Gamma Canis Minoris

GAMMA CMI (Gamma Canis Minoris). The classic outline of Canis Minor, Orion's smaller hunting dog, is among the simplest in the sky, consisting of a line between bright, first magnitude Procyon and third magnitude Gomeisa (Beta CMi). The other stars, Gamma through Zeta (plus a few Flamsteed stars) within this small figure, do not much seem to count. Gamma maybe even less so, the fourth magnitude (4.32) class K (K3) giant rather hiding just two- thirds of a degree northeast of Beta, though at 318 light years (give or take 15) considerably farther. What Gamma lacks in luster, it makes up for in confusion, as it is a very close unresolved "spectroscopic" double, the fainter one (Ab, "A" since there are also much fainter Gamma B, C, and D) adding a very uncertain component to the brighter (Aa), making Gamma itself difficult to assess. Rightly or wrongly, we make the assumption that the two have the same (estimated) temperature of 4400 Kelvin. After accounting for some infrared radiation from the coolish surface, we find a total luminosity of 265 times that of the Sun. A measure of the light ratio, with Gamma Ab a quarter the visual brightness of Gamma Aa (the actual K giant) gives 212 Suns for the brighter of the pair and 53 for the fainter. The problem is that if the temperatures are different, the visible brightness ratio will not be the same as the ratio considering the TOTAL luminosity. The giant's radius then comes in at 25 solar and the mass at an approximate 3 times that of the Sun, with an age of 400 million years. One source suggests that the other star is also evolved from hydrogen-fusing dwarfhood, with a mass ratio Ab/Aa of 0.90, but we really do not know. There is no measure of rotation speed, though there is a suggestion that the iron abundance is a bit low. What we DO know is that the mutual orbital period of the pair is 389.32 days, the synchrony to a year making full observational coverage of the orbit rather difficult. Assuming 3 solar masses for the brighter and 2.7 for the fainter, the average separation is 1.9 Astronomical Units, a precisely known eccentricity of 0.259 taking them as close as 1.4 AU and as far from each other as 2.3 AU. Other than all this, the star presents itself as a fine example of a fake multiple. At separations of 27, 115, and 138 seconds of arc lie 13th magnitude Gamma CMi B, 12th magnitude Gamma C, and 10th magnitude Gamma D. Observations over long periods of time show each moving far too much relative to Gamma A (our confusing close binary), clearly showing all to be just "optical" line-of-sight coincidences.

Written by Jim Kaler 3/19/10. Return to STARS.