A New Overview of Secular Period Changes of RR Lyrae stars inM5

Abstract

Secular period variations, β = Ṗ , in 76 RR Lyrae stars in the globular cluster M5 are analysed using our most recent CCD V photometry and the historical photometric database available in the literature since 1889. This provides a time baseline of up to 118 years for these variables. The analysis was performed using two independent approaches: first, the classical O − C behaviour of the time of maximum light, and second, via a grid (P, β), where the solution producing the minimum scatter in the phased light curve is chosen. The results of the two methods agree satisfactorily. This allowed a new interpretation of the nature of the period changes in many RR Lyrae stars in M5. It is found that in 96% of the stars studied no irregular or stochastic variations need to be claimed, but that 66% of the population shows steady period increases or decreases, and that 34% of the periods seem to have been stable over the last century.The lack of systematic positive or negative period variations in RR Lyrae stars in other globular clusters is addressed, and the paradigm of period changes being a consequence of stellar evolution is discussed in terms of chemical variations near the stellar core and of multiple stellar populations. In M5 we found a small positive average value of β and a small surplus of RRab stars with a period increase. Nevertheless, in M5 we have been able to isolate a group of likely evolved stars that systematically show positive, and in some cases large, period change rates.

Standard Galactic Field RR Lyrae. I. Optical to Mid-infrared Phased Photometry

“…Recent efforts have combined temporally well-sampled, long-baseline photometric data sets in Galactic star clusters to explore these effects and find that most period changes ascribed to evolutionary effects are consistent with being gradual when visualized with semicontinuous sampling over very long timescales. Period changes attributed to nonevolutionary origins are also observed, and in contrast to evolutionary effects, these are identified as sudden nonlinear or chaotic evolution of the light curve over time (for example see Arellano Ferro et al 2016, for a century-long study of period changes in M5). In our sample, nine stars show the Blazhko effect from Smith (1995, 16%;their Table 5.2), an additional four stars show the Blazhko effect from the compilation of Skarka (2013, 7%;), and two stars show the Blazhko effect in Skarka (2014, 4%), for a total of 15 stars showing the Blazhko effect.…”