The light curve of the semiregular variable L2 Puppis: II. Evidence for solar-like excitation of the oscillations

We analyse visual observations of the pulsations of the red giant variable L 2 Puppis (L 2 Pup). The data cover 77 yr between 1927 and 2005, thus providing an extensive empirical base for characterizing properties of the oscillations. The power spectrum of the light curve shows a single mode resolved into multiple peaks under a narrow envelope. We argue that this results from stochastic excitation, as seen in solar oscillations, with a mode lifetime of about 5 yr. The random fluctuations in phase also support this idea. A comparison with X Cam, a true Mira star with the same pulsation period, and W Cyg, a true semiregular star, illustrates the basic differences in phase behaviours. The Mira shows very stable phase, consistent with excitation by the κ-mechanism, whereas W Cyg shows large phase fluctuations that imply stochastic excitation. We find L 2 Pup to be intermediate, implying that both mechanisms play a role in its pulsation. Finally, we also checked the presence of low-dimensional chaos and could safely exclude it.