Reprocessing the Hipparcos data for evolved giant stars II. Absolute magnitudes for the R-type carbon stars

The Hipparcos Intermediate Astrometric Data for carbon stars have been reprocessed using an algorithm which provides an objective criterion for rejecting anomalous data points and constrains the parallax to be positive. New parallax solutions have been derived for 317 cool carbon stars, mostly of types R and N. In this paper we discuss the results for the R stars. The most important result is that the early R stars (i.e., R0 – R3) have absolute magnitudes and V − K colors locating them among red clump giants in the Hertzsprung-Russell diagram. The average absolute magnitude MK for early R-type stars (with V −K < 4) has been derived from a Monte-Carlo simulation implicitly incorporating all possible biases. It appears that the simulated magnitude distribution for a population with a true Gaussian distribution of mean MK = −2.0 and intrinsic standard deviation 1.0 mag provides a satisfactory match to the observed distribution. These values are consistent with the average absolute magnitude MK = −1.6 for clump red giants in the solar neighborhood (Alves 2000). Further, early R-type stars are non-variable, and their infrared photometric properties show that they are not undergoing mass loss, properties similar to those of the red clump giants. Stars with subtypes R4 – R9 tend to be cooler and have similar luminosity to the N-type carbon stars, as confirmed by their position in the (J −H,H −K) color-color diagram. The sample of early R-type stars selected from the Hipparcos Catalogue appears to be approximately complete to magnitude K0 ∼ 7, translating into a completeness distance of 600 pc if all R stars had MK = −2 (400 pc if MK = −1). With about 30 early R-type stars in that volume, they comprise about 0.04% (0.14% for MK = −1) of the red clump stars in the solar neighborhood. Identification with the red clump locates these stars at the helium core burning stage of stellar evolution, while the N stars are on the asymptotic giant branch, where helium shell burning occurs. The present analysis suggests that for a small fraction of the helium core burning stars (far lower than the fraction of helium shell-burning stars), carbon produced in the interior is mixed to the atmosphere in sufficient quantities to form a carbon star.