Supergiants in the field of the cluster M6, and the distribution of interstellar matter in the direction of the galactic center

Near-infrared eight-color classification photometry is presented for IRC stars in a field in Scorpius containing the open cluster M6. Distances and extinction values are computed from the photometric spectral types, luminosity classes, and colors. Of the six M stars observed, five are found to be supergiants lying at considerably greater distances than that of the cluster. A comparison of their distances with recent determinations of galactic structure in the direction of the galactic center indicates that the stars are likely to be associated with spiral-arm features approximately 2 and 4 kpc from the sun. This study shows that the dark clouds which produce the rift in the nuclear bulge lie predominantly at a distance of about 1.5 kpc and cause a visual extinction of about 8 mag. This screen of absorbing material is thus seen to be a major component of the Sagittarius arm.