PISMIS-20 - CASE-STUDY OF A YOUNG CLUSTER

Existing photometric (photoelectric and CCD) and spectroscopic data fo r stars in the young open cluster Pismis 20 have been reanalyzed for t he purpose of examining the cluster extinction law as well as for rede riving the intrinsic parameters for the cluster and its more exotic me mber stars, such as the luminous B2 Ia-O supergiant HD 134959 and the Wolf-Rayet star WR 67. The line of sight to Pismis 20 is remarkably cl ear of reddening material out to distances of similar to 1.25 kpc, whe re dust clouds, presumably associated with the Sagittarius-Carina (-I) arm of the Galaxy, add more than 3(m) of extinction to more distant s tars, including members of Pismis 20. The cluster extinction law is de scribed by a reddening relation of slope E(U-B)/E(B-V)=0.767+/-0.023 s .e. and by a value of R=A(V)/E(B-V)=2.99+/-0.16 s.e. The derived clust er distance of 3272+/-303 pc (V-0-M(V)=12.57+/-0.20), or 3272+/-33 pc with fixed R value, is 11% smaller than the value obtained by Vazquez et al. [A&AS, 111, 85 (1995)] in their recent study, the difference be ing accounted for mainly by the different approaches used to correct f or the effects of extinction on the photometric data. The derived lumi nosity of M(V)=-7.88+/-0.06 for the B2 Ia-O star HD 134959 confirms it s hypergiant status, while for WR 67 the inferred luminosity is M(V)=- 4.40+/-0.10 (M(v)=-4.48+/-0.10). This WN6 star lies on the edge of the cluster where the extinction is visibly larger and less well establis hed than near the cluster center. However, its derived space reddening of [E(B-V)]=1.28+/-0.03 leads to stellar parameters that are consiste nt with those inferred for other Wolf-Rayet stars of the nitrogen sequ ence. (C) 1996 American Astronomical Society.