The sample and mass loss rate of red supergiants in M31 and M33
Red supergiants (RSGs) are important contributors to the interstellar medium through massive winds. Due to serious interstellar extinction, the complete sample of RSGs is extremely difficult to obtain in our Galaxy. As two of the largest spiral galaxies in the Local Group, M31 and M33 can serve as a reference to analyze the global contribution of RSGs to the galactic medium. By using evident branching of red dwarfs and red (super)giants in the near-infrared color-color (J-H/H-K) diagram, the foreground dwarf stars are excluded efficiently with the UKIRT observational data, The most complete and pure sample of RSGs in M31 and M33, consisting of 5498 and 3055 RSGs respectively, are obtained from the color-magnitude ( J-K/K) diagram of the member stars. The dust radiative transfer model from the DUSTY code is run to calculate the mass loss rate of RSGs. The average mass loss rate of RSGs is found to be around 2.0 × 10^-5 M⊙ yr^-1 with a gas-to-dust ratio of 100, which yields a total contribution to the interstellar dust of about 1.1 × 10^-3 M⊙ yr^-1 in M31 and 6.0 × 10^-4 M⊙ yr^-1 in M33, a non-negligible source in comparison with evolved low-mass stars.