Long Term Photometric and Spectroscopic Monitoring of Semiregular Variable Stars

The understanding of semiregular variable stars presents a number of challenges that can be addressed by consistent long term photometric and spectroscopic monitoring. The observing program at Grinnell College has generated a large body of such data that has been used to investigate modes of pulsation, the role of dust, the possible role of chaos, and other issues. This paper summarizes these efforts and encourages other observers to help maintain the continuity of these data sets. The application of Fourier analysis and other techniques in numerous studies (see, for example, Mattei et al. 1997; Kiss et al. 1999; Percy et al. 2003; Percy and Tan 2013) has shown that many of these stars are truly multiperiodic and the presence of periodicity suggests that systematic physical processes are at work. The exact nature of those processes is still a matter of some debate, but the prevailing interpretation of the primary light variations in semiregular variables is that these stars are radially pulsating, as is believed to be the case for some other classes of variable stars. Radial velocity measurements (Cummings et al. 1999; Lebzelter et al. 2000) strongly suggest that this is the case, as do the observations of variations in radius (Perrin et al. 1999) and theoretical models (Xiong and Deng 2007; Ireland et al. 2011). Furthermore, the spectra of these stars sometimes include hydrogen emission lines that appear to be the result of pulsation-induced shocks (Wood 1974; Willson 1976). After an extended debate (Wood 1995; Percy and Polano 1998; Willson and Marengo 2012) a consensus emerged that the dominant mode of pulsation is the fundamental for the Miras and an overtone for the semiregulars. Early work at Grinnell contributed to this conclusion (Cadmus et al. 1991). More recent observational and theoretical work has refined this picture but the complexity of the light variations and other characteristics of these stars leave a number of questions unresloved. For example, the association of modes with the Mira and semiregular classifications seems not to be absolute (Soszyński et al. 2013a), there are variations in the amplitudes of the light curves (Kiss et al. 2000; Percy and Abachi 2013), the origin of the long secondary periods is still unclear (Wood et al. 2004), and the degree to which the shorter-period variations are associated with overtone pulsation is still an interesting question. Theorists have made a noble effort to include many of the complex physical processes that are simultaneously in play in a radial pulsation model, but a complete description is not yet in hand. In addition, the presence of several processes in addition to those directly related to radial pulsation has been suggested and debated, often in the context of the vexing long secondary periods discussed below. These include non-radial pulsation (Stello et al. 2014), convection (Stothers 2010), rotation (Wood et al. 2004, which also discusses other effects), binarity (Nicholls et al. 2009), planets (Berlioz-Arthaud 2003), and chaos (Buchler et al. 2004). There is clearly much work to be done on these stars by both observers and theorists. Cadmus, JAAVSO Volume 43, 2015 4 The existence of a period-luminosity (P-L) relation for long period variables was established long ago (Gerasimovič 1928) but the availability of large surveys such as OGLE and MACHO in the 1990s revolutionized this area of research by generating vast archives of light curves. These observations of stars in the Large Magellanic Cloud have been especially valuable because those stars are all at approximately the same distance so their relative luminosities can be determined with good accuracy. These new data led to the discovery of multiple P-L relations, called sequences, for various groups of stars, including the semiregulars (Wood et al. 1999). The Hipparcos mission, which generated greatly improved parallaxes for many stars, led to improvements in the P-L relations for galactic stars and showed that the same sort of sequences are present in that population (Soszyński et al. 2013b). A significant challenge has been to identify with certainty what distinguishes the stars that make up each of the handful of sequences. There is evidence that some of the sequences correspond to stars pulsating in different modes and others correspond to other phenomena (Wood 2000; Soszyński et al. 2007), but the distinctions are not always as sharp as one might wish. For example, the semiregulars appear to populate two of the sequences (Soszyński et al. 2013a). One of the sequences in the P-L diagram is associated with the long secondary periods—light variations on a time scale that is roughly ten times longer than the likely pulsational periods (Wood et al. 1999). These variations in some semiregulars have been known for a long time (Payne-Gaposchkin 1954) and have been the subject of much research in recent years. A number of explanations for this phenomenon have been offered (Nicholls et al. 2009; Wood et al. 2004) but none has gained widespread acceptance. While much progress has been made, understanding these aspects of the nature of the semiregular variables has been challenging, leaving significant questions only partially answered and offering ample opportunities for further research.