Properties of long -period variables from the MACHO Project

We use the eight-year light-curve database from the MACHO (MAssive Compact Halo Objects) Project, together with infrared colors and magnitudes from 2MASS and the Spitzer SAGE survey, to characterize long-period variables (LPVs) in the Large Magellanic Cloud. We examine the properties of both a period-selected sample of LPVs, and a color- and magnitude-selected sample of evolved stars. Both samples show the multiple period-luminosity sequences of LPVs that the MACHO Project discovered, as well as large numbers of stars with non-periodic variability. We have re-analyzed the light curves of the evolved stars with a Fourier method that is more sensitive to very low-amplitude variability, and find that three-quarters of our sample of evolved, variable stars have periodically varying light curves. We find that stars on the red giant branch often show non-periodic variability, but above a certain luminosity they are likely to pulsate with low amplitudes and relatively short periods. As these stars evolve to the top of the asymptotic giant branch (AGB), their periods become longer and stronger. We use the multi-periodic nature of these stars to identify period ratios that are characteristic of each period-luminosity sequence, allowing classification using only the star's light curve. These results are compared to our analysis of the MACHO Variable Star Catalogs for the Small Magellanic Cloud and Galactic Bulge, as well as AAVSO light curves for some well-known Galactic LPVs.;The J-[8.0] mid-infrared color is shown to be an indicator of mass loss for these stars, and depends on both the period and the amplitude of pulsation. Thus the greatest mass-loss rates are from the "extreme" AGB stars, which are analogues to Galactic Miras.;The long secondary periods present in half of our sample exhibit many features that are consistent with an origin due to binarity. However, because of the large fraction of these variables---all of which have periods in the relatively small range of 200 to 3000 days---binarity alone isn't the most likely explanation.