PISMIS 20 - A Case Study of a Young Cluster

Details PISMIS 20 - A Case Study of a Young Cluster PISMIS 20 - A Case Study of a Young Cluster

Feb 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aj....111..828t&link_type=abstract

Astronomical Journal v.111, p.828

Astronomy and Astrophysics

Astronomy

13

Open Clusters And Associations: Individual: Pismis 20

Scientific paper

Existing photometric (photoelectric and CCD) and spectroscopic data for stars in the young open cluster Pismis 20 have been reanalyzed for the purpose of examining the cluster extinction law as well as for rederiving the intrinsic parameters for the cluster and its more exotic member 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 clear of reddening material out to distances of ˜1.25 kpc, where dust clouds, presumably associated with the Sagittarius-Carina (-I) arm of the Galaxy, add more than 3m of extinction to more distant stars, including members of Pismis 20. The cluster extinction law is described by a reddening relation of slope EU-B/EB-V=O.767±O.O23 s.e. and by a value of R=Av/EB-V=2.92±0.16 s.e. The derived cluster distance of 3272±303 pc (V0-Mv=12.57±0.20), or 3272±33 pc with fixed R value, is 11% smaller than the value obtained by Vázquez et al. [A&AS, 111, 85 (1995)] in their recent study, the difference being accounted for mainly by the different approaches used to correct for the effects of extinction on the photometric data. The derived luminosity of Mv=-7.88±0.06 for the B2 Ia-O star HD 134959 confirms its hypergiant status, while for WR 67 the inferred luminosity is Mv=-4.40±0.10 (Mv=-4.48±O.10). This WN6 star lies on the edge of the cluster where the extinction is visibly larger and less well established than near the cluster center. However, its derived space reddening of =1.28±0.03 leads to stellar parameters that are consistent with those inferred for other Wolf-Rayet stars of the nitrogen sequence.

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