Membership in the extremely young open cluster NGC 6530 (M8).

Summary. Relative proper motions for 363 stars in the vicinity of the extremely young open cluster NGC 6530 (M8) have been determined from plates taken with the Yerkes Observatory 40-inch refractor. Based on these relative proper motions, which have an average mean error of + 0'.'0006/yr, probabilities of membership are determined with the result that of the 363 stars measured there are 76 expected members. The average reddening is found to be 0.35 mag with little scatter Adopting the ratio of total to selective absorption to be 3.0, a corrected distance modulus of 11.25 mag or a distance of 1780 pc is found. An age for the cluster of 2 x 106 yr has been determined based on the theoretical gravitational contraction isochrones of Iben, although the observations are perhaps fit slightly better by the 3He main sequence of Ulrich. The two yellow stars suggested as possible giant members of the cluster by Walker, as well as the five brightest red stars found by Blanco and Grant have zero probabilities of membership. Five probable nonmembers are suspected of being variable in light. Of the six Hoc emission line stars found by Herbig and measured by us, three are probable members. The two bright 0 stars, HD 164794 and HD 165052, have negligible relative proper motions. From an analysis of their proper motion errors, a minimum expansion age of 0.6 x 106 yr is derived, in agreement with the gravitational contraction age of 2 x 106 yr for the cluster. Herschel 36, the exciting star for the "hour glass" nebula, is possibly a cluster member. Key words: star clusters - contracting stars - proper motions - astrometry