Jing Wen

Asymptotic giant branch stars (AGBs) and red supergiant stars (RSGs) exhibit significant mass-loss phenomena and are considered important sources of interstellar dust. In this work, we employed a uniform method of spectral energy distribution fitting to analyze a large, and hence statistically significant, sample of approximately 40,000 RSGs and AGBs in the Magellanic Clouds (MCs), providing a new catalog of evolved stars that includes stellar parameters and dust properties. Our results reveal that the total dust-production rate (DPR) of the Large Magellanic Cloud is approximately 9.69 × 10⁻⁶ M _☉ yr⁻¹, while it is around 1.75 × 10⁻⁶ M _☉ yr⁻¹ for the Small Magellanic Cloud, with a few stars significantly contributing to the total DPR. No significant differences were observed in the contributions to DPR from carbon-rich and oxygen-rich (O-rich) evolved stars in the MCs. We explored the relations between stellar parameters (luminosity, infrared color, period, amplitude) and mass-loss rate (MLR) for evolved stars. A prominent turning point at log ( L / L ☉ ) ≈ 4.4 appears in the luminosity–MLR diagram of RSGs, potentially related to their mass-loss mechanism. The luminosity–MLR relation of AGBs is highly scattered. The DPR of AGBs shows a clear change with pulsation period and amplitude, with DPR exhibiting a drastic increase at pulsation periods of approximately 300 days and I-band amplitudes greater than 0.5 mag. Metallicity has some impact on the DPR of O-rich stars, with lower metallicity seeming to result in lower mean DPR and a higher proportion of optically thin stars.

The precise definition of the lower mass limit of red supergiant stars (RSGs) is an open question in astrophysics and does not attract much attention. Here, we assemble a spectroscopic evolved cool star sample with 6602 targets, including RSGs, asymptotic giant branch stars, and red giant branch stars, in the Large Magellanic Cloud based on Gaia DR3 and Sloan Digital Sky Survey IV/APOGEE-2. The reference spectrum of each stellar population is built according to the quantile range of relative intensity (1% ∼ 99%). Five different methods, e.g., χ ², cosine similarity, machine learning (ML), equivalent width, and line ratio, are used in order to separate different stellar populations. ML and χ ² provide the best and relatively consistent prediction of a certain population. The derived lower limit of the RSG population is able to reach the K _s -band tip of the red giant branch (K _s ≈12.0 mag), indicating a luminosity as low as about 10^3.5 L _⊙, which corresponds to a stellar radius of only about 100 R _⊙. Given the mass–luminosity relation of L / L ⊙ = f ( M / M ⊙ ) 3 with f ≈ 15.5 ± 3 and taking into account the mass loss of faint RSGs up to now, the minimal initial mass of the RSG population would be about 6.1 ± 0.4 M _⊙, which is much lower than the traditional threshold of 8 M _⊙ for the massive stars. This is the first spectroscopic evidence, indicating that the lower mass limit of the RSG population is around 6 M _⊙. However, the destinies of such faint RSGs are still elusive and may have a large impact on stellar evolutionary and supernova models.

Mass loss is a crucial process that affects the observational properties, evolution path, and fate of highly evolved stars. However, the mechanism of mass loss is still unclear, and the mass-loss rate (MLR) of red supergiant stars (RSGs) requires further research and precise evaluation. To address this, we utilized an updated and complete sample of RSGs in the Large Magellanic Cloud (LMC) and employed the 2-DUST radiation transfer model and spectral energy distribution fitting approach to determine the dust-production rates (DPRs) and dust properties of the RSGs. We have fitted 4714 selected RSGs with over 100,000 theoretical templates of evolved stars. Our results show that the DPR range of RSGs in the LMC is 10⁻¹¹ M _⊙ yr⁻¹–10⁻⁷ M _⊙ yr⁻¹, and the total DPR of all RSGs is 1.14 ×10⁻⁶ M _⊙ yr⁻¹. We find that 63.3% RSGs are oxygen-rich, and they account for 97.2% of the total DPR. The optically thin RSG, which comprise 30.6% of our sample, contribute only 0.1% of the total DPR, while carbon-rich RSGs (6.1%) produce 2.7% of the total DPR. Overall, 208 RSGs contributed 76.6% of the total DPR. We have established a new relationship between the MLR and luminosity of RSGs in the LMC, which exhibits a positive trend and a clear turning point at log L / L ⊙ ≈ 4.4 .