Mira Variables
Mira stars are the most spectacular type of long period variables due to their dramatic changes in brightness at visual wavelengths. The General Catalogue of Variable Stars defines Miras as stars with peak-to-peak V-band amplitudes larger than 2.5 magnitudes. The amplitudes of the Miras' light curves at longer wavelengths are much smaller. For example in the I-band (used by the OGLE survey), we adopted the boundary amplitude of 0.8 magnitudes to separate Miras from semiregular variables. However, the essential difference between these two classes of long-period variables lies in the number of excited pulsation modes. Mira stars are generally single-mode pulsators, oscillating solely in the fundamental mode, while semiregular variables usually exhibit two (or less often three) radial modes.
Mira variables are cool giant stars near the tip of the asymptotic giant branch. Their pulsation periods range from about 80 to over 1000 days. The amplitudes can be as large as 8-9 magnitudes in the V-band (4-5 magnitudes in the I-band). Miras' light curves do not repeat precisely from cycle to cycle, although they are usually much more stable than the light curves of semiregular variables. The pictures below show I-band light curves of several Mira stars obtained over a decade of the OGLE-IV observations. Left panels present unfolded time-series photometry, right panels show the same data points folded with the pulsation periods.
Mira variables can be divided into oxygen-rich and carbon-rich stars, depending on whether their surface C/O abundance ratios are lower or greater than 1, respectively. All the light curves presented above belong to the oxygen-rich giants. Carbon-rich Miras are very rare in the Milky Way, but they make up the majority of Mira population in the Magellanic Clouds, which can be attributed to the lower metallicity of stars in these galaxies. Carbon-rich Miras usually exhibit slow irregular variations of their mean brightness, probably caused by obscurations by the circumstellar dust shells made from the material ejected by the giant star. Several typical light curves of carbon-rich Mira stars in the Galactic bulge and Large Magellanic Cloud are displayed below.
A large fraction of Miras show smaller or larger humps (or, if you prefer, bumps) on the ascending branch of the light curve. This feature can be attributed to the 2-to-1 period resonance between the fundamental and the first-overtone mode. Sometimes the humps are so large that the light curves can be considered to have double maxima. In addition, cycle-to-cycle variations of the hump height are not uncommon. Several different variants of the folded light curves of Mira stars are showed in the pictures below.
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