations, including a better distance, will eventually reconcile the results. Adopting the more reasonable evolutionary masses, the more massive star of the pair is right at the limit above which stars explode as supernovae, while the lesser star is probably just below it, though not by much. Various future scenarios include a double neutron star (if they both explode, which is unlikely), an ejected runaway star that leaves a neutron star behind, a white dwarf-neutron star combination, a runaway neutron star, or a double white dwarf (both near the allowed upper limit of 1.4 solar masses), depending on how the stars lose mass and interact as they age.
Written byJim Kaler 3/11/16. Return to STARS.