VLBA Observation of SiO Masers in the M Giant IRC –10414

We have mapped an M giant star, IRC ⤓10414, in the SiO J = 1–0 v = 1 line at 43.1 GHz with the VLBA with a spatial resolution of better than 0.5 mas. The maser spots spread over on a circle having a radius of about 7.1 mas. A velocity gradient in the east-west direction of ∼ 0.7 km s⁻¹ mas⁻¹ is found. Monitoring observations with Nobeyama 45-m telescope have shown that the intensity ratio of the SiO J = 1 – 0 v = 1 to v = 2 maser line was quite unusual (5–40) for the period of 1997–1999 in this object; this ratio was known to be close to unity in many late-type stellar objects, but was known to deviate strongly from unity in young stellar objects. The unusual line intensity ratio of this star may suggest a chemical or geometrical anomaly of the envelope of IRC –10414. A possible ring/disk model is discussed in order to explain the distribution of the maser spots in this object.