This project will analyze brightness data for luminous, cool red giants of the Mira type. These stars show complicated patterns of low-frequency variability, and have been the target of long-term study by the professional and amateur community. Dr. Templeton will assemble archival and ongoing observations of several hundred Mira stars to explore empirically the frequencies of pulsation and whether these correlate with other properties of the stars such as temperature or luminosity.
The project is expected to contribute to public outreach through collaborations between amateur and professional astronomers. The data for this project will be made available to the community, especially to provide input to future numerical modeling of Mira pulsations.
Mira variables are very evolved stars nearing the ends of their lives. They are among the most prominent and longest-observed variable stars in our skies, and both amateur and professional astronomers have observed some individual Mira variables for well over a century. Their variations are primarily due to pulsation -- the surface of the star expanding and contracting in a spherically symmetric way -- and for the most part this variation is very regular, well-behaved and well-understood. But there are subtle changes that occur in addition to pulsation over time that aren't understood. Our research was designed to quantify and understand this "noise". Our project involved examining hundreds of "light curves" -- records of how bright a star was at a moment in time -- using various statistical methods that helped us understand what was happening over time. Our hypothesis was that the irregular behaviors of the star correlated in some way to other fundamental properties such as pulsation period, spectral type, or some other parameter. We found few clear correlations between the variations and these properties, with two exceptions: we see more irregularity in stars with larger pulsation amplitudes, and we find a modest correlation between the pulsation period and the timescales above which the low-frequency noise changes in character. We presented many of the results in two conference proceedings during the span of the award, and are preparing a paper for publication on the final discovery now that the project has ended. Our work has not determined the physical causes of why Mira variables show this irregularity. Our underlying hypothesis is that the irregularity is caused by the presence of large-scale convection and supergranulation in these stars, a much larger-scale manifestation of convective granulation seen on the Sun and other stars. Our results have not established a physical cause, but if the timescales for the noise in Mira variables correlate with the pulsation period -- which is determined by the radius of the star -- this may suggest that the size of the star constrains the character and timescale of the noise. This would be entirely consistent with theories of supergranulation. As part of our research work, we re-examined the historical data archives of the AAVSO with an eye toward looking for (and in some cases flagging) errors and other obvious problems with our data sets. AAVSO staff validated hundreds of Mira and semiregular variable light curves from our archives, and these newly-examined data sets are all freely available from the AAVSO website ( www.aavso.org/data-download ). We are also preparing a set of educational exercises for middle- and high-school classrooms that explore the concepts of stellar variability, light curve modeling and analysis. These exercises are currently in development, and are being tested and evaluated by project collaborators in a classroom setting.
