Detection of the SiO /v = 2, J = 2-1/ maser

Abstract

Masers in one or more rotational transitions of the first three excited vibrational states of the SiO molecule have now been detected toward over 60 late-type Mira variables and supergiants and toward the infrared source IRc2 in the KL Nebula. The precise geometry and pump mechanism for the SiO masers, however, are not yet well understood. Buhl et al. (1974) found the v = 2, J = 2-1 transition to be undetectable in a search involving four stars, although similar transitions are usually strongly masing. A description is presented of the results of a sensitive search for this line in a number of late-type stars and Ori A, taking into account its final detection in Chi Cyg. The only detected SiO, v = 2 line maser star, Chi Cyg, has the largest known visual range of any Mira variable and is also one of the nearest (90 pc). It is one of the coolest stars yet identified, with an effective temperature of about 1800 K at minimum.