Period evolution of Very Long Period Variables: The Semiregulars L. SABIN , A. A. ZIJLSTRA

1. Period evolution of Very Long Period Variables: The Semiregulars L. SABIN , A. A. ZIJLSTRA University of Manchester, School of Physics  Astronomy P.O Box 88 Manchester M60 1QD, United Kingdom Email: laurence.sabin@manchester.ac.uk aaz@iapetus.phy.umist.ac.uk Introduction Like the Miras , the Semiregulars are red giant or supergiant pulsating stars, but oscillating with less regularity as indicated by their name. They are divided in different classes: SRa, SRb, SRc, SRd (super giants), from the most to the less regular shape of their light curve. The study of the period changes in these variable stars will not only reveal the action (or not) of physical processes such as thermal pulses, magnetism, convection . . . etc via the observation of stable or unstable behavior in the long run, but also we could be able to notice the differences or similarities with the Mira-type stars. The effect on the mass loss which is characteristic of the AGB stars may therefore be investigated. The study will concern stars with periods greater than 450 days as their long-period evolution is more likely to permit us to see perturbations in their behavior. The Sample The Semiregulars with period longer than 450 days are not in high number: using the database of the AAVSO and the AFOEV we found 26 stars corresponding to our criterion. The study of the light curves of these stars involving the use of the wavelet analysis (developed by Foster- 1996) allows the correct visualization of periodic variations in only 11 of them. This is due, in part, to the fact that the stars present close modes of pulsation ( believed to be the 1st and 2nd overtone) and there is a phenomenon of aliasing preventing us to have a correct analysis. Comparative study The case of ST Psc is not common as the star presents a nearly straight increase of 90 days over 33 years. Other SRs (S Aur, S Per for example) show that kind of rise or fall but the evolution is characterised by oscillating motions. They will therefore be put in the category of meandering stars like the V Del-type Miras. Meandering behaviour: V Cam-type trend The period evolution We used a linear fitting to define the change (by dlnP/dt: the slope over the mean period). In order to compare these variations to the ones of the Miras we define the same method of calculation and the same significant threshold for the period change : 2 (1σ dlnP/dt error: 0.0002, in days) (Templeton, Mattei and Willson 2005). It appears that the SRs are very unstable and more than the miras. We can see that the Semiregulars are more unstable than the Miras. It may be due to the fact that the SRs (may) pulsate according the first overtone (and are most often accompanied with a second one) while the Miras are believed to pulsate in the fundamental mode. Both groups present almost the same trends: continuous changes, (possible) sudden drops and more often meandering behaviours. The particular study of the period evolution for both types of stars shows that most of the SRs and nearly half of the O-rich and C-rich Miras studied; show an increasing trend in their behaviour: the period is rising. So if the process responsible for this phenomenon is a thermal pulse, we can say that the stars are experiencing the consequences of one which has occurred some times ago. The stars showing decreasing period changes, may present the short moment after the appearance of the pulse when the temperature is still high, and the luminosity drops. The small variations (meandering stars) could be due to other phenomena. The Very Long Period Variables can be seen, regarding their period instability, as the witnesses of the physical changes occurring in AGB stars . We must nevertheless also quote the particularly unstable behaviour of 52 Mira-type stars with a period between 200 and 450 days. They have been taken out of a panel of 547 Mira stars (Templeton et al. 2005). The sign of a sudden change? RW Cyg presents nearly the same period evolution as the Mira R Cen, but we must underline the fact that the star has been observed over only 30 years regarding to 88 for R Cen. So we have to be careful while dealing with this object. Moreover, while for R Cen the decrease in period is accompanied by a decrease in amplitude, this is not the case for RW Cyg. The different classes of evolutionary trend. The high level of perturbation observed while studying the Semiregulars’ period evolution, prevents us to establish a very good classification of the different observable trends. Nevertheless, even with our small sample, we can find nearly the same categories as found for the Miras. Interpretation We can see that the period evolution of the Semiregular stars can be described nearly in the same way as for the Miras. The meandering and continuous trends are the most represented, part from the time we consider as truthful the analysis for such a biased type of signal. The denomination of the stars (SRa., SRb...etc) does not seem to play a role in the type of period evolution. The correlation between these changes and thermal pulses is not so obvious: among the stars studied by Lebzelter and Hron (2003), SW Gem and TW Peg do not present Technetium; the other stars have not been studied. Moreover the presence of close modes of pulsation prevents us to have a correct analysis of the light curve. Continuous decrease or increase References Lebzelter T. and Hron J., 2003, AA 411,533-542. Templeton M.R., Mattei ,J.A.,Willson L.A., 2005, AJ, 130:776-788 Zijlstra A.A and Bedding T.R, 2002,JAAVSO,31,2 We acknowledge the AAVSO and the AFOEV for the data as well as all the observers who provide them.