Infrared
J and
K photometry has been obtained of all red giants in the half
square degree region of the LMC bar searched for variables using the MACHO
database by Wood et al. (1999). The
K-
diagram shows the five
distinct period-luminosity sequences which are also evident in the MACHO
photometry. The
K-
plot allows identification of the Mira sequence
from among the five observed sequences. As shown in Wood et al. (1999),
comparison of observed periods, luminosities and period ratios with theoretical
models identifies Miras unambiguously as radial fundamental mode pulsators,
while semi-regular variables can be pulsating in the 1st, 2nd or 3rd overtone,
or even the fundamental mode. All these variables lie on just 3 of the 5
distinct sequences, and they all appear to be on the AGB. The nature of the
variability of stars on the other two sequences is currently unknown.
Possibilities include contact and semi-detached binaries, rotating stars with
giant star spots, episodic dust formation, rotationally stabilized non-radial
g- modes and strange pulsation modes caused by convection-pulsation
interaction. These possibilities are discussed.
binaries: close -- Magellanic Clouds -- stars: AGB and post-AGB --
stars: variables: other
