Phase-dependent Photometric and Spectroscopic Characterization of the MASTER-Net Optical Transient J212444.87+321738.3: An Oxygen-rich Mira

We describe the time-dependent properties of a new spectroscopically confirmed Mira variable, which was discovered in 2013 as MASTER-Net Optical Transient J212444.87+321738.3 toward the Cygnus constellation. We have performed long-term optical/near-infrared (NIR) photometric and spectroscopic observations to characterize the object. From the optical/NIR light curves, we estimate a variability period of 465 +/- 30 days. The wavelength-dependent amplitudes of the observed light curves range from Delta I similar to 4 mag to Delta K similar to 1.5 mag. The (J - K) color index varies from 1.78 to 2.62 mag over phases. Interestingly, a phase lag of similar to 60 days between optical and NIR light curves is also seen, as in other Miras. Our optical/NIR spectra show molecular features of TiO, VO, CO, and strong water bands that are a typical signature of oxygen-rich Mira. We rule out S- or C-type as ZrO bands at 1.03 and 1.06 mu m and C-2 band at 1.77 mu m are absent. We estimate the effective temperature of the object from the Spectral Energy Distribution, and distance and luminosity from standard Period-Luminosity relations. The optical/NIR spectra display time-dependent atomic and molecular features (e.g., TiO, Na I, Ca I, H2O, CO), as commonly observed in Miras. Such spectroscopic observations are useful for studying pulsation variability in Miras.