This work establishes the most complete sample of red supergiants (RSGs) in 12 low-mass galaxies (WLM, IC 10, NGC 147, NGC 185, IC 1613, Leo A, Sextans B, Sextans A, NGC 6822, Pegasus Dwarf, SMC, and LMC) of the Local Group, which forms a solid basis to study the properties of RSGs as well as the star formation rate and initial mass function of the galaxies. After removing the foreground dwarf stars by their obvious branch in the near-infrared color–color diagram ( J − H 0 / H − K 0 ) with the UKIRT/WFCAM and 2MASS photometry as well as the Gaia/EDR3 measurements of proper motion and parallax, RSGs are identified from their location in the color–magnitude diagram J − K 0 / K 0 of the member stars of the specific galaxy. A total of 2190 RSGs are found in 10 dwarf galaxies, and additionally, 4823 and 2138 RSGs are found in LMC and SMC, respectively. The locations of the tip of the red giant branch in the J − K 0 / K 0 diagram are determined to serve as an indicator of the metallicity and distance modulus of the galaxies.

The aim of this paper is to establish a complete sample of red supergiants (RSGs) in M31 and M33. The member stars of the two galaxies are selected from the near-infrared (NIR) point sources after removing the foreground dwarfs from their obvious branch in the J − H/H − K diagram with the archival photometric data taken by the UKIRT/WFCAM. This separation by NIR colors of dwarfs from giants is confirmed by the optical/infrared color–color diagrams (r − z/z − H and B − V/V − R) and the Gaia measurement of parallax and proper motion. The RSGs are then identified by their outstanding location in the members’ J − K/K diagram due to high luminosity and low effective temperature. The resultant sample has 5498 and 3055 RSGs in M31 and M33 respectively, which should be complete because the lower limiting K magnitude of RSGs in both cases is brighter than the complete magnitude of the UKIRT photometry. Analysis of the control fields finds that the pollution rate in the RSG sample is less than 1%. The by-product is the complete sample of oxygen-rich asymptotic giant branch stars (AGBs), carbon-rich AGBs, thermally pulsing AGBs, and extreme AGBs. In addition, the tip-RGB is determined together with its implication on the distance modulus to M31 and M33.

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Mass loss is an important activity for red supergiants (RSGs) and can influence their evolution and final fate. Previous estimations of mass-loss rates (MLRs) of RSGs exhibit significant dispersion due to differences in method and the incompleteness of samples. With the improved quality and depth of surveys including the UKIRT/WFCAM observations in the near-infrared, and LGGS and PS1 in the optical, a rather complete sample of RSGs is identified in M31 and M33 according to their brightness and colors. For about 2000 objects in either galaxy from this largest ever sample, the MLR is derived by fitting the observational optical-to-mid-infrared spectral energy distribution with the DUSTY code of a 1D dust radiative transfer model. The average MLR 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 from RSGs 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. The MLRs are divided into three types by the dust species, i.e., amorphous silicate, amorphous carbon, and optically thin, and the relations between MLR and stellar parameters, infrared flux, and colors are discussed and compared with previous works for the silicate and carbon dust groups.

The infrared (IR) excess from OB stars is commonly considered to be a contribution from ionized stellar wind or circumstellar dust. With the newly published Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST)-OB catalog and Galactic O-Star Spectroscopic Survey data, this work steps further on understanding the IR excess of OB stars. Based on a forward-modeling approach comparing the spectral slope of observational spectral energy distributions and photospheric models, 1147 stars are found to have IR excess out of 7818 stars with good-quality photometric data. After removing the objects in the sightline of dark clouds, 532 (∼7%) B-type stars and 118 (∼23%) O-type stars are identified to be true OB stars with circumstellar IR excess emission. The ionized stellar wind model and the circumstellar dust model are adopted to explain the IR excess, and Bayes factors are computed to quantitatively compare the two. It is shown that the IR excess can be accounted for by the stellar wind for about 65% cases, of which 33% by free–free emission and 32% by synchrotron radiation. Other 30% sources could have and 4% should have a dust component or other mechanisms to explain the sharp increase in flux at λ > 10 μm. The parameters of the dust model indicate a large-scale circumstellar halo structure, which implies the origin of the dust from the birthplace of the OB stars. A statistical study suggests that the proportion with IR excess in OB stars increases with the stellar effective temperature and luminosity, and that there is no systematic change in the mechanism for IR emission with stellar parameters.