Phase Lags in the Optical-Infrared Light Curves of Asymptotic Giant Branch Stars

To search for phase lags in the optical-infrared light curves of asymptoticgiant branch stars, we have compared infrared data from the COBE DIRBEsatellite with optical light curves from the AAVSO and other sources. We found17 examples of phase lags in the time of maximum in the infrared vs. that inthe optical, and 4 stars with no observed lags. There is a clear differencebetween the Mira variables and the semi-regulars in the sample, with themaximum in the optical preceding that in the near-infrared in the Miras, whilein most of the semi-regulars no lags are observed. Comparison to publishedtheoretical models indicates that the phase lags in the Miras are due to strongtitanium oxide absorption in the visual at stellar maximum, and suggests thatMiras pulsate in the fundamental mode, while at least some semi-regulars arefirst overtone pulsators. There is a clear optical-near-infrared phase lag inthe carbon-rich Mira V CrB; this is likely due to C2 and CN absorptionvariations in the optical.Comment: AJ, in pres