THE PROPERTIES OF LONG-PERIOD VARIABLES IN THE LARGE MAGELLANIC CLOUD FROM MACHO

Abstract

We present a new analysis of the long-period variables in the Large\nMagellanic Cloud (LMC) from the MACHO Variable Star Catalog. Three-quarters of\nour sample of evolved, variable stars have periodic light curves. We\ncharacterize the stars in our sample using the multiple periods found in their\nfrequency spectra. Additionally, we use single-epoch Two Micron All Sky Survey\nmeasurements to construct the average infrared light curves for different\ngroups of these stars. Comparison with evolutionary models shows that stars on\nthe red giant branch (RGB) or the early asymptotic giant branch (AGB) often\nshow non-periodic variability, but begin to pulsate with periods on the two\nshortest period-luminosity sequences (3 & 4) when they brighten to K_s ~ 13.\nThe stars on the thermally pulsing AGB are more likely to pulsate with longer\nperiods that lie on the next two P-L sequences (1 & 2), including the sequence\nassociated with the Miras in the LMC. The Petersen diagram and its variants\nshow that multi-periodic stars on each pair of these sequences (3 & 4, and 1 &\n2) typically pulsate with periods associated only with that pair. The periods\nin these multi-periodic stars become longer and stronger as the star evolves.\nWe further constrain the mechanism behind the long secondary periods (LSPs)\nseen in half of our sample, and find that there is a close match between the\nluminosity functions of the LSP stars and all of the stars in our sample, and\nthat these star's pulsation amplitudes are relatively wavelength independent.\nAlthough this is characteristic of stellar multiplicity, the large number of\nthese variables is problematic for that explanation.\n