Rotation in the envelope of an evolved star: Observations of the SiO masers toward NML Cygni

We have discovered a new peak of emission in the v = 1, J = 1-0, SiO maser spectrum of the supergiant NML Cygni. This new peak of emission is blueshif ted by similar to 18 km s(-1) from the previously detected SiO masers aroun d V-LSR approximate to 0 km s(-1). A double-peaked SiO maser profile makes NML Cyg unique among evolved stars. We present the first images of the SiO masers toward NML Cyg observed using the Very Long Baseline Array. These im ages span the entire spectrum of NML Cyg, including the new blueshifted pea k. We find that the masers form an elliptical ring of emission approximatel y 33 x 27 mas with the major axis oriented at a position angle of similar t o 141 degrees. The orientation of the ring is consistent with the northwest -southeast elongation previously observed in both the OH and H2O maser shel ls. The elongation axis of the SiO maser shell also provides an axis of sym metry for the velocity structure of the masers. The observed velocity struc ture is interpreted as rotation of the SiO maser shell with a velocity V-m sin i approximate to 11 km s(-1) relative to a systemic velocity of -6.6 km s(-1).