The Period-Luminosity Relation of Red Supergiant Stars in the Small Magellanic Cloud

The characteristics of light variation of red supergiant (RSG) stars in the Small Magellanic Cloud (SMC) are analyzed based on the nearly 8–10 year data collected by the ASAS and MACHO projects. The 126 identified RSGs are classified into five categories accordingly: 20 with poor photometry, 55 with no reliable period, 6 with semi-regular variation, 15 with a long secondary period (LSP) and distinguishable short period, and 30 with only an LSP. For the semi-regular variables and the LSP variables with distinguishable short period, the K S -band period–luminosity ( P – L ) relation is analyzed and compared with that of the Galaxy, the Large Magellanic Cloud, and M33. It is found that the RSGs in these galaxies obey a similar P – L relation except for those in the Galaxy. In addition, the P – L relations in the infrared bands, namely, the 2MASS JHK S , Spitzer /IRAC, and Spitzer /MIPS 24 μm bands, are derived with high reliability. The best P – L relation occurs in the Spitzer /IRAC [3.6] and [4.5] bands. Based on the comparison with the theoretical calculation of the P – L relation, the mode of pulsation of RSGs in the SMC is suggested to be the first-overtone radial mode.