Six years of short-spaced monitoring of the v=1 and v=2, J=1-0 (SiO)-Si-28maser emission in evolved stars

We present the results from a monitoring of the upsilon = 1 and upsilon = 2 , J = 1-0 (SiO)-Si-28 maser emission in 21 objects, covering all types of k nown SiO maser emitters: 13 Mira variables, 2 long period semiregulars (SRG s), 3 variable supergiants (SGs), 2 OH/IR stars, and one young stellar obje ct. This study has been carried out with the 13.7 meter radiotelescope of t he Centro Astronomico de Yebes (Guadalajara, Spain), from July 1984 to May 1990, and represents the longest and most tightly sampled monitoring of SiO masers ever published. Our data show that for Mira-type (i.e. regular) variables, the SiO and opti cal light curves agree in period, and that the maxima of the SiO emission l ag the optical ones by about 0.1 - 0.2 periods. Since a similar lag charact erizes the near infrared (NIR) emission variability from these stars, we co nclude that for regular variables SiO and NIR vary in phase. This result wa s confirmed in three objects for which NIR variability curves are available . For SRGs and SGs, we found a less systematic behavior, but when the SiO e mission is periodic, its variability curve agrees with the optical one, als o showing a lag between maximum epochs similar to that of Mira-type stars. The data clearly reveal other interesting details on the SiO maser variabil ity, such as the strong intensity differences between different maxima and changes in the velocity distribution of the emission. Finally the SiO maser s associated to the young stellar object Orion IRc2 showed a double peaked spectrum with low amplitude, aperiodic variations.