Rotation in the Envelope of an Evolved Star: Observations of the S[CLC]i[/CLC]O Masers Toward NML Cygni

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Abstract
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 blueshifted by ~18 km s-1 from the previously detected SiO masers around VLSR ≈ 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 images span the entire spectrum of NML Cyg, including the new blueshifted peak. We find that the masers form an elliptical ring of emission approximately 33 × 27 mas with the major axis oriented at a position angle of ~141°. The orientation of the ring is consistent with the northwest-southeast elongation previously observed in both the OH and H2O maser shells. The elongation axis of the SiO maser shell also provides an axis of symmetry for the velocity structure of the masers. The observed velocity structure is interpreted as rotation of the SiO maser shell with a velocity Vm sin i ≈ 11 km s-1 relative to a systemic velocity of -6.6 km s-1.