The intrinsic colors of Wolf-Rayet stars

Abstract

Nine W-R stars observed in the Carina region have been added to the list of a l Wolf - Rayet stars with known colors in the UBV system. If all these stars (28 stars) are plotted in the (B -V , U-B) diagram, it shows that they are situated in reddening lines which ends in the main sequence between spectral types BO V and B5 V. The WC stars are always in a reddening line belonging to later spectral type than the WN stars. One W-R star, HD 5Ö896, was observed in the cluster Cr 121. This star with Mᵥ= -2, according to the color-magnitude diagram of the cluster (Feinstein, unpublished), is in the main sequence very near the turn-off point where the stars begin to evolve to the red giants. The cluster is very young and its diagram is the same as that of h and χ Persei. In NGC 6321 there are two W-R stars. This cluster observed by Kron and Breckinridge in the PV system, and by Feinstein in the UBV system (unpublished), shows that these two stars are also at the turn-off point. The absolute magnitude of both are around Mᵥ= -4.5. Two of those stars are present above both W-R objects. The fact that some W-R are members of clusters, is also confirmed by Westerlund who found this kind of stars in three clusters of LMC. All are on the main sequence and very near the point where the evolution proceeds to the giants. The absolute magnitude is around -6. Andrillat showed that the field W-R stars have absolute magnitudes in a range between Mᵥ= -1.5 and -5, so that these stars display a large spread in absolute magnitude. As this fact is confirmed in clusters, and they are also always near the turn-off point, it seems that the condition of W-R stage is connected in some way with the beginning of the evolution to the red giants. Then, any main sequence star may be a W-R in the range of absolute magnitude between -1 and -6.