GAMMA CRT (Gamma Crateris), one confused star. It's not really that the star is confused, though; it's more that WE are confused about IT. Another fourth magnitude (4.08) class A (A5) hydrogen- fusing dwarf in what is arguably the dimmest of ancient constellations, Crater (the Cup), is not going to get a lot of attention (just 75 references to it over the past 150 years), which is reflected by what is "known" about it. Class assignments span the full range of the A stars, running from as hot as A0 to as cool as A9! The star's color suggests more like A7. We'll stick with the nominal A5. Temperature measures are equally askew, and range from a low of 7750 Kelvin to a high of 10,700 K. Given that an A5 star should have a temperature of 8700 K, we adopt the average of 9000 Kelvin. Stars like this one do not vary significantly, so the problem rests with observation. At least the distance of 82 light years (good to better than a light year) is secure. And at least the temperature uncertainty plays but a small role in estimating the amount of ultraviolet radiation from the star. Given these parameters, Gamma Crt radiates a dozen Sun's-worth of light into space, yielding a radius of 1.5 times that of the Sun. A solidly-known (and rather high) projected equatorial rotation speed of 138 kilometers per second (apparently enough to keep the stellar atmosphere sufficiently stirred to prevent odd chemistry through separation of elements) yields a rotation period under 0.6 days. Theory subsequently gives a mass of 1.9 solar and suggests relative youth for a star with hydrogen-fusion lifetime of about two billion years. Adoption of a lower temperature of 8000 K (which slightly diminishes the amount of ultraviolet light) does not make much difference, dropping the mass to 1.75 Suns. As might by now be expected for Gamma Crt, it's not clear if the star has a dusty disk or not, some authorities saying yes, others no.

And it's also no surprise that the confusion extends to Gamma Crt's binary companion, Gamma B, which from the run of past observations should now be about 6 seconds of arc from Gamma A. In the 19th century, Smythe and Chambers called the pair "fine but delicate," ... "bright white and grey." Herschel, who discovered the duplicity, said the companion was "utterly obnubilated under the slightest artificial light." (Obnubilated? "To cloud, or obscure.") Magnitude estimates and measures range from as bright at 7.9 to as faint as 11. Going with a listed value of magnitude 9.6 makes the companion a warm-side class K dwarf with a mass of perhaps 0.8 Suns. A physical separation of at least 150 Astronomical Units (which, given the likely foreshortening, might be a lot bigger) leads from Kepler's laws to a period of at least 1150 years. So the bottom line seems to be that nobody really knows the class, the temperature, whether the star has a surrounding disk, or the magnitude of the companion. At least we get to use the word "obnubilated," which describes not just the companion, but the entire system!
Written by Jim Kaler 4/15/11. Return to STARS.