Parallaxes and proper motions of prototypes of astrophysically interesting classes of stars. I. R coronae borealis variables

Hipparcos data were obtained under the 1982 announcement of opportunity for the seven brightest R CrB stars then known. None of the parallax measurements is different from zero in a statistically significant way, though most of the proper motions are. The body of the data nevertheless suggests that the stars belong to two rather different populations in either luminosity, kinematics, or both. The best-known stars (R CrB itself, RY Sgr, and perhaps others) probably are the very luminous, MV = -4 to -5, objects they are generally accused of being, but others may belong to a fainter class for which there is some independent evidence among stars in the Large Magellanic Cloud. Alternatively, or in addition, the apparently fainter stars may belong to a higher-velocity population that we have had rather bad luck in sampling. Of two rash assumptions, the one that all seven stars are at the same distance of 1200 pc leads to a velocity ellipsoid 〈U2〉1/2, 〈V2〉1/2, 〈W2〉1/2 = 41, 30, 35 km s-1, which is "not inconsistent" with values typical of (other) old disk populations, like carbon stars, long period variables, and planetary nebulae. The seemingly less rash assumption that all stars have MV = -4.5 leads to 〈U2〉1/2, 〈V2〉1/2, 〈W2〉1/2 = 52, 60, and 104 km s-1, which resembles no known stellar population, at least not in our galaxy