The 3D Distribution of Long-period Mira Variables in the Galactic Disk*

Long-period Mira variable stars are considered to have relatively high initial masses and may be potentially useful as tracers of spiral arm structure of the Milky Way. From 2004 to 2017, we monitored long-period Mira candidates selected from the IRAS color-color diagram in the near-infrared K ' band. As an initial result of this study, we found 108 Mira variables and determined their periods, mean magnitudes, and amplitudes. Most of them are located between 0 degrees and 90 degrees in Galactic longitude. The peak of their period distribution is at around 500 days, which is longer than the typical value for Mira variables selected in optical surveys. Distances to our Mira variables have also been estimated using the period-luminosity relation (PLR) in 3.4 mu m with the help of a three-dimensional map of interstellar extinction. While the K-s-band PLR has a large scatter at longer periods (log P > 2.6), the PLR based on the Wide-field Infrared Survey Explorer 3.4 mu m data has a much smaller scatter. We compare the spatial distribution of our sample to the spiral arms in the literature, and discuss the possible association of the long-period Mira variables with the spiral arms although the limited spatial coverage and the limited distance accuracy of the current sample prevent us from drawing a firm conclusion.