Six years of short-spaced monitoring of the and , ²⁸SiO maser emission in evolved stars

¹ Observatorio Astronómico Nacional (OAN), Apartado 1143, E-28800 Alcalá de Henares, Spain
² DEMIRM/Observatoire de Paris-Meudon, 61 avenue de l'Observatoire, 75014 Paris, France
³ NASA-Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, U.S.A.
⁴ Departamento de Matemática Aplicada II (Biomatemática), Sección Departamental de Óptica, Escuela Universitaria de Óptica, Universidad Complutense de Madrid, Av. Arcos de Jalón s/n, E-28037 Madrid, Spain
⁵ Departamento de Física, Universidad de Alcalá de Henares, Campus Universitario, E-28871 Alcalá de Henares, Spain

Send offprint request to: J. Alcolea e-mail: j.alcolea@oan.es

Received: 26 January 1999
Accepted: 28 June 1999

Abstract

We present the results from a monitoring of the and , ²⁸SiO maser emission in 21 objects, covering all types of known SiO maser emitters: 13 Mira variables, 2 long period semi regu lars (SRGs), 3 variable super giants (SGs), 2 OH/IR stars, and one young stellar object. This study has been carried out with the 13.7 meter radiotelescope of the Centro Astronómico 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 optical light curves agree in period, and that the maxima of the SiO emission lag the optical ones by about 0.1-0.2 periods. Since a similar lag characterizes the near infrared (NIR) emission variability from these stars, we conclude that for regular variables SiO and NIR vary in phase. This result was 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 emission is periodic, its variability curve agrees with the optical one, also showing a lag between maximum epochs similar to that of Mira-type stars. The data clearly reveal other interesting details on the SiO maser variability, such as the strong intensity differences between different maxima and changes in the velocity distribution of the emission. Finally, the SiO masers associated to the young stellar object Orion IRc2 showed a double peaked spectrum with low amplitude, aperiodic variations.