29 CMA (29 Canis Majoris). Among the most brilliant of
constellations, Canis Major (Orion's Big Dog) holds the brightest star
of the sky, Sirius, as well as the faintest
of first magnitude, Adhara. In the
southern part of the constellation, to the
east of Adhara, Canis Major is also host to two rare naked eye class
O stars, Tau CMa, a nominal fourth
magnitude O9 supergiant
that inhabits the open
cluster NGC 2362 and, less than half a degree to the north,
29 CMa, which is listed as a hotter fifth magnitude (4.98) O7
supergiant (but see below). Little is certain about the star. It's
been suggested to be a runaway from the aforementioned cluster, but
at a distance of 1900 light years it's much closer, Tau an uncertain
3000 light years away, the cluster 4500 or so. But 29 CMa could
statistically be as close as 1650 or as far as 2260, and there are
uncertainties associated with the other distances, so it's certainly possible. At
first our star looked so simple, an eclipsing binary of equal
components with a period of 4.39341 days that clip each other to
produce a variation of a few tenths of a magnitude, which gives it the
variable star name UW CMa. But then it all
falls apart. There is another sixth magnitude (5.98) component at a separation of 0.2
or so seconds of arc, 29 Ab, which makes the brighter eclipsing double
29 Aa and lowers its magnitude to 5.53. And there may be yet another
member, tenth magnitude 29 CMa E at a full second of arc separation.
(Faint and more distant 29 B, C, and D seem to be just line-of-sight
coincidences.) So the picture after evaluation of the magnitudes,
including 0.58 magnitudes of dimming by interstellar dust, is that of an
inner pair of close O7 "supergiants" orbited by a B0 dwarf, the inner triple in turn orbited by a B7
dwarf. O stars first. The temperature, needed to evaluate a
whopping amount of ultraviolet radiation, falls
between 33,200 and 38,000 Kelvin; we'll adopt a weighted mean of
36,400. Each of the O stars then has a luminosity of around 35,400
Suns and a radius of 4.7 solar. Projected
equatorial rotation velocities for one of the O dwarfs is 128
kilometers per second, 176 for the other, which yields rotation
periods under 1.8 and 1.3 days, faster than required for synchronous
rotation. Theory shows the masses of each to be around 20 Suns and
that the stars are young dwarfs just starting their hydrogen-fusing
lifetime of 8 million years. However, the stars fall into a
difficult fix. They are either too hot for their luminosities or
too faint for their temperatures, so something is wrong, which is
no surprise. In any case, Kepler's laws suggest that the two O7 stars
of 29 CMa Aa orbit with a separation of just 0.14 Astronomical Units.
They may in fact transfer some mass, one to the other, as one is given
as O7e, for emission of radiation, suggesting free-flowing gas. Ab,
the B0 dwarf, orbits these at maybe 120 AU, and from a probable mass
of 13 Suns takes 175 years to go around. 29 E, the outer B7 dwarf,
should come in at around 3 Suns. At a distance from the inner triple
of some 600 AU, it would take 2500 years to make a full circuit. Or
not, as none of the orbits are really known. All is uncertain, not
surprising for stars of this sort. On a minor note, 29 Uma might
be the exciting star of a local diffuse nebula, S 310. Or not.
One thing is for sure, however, that the inner O stars will blow up
as supernovae. Unless
one is ejected by the explosion, they might wind up as an exceedingly
rare double neutron
star. The B0 star, which may well be ejected, should explode
too. 29 CMa E, which will almost certainly be ejected, can watch
the fun.
Written byJim Kaler 4/24/15. Return to STARS.