6. Conclusions

We present the results of the most complete monitoring of ²⁸SiO maser emission in evolved stars carried out up to date, due to the regularity and short-spacing of the sampling ($\sim$ 30 days), the wide set of sources observed, and the long period of time covered (6 years). The observed transitions are the v=1 and v=2, J=1-0 lines. The sample contains a total of 21 objects, including red supergiants, semi-regular variable stars, Mira-type variables, OH/IR objects and one young stellar object. Special care has been taken to optimize the relative calibration of the data during the whole observing period. We conclude that, for each object, the relative flux uncertainty remains within $\pm$ 10%.

The data have been compared with optical and NIR light-curves. For objects known to be regular variables in the visible and/or in the IR (i.e. Mira-type and OH/IR stars), the SiO masers also appear to vary periodically, showing a period equal to the optical one in agreement with previous works, see Hjalmarson & Olofsson (1979), and Martínez et al. (1988). We have confirmed this result by a preliminary Fourier analysis of the SiO variability curves; no secondary periods have been found. We also confirm the existence of a systematic phase lag between the optical and the SiO maximun epochs, of about 0.1-0.2 periods. Of the 59 optical or IR maxima of Mira-type stars in our monitoring (excluding the S-type star $\chi$ Cyg), in 53 cases (90%) we find a well defined maximum in at least one of the SiO lines between phase 0.0 and 0.3 (with respect to the optical maximum). This phase lag is comparable to that found between optical and IR maxima. Therefore, the SiO masers vary systematically in phase with the IR. This result is directly confirmed, in particular, for the stars R Aqr, IK Tau, and IRC +10011, for which variability curves at 4 $\mu$m simultaneous with our SiO monitoring are available. The IR maxima of these objects show a striking coincidence with the SiO ones in all cases.

However, the reproducibility of the SiO curves is often poor. The contrast (intensity ratio between consecutive maximum and minimum epochs) is strongly variable in most objects, with an average value of about 3. In o Cet (Mira) this parameter shows particularly high values, ranging from 4 up to 20. U Her, W Hya, and R Leo show contrasts between 1 and 3; a contrast of 1 is the extreme case in which an SiO maximum is missed, i.e. it is not clearly detected between two consecutive minima. Only 3 or 4 missing SiO maxima can be identified in Mira-type stars, in the curves of U Her, W Hya, and R Leo. On the other hand, the SiO emission of IK Tau shows a remarkably regular variability.

In supergiants and semiregular giants the variability of the SiO masers is much less regular than for Miras. In two objects, the supergiant VX Sgr and the semiregular variable GY Aql, we find nevertheless a periodic behavior quite well correlated with the optical variability (VX Sgr and GY Aql are relatively regular variables also at other wavelengths). The SiO emission from the young object Orion IRc2 shows low-amplitude, unpredictable variations.

The shape and centroid of the line profiles are also variable. However, no clear periodicity or trend is found, even for Mira variables. We notice centroid variations larger than about 5 km$\,$s$^{\rm -1}$ in periods of time shorter than 200 days for the stars R Aqr, U Her, and W Hya. In o Cet and R Leo, the line shape completely changed in time scales of this order. However in the supergiant VY CMa and the Mira variable IK Tau, the line shape has been remarkably constant during the whole observing period.

Acknowledgements

The pure observational work in this paper amounts to more than 2000 hours of telescope time. Technical works can be estimated as taking at least the same amount of time. Such an effort would have never been possible without the participation of all the astronomers and the whole technical staff of the Yebes Observatory. In this research, we have used, and acknowledge with thanks, data from the AAVSO International Database, based on observations submitted to the AAVSO by variable star observers worldwide. We also acknowledge T. Le Bertre for providing us with the NIR observations presented in this paper. This work has been financially supported by several projects of the Spanish DGES, the last one under code PB96-0104.